Adam Jeziorski

The Widespread Threat of Calcium Decline in Fresh Waters

Calcium concentrations are now commonly declining in softwater boreal lakes. Although the mechanisms leading to these declines are generally well known, the consequences for the aquatic biota have not yet been reported. By examining crustacean zooplankton remains preserved in lake sediment cores, we document near extirpations of calcium-rich Daphnia species, which are keystone herbivores in pelagic food webs, concurrent with declining lake-water calcium. A large proportion (62%, 47 to 81% by region) of the Canadian Shield lakes we examined has a calcium concentration approaching or below the threshold at which laboratory Daphnia populations suffer reduced survival and fecundity. The ecological impacts of environmental calcium loss are likely to be both widespread and pronounced.

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.

A Multi-Trophic Exploratory Survey of Recent Environmental Changes using Lake Sediments in the Hudson Bay Lowlands, Ontario, Canada

Abstract A multi-proxy paleolimnological survey was performed on 13 lakes in the Hudson Bay Lowlands (HBL) of northern Ontario in order to provide a regional analysis of recent environmental changes in this poorly studied sub-Arctic region. In contrast to the amplified warming experienced by most of the circumpolar Arctic since the mid-19th century, the climate of the Hudson Bay (HB) region has remained relatively cool and stable for hundreds of years. However, since approximately the 1990s, the HBL has experienced rapid and large increases in air temperature and declines in sea ice. Diatom, cladoceran, and chironomid remains preserved in the recent (surface) and pre-1850 sediments of 13 lakes were used to examine whether this new climate regime has resulted in species assemblage changes across multiple trophic levels. Our results indicate clear limnological responses to warming among the freshwater biota of HBL lakes; however, the magnitude of this change varied among both biological indicators and sites. As expected, diatoms exhibited the greatest degree of change, closely followed by chironomids, with relatively little change observed among cladoceran assemblages. Planktonic diatoms were more common in modern assemblages, often including plankters that were previously not recorded in the bottom sediments, and in fact all indicator groups recorded a change in benthic/littoral taxa in the recent sediments indicative of warming-induced increases in habitat availability due to decreased lake ice cover.

Aquatic Ecosystem Responses to Rapid Recovery from Extreme Acidification and Metal Contamination in Lakes Near Wawa, Ontario

In the region northeast of Wawa, Ontario (Canada), many circumneutral lakes downwind of a nearby iron-sintering plant were strongly acidified (pH 3–4) in response to the emissions of large amounts of sulfur dioxide from 1939–1998. Following closure of the plant in 1998, lakewater pH has returned to circumneutral conditions due to the high buffering capacity of the local geological substrate. Prior paleolimnological analyses of dated sediment cores have detected some biological recovery among algal communities (diatoms and chrysophytes), although they have not returned to their pre-impact assemblages. Here we take a broader ecosystem approach, and build upon the algal analyses by examining cladoceran sedimentary assemblages, and spectrally-inferred chlorophyll a and dissolved organic carbon (DOC) from the same dated sediment cores. Similar to the algal communities, recent cladoceran sedimentary assemblages from three impacted lakes remain in an altered state relative to the pre-impact period (for example, increased relative abundances of Chydorus brevilabris and reduced cladoceran density in sediments). However, trends in the spectrally-inferred chlorophyll a and DOC were mixed, with long-term decreases in the study lake closest to the plant and long-term increases within the other lakes. Collectively, the multi-proxy paleolimnological analyses of these markedly acidified lakes demonstrate the delayed biological recovery from acidification (and differences in timing) across multiple trophic levels, despite the near-elimination of acid deposition almost a decade previously, which led to a striking recovery in lakewater pH and increased food availability.

Diatoms indicate that calcium decline, not acidification, explains recent cladoceran assemblage changes in south-central Ontario softwater lakes

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.

Assessing variability in total phosphorus measurements in Ontario lakes

Abstract In most Ontario lakes, phosphorus is present in trace quantities, making the precise measurement of concentrations difficult. The considerable variation that results in many datasets can be attributed to imprecise analysis. Even with precise analysis, substantial variation in ice-free whole-lake, mean mixed layer, and spring turnover total phosphorus (TP) concentrations often remain, both between years and within a given year. Lake managers have adopted many strategies to address these issues. Whole-lake TP concentrations expressed as ice-free season means are difficult to derive because they require the collection of numerous, volume-weighted samples often from multiple lake layers. As a result, spring turnover or ice-free-mean, mixed layer TP concentrations are often used to characterize the trophic status of a lake or to describe the nutrients available for primary production. These different phosphorus characterizations will vary between years, leading many lake managers to use long-term (multiple year) means to describe trophic status. The ability to interpret natural variation or trends in these data can be reduced by any bias introduced through sample collection and storage (container) methods or as a result of sample contamination by zooplankton, which can produce high sample TP concentrations. Much variability can be reduced through precise analysis, and most sources of error can be eliminated by sampling directly into the same borosilicate glass tubes used to digest samples prior to analysis and by coarse filtering water samples in situ to 80 μ m to eliminate large zooplankton. We demonstrated the importance of collecting precise TP data and presented the variation associated with 25 years of TP measurements in Ontario, Precambrian Shield lakes. We also demonstrated various sample collection strategies that can be used to characterize the nutrient status of lakes and explained how the interpretation of these can be affected by variability in the results.

Managing lake trout lakes in a warming world: a paleolimnological assessment of nutrients and lake production at three Ontario sites

ABSTRACT Nelligan C, Jeziorski A, Rühland KM, Paterson AM, Smol JP. 2016. Managing lake trout lakes in a warming world: A paleolimnological assessment of nutrients and lake production at three Ontario sites. Lake Reserve Manage. 32:315−328. Recent declines in hypolimnetic dissolved oxygen (DO) concentrations in many lakes throughout Ontario have prompted concern regarding the health of lake trout (Salvelinus namaycush) populations. We used diatom assemblages and spectrally inferred chlorophyll a from the dated sediments of 3 lake trout lakes to track trends in total phosphorus (TP) and whole-lake production over the past ∼150 years. Eagle Lake is considered a sensitive lake trout lake because DO concentrations have been lower than the provincial standard (>7 mg/L) since the early-2000s; however, a lack of monitoring data impedes the evaluation of current conditions within a historical context. For a regional perspective, we compared trends in Eagle Lake with those of 2 additional lake trout lakes ∼200 km away. The diatom assemblages of Eagle Lake underwent a shift in the early-1980s, marked by a decline in the relative abundance of Stephanodiscus minutulus and an increase in small Cyclotella (sensu lato) taxa. This change is indicative of nutrient reductions and is consistent with a decline in both monitored and diatom-inferred TP. Sedimentary chlorophyll a peaked after the 1980s, suggesting whole-lake production has increased independent of nutrient loading, likely due to regional warming and a longer growing season. Diatom compositional changes at the 2 other sites are indicative of climate warming, with increases in the relative abundance of Discostella stelligera and spectrally inferred chlorophyll a. Our data suggest climate is likely the dominant driver of the observed algal changes, highlighting that the effects of warming on habitat quality should be considered in lake management strategies.

Long-term reconstruction of deep-water oxygen conditions in Osoyoos Lake (British Columbia, Canada): implications for Okanagan River sockeye salmon

Abstract Simmatis B, Jeziorski A, Zemanek A, Selbie DT, Hyatt K, Fryer JK, Cumming BF, Smol JP. 2018. Long-term reconstruction of deep-water oxygen conditions in Osoyoos Lake: implications for Okanagan River sockeye salmon. Lake Reserve Manage. 34:392–400. Osoyoos Lake is the primary nursery lake supporting sockeye salmon (Onchorhynchus nerka) originating from the Okanagan River system in south-central British Columbia, Canada. By the early 1960s, Osoyoos Lake had experienced significant cultural eutrophication related to rapid human development of the watershed, increasing total phosphorus concentrations, and frequent algal blooms. Concern exists regarding recent (post-1990s) declines in hypolimnetic oxygen concentrations and the potential degradation of salmon habitat in this multi-basin lake. However, reliable observations are largely unavailable for oxygen conditions prior to the mid-1990s. We reconstructed late-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in Osoyoos Lake from the late 1800s to 2009 by examining past chironomid (Diptera: Chironomidae) subfossil assemblages from 210Pb-dated lake sediment cores. Chironomid-inferred VWHO revealed only slight variations since the late 1800s (mean values for the north and south basins are 4.4 and 2.9 mg/L, respectively), with seemingly little response to the cultural eutrophication of the mid-20th century. The chironomid-inferred VWHO trends suggest that the current management target of 5.8 mg/L dissolved oxygen for both basins of Osoyoos Lake may require revision to better reflect reference conditions and natural seasonal anoxia.