Layout 1 INTRODUCTION Palaeolimnological tools can be applied to reveal in- formation retained in aquatic sediments (Smol 1992, Eu- ropean Union 2000). Cladocerans (Branchiopoda: Phyllopoda), which have been demonstrated to be excel- lent indicators of environmental change (Eggermont and Martens, 2011), have been used in palaeolimnological studies to reconstruct past environmental conditions of lake ecosystems since the 1950s (Frey, 1986). Despite their obvious value, methodological limitations and sources of error still exist. Cladoceran remains, for exam- ple, preserve selectively in lake sediments, as some species and some components preserve better than others. In the case of Daphniids O.F. Müller, only postabdomal claws and resting spores (ephippia) are typically found in sediments (Schmidt et al., 1998; Korhola, 1999; Sarmaja- Korjonen, 2002), while the small size of their claws in- flicts an elevated risk of losing claws during sieving. This may lead to an underestimation of Daphnia O. F. Müller abundance when it is based on the number of claws (Nykänen et al., 2009). The poor preservation of Daphnia is particularly problematic, because Daphnia is by far the most studied cladoceran taxon (Ebert, 2005). For exam- ple, Peters and de Bernardi (1987) and Lampert (2011) published comprehensive reviews of Daphnia ecology, and the number of scientific articles on Daphnia indexed in the Thomson Reuters Web of Science exceeds 10 000 (Seda and Petrusek, 2011). Nevertheless, Daphnia re- mains do preserve well in some lakes, such as in Lake Kivijärvi (Finland), although the reasons behind this vari- ation in preservation are not yet fully understood. The preservation issues in sedimentary cladocerans have been studied before (Deevey, 1964; Kerfoot, 1981), but we are unaware of previous research attempting to assess the variations in Daphnia degradation in a sediment core. In this study we combined data on Daphnia preserva- tion to sedimentary geochemistry, diatom-inferred lake water pH, predation indices and known land use and fish- ing history of Lake Kivijärvi in order to shed new light on the preservation issues of Daphnia remains. METHODS Study area Lake Kivijärvi is a small (1.8 km2), relatively deep (max depth 12 m), brown-coloured lake, which is sur- rounded by mires and coniferous forests. It is located at 165 m asl in central northern Finland (N 63° 55.998’ E 27° 54.281’). During the summer stratification in August 2006, which corresponds to the age of our topmost sedi- Advances in Oceanography and Limnology, 2016; 7(2): 142-150 ARTICLE DOI: 10.4081/aiol.2016.6293 This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). Varying degradation of subfossil Daphnia longispina during the past 250 years and the discovery of fossil helmet-type head shields: preliminary results Jaakko Johannes Leppänen,* Jan Weckström Department of Environmental Sciences, University of Helsinki, P.O. Box 65, FIN-00014, Finland *Corresponding author jaakko.leppanen@helsinki.fi ABSTRACT Zooplankton are regarded as a good indicator of environmental change, but comprehensive monitoring programs including zoo- plankton are uncommon and only rarely extend over longer periods of time. A part of the zooplankton community can be reconstructed using palaeolimnological methods, yet challenges remain. For example, cladoceran subfossil remains preserve selectively in sediments. In particular, the remains of Daphnia spp. are known to usually exhibit poor level of preservation; the reasons for this are still unclear. In the rural Lake Kivijärvi, located in central Finland, Daphnia subfossil remains preserve extraordinary well and multiple fossil com- ponents are found. However, the preservation level is not uniform and exhibits directional change throughout the sediment record. To investigate the changes in Daphnia preservation in lake sediments, we graded caudal spines from 20 fossil sediment samples into three taphonomic groups. A dataset of sediment geochemistry, diatom-inferred lake water pH, predation indices, and the catchment land use history was used to assess the environmental history of our study lake. In Lake Kivijärvi, the most significant change in Daphnia preser- vation seems to correspond best with the historical fishing activities. Additional explanatory variables include forestry in the catchment area, and pH, which, however, had contradicting effects on the preservation of Daphnia remains in this study. Finally, a fossil Daphnia longispina helmet type head shield derived from the lake sediment is presented for the first time. Key words: Daphnia; preservation; subfossil; palaeolimnology; cladocera. Received: September 2016. Accepted: November 2016. No n c om me rci al us e o nly Varying preservation of subfossil Daphnia longispina 143 ment sample, the lake surface water (1 m) was slightly acidic (pH 6.2), with an alkalinity of 59 µmol L–1 and con- ductivity of 19 µS cm–1. The total phosphorus and total nitrogen concentrations measured during this period were 33 µg L–1 and 540 µg L–1, respectively, and the chloro- phyll-a concentration 22 µg L–1. The lake is fed by River Lumijoki and the brook Myllypuro. Human activities, such as land use and fishing have affected the lake since the beginning of the 20th century. The catchment area (53 km2) has been heavily ditched during forestry practices, which were most intense during the 1960s and 1970s. In addition, one peat extraction area (33 ha) located in the western part of the Kivijärvi catchment operated between 1986 and 2012 (N. Huotari, personal communication). Today the proportion of the ditched area in the lake catch- ment is over 40%. More recent disturbances (since 2008) include sulphate and metal pollution from the Talvivaara Ni-Cu-Zn-Co mine, which is connected to Lake Kivijärvi via Lumijoki River and is located approx. 6 km NE from Lake Kivijärvi (Kauppi et al., 2013). The average (1971- 2000) yearly air temperature and precipitation are +2°C and ~600 mm, respectively (Kersalo and Pirinen, 2009). Annual precipitation has increased by an estimated 0.92 mm±0.50 mm year–1 (Irannezhad et al., 2014) during the last century and mean air temperature has increased by 2°C during the past 166 years (Mikkonen et al., 2014). Bathymetric data was obtained from the Finnish Envi- ronment Institute electronic GIS library (http://wwwd3.ym- paristo.fi/d3/wmsrajapinta.htm), the catchment area was determined using the online tool VALUE provided by the Finnish Environmental Institute (http://paikkatieto.ympar- isto.fi/value/) and was analysed using the geographical in- formation system program ESRI ArcMap 10.2.1. Water chemistry data was obtained from the OIVA database (Finnish Environment Institute, https://wwwp2.ymparisto. fi/scripts/oiva.asp). Data regarding forestry activity be- tween 1966 and 1994 was obtained from historical map archives of the National Land Survey of Finland (http://vanhatpainetutkartat.maanmittauslaitos.fi/). Coring Two short sediment cores were retrieved from the northern section of Lake Kivijärvi (N 63° 55.998’E 27° 54.281’ WGS84, at the depth of 8 m) using a HTH-Kajak sediment corer with a diameter of 9 cm (Renberg and Hansson, 2008). Core A (26 cm length) was used for sediment dating, and for analyses of sub-fossil cladocerans and diatoms, core B (24 cm) was used for elemental analysis. The cores were subsampled at 0.25 cm intervals for the first 5 cm and at 1 cm intervals between 5 and 26 cm. The initial rea- son for different subsampling for the first 5 cm was to pro- vide possibility for high resolution assessment of known mine pollution, which is affecting lake water quality since 2008. Mining effluent has resulted in elevated lake metal (Fe, Mn, Ni, Zn) and salt (NaSO) concentrations, and has turned the lake meromictic (Kauppi et al., 2013). Subsam- pling was conducted in the field and the samples were stored in plastic zip lock bags and stored in a dark cold room at 4°C. As the two cores were retrieved from the flat and large center of the lake and less than 1.5 m apart from each other, the temporal resolution of these two cores can be assumed as comparable. Nevertheless, the cores were correlated based on their visual features. Both cores ex- hibited analogous and clear changes in sediment compo- sition in the top first cm, where the sulfur rich layer originating from the mine pollution disappeared strictly at 0.75-1 cm level. Moreover, both cores showed a clear tran- sition layer between 4 and 6 cm, where the sediment color changed to darker brown. Below this, both cores showed unchanged sediment color and texture. The top 0-1 cm core samples were omitted from the study, as they were deposited after the onset of mine pollution and therefore strongly affected by metals and sulphur, which caused the almost complete disappearance of Daphnia remains. Dating and sediment chemistry Freeze dried sediment samples from core A were ra- diometrically dated at the Liverpool University Environ- mental Laboratory, using Ortec HPGe GWL well-type coaxial low background intrinsic germanium detectors (Appleby et al., 1986). The samples were analysed for 210Pb, 226Ra, 137Cs and 241Am by direct gamma assay. Ele- mental concentrations of erosion indicators magnesium (Mg), potassium (K) and sodium (Na) were analysed in the acid soluble (NHO3) sediment fractions in the Metropolilab Environmental laboratory at Helsinki, which is an accredited testing laboratory (FINAS T058). Daphnia analysis Each sediment subsample (~1 cm3 of wet material) was treated using 10 % KOH solution, heated (80°C) on a hotplate for 30 min, and sieved through 48 µm mesh size with tap water. Samples were mounted with safranine stained glycerol. For more information regarding the pro- tocol, see Szeroczyñska and Sarmaja-Korjonen (2007). Daphnia remains were identified and counted using both 200 and 400 x magnifications. During the preliminary mi- croscopic screening, a broad variation in the preservation of postabdominal claws was observed, e.g. notable varia- tions in type and magnitude of damage in claws, denticles and setae. In addition, also the size of the claws showed large variability, which, in turn, has a direct impact on their resistance to damage. Classification of claws was thus found to be extremely difficult, whereas the differ- ences in degree of preservation were clearer for caudal spines. Caudal spines were therefore identified as the most No n c om me rci al us e o nly 144 J.J. Leppänen and J. Weckström suitable component to study the variance in preservation of subfossil Daphnia. To evaluate these preservation dif- ferences, 100 spines from each one of the 20 sediment subsamples were identified and assessed for a taphonomic grade. Taphonomic grading is used in palaeoecology to assess the deposition speed and reworking of fossil as- semblages (Brandt, 1989). The studies regarding micro- bial decomposition of Daphnia remains are rare, but it has been reported that Daphnia remains are rapidly and heav- ily colonized by chitin degrading microbes (Tang et al., 2009) and fungi (Czezuga et al., 2002). The colonization by degrading organisms would imply a rather even and comprehensive degradation within a certain sediment layer than mechanical impacts. In the present work each spine was graded in relation to its stiffness, folding and breakage (Tab. 1; Fig. 1). We assumed that folded or bro- ken spines have been damaged due to mechanical impact, and spines that have completely lost their form (slack spines) have been affected by microbial or chemical degradation. Predation assessment To assess changes in predation on cladocerans in Lake Kivijärvi, we conducted morphological measurements of Eubosmina longispina Leydig exoskeletal remains (36-46 measurements of carapace and mucro length per sample), and counted the Chaoborus spp. Lichtenstein, mandibles detected in the sediment subsamples to calculate the ratio (number of Chaoborus spp. mandibles: 100 Daphnia spines). The value as an ecological indicator of morpho- logical changes within the genus Bosmina has been recently reviewed by Korosi et al. (2013), but the applicability of subgenera Eubosmina morphology is not so straight for- ward due to contrasting results in literature (Sprules et al., 1984; Johnsen and Raddum, 1987). In contrast, the abun- dance of Chaoborus larvae has been demonstrated to reflect the abundance of fish and to affect the cladoceran commu- nity structure (Sweetman and Smol, 2006). Numerical analysis The non-parametric Mann-Kendall trend test (Gilbert, 1987) was used to detect significant monotonic trends in the taphonomic grade data of sedimentary Daphnia re- mains. Constrained optimal sum of squares partitioning with untransformed species percentage data (Birks and Gordon, 1985) was used to detect significant zones in taphonomic stratigraphy. The number of statistically sig- nificant zones was calculated using the broken-stick model described in Bennett (1996). Optimal partitioning was conducted using the program ZONE 1.2 (Lotter and Juggins, 1991). The Mann-Kendall test was conducted with PAST statistics 3.10 software (Hammer et al., 2001). Tab. 1. Taphonomic grades for Daphnia caudal spines. Excellent preservation Fair preservation Poor preservation Stiff and straight spines with Stiff and straight spines with Slack spines. Initial shape is no visible damage breakage and/or folding no longer sustained Fig. 1. A) An example of a well-preserved caudal spine graded as excellent. B) A relatively well preserved, but visibly damaged caudal spine graded as fair. C) A poorly preserved caudal spine graded as poor. No n c om me rci al us e o nly Varying preservation of subfossil Daphnia longispina 145 Diatom-inferred lake water pH (DI-pH) The pH history of Kivijärvi was quantitatively recon- structed using an independent modern diatom-water pH calibration data set consisting of 98 surface-sediment di- atom assemblages from northern Finland and correspon- ding pH measurements (for more details see Seppä and Weckström, 1999; Väliranta et al., 2011). After testing dif- ferent models (WA-inverse deshrinking, WA-classical deshrinking, Partial-Least-Squares) the 1-component weighted average partial least squares (WA-PLS) model provided the best performance with a coefficient of deter- mination (r2) of 0.68 and a root mean square error of pre- diction (RMSEP) of 0.31 pH units. This diatom-based quantitative pH model was then applied to the fossil di- atom data analyzed from Lake Kivijärvi. Methods and taxonomic literature used for diatom analyses are de- scribed in Weckström et al. (1997). The quantitative DI- pH is inferred only for the depths of 1-16 cm, as the original reason for constructing the DI-pH model was to study the recent impact of the Talvivaara mine on the Lake Kivijärvi water chemistry (unpublished data). RESULTS Dating and sediment geochemistry According to the radiometric dating, the sediment depth of 2.5 cm records the fallout from Chernobyl reactor accident (AD 1986) and the depth of 4.5 cm records the fallout maximum from atmospheric nuclear weapons test- ing (AD 1963, Appleby et al., 1991). The dating provided a sedimentation rate of approximately 0.1 cm year–1. Un- supported lead concentrations reach zero values at the depth of 10 cm, which corresponds to the beginning of the 20th century (1910±8). Concentrations of K and Mg increased after 1925 and reached maximum values in the late 1990s (Fig. 2). Fig. 2. Results of taphonomical stratigraphy, geological characteristics, predation indices and diatom-inferred lake pH. Eubosmina measurements are in µm (bar, average carapace length or mucro length; vertical line inside bar, standard deviation). Note that Eubosmina were not measured between depths 1-5 cm. Chaoborus spp., number of Chaoborus spp. claws counted in each sample per 100 Daphnia spines. Horizontal line indicates the statistically significant shift in the preservation pattern. No n c om me rci al us e o nly 146 J.J. Leppänen and J. Weckström Daphnia remains Many Daphnia remains were detected throughout the core, except for the top 0-1 cm section, where they were extremely rare, likely in relation to the impact of the mine pollution. In the other sediment layers, postabdominal claws, carapace, ephippia, caudal spines and head shields were present in large numbers. Only Daphnia longispina O.F. Müller -type postabdominal claws were detected. Head shields showed various degrees of preservation in- cluding many very well preserved helmet-type head shields (Fig. 3A), with one head shield still attached to the carapace (Fig. 3B). There was a significant increasing trend towards present in the proportion of caudal spines graded as excellent (Mann-Kendall: Z=3.88, S=103, P<0.001) and fair (Mann-Kendall: Z=3.53, S=94, P<0.001), and a significant decreasing trend in the pro- portion of caudal spines graded as poor (Mann-Kendall: Z=3.94, S=-105, P<0.001). Zoning revealed two signifi- cant zones at depths 1-9 and 9-26 cm (Fig. 2). Predation assessment The length of the mucro and carapace of E. longispina exhibited no clear shifts within the core (Fig. 2). Measure- ments were terminated at the depth of 5 cm because of the appearance of Bosmina longirostris O. F. Müller in the sed- iment, as carapaces of E. longispina and B. longirostris are extremely difficult to be discriminated from each other. Chaoborus mandibles were detected in very small numbers (0-2 mandibles per sample) throughout the core. DI-pH The DI-pH of the deepest core layers (which were likely deposited during the 18th century), was comparable to DI-pH values around year 2000 (Fig. 2). However, DI- pH started to decrease since the late 19th century reaching the lowest values during the 1960s (Fig. 2). DI-pH re- mained at lower values between ca. 1920 and 1995, but later increased towards modern levels. DISCUSSION Daphnia remains in Lake Kivijärvi Usually only Daphnia postabdomal claws and ephip- pia are found in lake sediments. Sometimes, as reported by Frey (1991), Mancini et al. (1999), and Sarmaja-Kor- jonen (2007), different components of Daphnia subfossils can stay relatively intact in the sediment for extended pe- riods of time. Also Daphnia head shields have been de- tected in sediment samples (Frey, 1991; Mancini et al., 1999), but to our knowledge, helmet-type head shields of Daphnia longispina –group species have not been re- ported from lake sediment samples before. In the sedi- ments of Lake Kivijärvi sediments, the Daphnia head shields appeared folded with fornices clearly visible, and Fig. 3. A) Daphnia helmet type headshields. Scale bar: 100 µm. B) Daphnia headshield attached to carapace. No n c om me rci al us e o nly Varying preservation of subfossil Daphnia longispina 147 a number of different structures were well preserved. However, the caudal spines showed the highest abun- dance. Even though postabdominal claws are usually used in research on subfossil Daphnia (Korosi et al., 2011), caudal spines were used in the present work, as they could be classified more easily due to their simpler form, while the classification of claws in relation to physical condition would have been very difficult due to the higher variabil- ity in their characteristics and damage magnitude. In ad- dition, caudal spines exhibited one additional character of degradation, namely the total loss of form (slack spines). The spines graded as poorly preserved were slack, sug- gesting that their structure had been altered. According to Tang et al. (2009), cladoceran carcasses are rapidly and heavily colonized by chitin-degrading bacteria. Loss of form may therefore refer to microbial or chemical degra- dation, rather than to a mechanical impact. In addition, the loss of form can be considered as linked to autochtho- nous in-sediment processes, because, if the loss of form were a result of sample preparation, it should have af- fected similar proportions of spines in all samples. In con- trast, spines graded as fair good preserved were almost intact apart from small breakages likely resulting from sample treatment (e.g., sieving) or from predator or scav- enger attacks. Due to the uncertainty in pointing out whether the damage originates from laboratory treatment or from autochthonous processes, the indicator value of fair graded spines in this study is negligible. Changes in Daphnia preservation in Lake Kivijärvi According to the statistically significant core zonation, the greatest shift in Daphnia preservation occurred at the depth of 9 cm (~1925) and was clearly reflected by changes in proportions of different grades. Though not statistically significant, also the depth of 12 cm (~ pre 1900) clearly emerged as the level where early changes in Daphnia preservation onset. The 9 cm shift is most pro- nounced for the excellent and poor grades, whereas the change regarding spines graded as fairly preserved is not as distinct. Earlier studies suggest that Daphnia preserva- tion is controlled by temperature (Szeroczyñska and Za- wisza, 2005) or water chemistry (Sarmaja-Korjonen, 2007). However, changes in hypolimnetic temperatures of Lake Kivijärvi during the 1920s are not likely, because a most notable warming occurred in N-Finland only after the 1960s (Mikkonen et al., 2014). Moreover, hypolim- netic temperatures during stratification are not easily al- tered by relatively small scale increase in summer air temperatures (Arvola et al., 2010). DI-pH showed a slight decrease since ~1910, which may have been related to natural acidification i.e. contin- uous leaching of buffering mineral elements like calcium (Ca), potassium (Na), magnesium (Mg), and sodium (K) (Pennington, 1984). In fact, the proportion of naturally acidified lakes is higher in central and norther Finland compared to S Finland, where anthropogenic acidification is more common (Meriläinen and Huttunen, 1990). The decrease in DI-pH since ca. 1910 corresponds relatively well to the change in Daphnia preservation, but effects of pH on preservation changes is not straight forward. In fact, preservation was not affected when pH increased since ca. 1995, thus contradicting the response during slight acidification at the beginning of the 20th century. The lowest DI-pH values around 1960 could be due to the increased impact of acid rain on small freshwaters as sul- phur emission in Finland increased substantially during the period of 1950-1970 (Kauppi et al., 1990). However, this period of lower pH seemed not to affect the preser- vation of Daphnia remains (Fig. 2). The level of preservation of all subfossil material is usually related to accumulation rates, with good preser- vation (high taphonomic grade) reflecting fast accumula- tion and burial (Brandt, 1989). Since the core was only 210Pb-dated, we lack dates previous to 1900 (i.e., below the depth of 10 cm). As a consequence, the possibility of low sedimentation rates and subsequent poor preservation of Daphnia before ca. 1925 cannot be ruled out. Possible reasons behind major changes in the sedimentation rate during the pre-industrial era should be principally related to hydrological perturbations, such as water level manip- ulation and channel building. However, according to the map by Gylden (1848), shape and size of Kivijärvi did not change since the mid-19th century, which excludes any large-scale water works during the early 20th century. Cladoceran remains are directly affected by microbial degradation of chitin, which occurs both in the water and in the sediment (Swiontek Brzezinska et al., 2008). Bac- teria are the most important responsible of chitin decom- position in aquatic environments (Aumen, 1980; Gooday, 1990), but there are indications that particularly Daphnia remains are attacked also by aquatic fungi (Czezuga et al., 2002). In addition, chemical changes of the sediments (di- agenesis) may alter the conditions within the sediment and the degree of ongoing degradation of chitinous subfossil remains (Kidwell and Flessa, 1995). Forestry activities may affect lake water characteristics such as chemical oxygen demand (Ahtiainen, 1992; Rask et al., 1998), which in their turn may affect the fungal community (Wurzbacher et al., 2010) and slow down the bacterial de- composition of chitin (Köllner et al., 2012) at the sedi- ment-water interface. However, documented forestry activities occurred well after the 1920s, while the recent increasing concentrations of erosion indicators (Virkka- nen and Tikkanen, 1998) in the studied sediment core started only during the last decades of the 19th century. In addition to forestry actions, also changes in lake produc- tivity could affect microbial community (Naeher et al., 2012). However, according to the available maps and per- No n c om me rci al us e o nly 148 J.J. Leppänen and J. Weckström sonal communication by S. Peronius, no farmland existed within the catchment during the early 20th century. More- over, the first residents occupying the lake´s shoreline in the early 20th century were not farmers, but employed, thus the agricultural land use was minimal. This is also supported by the diatom data (not shown here), which did not show any indication of eutrophication as no taxa pre- ferring elevated nutrient concentrations occurred during the last centuries. The sole known potentially relevant event that oc- curred in Lake Kivijärvi during the 1920s was the begin- ning of intense fishing activity, which resulted in large vendace (Coregonus albula Linneaus) catches since the 1920s (S. Peronius, personal communication). Plank- tivorus fish are known to target large-size zooplankton Zaret, 1980), and vendace has been noted to prefer large cladocerans, such as Eubosmina and Daphnia, in Swedish (Hamrin, 1983) and Finnish (Viljanen, 1983) forest lakes. Cladoceran remains are poorly digested during their pas- sage through fish guts (Sutela and Huusko, 1993; Ric- cardi, 2000) and Daphnia remains have been proved to survive the passage through vendace and whitefish diges- tive apparatus (Sutela and Huusko, 2000). The poor preservation level of Daphnia spines before the 1920s may therefore reflect a higher proportion of partly di- gested Daphnia in the correspondent sediment samples. The intensive removal of fish may have also increased the abundance of invertebrate predators (Milardi et al., 2016). Elevated invertebrate predation has been noted to induce thickening and hardening of Daphnia carapaces (Rabus et al., 2013). This might be the reason for the appearance of hard and thick remains, which are more resistant to post-mortem degradation. However, the lack of variations of Eubosmina size suggests that no high magnitude changes in invertebrate predation have occurred during the shift in Daphnia preservation. Moreover, the very low number of Chaoborus mandibles in our samples (<2 per sample) allow no assumptions regarding changes of pre- dation regime in Lake Kivijärvi, because according to Quinlan and Smol (2010) a minimum of 5 to 10 mandibles is needed to reliably assess Chaoborus assem- blages. High head helmet and a long spine are also con- sidered as general defensive structures, which are grown by Daphnia when they are subjected to invertebrate pre- dation (Laforsch and Tollrian, 2004). The fact that only Daphnia remains with long spine and high helmet were identified throughout the core studied, further supports the hypothesis that only little change in the intensity of invertebrate predation occurred at Lake Kivijärvi during the last ~250 years. The results presented here suggest the potential of Daphnia caudal spine to be used as indicator of clado- ceran degradation in sediments. However, more work is still necessary to further evaluate this potential. For ex- ample, type and degree of degradation of caudal spines caused by fish ingestion, should be experimentally as- sessed. Similarly, the microbial degradation of Daphnia remains should be tested in a controlled environment, in order to clarify the degradation process and the actual mi- crobial community involved in Daphnia degradation. CONCLUSIONS This work provides first evidence for the potential use of caudal spine as indicator of Daphnia degradation in sediments. Historical information and sediment geochem- istry suggest that no large-scale environmental or hydro- logical changes have affected the remote Lake Kivijärvi during the early decades of the 20th century, when major changes in Daphnia preservation occurred. On the other hand, the historical data and the diatom-inferred pH pro- file indicate the increase in fishing activity after 1920 and the steady decrease in water pH from the beginning of 20th century till the 1960s as possible drivers of spine preser- vation of Daphnia. Though more work is clearly needed to experimentally verify the role of these factors in affect- ing Daphnia preservation, these preliminary results shed new light on the issue of large cladoceran preservation in lake sediments. This aspect is still poorly explored, but might have the potential to improve the reliability of palaeolimnological reconstructions. ACKNOWLEDGMENTS This work was funded by Tellervo and Jussi Walden foundation, Soil protection and environmental protection technology (MUTKU) society and Kainuu Centre for Economic Development, Transport and the Environment. We are grateful to Jussi Leppänen for assistance in the field and to two anonymous reviewers for their valuable comments and constructive suggestions which greatly im- proved this manuscript. REFERENCES Ahtiainen M, 1992. The effects of forest clear-cutting and scar- ification on the water quality of small brooks. Hydrobiologia 243/244:465-473. Appleby PG, Nolan PJ, Gifford DW, Godfrey MJ, Oldfield F, Anderson NJ, Battarbee RW, 1986. 210Pb dating by low background gamma counting. Hydrobiologia 141:21-27. Appleby PG, Richardson N, Nolan PJ, 1991. 241Am dating of lake sediments. Hydrobiologia 214:35-42. Arvola L, George G, Livingstone DM, Järvinen M, Blenckner T, Dokulil MT, Jennings E, Aonghusa CN, Noges P, Noges T, Weyhenmeyer GA, 2010. The impact of the changing cli- mate on the thermal characteristics of lakes, p. 85-101. In: G. George (ed.), The impact of climate change on European No n c om me rci al us e o nly Varying preservation of subfossil Daphnia longispina 149 lakes, Aquatic Ecology Series 4. Springer Science+Business Media B.V. Aumen NG, 1980. Microbial succession on a chitinous substrate in a woodland stream. Microbial Ecol. 6:317-327. Bennett K, 1996. Determination of the number of zones in a biostratigraphical sequence. New Phytol. 132:155-170. Birks HJB, Gordon AD, 1985. Numerical methods in Quater- nary pollen analysis. Academic Press, London: 317 pp. Brandt DS, 1989. Taphonomic grades as a classification for fos- siliferous assemblages and implications for paleoecology. Palaios 4:303-309. Czeczuga B, Kozłowska M, Godlewska A, 2002. Zoosporic aquatic fungi growing on dead specimens of 29 freshwater crustacean species. Limnologica 32:180-193. Deevey ES, 1964. Preliminary account of fossilization of zoo- plankton in Rogers Lake. Int. Ver. Theor. Angew. Limnol. Verh. 15:981-992. Ebert D, 2005. Ecology, and evolution of parasitism in Daphnia. National Library of Medicine (US), National Center for Biotechnology Information, Bethesda: 110 pp. Eggermont H, Martens K, 2011. Preface: Cladocera crustaceans: sentinels of environmental change. Hydrobiologia 676: 1-7. European Union, 2000. Directive 2000/60/EC of the European Parliament and the council of 23 October 2000 on establish- ing a framework for community action in the field of water policy. Eur. J. Commun. L327:1-72. Frey DG, 1986. Cladocera analysis, p. 667-692. In: B.E. Berglund (ed.), Handbook of palaeoecology and palaeohy- drology. J. Wiley & Sons Ltd. Frey DG, 1991. First subfossil records of Daphnia headshields and shells (Anomopoda, Daphniidae) about 10 000 years old from northernmost Greenland, plus Alona guttata (Chydori- dae). J. Paleolimnol. 6:193-197. Gilbert RO, 1987. Statistical methods for environmental pollu- tion monitoring. Van Nostrand Reinhold, New York: 336 pp. Gooday GW, 1990. The ecology of chitin degradation. Adv. Mi- crob. Ecol. 11:387-430. Gylden CW, 1863. [Sydöstra delen af Uleåborgs län med norra delen af Kuopio län].[Map in Finnish]. Sektionen D4. Karta öfver Finland. Hammer Ø, Harper DAT, Ryan PD, 2001. PAST: Paleontologi- cal statistics software package for education and data analy- sis. Palaeontol. Electron. 4:1-9. Hamrin SF, 1983. The food preference of vendace (Coregonus albula) in South Swedish forest lakes including the preda- tion effect on zooplankton populations. Hydrobiologia 101:121-128. Irannezhad M, Marttila H, Kløve B, 2014. Long-term variations and trends in precipitation in Finland. Int. J. Climatol. 34:3139-3153. Johnsen GH, Raddum GG, 1987. A morphological study of two populations of Bosmina longispina exposed to different pre- dation. J. Plankton Res. 9:297-304. Kauppi S, Mannio J, Hellsten S, Nysten T, Jouttijärvi T, Huttunen M, Ekholm P, Tuominen S, Porvari P, Karjalainen A, Sara- Aho T, Saukkoriipi J, Maunula M, 2013. [Arvio Talvivaaran kaivoksen kipsisakka-altaan vuodon haitoista ja riskeistä vesiympäristölle. Suomen ympäristökeskuksen raportteja 11/2013].[Book in Finnish]. Suomen Ympäristökeskus, Helsinki: 90 pp. Kerfoot WC, 1981. Long-term replacement cycles in cladoceran communities: A history of predation. Ecology 62:216-233. Kersalo J, Pirinen P, 2009. [The climate of Finnish regions].[Book in Finnish]. Finnish Meteorological Institute. Yliopistopaino, Helsinki: 185 pp. Kidwell SM, Flessa KW, 1995. The quality of fossil record: pop- ulations, species and communities. Annu. Rev. Ecol. Syst. 26:269-299. Korhola A, 1999. Distribution patterns of Cladocera in subarctic Fennoscandian lakes and their potential in environmental re- construction. Ecography 22:357-373. Korosi JB, Jeziorski A, Smol JP, 2011. Using morphological characters of subfossil daphniid postabdominal claws to im- prove taxonomic resolution within species complex. Hydro- biologia 676:117-128. Korosi JB, Kurek J, Smol JP, 2013. A Review on utilizing Bosmina size structure archieved in lake sediments to infer historic shifts in predation regimes. J. Plankton. Res. 35:444-460. Köllner KE, Carstens D, Keller E, Vazquez F, Schubert CJ, Zeyer J, Bürgmann H, 2012. Bacterial chitin hydrolysis in two lakes with contrasting trophic statuses. Appl. Environ. Microbiol. 78:695-704. Laforsch C, Tollrian R, 2004. Inducible defenses in multipreda- tor environments: cyclomorphosis in Daphnia cucullata. Ecology 85:2302-2311. Lampert W, 2011. Daphnia: Development of a model organism in ecology and evolution. In: O. Kinne (ed.), Excellence in Ecology, Book 21. International Ecology Institute. Kauppi P, Anttila P, Kenttämies K, 1990. Introduction to this book, p. XIII-XVIII. In: P. Kauppi, P. Anttila, K. Kenttämies (eds.), Acidification in Finland. Springer-Verlag. Lotter, AF, Juggins S, 1991. PLOPROF, TRAN and ZONE. Pro- grams for plotting, editing and zoning of pollen and diatom data. INQUA Commmission for the study of the Holocene, Working Group on Data Handling Methods, Newsletter 6. Mancini M, Comoli P, Margaritora FG, 1999. An unusual type of Daphnia head shields from plankton and sediments of Hi- malayan lakes. J. Limnol. 58:29-32. Meriläinen J, Huttunen P, 1990. Lake acidification in Finland. Phil. Trans. R. Soc. Lond. B 327:423-425. Mikkonen S, Laine M, Mäkelä HM, Gregow H, Tuomenvirta H, Lahtinen M, Laaksonen A, 2014. Trends in the average tem- perature in Finland, 1847-2013. Stoch. Env. Res. Risk. A. 29:1521-1529. Milardi M, Siitonen S, Lappalainen J, Liljendahl A, Weckström J, 2016. The impact of trout introductions on macro-and micro-invertebrate communities of fishless boreal lakes. J. Paleolimnol. 55:273-287. Naeher S, Smittenberg RH, Gilli A, Kirilova EP, Lotter AF, Schubert CJ, 2012. Impact of recent lake eutrophication on microbial community changes as revealed by high resolution lipid biomarkers in Rotsee (Switzerland). Org. Geochem. 49:86-95. Nykänen M, Vakkilainen K, Liukkonen M, Kairesalo T, 2009. Cladoceran remains in lake sediments: a comparison be- tween plankton counts and sediment records. J. Paleolimnol. 42:551-570. Pennington W, 1984. Long-term natural acidification of upland sites in Cumbria: Evidence from post-glacial lake sediments, p. 28-46. In: Freshwater Biological Association (eds.), Fifty- No n c om me rci al us e o nly 150 J.J. Leppänen and J. Weckström second annual report for the year ended 31st March 1984. Freshwater Biological Association, Ambleside. Peters RH, de Bernardi R, 1987. Daphnia. Mem. Ist. Ital. Idro- biol. 45:1-502. Quinlan R, Smol JP, 2010. The extant Chaoborus assemblage can be assessed using subfossil mandibles. Freshwater Biol. 55:2458-2467. Rabus M, Söllardl T, Clausen-Schaumann H, Laforsch C, 2013. Uncovering ultrastructural defences in Daphnia magna – an interdisciplinary approach to assess the predator-induced fortification of the carapace. PLoS One 8:e67856. Rask M, Nyberg K, Markkanen S-L, Ojala A, 1998. Forestry in catchments: effects on water quality, plankton, zoobenthos and fish in small lakes. Boreal Env. Res. 3:75-86. Renberg I, Hansson H, 2008. The HTH sediment corer. J. Pale- olimnol. 40:655-659. Riccardi N, 2000. Comparison of different stoichiometric methods for the estimation of proximate biochemical composition of crustacean zooplankton and some considerations on energy transfer to planktophagous fish. J. Limnol. 59:179-185. Sarmaja-Korjonen K, 2002. Multi-proxy data from Kaksoislammi Lake in Finland: dramatic changes in the late Holocene clado- ceran assemblages. J. Paleolimnol. 28:287-296. Sarmaja-Korjonen K, 2007. Subfossil shell margins and caudal spines of Daphnia in Finnish lake sediments – Is Daphnia underrepresented in cladoceran analysis? Studia Quaternaria 24:61-64. Schmidt R, Wunsam S, Brosch U, Fott J, Lami A, Löffler H, Marchetto A, Müller MW, Pražáková M, Schwaighofer B, 1998. Late and post-glacial history of meromictic Längsee (Austria), in respect to climate change and anthropogenic impact. Aquat. Sci. 60:56-88. Seda J, Petrusek A, 2011. Daphnia as a model organism in lim- nology and aquatic ecology: introductory remarks. J. Lim- nol. 70:337-344. Seppä H, Weckström J, 1999. Holocene vegetational and lim- nological changes in the Fennoscandian tree-line area as documented by pollen and diatom records from Lake Tsuolbmajavri, Finland. Ecoscience 6:621-635. Smol JP, 1992. Paleolimnology: an important tool for effective ecosystem management. J. Aquat. Ecosyst. Health 1:49-58. Sprules WG, Carter JCH, Ramcharan CW, 1984. Phenotypic as- sociations in the Bosminidae (Cladocera): zoogeographic patterns. Limnol. Oceanogr. 29:161-169. Sutela T, Huusko A, 1994. Digestion of zooplankton in the ali- mentary tract of vendace (Coregonus albula) larvae. J. Fish. Biol. 44:591-596. Sutela, T, Huusko A, 2000. Varying resistance of zooplankton prey to digestion: Implications for quantifying larval fish diets. T. Am. Fish. Soc. 192:545-551. Sweetman JN, Smol JP, 2006. Reconstructing fish populations using Chaoborus (Diptera:Chaoboridae) remains – a review. Quat. Sci. Rev. 25:2013-2023. Swiontek Brzezinska M, Lalke-Porczyk E, Donderski W, 2008. Utilization of shrimp waste as a respiration substrate by planktonic and benthic microorganisms. Pol. J. Environ. Stud. 17:273-282. Szeroczyñska K, Zawisza E, 2005. Daphnia remains from the sediment of lake Somaslampi (NW Finnish Lapland) and Lake Wigry (NE Poland). Studia quaternaria 22:55-57. Szeroczyñska K, Sarmaja-Korjonen K, 2007. Atlas of subfossil Cladocera from Central and Northern Europe. Friends of the Lower Vistula Society: 84 pp. Tang KW, Bickel SL, Dziallas C, Grossart H-P, 2009. Microbial activities accompanying decomposition of cladoceran and copepod carcasses under different environmental conditions. Aquat. Microb. Ecol. 57:89-100. Viljanen M, 1983. Food and food selection of cisco (Coregonus albula L.) in a dysoligotrophic lake. Hydrobiologia. 101: 129-138. Virkkanen J, Tikkanen M, 1998. The effects of forest ditching and water level changes on sediment quality in a small lake, Perhonlampi, Central Finland. Fennia 176:301-317. Väliranta M, Weckström J, Siitonen S, Seppä H, Alkio J, Juuti- nen S, Tuittila E-S, 2011. Holocene aquatic ecosystem change in the boreal vegetation zone of northern Finland. J. Paleolimnol. 45:339-352. Weckström J, Korhola A, Blom T, 1997. The relationship be- tween diatoms and water temperature in thirty subarctic Fennoscandian lakes. Arctic Alpine Res. 29:75-92. Wurzbacher CM, Barlocher F, Grossart HP, 2010. Fungi in lake ecosystems. Aquat. Microb. Ecol. 59:125-149. Zaret TM, 1980. Predation and freshwater communities. Yale University Press, New Haven: 185 pp.No n c om me rci al us e o nly