Paleoecological assessment of cladoceran community dynamics in two subarctic peatlands
Crustacean community structure and dynamics are very well studied in lakes, rivers and oceanic systems but wetlands, where moisture conditions fluctuate, have not received equal attention in research. For example, cladoceran communities in peatland systems in the subarctic region have not been fully investigated. We used paleolimnological and paleoecological methods to study plant and cladoceran assemblages and the community dynamics in two subarctic peatlands, which differ in their hydrological characteristics. At the first site, Iitto, river floods introduce planktonic species to fen pools and the steep topography of the catchment induces rapid but relatively short flooding periods. Fluctuating environmental conditions result in a high amount of cladoceran resting stages in the samples. At the other site, Kaamanen, the cladoceran assemblage goes through clear directional changes, which could be attributed to changes in fen hydrology and ultimately to climatic changes during the past two millennia.
KeywordsPeatland hydrology Cladocera Peat plants Plant macrofossils Climate Flooding Paleolimology
Subarctic fens are challenging habitats for aquatic biota due to fluctuating water levels and a long season of ice and snow cover. In Northern Finland, many peatlands are flooded in spring due to snowmelt, which is followed by gradual shrinking of the open water area in late summer and autumn. Lakes in the northern Finnish Lapland are covered by ice from late October to early June. For aquatic animals, which are capable of inhabiting temporary wetland pools, rapid emergence and fast reproduction are key survival characteristics. Cladocera, a group of crustacean zooplankton, are known to bypass drought or winter by diapause and are able to develop large populations in short period of time. The group is of pivotal importance in nearly all types of aquatic systems as they establish a link from primary producers to higher trophic levels. In addition, crustacean zooplankton are an important component in microbial loops, linking the group to aquatic carbon dynamics (e.g. Gaedge 2009; Matveev and Robson 2014). Moreover, cladocerans are regarded as an excellent indicator of environmental changes (Jeppesen et al. 2011) and the animals leave well preserved, identifiable remains upon death or molting, allowing the utilization of paleolimnological methods (Frey 1960). In addition to cladocerans, plant macrofossils are a paleoecological indicator for past habitat changes and changes in carbon accumulation patterns in Subarctic peatlands (Väliranta et al. 2007, 2017, Swindles et al. 2015, Teltewskoi et al. 2016, Gałka et al. 2017).
Flood frequency and the longevity of inundation period are known to control zooplankton communities in floodplain ponds (Medley and Havel 2007) and cladoceran community structure in different types of lakes in northern Finnish Lapland has been studied numerous times (Rautio 1998; Korhola 1999; Siitonen et al. 2011; Leppänen et al. 2017). Despite this, long-term monitoring studies or paleolimnological studies assessing the cladoceran community dynamics in subarctic peatland pools have not been conducted before. Similarly to cladocerans, the plants in subarctic peatlands are exposed to cold winters and occasional floods. The plant community reflects the average water level, nutrient status and acidity in northern peatlands (Mauquoy and Geel 2007; Väliranta et al. 2007). Slow decomposition in waterlogged and anoxic peat permits the usage of plant macrofossils (i.e. undecomposed plant remains) in paleoecological studies (Mauquoy and Geel, 2007; Väliranta et al. 2007; Mathijssen et al. 2014).
Here, we use subfossil cladocera and plant macrofossils to study the cladoceran community structure and historical community dynamics in two subarctic fen environments which vary in their hydrological regimes. Because peatland characteristics are strongly controlled by hydrology (e.g. Holden 2006) and peatlands are regarded highly as important ecosystems in terms of carbon dynamics and climate change (e.g. Gallego-Sala et al. 2018), we feel that faunal the aquatic component of peatland systems deserves more attention in research. Our main research question is whether cladoceran community structure is related to moisture conditions in subarctic peatlands. Ultimately, we attempt to assess whether cladoceran paleolimnology holds potential as additional proxy tool for peatland moisture conditions. In addition, our results contribute to current knowledge on subarctic peatland biodiversity.
Geographical characteristics of study sites
Latitude N (WGS84)
Longitude E (WGS84)
Altitude (m a.s.l.)
Peatland area (km2)
Catchment area (km2)
Elevation difference (m)
Average temperature (°C)
Average annual precipitation (mm)
Kaamanen peatland is a fen with strings, flarks, and non-permanently frozen hummocks. The catchment area is characterized by relatively small elevation difference and numerous lakes and ponds. The southern end of the catchment is flat whereas the northern part is divided by esker formations. The nearest permanent waterbodies are a small, meandering brook at a distance of ~50 m, with an eutrophic riparian zone (Aurela et al. 1998) and a small lake (1.8 ha) at a distance of ~300 m (Fig. 1c; Online Resource 1). The fen is located in an active groundwater region and there are numerous springs in the catchment suggesting a possibility for groundwater impact on water availability and water chemistry. According to available water chemistry data (OIVA database 2018), the lakes and rivers (n=5) in Kaamanen are nearly neutral pH (6.3-7.3), have relatively low nutrient concentrations (total phosphorus 4-12 μg l-1, total nitrogen 270-520 μg l-1) and low conductivity (2-11 mS m-1) and Ca (3.5-4 mg l-1).
Iitto is a palsa peatland, i.e. a fen complex with individual peat mounds (palsas) that have been uplifted from the surrounding fen surface by permafrost upheaval. On the studied peatland, separate palsa mounds rise approximately two to four meters above the surrounding fen complex. The Iitto catchment is characterized by the steep slopes of fells Latnjavarit (720 m.a.s.l) and Coreheapmi (605 m.a.s.l) resulting an elevation difference of over 300 m between the lowest and the highest point. In addition, a small lake (1.7 ha) and River Könkämeno are located in the vicinity of the sampling site at distances of 100 m and 300 m, respectively (Fig. 1b and Online Resource 2). According to water level monitoring data recorded at Kaaresuvanto monitoring station, 50 km downstream, the water level at River Könkämäeno can fluctuate by 4 meters, reaching its highest level during May-July. Surface waters (n=2) in the Iitto area are nearly neutral (pH 6.8-7.1), oligotrophic (total phosphorus 3-6 μg l-1, total nitrogen 200-250 μg l-1) and electric conductivity values are low (1.9-3.9 mS m-1) (OIVA database 2018).
Coring and subsampling
Two peat cores were collected with a box corer (60 cm x 7 cm) in September 2016, one from each peatland. Selected Sphagnum covered high lawn habitats (less than 30 cm from the surrounding wet peatland surface) represent transitional habitat between wet fen and dry hummock. Selecting these microhabitats, enables us to assess how cladoceran communities response to changes in moisture conditions and changes in vegetation assemblages, when there is a shift from wet to dry. Peat cores were wrapped in plastic film and placed inside PVC gutters for transportation to the University of Helsinki, where they were stored in a freezer until further analysis. The 60-cm long peat cores were defrosted and sliced in 1-cm intervals. The subsamples were stored in a cold room.
Plant macrofossil analysis
Plant macrofossil analysis was conducted at 2-cm intervals, increased to 1 cm for periods with prominent plant community changes. Volumetric 5 cm3 samples were rinsed through a 140-μm mesh under running water. The residue on a sieve was analysed under a stereomicroscope for relative (%) abundances. Further species level identification was conducted with a light microscope. The plant macrofossil analysis follows Mauquoy et al. (2010) and Väliranta et al. (2007). The highly decomposed plant matrix is called UOM, i.e. unidentified organic matter. Identification of plant macrofossils follows e.g. Viramo (1992); Mauquoy and Geel (2007); Laine et al. (2009). In addition, a plant reference collection was available.
Cladoceran analysis was conducted using standard procedures published by Korhola and Rautio (2001). In short, approximately 1 cm3 of wet peat sample was heated (90 °C) in 10% KOH bath for 1 hour and sieved through 50 μm mesh. The residue was stained using safranin and slides were prepared with gelatin glycerol jelly. Cladoceran remains were identified and counted using a light microscope at 100-400 x magnification. Cladoceran identification and nomenclature was based on (Szeroczyńska and Sarmaja–Korjonen 2007). A minimum number of 100 individuals was counted from each sample. Cladoceran resting eggs (ephippia) were counted but species were not specified.
Chronology was established based on five accelerator mass spectrometry (AMS) radiocarbon 14C dated samples (2 samples from Kaamanen core, 3 samples from Iitto core). 14C samples were sent to Poznan Radiocarbon Laboratory (Poznan, Poland) and to the Finnish Museum of Natural History (LUOMUS, Helsinki, Finland). The dated samples were bulk peat, with roots and rootlets picked apart (Holmquist et al. 2016). Age-depth models were created using a bayesian BACON package (Blaauw and Christen 2011) using weighted mean ages and age-depth models in R version 3.4.3.
Analysis of Similarities (ANOSIM; Clarke 1993) was used to assess the similarities between cladoceran assemblages between sites. ANOSIM analysis was conducted using Bray-Curtis dissimilarity matrix for square root transformed proportional data. Similarity percentages analysis (SIMPER; Clarke 1993) was used to quantify the overall average dissimilarity of cladoceran communities between sites and to pinpoint the species which contribute to the differences. SIMPER analysis was conducted on percentage data. Shannon H’ diversity index (for cladocerans) was calculated using percentage data. Species richness (for cladocerans) was calculated using rarefaction method for count data (Birks and Line 1992). All analysis were conducted in the programme PAST (Hammer et al. 2001).
Plant macrofossil results
Abrupt and clear changes from wet Cyperaceous fen vegetation assemblages to drier ligneous and/or lawn/hummock vegetation are characteristic for both peat records. The results of selected taxa are presented in Figs. 2 and 3. Ombotrophic plant assemblages are not presented (Fig. 2 and Fig. 3) because the cladoceran remains disappear after the abrupt changes in vegetation and hydrology.
The 57-cm long record is characterised by two distinct phases in plant assemblages. The first phase from ca. 755 cal. BC to ca. 1530 cal. AD is dominated by Scorpidium scorpioides and Cyperaceous fen species (Limosa type Carex spp. and Eriophorum spp.). As the fen peat is humified, the proportion of UOM is high. Diatoms and Chrysophyceae cysts were detected until ca. 1340 cal. AD together with high percentage of Scorpidium scorpioides. Ombrotrophic S. fucum appears after ca. 1530 cal. AD and abruptly replaces the fen plant assemblages. Along with S. fuscum, S. capillifolium and S. magellanicum prevail until ca. 1815 cal. AD. Dark roots, indicating presence of (dwarf) shurbs, are found throughout this second phase. Vaccinium oxycoccos leaves and Empetrum nigrum leaves are abundant with few Andromeda polifolia leaves from ca. 1815 cal. AD towards present.
The 55-cm long record is dominated by wet fen species until a sharp change at ca. 1725 cal. AD. Cyperaceae species Eriophorum spp. and limosa-type Carex remains occur with oligo- to mesotrophic S. lindbergii that usually thrives from pools to lawns (Laine et al. 2009). Around 1560 cal. AD S. lindbergii is replaced with Scorpidium scorpioides which is a typical fen taxon growing in wet hollows. Cf. S. platyphyllum is found until the plant composition shifts to ligneous and dwarf shrub remains around 1760 cal. AD. Peat layers with sedges and woody material are accompanied by high UOM. The phase with woody remains is characterised by high percentages of dwarf shrub roots and UOM, also Empetrum nigrum leaves are present. Around 1905 cal. AD Sphagna reappear with S. russowii (ombrotrophic to weakly minerotrophic) with Polytrichum strictum (from lawns to hummocks). Betula nana leaf remains are found throughout the ligneous phase to present.
Cladoceran remains were well preserved but relatively scarce in peat samples and in the top samples the remains became increasingly rare. An adequate number of subfossil remains for assemblage characterization (~100 individuals; Kurek et al. 2010) was found from 10 samples from each site. A total of 15 taxa were identified from the samples (min 3, max 11 in Kaamanen; min 6, max 12 in Iitto). Average Shannon (H´) diversity for Kaamanen was 1.48 (0.6 to 2.0) and 1.67 (1.16 to 1.91) for Iitto. The most common species in Kaamanen were Chydorus sphaericus (average relative abundance 39 % SD 25 %) and Alonella excisa (average relative abundance 18 % SD 11 %). The most common species in Iitto were Unapertura latens (average relative abundance 34 % SD 14 %) and Alonella nana (average relative abundance 30 % SD 8 %). Most common species in both sites were present in all samples. The cladoceran communities in Kaamanen and Iitto are not similar (ANOSIM R 0.74, p<0.001) and according to SIMPER analysis, the overall average dissimilarity is 69 %. The most important taxa which contributes to site-wise differences are U. latens (23 %), C. sphaericus (23 %) and A. nana (17 %) (Online Resource 2). The number of cladoceran resting eggs varied in Kaamanen from 0 to 15 per 100 cladoceran individuals (average 6) and from 14 to 26 per 100 cladoceran individuals (average 23) in Iitto. The results are presented in Fig. 2 and 3.
At the beginning of the record (57 cm level), the most abundant species are C. sphaericus, A.excisa and A. nana. The section between ca. 760 cal. BC (57 cm) and ca. 815 cal. AD (43 cm) is characterized by high cladoceran diversity and declining abundance of A. nana. Beginning at ca. 815 cal. AD (43 cm) the cladoceran diversity starts to decline and the next sample at ca. 1130 cal. AD (40 cm) is characterized by nearly total domination of Alona rustica. In the following samples ca. 1210 cal. AD and ca. 1365 cal. AD (39 cm and 37 cm) the most abundant species is C. sphaericus.
The cladoceran community in Iitto remains relatively stable through the record. The most distinct changes in the Iitto record are dated to ca. 1485 cal. AD (49 cm) and to ca. 1590 cal. AD (39 cm). 1485 AD pinpoints the beginning of an increase in diversity (H’) and an increase in proportional abundance of Eurycercus spp. and A. rustica. At ca. 1590 cal. AD (39 cm) and subsequent samples, Alona affinis exhibits a slight but visible increase in proportional abundance.
Cladoceran communities in Kaamanen and Iitto
In general, cladoceran species detected in both study sites are common in Finnish Lapland lakes (Leppänen et al. 2017) and ponds (Rautio 1998). In addition, many of the species detected in both sites are regarded as common in peat bog ponds (Glime 2017) and some of the species (A. nana, C. sphaericus, A. excisa, A. affinis, Acroperus harpae) were reported during the late stages of transition from hilltop lake to an acidic ombrotrophic bog in southern Finland (Korhola 1992). Moreover, A. excisa and C. sphaericus have been reported to be common in shallow peatland pools in Romania (Battes et al. 2014), Hungary (Vad et al. 2012) and Bolivia (A. excisa and Chydorus breivilabris; Coronel et al. 2007) which further confirms the ability of those species to inhabit temporary shallow water environments. In both sites, the amount of cladoceran remains declines abruptly in top samples and probably reflects the formation of a palsa mound or peat hummock, as also recorded in the plant macrofossil assemblages. The increase in height results in disappearance of aquatic habitat and explains the lack of cladoceran remains. In addition, in both sites, cladoceran communities are clearly connected to hydrological characteristics of the environment. The relationship between plant and cladoceran communities is visible in the hydrological preferences of plant species as the samples with high cladoceran diversity occur systematically together with high abundance of Scorpidium scorpioides, which is reported to inhabit the wettest microhabitats in Kaamanen (e.g. Heikkinen et al. 2002).
While the cladoceran species assemblages in both sites are largely comprised of common and well known littoral species (e.g. A. nana, A. excisa and C. sphaericus), the largest species-wise difference between the sites is related to high proportional abundance of U. latens in Iitto. Interestingly, U. latens, is a relatively newly described species (Sarmaja-Korjonen et al. 2000) and its taxonomy and ecology are not well resolved. Another species worth mentioning is Eubosmina longispina, detected only in Iitto, which is a planktonic species and common in large and deep lakes. Thus, the explanation for the presence of E. longispina in Iitto is most probably related to the flooding of nearby River Könkämäeno. Based on satellite images, the river floods regularly and may therefore introduce planktonic species into the fen pools. The third distinct difference between the sites is the high number of cladoceran resting eggs in the Iitto samples. The large numbers of resting eggs is a result of intense sexual reproduction and reflects a high frequency of disturbance because cladocera survive ecological hardships (e.g. winter or drought) by diapause. The disturbances in Iitto are probably related to catchment characteristics, namely, to the river flooding and to melt water dynamics. Due to the steep topography of the Iitto catchment and the absence of lakes, melt water flooding is probably ephemeral, resulting in rapid variations in peatland water level. In contrast, the Kaamanen region is characterized by vast wetlands and numerous lakes which slow down the water flow increasing the longevity of the flood phase (Bring et al. 2016). In Kaamanen, the major flood lasts from May to June (Väliranta et al. 2014) but flarks stay submerged longer (e.g. in 1995, until September; Heikkinen et al. 2002). Despite the direct contact to larger waterbodies, the absence of planktonic species in our samples suggests that the influx of cladocerans from upstream lakes into the Kaamanen fen pools is practically non-existent. This suggests that the flood water originates from regional snow melt rather than natural waterbodies upstream.
Thus, the major difference between the sites is therefore the longer flooding periods of the Kaamanen pool habitat during the growing season whereas the Iitto site is characterized by continuous influx of planktonic species and the fluctuating conditions.
Cladoceran community dynamics in Kaamanen
Cladoceran community dynamics in Iitto
Our results clearly indicate that cladoceran communities in subarctic peatlandsare relatively diverse. Species richness and diversity is related to water availability which, in turn is controlled mainly by climatic factors. This observation is important in terms of future climate change because the hydrological shifts may have high-magnitude impacts on aquatic diversity in subarctic peatlands. This study confirms the ability of crustacean zooplankton to thrive in harsh subarctic environments and identifies hydrological dynamics as the most important drivers of peatland cladoceran communities. In addition, our findings highlight the applicability of cladoceran remains in peatland studies and namely their potential as additional indicators of hydrological conditions. In addition, this potential could be further utilized in e.g. food web and carbon dynamics studies in flood prone peatland systems.
Open access funding provided by University of Helsinki including Helsinki University Central Hospital. This study was funded by Academy of Finland, Tellervo and Juuso Walden Foundation and University of Helsinki Funds. In addition, H.Z. acknowledges China Scholarship Council (grant no. 201404910499) and Y.L. acknowledges China Scholarship Council (grant no. 201706180087).
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