Jennifer B. Korosi

An illustrated guide to the identification of cladoceran subfossils from lake sediments in northeastern North America: part 1—the Daphniidae, Leptodoridae, Bosminidae, Polyphemidae, Holopedidae, Sididae, and Macrothricidae

Cladocera identifications based on subfossil remains differ considerably from identifications based on whole animals, yet despite this very few subfossil Cladocera identification guides exist. In particular, taxa belonging to the families Daphniidae, Leptodoridae, Polyphemidae, Holopedidae, Sididae, and Macrothricidae tend to preserve poorly, with only a few, small body parts available for identification within the sediments. Since many of these taxa are important environmental indicators in lakes, subfossil identification guides that enable them to be recognized based on only a few body parts are critical for paleolimnological investigations utilizing Cladocera. In this illustrated identification guide, we provide detailed descriptions of subfossil remains commonly recovered for 14 species belonging to the families Daphniidae, Leptodoridae, Polyphemidae, Holopedidae, Sididae, Macrothricidae, and Bosminidae for lakes in eastern Canada and northeastern USA, where several recent subfossil Cladocera-based investigations of environmental change have been focused. It is our hope that this taxonomic guide will improve consistency in subfossil Cladocera taxonomy among laboratories, and help to stimulate additional research in this ecologically significant region. The diverse taxa belonging to the family Chydoridae are considered separately.

An illustrated guide to the identification of cladoceran subfossils from lake sediments in northeastern North America: part 2—the Chydoridae

The diverse members of the cladoceran family Chydoridae are typically well preserved in lake sediments, although identifications of chydorids using subfossil remains still differ from identifications using whole animals. To address this issue, we analyzed and photographed chydorid subfossil remains from lakes in eastern Canada and northeastern USA and compiled this information into an illustrated identification guide for this region, which is characterized by high chydorid species richness. We provide detailed descriptions of commonly recovered body parts for each species, including headshields, carapaces, postabdomens, and postabdominal claws. We describe 29 chydorid taxa, discuss the distribution of these taxa within North America, and compare morphology with related species described from Alaska or northern Europe, the only other regions for which subfossil chydorid identification guides are available.

Zooplankton (Cladocera) species turnover and long-term decline of Daphnia in two high mountain lakes in the Austrian Alps

We investigated long-term succession of sedimentary cladoceran assemblages in two morphologically divergent mountain lakes by utilizing sediment traps and previously available sediment data. We aimed to detect lake-specific changes in pelagic communities potentially attributable to climate warming under the presumption that lakes and biotic communities may respond individually to the same regional climatic forcing. Both lakes showed a similar community turnover, as Daphnia was first replaced by Chydorus cf. sphaericus and during the twentieth century by the latest colonizer Bosmina. The community succession was similar among the lakes and consistent with the regional temperature increase, although the timing of community shift, the magnitude of change, and taxa in question differed. Decline of Daphnia mismatched with historical fish stockings, but the eventual extirpation of Daphnia in one of the lakes corresponded to the start of fish introductions. We propose that the observed shifts were mainly governed by increasing temperatures and its limnoecological consequences (e.g., deeper mixing). We suggest that Bosmina distribution may be extending to lakes at higher altitudes as a response to climate warming, and that it may replace Daphnia as the key component of pelagic alpine food webs by coping in interspecific resource competition under changed limnological regimes.

The jellification of north temperate lakes

Calcium (Ca) concentrations are decreasing in softwater lakes across eastern North America and western Europe. Using long-term contemporary and palaeo-environmental field data, we show that this is precipitating a dramatic change in Canadian lakes: the replacement of previously dominant pelagic herbivores (Ca-rich Daphnia species) by Holopedium glacialis, a jelly-clad, Ca-poor competitor. In some lakes, this transformation is being facilitated by increases in macro-invertebrate predation, both from native (Chaoborus spp.) and introduced (Bythotrephes longimanus) zooplanktivores, to which Holopedium, with its jelly coat, is relatively invulnerable. Greater representation by Holopedium within cladoceran zooplankton communities will reduce nutrient transfer through food webs, given their lower phosphorus content relative to daphniids, and greater absolute abundances may pose long-term problems to water users. The dominance of jelly-clad zooplankton will likely persist while lakewater Ca levels remain low.

Anomalous rise in algal production linked to lakewater calcium decline through food web interactions

Increased algal blooms are a threat to aquatic ecosystems worldwide, although the combined effects of multiple stressors make it difficult to determine the underlying causes. We explore whether changes in trophic interactions in response to declining calcium (Ca) concentrations, a water quality issue only recently recognized in Europe and North America, can be linked with unexplained bloom production. Using a palaeolimnological approach analysing the remains of Cladocera (herbivorous grazers) and visual reflectance spectroscopically inferred chlorophyll a from the sediments of a Nova Scotia (Canada) lake, we show that a keystone grazer, Daphnia, declined in the early 1990s and was replaced by a less effective grazer, Bosmina, while inferred chlorophyll a levels tripled at constant total phosphorus (TP) concentrations. The decline in Daphnia cannot be attributed to changes in pH, thermal stratification or predation, but instead is linked to declining lakewater [Ca]. The consistency in the timing of changes in Daphnia and inferred chlorophyll a suggests top-down control on algal production, providing, to our knowledge, the first evidence of a link between lakewater [Ca] decline and elevated algal production mediated through the effects of [Ca] decline on Daphnia. [Ca] decline has severe implications for whole-lake food webs, and presents yet another mechanism for potential increases in algal blooms.

A comparison of pre-industrial and present-day changes in Bosmina and Daphnia size structure from soft-water ontario lakes.

Understanding the long-term controls on cladoceran size structure has important implications for aquatic ecosystems. Although there has been considerable interest in zooplankton size trends for Canadian Shield lakes, data are not available for zooplankton size structure prior to the period of anthropogenic disturbances. Here, we present pre- and post-impact size data for the common pelagic cladocerans Bosmina and Daphnia for 44 well-studied Shield lakes in south-central Ontario (Canada). We show that Daphnia were larger and that the length of Bosmina body appendages (mucrones and antennules) was longer in pre-industrial times than they are today. The reduction in Bosmina appendage length we observed may suggest a reduction in copepod predation pressure since pre-industrial times. Reduced maximum body size in Daphnia is a predicted response to a warming climate in north temperate lakes; however, we suggest that alternate explanations, specifically acidification and subsequent recovery following emission re...

Distribution of cladoceran assemblages across environmental gradients in Nova Scotia (Canada) lakes

Cladocera sub-fossils have widely been recognized as useful environmental indicators. Nevertheless, investigations into the distribution and environmental controls on Cladocera are scarce in North America, hindering their use in paleolimnological studies. Here, we examine cladoceran assemblage and size structure from the surface sediments of 49 soft-water Nova Scotia (Canada) lakes to provide ecological data on this key taxonomic group, the first study of its kind for the northern Atlantic coast of North America. We found that Nova Scotia lakes contain a diverse chydorid assemblage, but were generally dominated by the smaller pelagic herbivore Eubosminalongispina. Daphnia, a larger pelagic herbivore, was relatively uncommon in these lakes. Redundancy analyses (RDA) identified maximum lake depth and dissolved organic carbon (DOC) as the environmental variables that best explained the structuring of these cladoceran assemblages. Generalized linear models were then used to better characterize the ecological associations for individual taxa. Body size of the bosminids in these lakes was significantly correlated only to total nitrogen (TN), with larger bosminids in lakes with lower TN values. Bosminid mucro length, an indication of invertebrate and fish predation pressure, was significantly related to TN, maximum lake depth, and lake surface area. The ecological information provided by this study should assist the interpretation of paleolimnological assessments of environmental change in these and other similar lakes.

Localized enrichment of polycyclic aromatic hydrocarbons in soil, spruce needles, and lake sediments linked to in-situ bitumen extraction near Cold Lake, Alberta.

The extraction of bitumen from the Alberta oil sands using in-situ technologies is expanding at a rapid rate; however, investigations into the environmental impacts of oil sands development have focused on surface mining in the Athabasca region. We measured polycyclic aromatic hydrocarbons (PAH) in soils, spruce needles, and lake sediment cores in the Cold Lake oil sands region to provide a historical and spatial perspective on PAH contamination related to in-situ extraction activities. A pronounced increase in PAH concentrations was recorded in one of two study lakes (Hilda Lake) corresponding to the onset of commercial bitumen production in ~1985. Distance from extraction rigs was not an important predictor of PAH concentrations in soils, although two samples located near installations were elevated in alkyl PAHs. Evidence of localized PAH contamination in Hilda Lake and two soil samples suggests that continued environmental monitoring is justified to assess PAH contamination as development intensifies.

A comparison of present-day and pre-industrial cladoceran assemblages from softwater Nova Scotia (Canada) lakes with different regional acidification histories

We compared cladoceran assemblages in modern and pre-industrial sediments from cores taken in 48 softwater lakes located in four main regions of Nova Scotia (Canada) to evaluate the impacts of acidification and other recent environmental stressors. Lakes in Kejimkujik National Park showed significant increases in Holopedium relative abundances and significant decreases in Alona and other chydorids since pre-industrial times, which appear to be related to declines in pH and calcium (Ca) concentrations caused by acidic deposition. Lakes in Bridgewater also showed a significant decrease in Alona, as well as a significant increase in macrothricid (Acantholeberis and Ophryoxus) taxa that cannot be explained by declines in pH, although declines in [Ca] have been recorded. Lakes in Yarmouth did not show any significant regional changes in major cladoceran species groups. Still, pre-industrial assemblages in these lakes significantly differed from modern assemblages, with assemblage changes being lake-specific and likely related to interactions between local and regional stressors acting on individual lakes. Finally, lakes in Cape Breton Highlands National Park, located on the taiga plateau, have received historically lower levels of sulphate deposition relative to other regions in the province, and recorded a significant decrease in Alona similar to Bridgewater lakes. The province-wide decrease in Alona across different acid deposition rates suggests that limnological changes related to climate warming may be responsible. Overall, this study shows that acidification history is an important predictor of cladoceran assemblage changes since pre-industrial times in Nova Scotia lakes, although multiple stressors result in complex Cladocera responses in some regions.

Investigating the response of Cladocera to a major saltwater intrusion event in an Arctic lake from the outer Mackenzie Delta (NT, Canada)

An increase in the frequency and intensity of marine storm surges is a predicted consequence of climate warming, and therefore it is important to better understand the biological responses to such events in coastal regions. In late September 1999, a major storm surge resulted in a saltwater intrusion event over a large area of the Mackenzie Delta (NT, Canada) front, causing rapid salinization of lakes on the alluvial plain. Due to a lack of long-term ecological monitoring data in the region, the impacts that the saltwater intrusion event had on the biota of affected lakes were unknown. We used high-resolution paleolimnological approaches to reconstruct past assemblage changes in Cladocera from impacted Lake DZO-29 (unofficial name) in order to determine how different cladoceran species responded to a major increase in lake salinity following the 1999 storm surge. Camptocercus were extirpated from the lake following the saltwater intrusion and have not recovered. We also observed an initial decrease in Alona relative abundance following the marine flooding, likely reflecting a loss of A. quadrangularis, A. barbulata, and A. costata from the lake. A. circumfimbriata, Chydorus biovatus, C. brevilabris, and Bosmina spp. were abundant both before and after the saltwater intrusion, and Paralona pigra was present following the storm surge, but not prior to it. The most notable shift in Cladocera in the recent sedimentary record, however, occurred much earlier, with an increase in pelagic Bosmina taxa and a subsequent decrease in the benthic/littoral taxa Chydorus and Camptocercus, an assemblage shift that is consistent with a response to climate warming in this region, and strongly correlated to other changes in the lake inferred to be as a result of regional warming.