

































In the summer of 2012, the oligotrophic waters of Lake Superior experienced the region’s !rst cyanobacterial bloom 
along the west shoreline of the Wisconsin Apostle Islands. While cyanobacterial blooms are common in other 
Laurentian Great Lakes and warmer inland lakes, this was the !rst event recorded in Lake Superior. In contrast, 
Siskiwit Lake, WI, which drains into Lake Superior near the bloom location, has reported semi-annual cyanobacterial 
blooms each summer. "is study focused on a more recent 2021 bloom in Siskiwit Lake, along with the sediment 
deposits in Siskiwit Lake and Siskiwit Bay, near the !rst 2012 bloom event. Using elemental and isotopic indicators 
(%OC, %N, and #13C), and High-Performance Liquid Chromatography (HPLC), the contribution of organic carbon 
sources over time in these locations was analyzed and compared with the carotenoid and pigment portrait of the 2021 
bloom in Siskiwit Lake. Findings suggested that OC sources in the sedimentary record of Siskiwit Bay were primarily 
lacustrine algae, demonstrating periods of high algal matter productivity, while Siskiwit Lake exhibited higher 
terrestrial organic matter input in comparison. Furthermore, carotenoid pigments identi!ed in the Siskiwit Lake 
bloom indicated a large presence of cyanobacteria amongst other algae, corroborating the presence of cyanobacteria 
paleo-indicators within the sediment. With this data, future investigations of algal pigments in the Lake Superior 
lacustrine deposits will provide additional reconstruction of high cyanobacteria productivity intervals and identify 
potential bloom periods in the region's history.

Aisthesis      Volume 15,  202445

Pigment and Geochemical Markers of Past and Present 
Cyanobacterial Presence in the Southwestern Lake Superior 
Region

by Lily Karg

I. Introduction 
 Cyanobacteria, also known as blue-green algae, 
are ancient photosynthetic bacteria that use sunlight 
and carbon dioxide (CO$) to produce biomass 
(Paerl, 2017). "ese organisms are responsible for 
stimulating the oxidation of Earth’s atmosphere 
billions of years ago and are naturally present in 
almost all aquatic ecosystems (Huisman et al., 2018). 
Some cyanobacteria species, such as those within 
the Dolichospermum genus, have also developed 
the unique ability to !x atmospheric nitrogen (N$) 
over their extensive evolutionary history (Huisman 
et al., 2018). Nitrogen !xation allows cyanobacteria 
to capture atmospheric N% gas and convert it into 
ammonia through catalysis via the nitrogenase 
enzyme complex, giving them competitive advantage 
over other non-nitrogen-!xing phytoplankton 
communities (Huisman et al., 2018). 
 As cyanobacteria have adapted to survive in 
adverse conditions, a warm and chemically nutrient-
rich environment promotes their proli!c biomass 
production (Huisman et al, 2018). When favorable 
conditions stimulate cyanobacteria to rapidly 

reproduce, they form thick mats on the surface 
of the water that are dense enough to extinguish 
the photosynthetic activities of aquatic plants 
(Huisman et al, 2018). "e ultimate degradation of 
this large amount of biomass also deprives !sh and 
benthic invertebrates of oxygen within the water 
column (Huisman et al, 2018). Furthermore, some 
cyanobacteria blooms can produce harmful toxins 
that are not only dangerous to the aquatic ecosystem 
but also humans, livestock, and other organisms that 
encounter them (Huisman et al., 2018; Sterner et al., 
2020). 
 Recent accumulation of cyanobacteria blooms 
in Lake Superior, the largest and most oligotrophic 
of the Laurentian Great Lakes, have raised concerns 
for the health of the region’s waters, as well as 
questions about the connection between these 
bloom events and climate change. In the summer of 
2012, Lake Superior experienced the !rst recorded 
cyanobacterial bloom along the western shoreline of 
the Wisconsin Apostle Islands. While cyanobacterial 
blooms are common in other Laurentian Great 
Lakes and warmer inland lakes, this was the !rst 



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  202446

event documented in Lake Superior. Several other 
blooms have occurred periodically in the lake since 
2012, and such uncharacteristic algal proliferation 
has prompted investigation into the paleolimnology 
record within the lake’s sedimentary deposits.
 Sediment geochemistry and phytoplankton 
photopigments are both proxies used in the 
interpretation and reconstruction of past ecological 
conditions of lake environments. More speci!cally, 
organic matter (OM) and carotenoid biomarkers 
preserved within the sedimentary record can be 
analyzed for indicators of di&erent algal communities. 
"rough chemical analysis, elemental carbon to 
nitrogen ratios (C:N) in correlation with isotopic 
carbon signatures (#13C) can be used to distinguish 
between primary sources of OM: C4 land plants, 
C3 land plants, marine algae, and lacustrine algae 
(Meyers, 1994). "ese signatures can then be paired 
with biomarker identi!cation and quanti!cation to 
reconstruct the organismal composition of historic 
ecosystems.
 According to a review by Meyers (1994), algae 
lack the abundant amounts of cellulose that vascular 
land plants possess. "ese compositional di&erences 
are re'ected in their carbon to nitrogen ratios: 
C:N ratios between 4 and 10 are typically seen in 
algae, while ratios greater than or equal to 20 are 
observed in vascular land plants (Meyers, 1994). 
Diagenesis related degradation of OM has been 
shown to alter C:N ratios in soils but remain reliably 
preserved in underwater environments (Meyer, 
1994). Furthermore, carbon isotope signatures 
(#13C) are used as markers of land plant types and 
di&erentiating marine and continental plant sources 
(Meyers, 1994). #13C signatures that fall within the 
-25‰ to -30‰ range are typically identi!ed as 
freshwater OM, identi!ed by Meyers in 1990 and 
1992. It should be noted, however, that these signals 
are usually indistinguishable from C( land plants 
(Meyers, 1994). C4 land plants, on the other hand, 
show much more enriched #13C signatures, around 
-10‰ to -14‰ (Meyers, 1994). 
 Cyanobacteria synthesize carotenoids, which 
are hydrophobic isoprenoid compounds that act 
as both accessory light-harvesting pigments and 
protection against harmful photooxidation (Tóth 
et al., 2015). "e speci!c carotenoid pigments 
synthesized by di&erent types of cyanobacteria can 
be used to identify speci!c taxon present in both 

paleo and contemporary blooms. Generally, the 
literature reports that freshwater cyanobacteria 
genera display carotenoid suites predominantly 
composed of echinenone and canthaxanthin, as well 
as the photoprotective pigments myxoxanthophyll 
and zeaxanthin (Hertzberg et al., 1971; Hirschberg 
& Chamovitz, 1994). In conjunction with elemental 
and isotopic analysis, pigment suites can be surveyed 
to identify the organic material sources within 
historic and present environments.

A. Using Speci!c Photopigments as Biomarkers of 
Phytoplankton Communities
 "e analysis of organic material within natural 
samples uses several proxies as source indicators. 
In studies of sediment and water surface samples, 
photosynthetic pigment and carotenoid biomarkers 
can be separated and used to identify speci!c 
taxon present in both paleo and contemporary 
environments (Hertzberg et al., 1971; Hirschberg 
& Chamovitz, 1994). "eir quanti!cation also 
serves as a means to interpret productivity. Highly 
referenced research, such as Bianchi et al. (1991), 
indicates that concentrations of chlorophyll-a 
and total photopigments relative to total organic 
carbon within a sample site are speci!c measures of 
autochthonous community contributions.
 Several instances have shown that this data 
can be used to infer bloom periods within the 
sediment record of lakes (Baustain et al., 2020;   
Hobbs et al., 2021; Leavitt, 1993). Cyanobacteria 
have typically been identi!ed based on the 
presence of myxoxanthophyll (1 & 2), echinenone, 
canthaxanthin, zeaxanthin, and aphanizophyll 
(Descy et al., 2000 Hertzberg et al., 1971; Hirschberg 
& Chamovitz, 1994; Schlüter et al, 2018). Detection 
of these markers within the sedimentary record 
has suggested the presence of algal communities 
consisting of cyanobacteria in several studies, and 
further interpretation of their concentrations has 
provided insight into the context of blooms events 
as we experience them today (Baustain et al., 2020; 
Bianchi et al., 1991; Hobbs et al., 2021; Leavitt, 1993; 
O’Bierne et al., 2017).
 Additionally, carotenoid concentrations in 
relation to total pigment composition and/or 
chlorophyll-a concentrations have been used to 
correspond characteristic ratios to algal genera 
(Je&ery et al., 2011; Schlüter et al., 2018; Hertzberg 



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  202447

et al., 1971; Egeland, 2016). While some pigments 
are especially unique to individual microorganism 
classes, several are shared, including diadinoxanthin, 
zeaxanthin, and neoxanthin (Bianchi & Canuel, 
2011; Egeland, 2016;  Hirschberg & Chamovitz, 
1994). Methods of distinguishing between the 
sources of these pigments, which are found in several 
organisms, continues to be researched and improved. 
"e analysis of pigment marker to Chl-a ratios has 
thus been used to further di&erentiate between types 
of cyanobacteria or other phytoplankton groups and 
their respective contributions in both culture and 
!eld measurements through a variety of techniques, 
as well as assessing post-depositional pigment 
preservation (Descy et al., 2000; Hobbs et al., 2021) 

B. "e Reliability of Pigments as Biomarkers
 Natural systems are o)en di*cult to study due 
to their dynamic interactions and complex cycles, 
emphasizing the need for examination of the 
reliability of biomarkers in analytical assessments. 
Runo&, winds, current, and other physical forces 
in'uence the exchange of water in estuaries and 
coastal regions (Baustian et al., 2020; Reinl et al., 
2020; Schlüter et al., 2014). Other environmental 
changes can alter the biochemical interactions within 
the water column, as well (Schlüter et al., 2014). 
"ese in'uences subject any organic biomarkers, 
including photopigments, to degradation forces. As 
they are susceptible to decay through in situ activities 
(including oxidation and herbivory) and post-
depositional degradation, some studies have claimed 
that relative pigment abundance should not be 
relied on as an accurate re'ection of phytoplankton 
community composition in the fossil record (Leavitt, 
1993; Ruess et al., 2005). Further degradation can 
occur before, during, or post extraction from both 
sediments and water surface samples (Hirschberg et 
al., 1994; Hertzberg et al., 1971; Chen et al., 2001). 
Moreover, other biochemical processes in vivo, such 
as chlorophyllase activity, can create problems when 
analyzing chlorophyll (Je&rey & Hallegrae&, 1987). 
Despite these selective losses, studies focusing on 
individual pigment fossil pro!les with respect to 
site-speci!c historical patterns show that population 
interaction interpretations are acceptable (Leavitt, 
1993; Baustian et al., 2020).
 Recent work by Baustian et al. (2020) has 
particularly demonstrated the utilization of 
paleolimnological indicators of long-term ecological 

changes within the Laurentian Great Lakes Basin to 
guide management practices towards site-speci!c 
measures. "eir quanti!cation of chlorophyll-a, 
carotenoid, and pigment derivatives was combined 
with elemental and isotopic analysis, in the !rst 
regional appraisal of the impacts of invasive mussel 
species and eutrophication via the fossil record 
(Baustian et al., 2020). Parallels between their results 
and those of related studies, correlations with nutrient 
level monitoring, and quality assurance of their data, 
both geochemical and pigment biomarkers were 
found to be applicable indicators of human impact 
on water quality (Baustian et al., 2020).
 Similarly, another recent study conducted by 
Hobbs et al. (2021) employed analogous methods 
in the investigation of the sedimentary record of 
a parkland lake known to host recurring harmful 
algal blooms. Hobbs et al. (2021) provided historical 
context of the algal community within the lake 
system and identi!ed impacts of the surrounding 
parkland and farming. Both of these independent 
studies by Baustian et al. (2020) and Hobbs et 
al. (2021) substantiated that despite natural 
interferences, pigments served as useful molecular 
markers with long-term monitoring and considerate 
examination of sedimentary records in site-speci!c 
interpretations.
 Another study by Descy et al. (2000) reported 
reliability in their methods of algal biomass 
prediction. "eir data advocated that marker 
pigment to Chl-a ratios showed enough agreement 
with calculated values to corroborate acceptable 
estimations of phytoplankton concentrations 
(Descy et al., 2000). Apart from their own empirical 
analysis, a coupled review examined several other 
techniques of phytoplankton biomass quanti!cation, 
with instances of unreliability reportedly due to light 
and nutrient availability changes by and cross-over 
within taxonomic classi!cations (Descy et al., 2000). 
Despite underlying limitations revealed through 
their review, Descy et al. (2000) detailed promising 
results from their own !ndings and other studies 
using CHEMTAX estimation of algal biomass and 
those utilizing the relationship of carbon biomass 
and xanthophyll concentrations.

C. Anthropogenic Climate Change in Connection with 
Cyanobacteria Productivity
 Anthropogenic activities not only impact 
the use of pigment biomarkers in assessments 



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  202448

of contemporary and historic phytoplankton 
communities, as reviewed in previous sections, 
but also the dominance of cyanobacteria in algal 
communities. Some studies have investigated 
potential drivers for increased cyanobacteria 
productivity in connection with climate change 
(Baustian et al., 2020; Hobbs et al., 2020; O’Bierne 
et al., 2017; Paerl & Paul, 2012; Reinl et al., 2020). 
As global temperatures warm and pollution from 
human activities contaminate water bodies and 
systems, for example, cyanobacteria and other 
phototrophic organisms easily proliferate (Paerl & 
Paul, 2012; Reinl et al., 2020). Moreover, changes in 
cyanobacterial dominance in aquatic communities 
as a result of climate conditions such as surface water 
temperatures, ice-free periods, nutrient loading, 
vertical strati!cation, and carbon dioxide availability 
have been correlated with anthropogenic in'uences 
(O’Bierne et al., 2017; Paerl & Paul, 2012).
 "e e&ects of climate change on the primary 
productivity within aquatic ecosystems have been 
observed through studies of bulk OM and pigment 
content within the sedimentary deposits of varying 
lakes (Leavitt, 1993; Reinl et al., 2020; O’Bierne et 
al., 2017; Paerl & Paul, 2012). An investigation by 
O’Bierne et al. (2017) reported that a rapid increase 
in autotrophic productivity within Lake Superior 
was found to be correlated to higher surface water 
temps and longer ice-free periods as a result from 
anthropogenic climate change, with the caveat 
that individual elemental markers should not be 
interpreted alone but in relation to other indicators. 
"ey proposed that changes in climate regimes 
observed recently within the lake system have been 
more in'uential than those experienced historically 
(O’Bierne et al., 2017). A similar review was published 
by Hobbs et al. in 2020, evaluating algal pigment and 
genetic biomarkers of a lake within the Olympic 
Peninsula that has been experiencing uncharacteristic 
cyanobacteria blooms. "eir !ndings also suggested 
that climate change in'uences on nutrient inputs 
and cycling have altered the ecological development 
of toxic cyanobacteria species (Hobbs et al., 2020).
 As mentioned previously, work by Baustian et al. 
(2020) had also investigated an array of damaging 
anthropogenic stressors on the Great Lakes region. 
Further synthesis of previously conducted studies 
highlighted damaging human interactions with 
freshwater ecosystems, including but not limited 
to the introduction of invasive species, agricultural 

development, suburbanization, and industrial 
chemical and biological pollutants (Baustian et 
al., 2020). Due to issues of eutrophication, algal 
proliferation, pollution, and su&ering !sh and wildlife 
populations, their study sites (within the western area 
of Lake St. Clair, Michigan, part of the Laurentian 
Great Lakes system) have been recognized by the 
Great Lakes Water Quality Agreement as an Area 
of Concern since 1987 (Baustian et al., 2020). Along 
with other studies within the nearby regions, the 
assessment of anthropogenic and climatic variables 
associated with declining water quality by Baustian 
et al. (2020) exemplify the exigency for further 
evaluation of the watershed’s health and potential 
mitigation solutions.

D. Knowledge Gaps and Current Work
 As has been reviewed here, a composite of 
internal and external factors in'uences both 
the incipient development of cyanobacterial 
communities and the use of biomarkers to identify 
them in aquatic ecosystems. Studies have evidenced 
that cross-examination of these interactions allows 
for analysis of contemporaneous and fossilized 
chemical and biological markers to reconstruct the 
ecological pro!les of cyanobacteria, interpret their 
compositional changes over time, and infer the 
relationship of anthropogenic climate change in 
the accumulation of harmful bloom events. Gaps 
in the literature call for further research of bloom 
event triggers and prediction frameworks, as well 
as the taxonomic record of speci!c phytoplankton 
species, markedly in the Great Lakes. Environmental 
assessments have also been recommended to be 
performed and/or taken into consideration speci!c 
to study sites when interpreting data. Considering 
Lake Superior, investigation into the lacustrine 
deposits and current phytoplankton communities 
will issue the groundwork for a comprehensive 
exploration of the lake’s dynamic biogeochemistry 
and how a changing climate could adversely a&ect it.
 "is study examined photopigment and 
geochemical markers of past and present 
cyanobacterial presence in the southwestern Lake 
Superior region in response to concerns around 
recent bloom events. Sediment cores were collected 
from Siskiwit Bay, WI, near the !rst recorded bloom 
events in Lake Superior, and Siskiwit Lake, WI, 
which is hydrologically connected to Siskiwit Bay via 
the Siskiwit River (Figure 1.).



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  202449

 
 Surface water samples were also collected from a 
recent 2021 bloom in Siskiwit Lake. Using elemental 
(%OC and %N) and isotopic indicators (#13C), the 
contribution of organic carbon sources over time 
in the sedimentary deposits of these locations 
was analyzed. Further, water column particulates 
including cyanobacteria were collected from a 2021 
bloom in Siskiwit Lake to determine the speci!c 
pigment makeup of cyanobacterial species in this 
region. 
      
I. Methods
 "e objective of this study was to provide 
historical context for modern-day primary 
productivity of oligotrophic Lake Superior. To 
achieve this, the organic carbon sources within 
the sedimentary record of the southwestern Lake 
Superior region and the presence of cyanobacteria 
throughout history were investigated by means of 
elemental and isotopic analysis. "e pigment suite of 
a modern-day algal bloom was pro!led, as well, for 
further interpretation of the ecological contribution 
of phytoplankton communities within this region of 
the watershed.

A. Elemental Analyzer-Isotope Ratio Mass 
Spectrometry
 Each sediment sample was analyzed for its 
elemental composition (%OC and %N) and stable 
isotopes (#13C) at 1-cm resolution using the EA-
IRMS instrument at the Large Lakes Observatory 
(LLO) of the University of Minnesota in Duluth. 
Ca&eine (0.25 mg), Acetanilide (0.40 mg), B-2153 
(25.0 mg), and B-2159 (0.50 mg) standards were 
utilized between every ten sample runs to ensure 
precision of the instrument and correct for dri). 
Percent organic carbon, nitrogen, and isotopic data 
were selected as proxies for the characterization of 

terrestrial versus aquatic OM within the study site 
sedimentary record. As was conducted in a review 
by Meyers (1994), carbon to nitrogen ratios were 
correlated with #13C values for both sites to further 
distinguish the deposited OM as lacustrine algae, 
marine algae, C3 land plants, or C4 land plants. 
Detection of organic carbon was improved by acid 
(12.0 M HCl) fumigation of carbonate contaminants 
prior to analysis. 

B. High-Performance-Liquid-Chromatography
 Water column samples were also taken from 
Siskiwit Lake during an algal bloom in June of 
2021, using porous micron !lters. 46 surface water 
!lter samples were freeze-dried a)er collection and 
stored in aluminum foil at -20ºC. "e samples were 
combined and homogenized into scintillation vials 
according to collection site as follows: Filter #1) 
Siskiwit Lake Boat Launch (SLBL), Filter #2) Siskiwit 
Lake Campground A (SLCA), Filter #3) Siskiwit 
Lake Campground B (SLCB), Filter #4) Siskiwit 
Lake Campground, Boat Launch, and Dock, Filter 
#5) Mixed Locations, Filter #6) Negative Di&erence 
(which showed loss in mass). Each homogenized 
sample was then mixed with approximately 3 mL 
acetone. "e resulting solutions were centrifuged 
for 5 minutes, sonicated for 1 minute in ice water to 
free the pigments from the cell membranes, syringe-
!ltered, and dried using nitrogen gas (Bianchi & 
Findlay, 1991; Chen et al., 2001; Schreiner et al., 
2013; Dr. Michael Shields, Texas A&M University). 
Finally, the samples were prepared for injection into 
the HPLC by resolving in 100 +L of acetone, with 100 
+L of trans-#-Apo-8’-carotenol internal standard 
(Dr. Michael Shields, Texas A&M University).
 "e HPLC analysis procedure was adapted from 
Dr. Michael Shields, Texas A&M University. Using 
a Dionex Ultimate 3000 for liquid chromatography 
with a DionexTM DAD 3000 detector (Chemistry 
and Biochemistry Department, University 
of Minnesota Duluth), the absorbance of the 
injected photosynthetic pigments was measured. 
Chromatographic separation of each sample was 
run for approximately 11 minutes, employing 
a linear gradient from 5% 70:30 (v:v) methanol 
and tetrabutylammonium acetate (TBAA) to 95% 
Optima-grade Methanol (Van Heukelem & "omas, 
2001), followed by a 4 minute isocratic hold and 8 
minute equilibration with Optima-grade Methanol.



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  202450

 Injections were run through an Acquity UPLC® 
VanGuard™ Pre-Column (BEH C8 1.7 +m, 2.1 x 5 
mm) and Zorbax Rapid Resolution High De!nition 
Eclipse Plus C8 analytical column (95Å, 2.1 x 100 
mm, 1.8 +m, Agilent Technologies), maintained at 
60°C with a 0.300-mL/min 'ow rate (Van Heukelem 
& "omas, 2001). Excitation and emission 
wavelengths were set at 450 nm (4-nm bandwidth) 
and 666 nm (4-nm bandwidth), respectively, with 
DAD collection at a rate of 20 Hz. "e detector’s 
linear range relevant to sample concentrations was 
evaluated by conducting calibration curves of mixed 
and individual pigment standards, acquired from 
DHI Water Quality Institute (Horsholm, Denmark), 
prior to the sample analysis. "e absorption pro!les 
of pigment standards were collected in advance 
to determine elution order for the carotenoids, 
chlorophylls, and degradation products of interest: 

a) Alloxanthin (0.948 mg/L)
b) Aphanizophyll (0.845 mg/L)
c) ,-Carotene (0.832 mg/L)
d) Canthaxanthin (1.648 mg/L)
e) Chlorophyll-a (1.707 mg/L)
f) Chlorophyll-b (1.190 mg/L)
g) Diadinoxanthin (0.690 mg/L)
h) Diatoxanthin (0.641 mg/L)
i) Echinenone (0.923 mg/L)
j) Fucoxanthin (1.490 mg/L)
k) Myxoxanthophyll (0.764 mg/L)
l) Neoxanthin (0.751 mg/L)
m) Pheophytin-a (3.208 mg/L)
n) Violaxanthin (1.118 mg/L)
o) Zeaxanthin (0.782 mg/L)

III. Results
A. Geochemical Evidence of Historic Cyanobacteria 
Organic Carbon Sources
 Total organic carbon content of the sediment 
cores was analyzed to obtain a relative percentage 
of OM deposited in the study sites over time (Fig. 
2). Sections of the core correspond to depositional 
time, with the surface of the core (0 cm) correlating 
to more recent years (Fig. 2). TOC in Siskiwit Lake 
ranged from 16.94% to 22.02% downcore, with a 
mean of 19.36% ± 0.04% (Fig. 2). 
 In Siskiwit Bay, increased sand deposition from 
55.5 to 63.5 cm manipulated bulk elemental and 
isotopic signatures. "is distinct layer of sand within 
the core was posited to have arti!cially lowered 

TOC, as sand poorly preserves organic matter 
(Yeasmin et al., 2017). Increasing TOC from ~50 
cm to 0 cm could re'ect increased loading of OM 
from primary producers within the bay. TOC values 
in the bay were lower overall compared to Siskiwit 
Lake, which further emphasizes a rising trend in 
concentration closer to the present day. Such could 
be indicative of rising productivity of cyanobacteria 
in Lake Superior, shown by previous work (O’Beirne 
et al., 2017). While higher inputs of terrestrial OM 
due to high magnitude storm events could also 
contribute to higher signals, the lake’s high surface 
area to watershed ratio mediates this contribution.

 
 
 

 

Results shown in Figures 3. and 4. exhibited #13C 
signatures that identi!ed lacustrine OM, with C:N 
ratios indicative of algal and C3 land plant sources. 
#13C values of -27.89‰ to -24.78‰ (mean of 
-25.84‰ ± 0.2‰) in Siskiwit Bay re'ected higher 
autochthonous sourcing than Siskiwit Lake (Fig. 3.). 
As observed in the TOC results, the e&ect of the sand 
layer was evidenced in signi!cantly lower isotopic 
composition from 55.5 to 63.5 cm in the bay. A 
less enriched isotopic average of -29.0‰ ± 0.1‰ in 
Siskiwit Lake implied slightly higher allochthonous 
contribution of OM (Fig. 3.). "is could be attributed 
to a higher ratio of the lake water to surrounding 
watershed area and more direct interactions between 
them, introducing terrestrial OM to the lake in high 
concentrations relative to its size, compared to 
Siskiwit Bay. 
 "e cross-plot of the elemental and isotopic 
data shown in Figure 4. was adapted from Meyers 
(1994) and further used to model the trends of 



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  2024

this freshwater algae OM with depth (Figures 4a. 
and 4b.). Isotopic signatures from 55.5 to 63.5 cm 
in Siskiwit Bay were included but the C:N ratios 
were not. Siskiwit Lake exhibited a more consistent 
C:N than Siskiwit Bay over time (Fig. 4b.), as was 
expected due to the regular frequency of blooms 
in the lake and watershed to lake surface area ratio, 
and higher overall C:N signature. Increasing TOC 
in Siskiwit Bay was re'ected in an increased C:N 
ratio close to the core surface (Fig. 4a.). C:N ratios in 
the bay were less consistent over time, where ratios 
decreased slightly from the bottom to middle of the 
core and increased from the middle to the surface 
(Fig. 4a.). As highlighted in Figure 2., C:N trends 
plotted against lacustrine algae isotope signatures 
could suggest increasing primary productivity 
within the bay (Fig. 4.; O’Bierne et al., 2017, Meyers 
et al., 1994). 

B. Pigments Indicative of Cyanobacterial Presence in 
2021 Algal Bloom 
 Results from the water !lter pigment extractions 
showed biomass concentrations predominating from 
cyanobacteria and chlorophyte sources within the 
water column during the 2021 Siskiwit Lake bloom. 
"e most abundant pigments, pheophytin-a (3.484 
mg/gOC) and Chl-a (2.868 mg/gOC) (Table 1.), are 
considered general indicators of productivity levels 
(Bianchi & Canuel, 2011; Huisman et al., 2018; Je&ery 
& Hallegrae&, 1987). Combined with ,-carotene 
(0.618 mg/gOC) (Table 1.), general productivity 
pigments accounted for 62.24% of bloom biomass 
(Fig. 5.). Cyanobacteria pigments (aphanizophyll, 
canthaxanthin, echinenone, and myxoxanthophyll) 
constituted 18.61%, notably higher than the 
7.44% (Chl-b and neoxanthin) contributed by 
only green algae/chlorophytes and higher plants 
(Fig. 5.; Bianchi & Canuel, 2011; Egeland, 2016; 
Huisman et al., 2018; Paerl, 2017). Diatoms and/
or dino'agellates (fucoxanthin, diadinoxanthin, 
diatoxanthin, and violaxanthin) and cryptophytes 
(alloxanthin) provided approximately 11.17% and 
0.80%, respectively (Fig. 5.; Bianchi & Canuel, 2011; 
Egeland, 2016).

IV. Discussion
 Elemental and isotopic data obtained from the 
Siskiwit Bay and Siskiwit Lake sediment deposits 
exhibited dominant signals of aquatic and terrestrial 
OM sources, respectively. Increasing trends of 
organic carbon concentrations were observed in 
the bay, withholding the arti!cially lowered TOC 
content of sand layer, while a 'uctuating deposition 
of OM was found within Siskiwit Lake (Fig. 2). 
Isotopic carbon signatures were predictably low, 
or less enriched, overall and shi)ed towards a less 
enriched pro!le approaching present day in both 
sites (Fig. 3). Separately, before the recent decrease 
observed from approximately 30 cm to 0 cm, trends 
in Siskiwit Bay exhibited a general enrichment over 
time with distinct drops that could be explained 
by historic cyanobacteria blooms (Fig. 3). With 
concurrent trends in carbon ratios, the change 
towards less negative isotope signatures could depict 
increased algal derived sourcing (Meyers, 1994). 
Despite remaining near the algal range of 4 to 10, 
substantial C:N increases approaching the lakebed’s 
surface might posit rising contribution from C3 

51



Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  2024

lands plants and/or less degraded sedimentary OM 
(Fig. 4a.; Meyers, 1994). 
 Siskiwit Lake bulk elemental and isotopic 
signatures were expected to reveal sedimentary OM 
largely derived from algal sources, based on the 
lake’s higher frequency of cyanobacteria blooms. 
While isotopic signatures alone could be interpreted 
as supporting this hypothesis (Fig 3.), relatively 
sustained C:N ratios downcore revealed higher 
inputs of carbon-rich material than nitrogen-rich 
materials, likely contributed by C3 plants in the 
surrounding watershed (Fig. 4.). 
 When reviewed alongside the characterization 
of Siskiwit Lake, interpretations of higher 
autochthonous sourcing in Lake Superior were 
supported by increasing TOC and #13C values and 
lower C:N ratios. Elevated C:N ratios near the 
lakebed surface do not completely denounce but 
marginally lower con!dence in these conclusions. 
Further di&erentiating whether increasing elemental 
ratios are truly due to increased terrestrial input, 
such as vascular land plants being deposited in the 
sediment through high magnitude discharge events, 
or higher abundance of autochthonous algal sources 
could be sought in historical environmental records. 
Moreover, further statistical modelling to distinguish 
these endmembers and cross-analysis of downcore 
phytoplankton pigment pro!les would provide more 
con!dence in bulk OM proxy interpretations.   
 While data for the downcore pigments in 
Siskiwit Bay and Siskiwit Lake were not yet available, 
pigment identi!cation and quanti!cation of the 
2021 Siskiwit Lake algal bloom showed a dominance 
of cyanobacteria, diatoms, and green algae. "e 
unique pigment suite also indicated the presence 
of photosynthesis-requiring and other pigments 
produced by several species, along with other 
pigment degradation products (Table 1.). High 
concentrations of chlorophyll-a suggested high 
primary productivity overall (Fig. 5.), as expected. 
Equally high concentrations of pheophytin-a 
were also expected and likely originated from 
bacterial decay or grazing activities (Fig. 5.) 
(Leavitt, 1993). Considerable cyanobacteria and 
diatom/dino'agellate sourcing was implied, with 
lower contribution from chlorophytes and other 
cryptophyte phytoplankton communities (Fig. 5.). 
While toxicity has not been assessed, these data 
suggested the presence of potentially nuisance 

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Pigment and Geochemical Markers of Past and Present Cyanobacterial Presence

Aisthesis      Volume 15,  2024

cyanobacteria species present in Siskiwit Lake, WI, 
and that these inland lake blooms could be potential 
drivers of cyanobacteria accumulation in Lake 
Superior via 'uvial seeding (i.e. Siskiwit River) (Fig. 1.; 
Sterner et al., 2020). "e data would also corroborate 
the presence of cyanobacteria paleo-indicators in the 
sediment cores, providing a modern-day reference 
suite for ancient algal communities.

V. Contextualization of Work
 "e recent rise of cyanobacterial blooms in 
Lake Superior in'uences much more than the 
health of the immediate aquatic ecosystem. Not 
only one of the largest freshwater lakes but also one 
of the cleanest, Lake Superior supplies important 
environmental services, particularly potable water, 
a&ecting an expansive population (Sterner et al., 
2020). One of many projects examining the lake’s 
health at the Large Lakes Observatory (LLO), this 
study contributes a cornerstone to improving our 
understanding of climate change interactions within 
Lake Superior’s watershed. An initial pro!ling of 
the lake’s sedimentary record and current bloom 
biomass composition provides temporal insight into 
the region’s cyanobacteria communities. It could also 
reveal spatial connections to inland lakes and rivers 
with the hydrological potential to seed blooms along 
the southern shores. In further research through the 
LLO, this preliminary data will be paired with the 
presence of algal pigments within the sedimentary 
record to reconstruct intervals of high productivity 
and identify potential bloom periods within the 
Lake Superior region. Continued work may include 
genomic sequencing and species identi!cation, 
providing the structure for HAB prediction models 
and water quality management strategies. 

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