C:\Users\gnuwi\AppData\Local\Temp\msoBBB1.tmp 1 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2023 Utah Geological Association Publication 51 ABSTRACT The elevation of Great Salt Lake has fallen to historic lows in recent years, exposing once submerged mi- crobialites along the lake’s shores. Although prior studies have attempted to map microbialite locations, this has proved challenging, with mapped microbialite areas limited to accessible shoreline locations or via indi- rect sonographic evidence. Meanwhile, the importance of Great Salt Lake’s microbialites to the lake’s food chain has made quantifying the extent of microbialites exposed versus submerged at different lake elevations critical to lake management decisions. Low lake levels combined with seasonal high-water clarity have ena- bled microbialite reefs to be spotted in aerial and satellite imagery, even in deeper areas of the lake. In this study, satellite images were used to identify and map microbialite reef areas in Great Salt Lake and along its dry shores. In the south arm, submerged microbialites were easily recognized as dark green reefs against a light-colored benthic background (primarily ooid sand). Stationary microbialite mounds were distinguished from rip-up clasts or other dark-colored mobile material by comparing potential microbialite regions across several high-visibility timepoints. In this way, we identified 649 km2 (251 mi2) of putative microbialite reef area: 288 km2 (111 mi2) in the north arm, 360 km2 (139 mi2) in the south arm, of which 375 km2 (145 mi2) was mapped at a high degree of confidence. We also produced geospatial shapefiles of these areas. This map, combined with currently available lake bathymetric data, permits the estimation of the extent of microbialite reef exposed vs. submerged in various parts of the lake at different lake elevations. At the end of fall 2022, when lake level dipped to 1276.7 masl (4188.5 ft-asl) in elevation, we estimate that ~40% of the south arm microbialite reef area was exposed. Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake, Utah: Implications for Microbialite Exposure Laura Wilcock1,2, Carie M. Frantz1, and Michael D. Vanden Berg3 1Department of Earth & Environmental Sciences, Weber State University, Ogden, Utah; Laura.wilcock@utah.edu 2Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 3Energy & Minerals Program, Utah Geological Survey, Salt Lake City, Utah INTRODUCTION Microbialites cover substantial portions of the Great Salt Lake benthos, and host microbial commu- nities are believed to be important to the Great Salt Lake ecosystem. Models of the lake’s ecosystem, therefore, must necessarily incorporate estimates of microbialite extent (Belovsky and others, 2011; Bar- rett, 2020), which need refining, particularly in the face of recent lake level decline and microbialite ex- posure. Microbialites in Modern Great Salt Lake Great Salt Lake is the largest saline lake in the western hemisphere. Unlike other terminal lakes in the Basin and Range of the western United States, which tend to be alkaline, Great Salt Lake is a Na-Mg -Cl-SO4-dominated system with relatively low levels of alkalinity (Domagalski and others, 1989; Jones and others, 2009). High rates of Ca2+ and HCO3 - delivery, slightly alkaline surface waters, the lake’s hypersalin- ity (which promotes CO2 degassing), and high levels of microbial activity produce conditions that approach or exceed aragonite saturation in much of the lake, despite relatively low lake water concentrations of Ca2+ and CO3 - (Pace and others, 2016; Ingalls and others, 2020; Bouton and others, 2020). These factors have made Great Salt Lake (as well as its predeces- sors) a “carbonate factory,” with carbonates making up a major portion of lake sediments, especially since the draining of Pleistocene Lake Bonneville (Jones and others, 2009; Vennin and others, 2019). Car- bonate deposits blanket the modern bed of the lake, and include organic-rich carbonate mud, oolitic sand, and microbialite reefs (Eardley, 1938; Chidsey and others, 2015; Vanden Berg, 2019; Ingalls and others, 2020; Bouton and others, 2020; Baskin and others, 2022). Microbialites are “organosedimentary deposits formed from interaction between benthic microbial communities…and detrital or chemical sedi- ment” (Burne and Moore, 1987). They are typically formed by processes of trapping and binding by mi- crobial mats (for example, Frantz and others, 2015), 10.31711/ugap.v51i.136 2 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake induction of mineral precipitation via metabolic activ- ities of microbial communities (for example, Dupraz and others, 2009), and/or inorganic calcification (for example, Shen and others, 2022). An aside on termi- nology: the term “bioherm,” ostensibly coined by Cumings and Shrock (1928), broadly refers to any reeflike mound built by living organisms. “Microbialite,” meanwhile, refers to a sedimentary rock built at least in part by the activities of microor- ganisms (Burne and Moore, 1987). Thus, “microbialite reef” is subtly different from “bioherm,” indicating that microorganisms are in- volved in the construction of the reefs, but also ac- knowledging potential abiogenic contributions. Great Salt Lake’s microbialites were first docu- mented by Eardley (1938) in his seminal tome de- scribing the lake’s chemistry and sediments, describ- ing in detail the ''extensive calcareous bioherms'' that were visible during a period of relatively low lake el- evation in the mid-1930s (Figure 1). He noted their dense mats (periphyton), dominated by the cyanobac- terium Aphanothece packardii (now identified as Eu- Figure 1. A) Modern Great Salt Lake south arm surface elevations as measured at USGS water monitoring locations 1001000 and 10010024. Green shaded areas indicate 1 ft elevation bands below 4200 ft-asl where microbialites were mapped (this study), with shade indicating the total percentage of microbialites that would be submerged at that lake ele- vation. B) Lake Bonneville-Great Salt Lake hydrograph (black line) showing ages and elevations of dated microbialite materials from Bouton and others, 2016a (light green circles) and Newell and others, 2017 (dark green circles). Hydro- graph prior to 13 ka modeled after Oviatt, 2015. Hydrograph after 13 ka modeled after Oviatt and others, 2021, with the dark gray horizontal bar indicating the uncertainty in lake elevation during the Great Salt Lake phase. 3 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 halothece spp.; Lindsay and others, 2019; Frantz and others, 2023), and attributed their formation to micro- bially-mediated carbonate precipitation. Even lower lake elevations in the early 1960s af- forded a second look at the lake’s microbialites. Ca- rozzi (1962) examined their morphological variability and spatial distribution and linked their occurrence to underlying topographic highs. Halley (1976) de- scribed in detail the high variability in their internal structure, with laminated and unlaminated microfab- rics existing within different portions of single micro- bialites (hence, “microbialite” vs. a more descriptive term such as thrombolite or stromatolites). He also noted a general lack of relationship between the living periphyton and observed calcified microstructure and microfossils, notably remarking that, “the organisms on the surface of the Great Salt Lake algal mounds are probably not those which are responsible for the internal structure.” By the late 1960s, the lake’s microbialites were once again submerged by a rise in lake level and all but forgotten until they reappeared in the early 2010s during the period of prolonged lake level fall after the 1986–1987 lake highstand. This ushered in a new era of Great Salt Lake microbialite research in which the microbialites were investigated as contributors to the lake ecosystem (Wurtsbaugh, 2009; Belovsky and others, 2011; Wurtsbaugh and others, 2011) and as geobiologic curiosities (Pedone and Folk, 1996; Baskin, 2014; Pace and others, 2016; Lindsay and others, 2017). Interest in the structures was further en- hanced by the discovery of the microbialite- associated pre-salt petroleum deposits of offshore Brazil in the mid-2000s, with interest in Great Salt Lake as a potential modern analog environment (Chidsey and others, 2015; Vanden Berg, 2019). Re- cent studies utilized new techniques and technology, including advanced microscopy (Pace and others, 2016), molecular biology (Lindsay and others, 2017), geospatial and marine acoustic technology (Baskin, 2014; Baskin and others, 2022), and drone imagery (Vanden Berg, 2019). While the bulk of academic focus on the lake’s microbialites (including that of this paper) has been on the extensive reefs that are submerged during “normal” levels of the modern lake, i.e., those below about 1280 meters above sea level (masl; 4200 feet above sea level, or ft-asl), microbialites and other pu- tative microbial carbonates are also found in discrete locations at higher elevations, associated with earlier phases of the lake system (Chidsey and others, 2015; Vennin and others, 2019; Homewood and others, 2022). However, in the remainder of this paper, we use “microbialites” to refer only to the reef-forming deposits below 1280 masl (4200 ft-asl) in Great Salt Lake and its recently exposed shores. The mega- and macrostructure (Shapiro, 2000) of Great Salt Lake’s microbialites includes roughly cir- cular domes ranging in size from ~15–300 cm in di- ameter, rings of the same scale with collapsed interi- ors, linear ridges up to several meters long, and mounds that outline the cracks of 30–75 m desicca- tion polygons at the lake margin (Vanden Berg, 2019) (Figure 2). The morphological diversity of the micro- bialites is presumably influenced by physical factors including substrate, bathymetry, tectonics, and hydro- dynamics. Correlations between these physical factors and microbialite growth suggest that microbialites tend to grow on underlying raised substrate (Eardley, 1938; Chidsey and others, 2015; Bouton and others, 2016b; Bouton and others, 2016a; Vennin and others, 2019; Vanden Berg, 2019; Kanik and others, 2020; Baskin and others, 2022). At the mesoscale, the inte- rior composition of the microbialites includes primar- ily clotted aragonite (posited to be of direct microbial origin (Pace et al, 2016; Vanden Berg, 2019), as well as trapped and cemented ooids, Artemia (brine shrimp) pellets, and some allochthonous grains (Chidsey and others, 2015). Many microbialites also include poorly-defined, laminated stromatolitic fab- rics as a minor interior component. Thus, the term mi- crobialite since the structures comprise a mix of fab- ric types, instead of using more specific terms such as stromatolite, thrombolite, or leolite. Radiocarbon (14C) dating of both solid carbonate and trapped organic material has yielded ages for mi- crobialite material of 12.7–2.7 ka (Figure 1A) (Bou- ton and others, 2016b; Bouton and others, 2016a; Newell and others, 2017). The reservoir effect in the modern lake appears to be on the order of several hundred years (Bowen and others, 2019; Paradis and others, 2023), however, it may have been greater in the past (Bowen and others, 2019), and carbonate for- mation in close association with groundwater may in- corporate a reservoir effect of over 5000 years (Homewood and others, 2022). Thus, there is a rather high degree of uncertainty in microbialite radiocarbon ages. Notwithstanding, to date, no modern ages have been measured from microbialite material, although dating is limited to only six microbialites from two locations at the northwest shore of Antelope Island, and none targeted periphyton-rich outer zones where modern carbonate precipitation appears to be happen- ing (for example, Pace and others, 2016). It also ap- pears that microbialites form over thousands of years, with a range from 7.6–12.7 cal ka measured from or- ganic material extracted from four zones within a sin- gle microbialite (Newell and others, 2017). This co- vers a period when the surface elevation of Great Salt Lake is poorly constrained within a rough range of L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake 4 Figure 2. Photographs of microbialites in and around Great Salt Lake. (A–B) Microbialites that grew at the boundaries of desiccation polygons at Promontory Point, north arm. Note the bright/light surface color (photosynthetic microbial mats are absent) of partially submerged microbialites in halite-saturated north arm water. (C–D) Microbialite reef at Ladyfinger Point on Antelope Island, showing transition from living periphyton to desiccated bright forms, (D) healthy mat and brine fly pupae visible on the surface of a collected microbialite sample; sample is roughly 14 cm across. (E–F) Microbialites at Bridger Bay off Ante- lope Island, showing (F) collapsed centers; area shown is roughly 1 m across. (G–H) Microbialite reef at Buffalo Point on Antelope Island, showing both ex- posed and partially-eroded structures, as well as (H) submerged structures with a dark, photosynthetic periphyton; area shown is roughly 0.8 m across. (I–J) Large and elongate microbialites off of Stansbury Island, with thrombolitic crust. Partially eroded crust visible in (J); area shown is roughly 1 m across. Loca- tions where each set of photographs were taken are shown as markers on the map in Fig. 8. 5 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 1271–1285 masl (4170–4216 ft-asl) (Oviatt and oth- ers, 2021) (Figure 1A). Regardless of their age and origin, microbialites play an important role in the modern Great Salt Lake. The exposure of vast expanses of microbialites with historically low lake elevation levels is threatening their preservation and keystone function in the Great Salt Lake ecosystem. Great Salt Lake comprises dis- tinct habitat types ranging from fresh– to brackish- water estuaries and wetlands where rivers enter the lake, to expansive mudflats and playas, to the hyper- saline open water of Gunnison Bay (the north arm) and the south arm of Great Salt Lake. Great Salt Lake has historically supported a simple but hemispherical- ly important ecosystem (Figure 3). Ten million birds rely on the lake, including 90% of the world’s Eared Grebes (Podiceps nigricollis), two species of Phala- ropes (Phalaropus lobatus and Phalaropus tricolor), and large nesting colonies of American White Peli- cans (Pelecanus erythrorhynchos) and California Gulls (Larus californicus) (Conover and Bell, 2020). The lake also supports an economically important brine shrimp cyst-harvesting industry, which supports global aquaculture (Marden and others, 2020). Great Salt Lake’s microbialites are a critical feature that supports this extreme ecosystem. Microbialites, the lithified structures, are distinct from microbialite pe- riphyton communities, which, in Great Salt Lake, are robust, productive, and diverse microbial communi- ties that blanket microbialite surfaces (Pace and oth- ers, 2016; Lindsay and others, 2017; Kanik and oth- ers, 2020; Ingalls and others, 2020). Microbialite pe- riphyton communities are conservatively estimated to be responsible for 30% of the lake’s primary produc- tivity (Wurtsbaugh and others, 2011; Anderson and others, 2020; unpublished data by B. Baxter and oth- ers, 2023), the remainder is attributed to planktonic algae. The significance of microbialites is as an- chored, solid substrates with substantial relief above the surrounding sediment in the Great Salt Lake ben- thos, providing islands of stability in otherwise mo- bile sediment where robust mats of photosynthetic microbes can develop. Microbialites can contribute biomass to pelagic zones via sloughing, wave action, Figure 3. A simplified Great Salt Lake food web illustrating the im- portance of the lake’s microbialites and associated periphyton, which feed brine fly larvae and the occasional brine shrimp, which in turn feed higher trophic levels of the ecosystem. Modified from Frantz and others (2023) (Licensed under CC4.0 and used with permission). 6 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake and/or bioturbation (MacIntyre and Melack, 1995; Barrett, 2020; Marden and others, 2020). Brine shrimp (Artemia franciscana) are filter feeders that prefer pelagic microalgae for nutrition, however, they will also graze on microbialite periphyton in shallow waters (Caudell and Conover, 2006; Lindsay and oth- ers, 2019; Brown and others, 2022). Indeed, stable isotope and gut content DNA evidence suggests that brine shrimp feed on microbialite surface communi- ties during summer months (Barrett, 2020; Marden and others, 2020), presumably because the shrimp re- duce the planktonic phytoplankton concentrations be- low the level at which they can efficiently feed (Be- lovsky and others, 2011), necessitating a supplemen- tary food source. Microbialites are also a critical part of the brine fly (Ephydra spp.) lifecycle, which depend on micro- bialites for habitat and food (Collins, 1980; Caudell and Conover, 2006; Belovsky and others, 2011; Wurtsbaugh and others, 2011; Conover and Bell, 2020; Brown and others, 2022), and are a critical nu- tritional source for both shorebirds and pelagic birds at Great Salt Lake (Conover and Bell, 2020; Sorensen and others, 2020). The overwhelming majority of brine flies appear to pupate on submerged micro- bialites (Collins, 1980; Wurtsbaugh, 2009), again, be- cause they offer a stable benthic substrate. Hatched brine fly larvae then feed primarily on microbialite periphyton communities (Collins, 1980; Barrett, 2020). In shore areas where submerged microbialites are nearby and salinity levels do not exceed 20% (which may be an upper survival limit for micro- bialite primary producers; Lindsay and others, 2019), the dense clouds of hatched brine flies in late summer are remarkable; walking through a microbialite reef disturbs innumerable thousands of flies that rise from the surface of microbialites and ponded water in swarms. Lake ecosystem models (for example, those de- scribed by Belovsky and others, 2011; Barrett, 2020) require accurate estimates of microbialite extent and relationships between lake elevation and the propor- tion of submerged vs. exposed microbialites. Lake Level Fall and Exposure of the Lake’s Microbialites Great Salt Lake elevation levels have dropped to historic lows in recent years, the result of mega- drought and overuse of water in the upstream water- shed (Null and Wurtsbaugh, 2020), with profound consequences to the lake ecosystem. Avian nesting grounds that were previously protected from preda- tion as islands have become connected to outer lake shores, disrupting bird populations (Kijowski and oth- ers, 2020; Sorensen and others, 2020). Increases in lake salinity have produced conditions that exceed levels at which keystone members of the ecosystem optimally survive and reproduce (Baxter and Butler, 2020; Great Salt Lake Salinity Advisory Committee, 2021). In addition, low elevation and consequent shoreline shift has exposed hundreds of kilometers of microbialite reefs, subjecting them—and their ecolog- ically-important periphyton communities—to desic- cation, negating their ecosystem function. Recent work by Frantz and others (2023) provid- ed some hope in the face of current mass microbialite exposure, showing that exposed and desiccated mi- crobialites can regain some of their periphyton com- munity in relatively short order once re-submerged in healthy lake water. However, their study was limited to a brief period of recovery, well before thick, car- bonate-rich mats began to reappear (which could take years to decades). Their results also indicated that re- covery is limited as lake level continues to fall and sa- linity continues to rise. In addition, they noted results that hint that individual microbialite areas harbor dis- tinct strains of Euhalothece, the primary microbialite phototroph; losing areas of reef may therefore disrupt natural microbial diversity and could make the lake’s microbialite-supported ecosystem less resilient to fu- ture change. Furthermore, they showed that subaerial- ly exposed microbialites are rapidly weathered. Ex- tended periods of exposure could reduce the height of microbialite reefs (and raise the surrounding sedi- ment), diminishing their value as habitat for periphy- ton and brine fly larvae, even if lake levels rebound. Mapping Great Salt Lake’s Microbialites The current threat to the lake’s microbialites with lake level fall, and consequent long-term impacts on the lake ecosystem, mean that management of Great Salt Lake and its watershed requires a quantitative un- derstanding of how different lake elevations affect microbialite exposure. This in turn depends on accu- rate maps of microbialite reef extent in Great Salt Lake, as well as refined relationships between lake bathymetry and microbialite exposure. Additionally, low lake levels and the exposure of the lake’s micro- bialites has presented new hazards for navigation of watercraft on the lake. Accurate mapping of micro- bialite extent also has scientific value, as illustrated by several recent publications that have linked micro- bialite locations and extent to topographic features, faults, tectonics, wave energy, depth bands, and groundwater availability (Bouton and others, 2016b; Bouton and others, 2016a; Vanden Berg, 2019; Baskin and others, 2022). 7 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 The first map of microbialite extent was from Eardley (1938), who took advantage of a period of relatively low lake level in the mid-1930s to map them roughly from shore, as well as observing them at depths up to 1 m during “considerable travel” via a boat, the appropriately named Hydrographer, near the shores of the lake and in transects between the lake’s islands. His paper includes both site and aerial photo- graphs at various locations around the lake shore. Im- portantly, he also noted that cores from previously conducted engineering studies indicated prior periods of microbialite formation in the lake in areas different from where he had observed them. He used a planim- eter and his map to determine a rough microbialite reef area of 398 km2 (154 mi2) within the lake (Figure 4). Due to limited mapping technology and limited field observations, Eardley’s map largely missed mi- crobialite reefs on the western side of the lake, as well as deeper-water areas, whereas extents on the east side of the north arm are overestimated. Overall, Eardley underestimated the extent of Great Salt Lake microbialites. For his 2014 Ph.D. dissertation, Baskin (Baskin, 2014) produced the first major update to Eardley’s map, utilizing single-beam sound-velocity soundings obtained during his work producing digital bathymet- ric surveys of the lake with the United States Geolog- ical Survey (USGS) (Baskin and Allen, 2005; Baskin and Turner, 2006). His method for identifying micro- bialites involved a calculation of rugosity from the sounding data that was truthed in select high-rugosity areas using dual-frequency 2D side scan sonar, swept- frequency Chirp sub bottom profiles, and videogra- phy (when lake visibility permitted), as well as in situ sampling in known microbialite locations. The ex- tents identified in his dissertation were then updated and refined with the publication of Baskin and others (2022). This newer publication identified an area of ~1000 km2 (~390 mi2) of putative microbialite reef, with >700 km2 (270 mi2) in the south arm and >300 km2 (~120 mi2) in the north arm (Figure 4), nearly tri- pling the extent mapped by Eardley (1938). In his the- sis, Baskin also noted the effect of the railroad cause- way, completed in 1959, that bisected the lake and cut off the north arm from most of the lake’s freshwater input, causing it to become rapidly salt-saturated and killing off the Euhalothece-based periphyton on north arm microbialites (this was also noted by Post, 1977, and verified with DNA sequencing by Lindsay and others, 2017). Although extensive, Baskin’s map was largely based on indirect data; due to time and re- source constraints he was only able to verify the pres- ence of benthic microbialites in limited areas of his reported mapped extent. Vanden Berg (2019) produced an alternative map of microbialite extent using Google Earth imagery and limited field mapping, yielding a microbialite reef aerial extent of 680 km2. However, the map and ex- tent estimates were limited by the availability of clear -water imagery and stated the need for further field verification. Bouton and others (2020) further amended micro- bialite extent estimates by merging the Eardley (1938) and Baskin (2014) maps and adding additional refinement based on limited remote imagery of west- ern Antelope Island from Bouton and others (2016a), yielding an expanded (and overestimated) micro- bialite reef aerial extent of 1261 km2 (487 mi2). In sum, maps of microbialite reef extent in the literature to date have given conflicting and highly variable re- sults (Figure 4). Recent low lake elevations and increasing resolu- tion of satellite and aerial imagery have made micro- bialite mapping via remote imaging more powerful and accurate than ever before. Water column visibil- ity in the lake varies greatly with season, biological activity, and weather, however, during clear-water pe- riods the Secchi disk depth typically exceeds 3 m (10 ft), making the lake bottom visible from aerial view in all but the deepest portions of the lake (Belovsky and others, 2011). Microbialites are visible to depths in excess of 4 m (13 ft) in some high-visibility images, a fact that several studies have utilized to identify ex- tents of microbialites against the lake bed (Bouton and others, 2016a; Vanden Berg, 2019). Advantages of using remote imagery over field-based mapping in- clude the ability to quickly map large regions across the full extent of the lake (vs. transects or areas only accessible from shore), and that dry, shallow-water, and deep microbialites can all be mapped using the same method. The varied estimates of microbialite extent from prior literature (Table 1) adds a large element of un- certainty to estimates of overall microbialite produc- tivity, microbialite exposure, and other factors influ- encing the management of Great Salt Lake. Thus, our study attempted to improve on previous estimates by (1) mapping microbialites using satellite imagery, taking advantage of historic low lake level and im- proved spatial and temporal resolution of available images, (2) confirming (or refuting) the presence of suspected microbialite areas from prior mapping ef- forts via aerial imagery and field checks, and (3) gen- erating shapefiles of microbialite reef extent that can be used in quantitative estimates of microbialite ex- tent and exposure. Here, we present our results, which include the most detailed map of Great Salt Lake mi- crobialite extent to date and a model of microbialite exposure at different lake elevations. L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake 4 Figure 4. Comparison of previously published microbialite reef extent maps for Great Salt Lake with our mapped reef extent. (A) Microbialite reef areas mapped by Eardley (1938; in purple), Baskin and others (2022; in blue), and this study (yellow), highlighting areas of overlap and major differences. (B) Quan- tified comparison of mapped reef areas in the three studies. Darker vs. lighter colors in the plot for this study indicate regions of high vs. low confidence. 9 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 METHODS Mapping Microbialites Using Satellite Imagery Data Acquisition Positive identification of microbialites through the application of remote sensing required high- resolution imagery with sufficient temporal resolution to permit analysis of areas of interest during favorable periods (i.e., periods without obscuring cloud cover, with low lake elevations, and with good water clari- ty). Imagery was collected through Esri’s World Im- agery Wayback (EWIW) archive. EWIW is a digital archive of published world imagery since 2014 that is stored as layer files that can be downloaded or viewed online through ArcGIS’s living atlas. The current ex- tent of the Great Salt Lake covers over 4000 km2 within the Great Salt Lake basin (within the quad 40.6 –41.8°N, 111.8–113.2°W). Imagery for the region is collected via multiple satellite constellations at differ- ent temporal sequences that are location-dependent. North and south arms of Great Salt Lake required sets of time series imagery that often come from different capture dates (Table 2). EWIW acquires imagery via Landsat, USDA NAIP, TerraColor, Digital Globe, GeoEye IKONOS and AeroGRID at 0.6–15 m spatial resolution depending on location and provider. Dates were selected to provide optimal below- water visibility, with favorable atmospheric condi- tions (especially low cloud cover), clear water periods (during the absence of water turbulence or algal blooms), and relatively low lake levels (permitting visibility in deeper areas of the lake), allowing good visual records of changing microbialite reef exposure (Figure 5). The analysis over multiple time points was vital for distinguishing loose debris from true reef, as illustrated in Figure 6. Google Earth Pro (GEP) was also utilized to compare and contrast visible reef zones with EWIW imagery. GEP utilizes Landsat and Copernicus satellite constellations for imagery collec- tion. Dates of available archival GEP imagery vary; imagery from 2016–2022 provided the best clarity for positive or negative identification of microbialites. Imagery in GEP varies based on location and scale, with each view of lake locations utilizing several re- mote sensing sources and acquisition dates. High-resolution historical imagery was collected from EWIW and downloaded as layer files. Once im- ported into ArcGIS Pro, each layer file was used for side-by-side comparison of microbialite structures. This side-by-side analysis of archived EWIW and GEP imagery was used to digitize areas that could be positively identified as reef zones via remote sensing. Identification and Mapping To develop criteria for microbialite reef identifi- cation, we first compared characteristics of known reef zones (from field studies by the authors) to our remote sensing imagery (Figure 7). We identified three reliable patterns for identifying microbialites in remote imagery. Reference Method Shortcomings and uncertain es Mapped microbialite extent (km2) South Arm North Arm Total Eardley, 1938 Field verifica on from shore and by boat Limited to primarily nearshore areas confirmed in the field, missed areas of deeper microbialite reef and areas in the western por ons of the lake 117 160 277 260 Baskin and others, 2022 Rugosity from acous c sound- ings during bathymetric surveys, par ally confirmed in the field Indirect measure with limited field confirma on 700 654 300 446 1000 1099 Vanden Berg, 2019 Remote imagery Limited image availability, limited field verifica on 56 92 147 Bouton and others, 2020 Merged prior maps with addi- onal areas from remote image- ry reported in Bouton and oth- ers, 2016a Inherited uncertain es from prior work, assumed variable regions were due to burial vs. rip-up clasts 1261 This study (high confidence) Remote imagery Limited field verifica on, some deep- water areas could not be mapped 288 360 648 Table 1. Summary of prior attempts to map lakebed microbialites in Great Salt Lake. Where given, reported values are non-italicized while values inferred from traced shapefiles are italicized. 10 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake First, “healthy,” submerged microbialites appear dark green in remote imagery and stand out against the brighter carbonate sediment background (Figs. 7A & 7D). In some instances of dark green submerged substrate, microbialite reefs were indistinguishable from loose microbialite debris (Figure 6) in single im- ages; for such regions, we compared images from at least three different dates to look for evidence of mo- bility, with only stationary features mapped as reef. Second, as lake levels fall, microbialites become exposed and “bleach” (Frantz and others, 2023), caus- ing exposed reef areas to appear bright in partially- exposed reef areas. Our second identified pattern was that of white reef areas (bleached microbialites) with patterned high-relief mounds (for example, Figs. 7C & D). Image Capture Date World Imagery Date Loca on Provider Resolu on (m) Accuracy (m) 2014-06-29 2015-07-08 South Arm NAIP 1 6 2014-08-31 2015-07-08 North Arm NAIP 1 6 2016-06-26 2017-05-03 South Arm NAIP 1 6 2016-07-15 2017-05-03 North Arm NAIP 1 6 2016-05-07 2018-01-08 North Arm Digital Globe 0.5 10.2 2013-08-29 2018-01-08 South Arm Digital Globe 0.5 10.2 2022-05-07 2022-11-02 South Arm Maxar (GEO1) 0.46 5 2021-10-15 2022-11-02 North Arm Maxar (WV02) 0.5 5 2021-04-08 2022-12-14 South Arm Maxar (GEO1) 0.46 5 2021-10-15 2022-12-14 North Arm Maxar (WV02) 0.5 5 Table 2. Summary of remote imagery utilized for this study. Image Capture Date is the date satellite images were cap- tured, while World Imagery Date is a date of availability in ArcGIS for the set of images. Figure 5. Comparison of satellite images of a specific location at northern Antelope Island (41.06°, -112.26°) using differ- ent image dates. In all images, the thin, white dashed line shows the area outlined as microbialite reef in this study. (A) Mi- crobialite reef can be seen as a dark green submerged region in June 2014 (Esri World Imagery Wayback). (B) In May 2016, visibility of the reef was limited due to poor water clarity and higher lake elevation (Esri World Imagery Wayback). (C) In September 2018, part of the visible reef was obscured due to image distortion and resolution issues (Google Earth Pro). (D) Waves on the lake in May 2020 obscure the reef (Google Earth Pro). (E) Waves and light reflection again ob- scure parts of the reef, with image stitching artifacts obscuring other portions (Esri World Imagery Wayback). (F) Exposed microbialite reef appear as bright/light regions during low lake level in May 2022 (Esri World Imagery Wayback). 11 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 Finally, microbialites tend to form on the perime- ters of “megapolygons”—polygonal structures rough- ly 30–75 meters in diameter (Vanden Berg, 2019) (Figs. 7A & 7B). Thus, megapolygons are our third identified pattern. In contrast, zones of smaller desic- cation–related polygons, averaging only 4–9 meters, are present along shoreline areas at higher elevation and are not associated with microbialites (Vanden Berg, 2019). These smaller polygons can be ephemer- al, appearing and disappearing with changes in lake level. In some areas, particularly north and northwest of Hat Island (112.586°W 41.071°N), we identified broad regions of megapolygons (some quite faint) at elevations above 4195 ft-asl, however, we excluded these from our map due to lack of field verification and their anomalously high elevations; if micro- bialites are found associated with these megapoly- gons, they might belong to an older generation. Regions positively identified as containing micro- Figure 6. Examples of mobile debris. (A) Field photograph of loose carbonaceous microbial mat debris between actual microbialite mounds at Buffalo Point in August 2021. (B) Google Earth Pro remote image showing a dark green region of potential microbialite reef in the southwest arm of the lake (40.983°, -112.709°) on 2019-08-17, and (C) Google Earth Pro remote image of the same location on 2015-06-27 showing shifted mobile debris. Scale bars in (B) and (C) are both 1 km. Figure 7. Examples of field-verified microbialite reef areas identified from remote imagery in the south arm of Great Salt Lake. (A) Dark green submerged microbialites and bright bleached megapolygons indicate the presence of micro- bialites in a nearshore area in the south arm of the lake (41.073°, -112.573°). (B) Submerged desiccated microbialite- edged polygons in the north arm of the lake (41.249°, -112.533°). (C) Bright exposed and desiccated microbialites stand out against green lake water at a site near the Antelope Island marina (41.064°, -112.237°). (D) Partially submerged microbialites between Buffalo Point and White Rock Bay (41.033°, -112.275°). Scale bar in all images is 100 m. Image locations are shown as markers on the map in Fig. 8. Images from Google Earth Pro. 12 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake bialite reefs were mapped in ESRI’s ArcGIS Pro. Separate feature classes were created for the north and south arms and were digitized using ArcGIS Pro by tracing shapes over downloaded imagery. Feature classes as well as bathymetric layers were imported and projected as UTM NAD83 zone 12N to minimize distortion and maximize location accuracy. Areas were initially digitized in large zones before being re- fined to greater resolution in a second stage of pro- cessing. Comparison to Prior Work Areas mapped by prior studies were given extra attention in our analysis, with maps by Eardley (1938), Vanden Berg (2019), Bouton and others (2020), and Baskin and others (2022) providing a framework for the mapping efforts described in this study (Figure 4). Some regions identified as reef zones by Baskin and others (2022) were not able to be conclusively analyzed using remote imagery due to their occurrence in deeper areas of the lake. We in- cluded some of these regions from Baskin in our map as low-confidence regions. Field Verification Many identified reef sites were confirmed with field verification, particularly in accessible shoreline areas (Figure 8); these regions are denoted as high- confidence regions in our map. The western shores of the lake are difficult to access due in part to military restrictions and private land ownership, thus most sites on the west side of the lake have not been field verified. Identified reef sites not yet confirmed with field verification are denoted as low-confidence re- gions except for those associated with megapolygons, which were classified as high-confidence even in the absence of field verification. Lake Elevation-Exposure Model In order to develop a model of microbialite expo- sure at different lake elevations, we used shapefiles for the mapped microbialites and determined overlap with lake bathymetry shapefiles (1 ft intervals) im- ported from Baskin and Allen (2005) and Baskin and Turner (2006). However, caution should be exercised when using the historical bathymetry data, especially in the nearshore environment: modern observations during extreme low lake level indicate that these con- tours are significantly incorrect in several nearshore environments around the lake. Inaccuracies in the bathymetric data will create inaccuracies in the expo- sure models presented in this study, but currently this is the only published bathymetric data available. Mi- crobialite reef area shapes were combined in distinct layers for the north vs. south arm of the lake, since the two arms can have independent water surface ele- vation levels and can be managed separately for eco- system function. Digitized microbialite reef zones were split based on bathymetric data. These clipped zones were used to identify areas of exposure as lake levels decline. Areas of mapped microbialite reef at elevations above bathymetric lines were considered exposed at that lake elevation, whereas areas of microbialite reef at or below bathymetric lines were considered sub- merged. The curve fit least-squares function in the scipy.optimize python package (Virtanen and others, 2020) was used to generate logistic regression models parameterized to fit the area vs. bathymetry elevation values for each arm of the lake using the least squares method. RESULTS Microbialite Reef Extent Our remote imagery-based mapping of micro- bialite extent indicates 360 km2 (139 mi2) of micro- bialite reef between 1271.6 and 1280.5 masl (4172– 4201 ft-asl) in the south arm of Great Salt Lake, of which 45% are high-confidence regions. In the north arm of the lake, we mapped 288 km2 (111 mi2) of mi- crobialite reef in the same elevation band, of which 74% are high-confidence regions confirmed with field observation (Figure 8). The distributions of mapped microbialites by elevation were similar in the north and south arms (Figure S1), although our mapped re- gion in the north arm was limited by limited field ver- ification, poor water visibility, and image resolution. Our mapped extent was somewhat similar with the Baskin and others (2022) map, with several im- portant differences. First, we were able to map micro- bialites in exposed shore environments that were in- accessible by boat and therefore unable to be mapped sonographically by Baskin, thus, our map extends to higher elevations than the Baskin and others (2022) map (for example, bottom left of Figure 9C). Second, in some regions, areas mapped by Baskin extended deeper into the lake than what we found, for example, on the western shore of the lake (Figure 9B–C). Third, our map is more spatially refined (Figure 9E). Also, some regions mapped by Baskin were exposed as dry shoreline in recent years, with no apparent mi- crobialites present (for example, Figure 9D). Most (95%) of the microbialites that we mapped lie in an elevation band between 1274.0 and 1278.6 13 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 Figure 8. Mapped extent of microbialites in Great Salt Lake (this study) showing regions of high confidence of microbialite occurrence (areas confirmed with field verification or presence of megapolygons) and regions mapped at low confidence of microbialite occurrence (areas of apparent microbialite reef in remote imagery). Stars indicate areas where field verification of microbialite reef existence (or non-existence) was verified. Tri- angles mark the approximate locations of photograph sets shown in Fig. 2. Circles mark the locations of re- mote imagery shown in Fig. 7. Basemap imagery provided by Earthstar Geographics. 14 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake masl (4180–4195 ft-asl) (Figure 10). Several notable deeper-water outlier areas are bounded by active Qua- ternary fault zones (Figure S2). Elevation-Exposure Model Our findings for microbialite exposure at different lake elevations are summarized in Table 3 and Figs. 11–12. Fitting a logistic regression line (Equation 1) us- ing the least-squares method to the lake elevation (elev, in masl or ft-asl) vs. microbialite exposure data (in km2 or mi2) gave r2 values ≥ 0.995 for all models (Figure 12). Equation 1: In Equation 1, is the area (in km2 or mi2) of microbialites exposed at a given lake elevation (elev, in masl), where L, k, x0, and b are model parameters defined in Table 4. DISCUSSION Refined Map of Microbialite Reef Extent for Great Salt Lake Our remote imagery-based map of microbialite extent yielded an extent of microbialites between the lower and upper bounds of prior work (Figure 4): at both low and high confidence levels, we mapped sig- nificantly more microbialite area than Eardley (1938), but substantially less than what was mapped by Baskin and others (2022). Because it relied on limited field observation and rough mapping tools available at the time, the Eardley (1938) map represents an understandable underesti- mate of microbialite extent. Meanwhile, the Baskin and others (2022) map covered the entire lakebed in relatively high resolution, however, by relying on in- direct measurements of lake-bottom rugosity, it could have overestimated true microbialite extent. In gen- eral, our map refines the spatial extent of reefs identi- fied by Baskin: 86% of our mapped regions were also mapped by Baskin, for both our high and low confi- Figure 9. Example detail areas where mapped microbialite extents in this study differed significantly from Baskin and others (2022). (A) Mapped microbialite extents in Baskin (blue) vs. this study (yellow) showing areas of detail (B–E). (B) Region along the northeastern lakeshore mapped as having microbialites by Baskin where we were unable to find evidence of microbialites in remote imagery or via field checks. Base image from Maxar 2015-07-08. (C) Area along the western shore of the lake where we identified a region of higher elevation microbialites visible in remote imagery but unmapped by Baskin. The Baskin map also extends into deeper water than we were able to confirm. Base image from Maxar 2015-04-27; mid-image color changes is an imagery artifact. (D) Area at the southwestern shore of the lake where the Baskin map includes microbialites where we only observed regions of mobile clasts. Base image from Maxar 2021-10-16. (E) Region off the northwest shore of Antelope Island where high-resolution imagery from Esri World Imagery Wayback and Google Earth Pro allowed for more precise mapping of microbialite reef zones in our study relative to the Baskin map. Base image from Maxar/Earthstar Geographics 2022-05-08. White scale bars in are- as of detail (B–E) are all 1 km. 15 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 Figure 10. Histograms of microbialite reef area identified at high and low confidence in different 1 ft elevation bands (labels show the lower bound of the band). (A) North arm (NA). (B) South arm (SA). (C) Both arms. L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribu on at Great Salt Lake 16 Eleva on band ( ‐asl) Area of mapped microbialite reef (km2) Total area exposed at lower eleva on bound (km2) High confidence Low confidence High confidence All mapped (high + low conf.) North Arm South Arm North Arm South Arm    North Arm South Arm Whole lake North Arm South Arm Whole lake 4172  –  4173  0.00  0.01  0.00  0.79  212.0  162.6  374.6  288.45  360.4  648.8  4173  –  4174  0.00  0.00  0.00  0.03  212.0  162.6  374.6  288.45  359.6  648.0  4174  –  4175  0.00  0.02  0.00  1.06  212.0  162.6  374.6  288.45  359.5  648.0  4175  –  4176  0.01  0.01  0.00  2.90  212.0  162.5  374.6  288.45  358.4  646.9  4176  –  4177  0.04  0.06  0.00  1.80  212.0  162.5  374.5  288.43  355.5  644.0  4177  –  4178  0.09  0.08  0.00  2.04  212.0  162.5  374.4  288.39  353.7  642.1  4178  –  4179  0.09  0.15  0.00  5.55  211.9  162.4  374.3  288.30  351.6  639.9  4179  –  4180  0.15  1.17  0.00  9.65  211.8  162.2  374.0  288.21  345.9  634.1  4180  –  4181  0.59  1.81  0.01  10.07  211.6  161.1  372.7  288.07  335.0  623.1  4181  –  4182  6.33  4.35  0.01  12.28  211.1  159.3  370.3  287.47  323.2  610.6  4182  –  4183  7.30  1.45  0.82  14.94  204.7  154.9  359.6  281.12  306.5  587.6  4183  –  4184  9.36  7.41  1.16  15.21  197.4  153.5  350.9  273.00  290.1  563.1  4184  –  4185  16.90  3.66  3.78  14.49  188.1  146.0  334.1  262.48  267.5  530.0  4185  –  4186  14.32  14.12  5.30  12.67  171.2  142.4  313.5  241.80  249.4  491.2  4186  –  4187  15.62  11.27  6.19  9.90  156.8  128.3  285.1  222.18  222.6  444.8  4187  –  4188  13.67  29.04  6.05  9.06  141.2  117.0  258.2  200.37  201.4  401.8  4188  –  4189  12.73  20.46  6.09  10.36  127.5  88.0  215.5  180.64  163.3  344.0  4189  –  4190  13.99  20.04  8.26  12.43  114.8  67.5  182.3  161.82  132.5  294.3  4190  –  4191  14.65  11.45  8.91  12.21  100.8  47.5  148.3  139.58  100.0  239.6  4191  –  4192  23.07  10.32  9.15  12.43  86.2  36.0  122.2  116.02  76.4  192.4  4192  –  4193  23.11  10.01  8.31  16.06  63.1  25.7  88.8  83.80  53.6  137.4  4193  –  4194  21.04  6.50  5.52  7.30  40.0  15.7  55.7  52.38  27.5  79.9  4194  –  4195  14.96  5.71  4.63  4.28  19.0  9.2  28.1  25.82  13.7  39.5  4195  –  4196  2.91  0.71  2.15  0.24  4.0  3.5  7.5  6.22  3.7  10.0  4196  –  4197  0.77  2.19  0.07  0.03  1.1  2.8  3.8  1.16  2.8  4.0  4197  –  4198  0.25  0.54  0.00  0.00  0.3  0.6  0.9  0.32  0.6  0.9  4198  –  4199  0.07  0.00  0.00  0.00  0.1  0.0  0.1  0.07  0.0  0.1  4199  –  4200  0.00  0.01  0.00  0.00  0.0  0.0  0.0  0.00  0.0  0.0  4200  –  4201  0.00  0.01  0.00  0.00  0.0  0.0  0.0  0.00  0.0  0.0  Table 3. Mapped microbialite reef area in different elevation bands, and area of microbialite exposure when lake level reaches the lower elevation bound. 17 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 Figure 11. Map of microbialite reef areas (this study) correlated with lake bathymetry, highlighting the areas of microbialite reef exposed at different lake surface elevations (in ft-asl). Basemap imagery provided by Earthstar Geographics. 18 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake Figure 12. Relationship between lake elevation and total cumulative microbialite exposure in Great Salt Lake. Data points for each elevation band that we mapped are shown as points along with corresponding logistic regression best-fit lines. Shaded areas represent the range of standard error for the regression mod- els. The dashed vertical line marks the lake elevation at the autumn 2022 minimum (4188.5 ft-asl). (A) Micro- bialites mapped in the north arm of Great Salt Lake at high (light) and high+low (dark) confidence. (B) Mi- crobialites mapped in the south arm of Great Salt Lake at high (light) and high+low (dark) confidence. (C) Values for the whole lake, with mapped microbialites at high (light) and high+low (dark) confidence. 19 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 dence maps. However, Baskin mapped ~350 km2 (135 mi2) more microbialite areas than we could con- firm, largely in deep-water areas of the lake. There are several key differences between our map and the Baskin map that warrant future field verification. First, our technique allowed for mapping of micro- bialites in shore environments that were not navigable and therefore unmapped by Baskin, for example, in an area north of Lakeside where we identified desic- cation megapolygons (Figure 9B). Second, areas mapped by Baskin frequently extended deeper into the lake than our remote imagery-based approach per- mitted, for example, on the western shore of the lake, and in the area between Antelope Island and Fremont Island (Figs. 4, 9B–C). We did not include these deeper-area regions of putative reef mapped by Baskin in our map or elevation-exposure model, how- ever, we cannot rule out that they exist. Also, our map only accounts for consistently unburied microbialites, which are more likely to contribute to lake productivi- ty than intermittently buried microbialites, which could have been included in the Baskin and others (2022) map. Heavily eroded microbialites may also have been missed by our map. Lake Elevation and Microbialite Exposure During the autumn 2022 historic lake lowstand of 1276.7 masl (4188.5 ft-asl), we estimate (from micro- bialites mapped at both high and low confidence in this study) that >294 km2 (114 mi2, or >45%) of the lake’s microbialites were exposed, >133 km2 (51 mi2) in the south arm (>37% exposure), and 162 km2 (63 mi2) in the north arm (>56% exposure). Microbialites in the lake’s north arm no longer support a robust mi- crobialite surface community because of the arm’s high salinity levels (Lindsay and others, 2019), thus, their exposure or submergence likely does not have much influence on the support of higher tropic levels in the Great Salt Lake food web. In the south arm, re- cent evidence suggests that microbialite photosynthet- ic (periphyton) communities can survive months of subaerial exposure, and that re-submerged micro- bialites appear to be rapidly recolonized by lake water microorganisms (Frantz and others, 2023). However, subaerially exposed microbialites cannot contribute to the benthic or planktonic food chains in the lake. Ad- ditionally, areas of microbialites that experienced fre- quent exposure in the past half century never fully re- developed a healthy periphyton (marked by thick ge- latinous mats) even when re-submerged for periods of several seasons to years, indicating that the damage caused by prolonged exposure is long-lasting. It is al- so important to note that microbialites in the hyper- saline north arm of the lake also lack the robust mats of primary producers that are present in “healthy” mi- crobialites (Lindsay and others, 2017); this is one of the reasons we clearly separate our maps of north vs. south arm microbialites. Finally, exposed micro- bialites are subjected to rapid weathering, and it could take decades or even centuries for the raised mounds that represent stable oases in an otherwise shifting lake benthos to re-form. Thus, the consequences of long-term subaerial exposure of the lake’s micro- bialites are profoundly concerning for the lake eco- system. Even in the short term, there are ecosystem conse- quences of microbialite exposure. If microbialite pe- riphyton communities conservatively represent 30% of primary production in Great Salt Lake, the expo- Logis c regression model parameters - metric units (masl, km²) Logis c regression model parameters - imperial units ( -asl, mi²) Arm Confidence r² L k x₀ b L k x₀ b North Arm high 0.9954 229 ± 6 1.14 ± 0.07 1277.0 ± 0.1 -14 ± 5 88 ± 2 0.35 ± 0.02 4189.6 ± 0.2 -5.4 ± 1.8 North Arm high+low 0.9967 308 ± 6 1.23 ± 0.06 1277.0 ± 0.1 -16 ± 5 119 ± 2 0.38 ± 0.02 4189.8 ± 0.2 -6.3 ± 1.9 South Arm high 0.9992 163 ± 1 1.71 ± 0.04 1276.7 ± 0.0 0 ± 1 63 ± 0 0.52 ± 0.01 4188.5 ± 0.1 -0.1 ± 0.4 South Arm high+low 0.9988 376 ± 5 1.12 ± 0.04 1276.4 ± 0.0 -14 ± 3 145 ± 2 0.34 ± 0.01 4187.5 ± 0.1 -5.5 ± 1.2 Whole lake 0.9988 390 ± 4 1.32 ± 0.04 1276.8 ± 0.0 -12 ± 3 151 ± 2 0.40 ± 0.01 4189.1 ± 0.1 -4.7 ± 1.3 Whole lake high+low 0.9986 684 ± 9 1.14 ± 0.04 1276.7 ± 0.0 -31 ± 7 264 ± 4 0.35 ± 0.01 4188.6 ± 0.1 - 12.0 ± 2.6 high Table 4. Logistic regression model results for microbialite exposure area at different lake elevations. To aid in the use of models for management, values are presented for use of both metric units (masl for lake elevation, km2 for area of exposed microbialites) and imperial units (ft-asl for lake elevation, mi2 for area of exposed microbialites). 20 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake sure of ~ 40% of them in the lake’s south arm may have equated to a > 10% reduction in overall lake pri- mary production in summer 2022 compared to “healthy” lake elevations (when microbialites are ful- ly submerged). If one assumes that the bulk of micro- bialite-supported primary productivity occurs in rela- tively shallow water (i.e., the year-round photic zone), it is possible that the relative aerial extent of microbialites that occupy this zone has been relatively stable over the past several years of lake level fall, however, further lake level decline would substantial- ly decrease the area of productive microbialites. Also significant to the ecosystem is the substantial de- crease in Ephydra pupa anchor sites that occurs when microbialites become subaerially exposed. The greatest change in submerged microbialites occurs between 1275.6 and 1278.0 masl (4185–4193 ft-asl; Figure 12) because of the large expanses and high density of microbialites in this zone (Figure 10). The lower bound for the lake elevation target range for management of 1279.5 masl (4198 ft-asl) (Utah DNR Forestry, 2013) ensures that nearly all of the lake’s microbialites are submerged. At 1278 masl (4193 ft-asl), 88% are submerged, while at 1275.6 masl (4185 ft-asl), only 24% remain submerged. Ad- ditionally, at lake elevation levels below ~1277 masl (4190 ft-asl), microbialite community health becomes threatened not only by exposure, but by salinity. At salinity levels above 15%, the primary productivity of Euhalothece—and, thus, microbialite-associated productivity—declines (Lindsay and others, 2019); this corresponds to a lake elevation of roughly 1277 masl (4191 ft-asl). Thus, due both to microbialite ex- posure and high salinity levels, elevations above 1277 masl (4191 ft-asl) should be a minimum for lake man- agement with respect to microbialite-supported eco- system survival, whereas elevations above 1278.6 masl (4195 ft-asl) keep nearly all of the lake’s micro- bialites submerged. Limitations of this Study Although we believe our map is a significant im- provement over previously published maps of micro- bialite extent, it has several limitations and caveats. First, our map is limited to visible reef areas. In regions where remote imagery is low resolution, we were unable to confidently map microbialites. We were also unable to conclusively confirm or refute microbialite reef areas in deep-water portions of the lake (generally, below 1275 masl, or 4183 ft-asl, alt- hough this varied somewhat by remote imagery avail- able), where water obscures reflected light. These deep-water portions of the lake represent an area of 1800 km2 (~700 mi2) and include 232 km2 (90 mi2) of microbialite reef mapped by Baskin and others (2022); we cannot rule out the existence of micro- bialites above surrounding lake sediment at depths below 1275 masl (4183 ft-asl), but we were only able to confirm the probable existence of microbialites in 53 km2 (20 mi2) of that area based on remote imagery and the methods of our study. This could account for some, but not all discrepancies between the Baskin map and ours. This caveat to our study could be rem- edied with a comprehensive field verification cam- paign. Deep-water areas may need to be verified by divers. Our study could also be used to help refine Baskin’s benthic rugosity-based mapping algorithm (Baskin, 2005). Second, we excluded regions of reef that were not consistently visible in remote imagery. We did this to exclude areas of shifting microbialite debris/rip-up clasts. However, the change in visibility could also be due to shifting ooid sands covering up and then re- exposing areas of active reef (as noted by Bouton and others, 2016). These regions of reef could still, when exposed, contribute to primary production in the lake. Roughly 59 km2 (23 mi2) of the lakebed we analyzed in this study comprised regions of variable brightness, i.e., either mobile clasts or varied exposure/covering by surrounding sediment, and it was not possible to distinguish mobile clasts from shifting sediment ob- scuring true reef areas. Third, our model of microbialite exposure vs. lake elevation is based on the bathymetry of Baskin and Allen (2005) and Baskin and Turner (2006), which was limited spatially to 1-km transects in the naviga- ble portions of the lake (Baskin, 2005; Baskin, 2006). Thus, the bathymetry, especially in the elevation band of 1276.5–1278.6 masl (4188–4195 ft-asl), which corresponds to one of the greatest expanses of micro- bialite reef (Figure 10), is poorly constrained, limiting the accuracy of our model. Bathymetry in this band can be improved with detailed lidar mapping, work that is currently being explored and, we hope, done more extensively in the near future. Finally, prolonged subaerial exposure of the lake’s microbialites results in their rapid weathering (Frantz and others, 2023), thus, microbialite extents at higher elevation bands are subject to change (decrease) during periods of low lake elevation. Addi- tional research is required to quantify and model rates of microbialite weathering. SUMMARY We mapped 649 km2 (251 mi2) of microbialite reef in Great Salt Lake by leveraging low lake levels and recent availability of high-resolution remote im- agery. Of that, 375 km2 (145 mi2) were either field- 21 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 verified or were identified as megapolygons, which are linked to microbialites in Great Salt Lake (Vanden Berg, 2019). We believe that our map of microbialite extents refines previously published maps. We have also produced shapefiles of microbialite extent at dif- ferent lake elevations (Supplemental Materials). Our model of microbialite exposure vs. lake elevation can be used to inform Great Salt Lake management: 1278.6 masl (4195 ft-asl) should be considered as a critical minimum lake elevation (with the understand- ing that higher lake levels provide greater protection) with respect to microbialites; at this depth, 98% of the lake’s microbialites are submerged. During the histor- ic lowstand in autumn 2022 of 1276.7 masl (4188.5 ft -asl), we estimate that >37% of the microbialites in the south arm of the lake were subaerially exposed, representing substantial damage to benthic primary productivity (which was likely already threatened by high salinity levels) and Ephydra larva habitat. ACKNOWLEDGEMENTS LW was supported by NSF RISE #1801760 to Elizabeth Balgord. CF was supported by NSF EAR #1826869. MVB was supported by the Utah Geologi- cal Survey. We thank Ryan Frazier and Michael Hernandez for their extensive help with GIS aspects of this pro- ject, and several reviewers for detailed and helpful comments on a prior version of this manuscript. REFERENCES Anderson, N.L., Barrett, K.L., Jones, S.E., and Belov- sky, G.E., 2020, Impact of abiotic factors on mi- crobialite growth (Great Salt Lake, Utah, USA): a tank experiment: Hydrobiologia, v. 847, no. 9, p. 2113–2122, doi: 10.1007/s10750-020-04235-9. 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W., VanderPlas, J., Laxalde, D., Perktold, J., Cimrman, R., Henriksen, I., Quintero, E. A., Harris, C. R., Archibald, A. M., Ribeiro, A. H., Pedregosa, F., van Mulbregt, P., and SciPy 1.0 Contributors, 2020, SciPy 1.0: Fundamental Algo- rithms for Scientific Computing in Python: Nature Methods, v. 17, no. 3, p. 261-272. Wurtsbaugh, W.A., 2009, Biostromes, brine flies, birds, and the bioaccumulation of selenium in Great Salt Lake, Utah: Saline lakes around the world: unique systems with unique values. Natu- ral Resources and Environmental Issues, vol XV, v. 15, p. 1–15. Wurtsbaugh, W.A., Gardberg, J., and Izdepski, C., 2011, Biostrome communities and mercury and selenium bioaccumulation in the Great Salt Lake (Utah, USA): Science of The Total Environment, v. 409, no. 20, p. 4425–4434, doi: 10.1016/ j.scitotenv.2011.07.027. 25 M.D. Vanden Berg, R. Ford, C. Frantz, H. Hurlow, K. Gunderson, G. Atwood, editors 2024 Utah Geological Association Publication 51 SUPPLEMENTARY INFORMATION . Mapped microbialite area shapefiles, data tables, Python code used for analysis, and supplemental fig- ures are available at Open Science Framework: https://osf.io/uf9yg/. 26 L. Wilcock, C.M. Frantz, and M.D. Vanden Berg Use of Remote Imagery to Map Microbialite Distribution at Great Salt Lake