Sushil S. Dixit

Diatom assemblages from Adirondack lakes (New York, USA) and the development of inference models for retrospective environmental assessment

Detrended canonical coreespondence analysis (DCCA) was used to examine the relationships between diatom species distributions and environmental variables from 62 drainage lakes in the Adirondack region, New York (USA). The contribution of lakewater pH, Alm (monomeric Al), NH4, maximum depth, Mg, and DOC (dissolved organic carbon) were statistically significant in explaining the patterns of variation in the diatom species composition. Twenty-three and sixteen diatom taxa were identified as potential indicator species for pH and Alm, respectively (i.e. a taxon with a strong statistical relationship to the environmental variable of interest, a well defined optimum, and a narrow tolerance to the variable of interest). Using weighted-averaging regression and calibration, predictive models were developed to infer lakewater pH (r2=0.91), Alm (r2=0.83), DOC (dissolved organic carbon) (r2=0.64), and ANC (acid neutralizing capacity; r2=0.90). These variables are of key importance in understanding watershed acidification processes. These predictive models have been used in the PIRLA-II (Paleoecological Investigation of Recent Lake Acidification-II) project to answer policy-related questions concerning acidification, recovery, and fisheries loss.

Decreased Acid Deposition and the Chemical Recovery of Killarney, Ontario, Lakes

Abstract Lakes in Killarney Park near Sudbury, Ontario, Canada, have shown dramatic water quality changes including general increases in pH and alkalinity, and decreases in SO2−4, base cations and metals. While some lakes have recovered to pH > 6.0, many are still highly acidic despite decades of improvement. Very high historical S deposition related to emissions from the Sudbury metal smelters dominated the acidification process in this region. However, since the implementation of substantial S emission controls (90%) at the smelters, the Sudbury emissions are no longer the major source of S deposition in the Sudbury area. Wet deposition of SO2−4 and SO2−4 concentrations in lakewaters at Killarney now approach values in the Dorset, Ontario, area, about 200 km from Sudbury. This suggests that the S deposition to the Killarney area is now primarily from long-range transport, not from local sources. Studies of Killarney lakes are revealing the complex nature of the chemical recovery process. As lake acidity decreases, other changes including decreased Ca2+ concentrations, increased transparency, and altered thermal regimes may potentially affect some of these ecosystems. It is clear that continuing assessments of the recovery of Killarney lakes, within a multiple-stressor framework, are needed.

Diatom and chrysophyte functions and inferences of post-industrial acidification and recent recovery trends in Killarney lakes (Ontario, Canada)

The surface sediment diatom and chrysophyte assemblages from 33 Sudbury lakes were added to our published 72 lake data set to expand and refine the diatom and chrysophyte-based inference models that we had earlier developed for this region. Our calibration data set now includes 105 lakes, representing gradients for multiple environmental variables (e.g., lakewater pH, metals, and transparency). The revised models are based on the weighted averaging calibration and regression approach and include bootstrap error estimates. The pH model was the strongest (r2boot = 0.75, RMSEboot = 0.50). The chrysophyte-inferred pH model (r2boot = 0.79, RMSEboot = 0.48) that we developed was as robust as the diatom pH model. Diatom and chrysophyte inferred pH models were then applied to ‘top’ (surface sediments representing current conditions) and ‘bottom’ (generally from > 30 cm deep representing pre-industrial conditions) sediment diatom and chrysophyte assemblages of 19 Killarney area lakes near Sudbury. The top and bottom inferred pH results were compared to early-1970s measured pH data. These data suggest that, although many of the poorly buffered Killarney lakes had experienced acidification, marked pH recovery has occurred in many lakes within the last 25 years. Despite the stunning pH recovery, the present-day diatom and chrysophyte assemblages are significantly different from assemblages present during pre-industrial times. Our results suggest that biological recovery may require more time than chemical recovery. It is also likely that these lakes may never recover biologically because other anthropogenic stressors (e.g., climate warming and increased exposure to UV-B radiation) may now have greater influence on biological communities in Killarney/Sudbury area lakes than acidification.

Variability in diatom and chrysophyte assemblages and inferred pH: paleolimnological studies of Big Moose Lake, New York, USA

We measured variability in the composition of diatom and chrysophyte assemblages, and the pH inferred from these assemblages, in sediment samples from Big Moose Lake, in the Adirondack Mountains of New York. Replicate samples were analyzed from (1) a single sediment core interval, (2) 12 different intervals from each of 3 separate cores, and (3) 10 widely spaced surface sediment samples (0–1 cm). The variability associated with sample preparation (subsampling, processing, and counting) was relatively small compared to between-core and within-lake variability. The relative abundances of the dominant diatom taxa varied to a greater extent than those of the chrysophyte scale assemblages. Standard deviations of pH inferences for multiple counts from the same sediment interval from diatom, chrysophyte, and diatom plus chrysophyte inference equations were 0.04 (n=8), 0.06 (n=32), and 0.06 (n=8) of a pH unit, respectively. Stratigraphic analysis of diatoms and chrysophytes from three widely spaced pelagic sediment cores provided a similar record of lake acidification trends, although with slight differences in temporal rates of change. Average standard deviations of pH inferences from diatom, chrysophyte and diatom plus chrysophyte inference equations for eight sediment intervals representing similar time periods but in different cores were 0.10, 0.20, and 0.09 pH unit, respectively. Our data support the assumption that a single sediment core can provide an accurate representation of historical change in a lake. The major sources of diatom variability in the surface sediments (i.e., top 1.0 cm) were (1) differences in diatom assemblage contributions from benthic and littoral sources, and (2) the rapid change in assemblage composition with sediment depth, which is characteristic of recently acidified lakes. Because scaled chrysophytes are exclusively planktonic, their spatial distribution in lake sediments is less variable than the diatom assemblages. Standard deviations of pH inferences for 10 widely spaced surface sediment samples from diatom, chrysophyte and diatom plus chrysophyte inference equations were 0.21, 0.09, and 0.16 of a pH unit, respectively.

Effects of Regional Reductions in Sulphur Deposition on the Chemical and Biological Recovery of Lakes within Killarney Park, Ontario, Canada

The lakes in KillarneyProvincial Park, located 40–60 km southwest ofSudbury, Ontario, were some of the first lakesin North America to be acidified by atmosphericpollutants. Acidification affected thousandsof fish and invertebrate populations in dozensof lakes. Since the 1970s, water quality hasimproved in response to atmospheric pollutionreductions and some lakes have alreadyrecovered to approximately their pre-industrialpH levels, as inferred from diatom microfossilsin lake sediments. Since the 1970s, fishspecies richness has not changed substantially,but zooplankton species richness has increasedin acidified lakes. The critical sulphur load,the amount of SO2-derived acid depositionthat can occur while still maintaining suitable water quality, was estimated to beexceeded in 38% of the park area in 1997. Depending on which of four possible NorthAmerican emission control scenarios (CLR =currently legislated reduction; CLR + 25%; CLR+ 50%; CLR + 75%) is achieved by 2010, theprojected critical loads will be exceeded inabout 0-30% of the park area in the future. There are many factors that can affectbiological recovery rates of damaged lakes, butit is expected that biological recovery willlag considerably behind observed chemicalrecovery rates.

Algal microfossils provide high temporal resolution of environmental trends

Sedimentary chrysophytes and diatoms from Baby Lake (Sudbury, Canada) were analyzed to assess lakewater trends associated with (i) the commencement of metal mining and smelting activity at the Coniston Smelter in 1913, and (ii) the closure of the smelter in 1972. We reconstructed several environmental variables (i.e. pH, total [Al], and [Ni]) by applying weighted-averaging regression and calibration models to our paleolimnological data. As a result of emissions from the Coniston Smelter, the lake started to acidify from a high pH of about 6.5 in 1940 to a low pH of 4.2 in 1975. During this period, inferred lakewater [Al] and [Ni] also increased dramatically. Following the closure of the smelter in 1972, inferred lakewater pH and metal concentrations recovered close to pre-industrial levels. The inferred pH recovery corresponds closely to the known lakewater pH recovery (a difference of about 0.4 and 0.6 of a pH unit was observed between measured pH, and chrysophyte and diatominferred pH, respectively). Our study demonstrates that algal microfossils can provide high resolution inferences of past environmental trends. This approach holds great potential for documenting a lakes response to decreased loadings of sulphate and metals, and in helping to establish and implement emission standards.

Paleolimnological assessment of damage to fish communities in three acidic, Canadian Shield lakes

Abstract Long-term fisheries data from small lakes are often sadly lacking. We employ three classes of sedimentary remains (mandibles of larval Chaoborus , diatom valves and chrysophyte scales) to verify that the fish communities of three small, acidic (pH 5.3–5.6) Canadian Shield lakes in south-central Ontario were richer 20–100 years ago at a time when the lakes were almost certainly less acidic. To do this, we first verify that Chaoborus americanus is never abundant in the presence of fish using data from a new survey of 50 lakes that corrects for methodological flaws of the past. We then examine changes with sediment depth in our three classes of indicators in Lakes Drummer, Little Eastern, and Shoelace. In all cases, appearrances of Chaoborus americanus and changes among algl remains in recent sediments suggest that fish were lost at a time when lake waters were acidifying, particulary because of pH depressions during spring melt. These results contribute to a growing body of literature indicating that the use of multiple sedimentary indicators can provide assessments of past limnological conditions that are of interest to both lake managers and aquatic ecologists. They also suggest that the fish communities of some small, currently acidic lakes in south-central Ontario were almost certainly richer in the past.

Setting Restoration Goals for an Acid and Metal-Contaminated Lake: A Paleolimnological Study of Daisy Lake (Sudbury, Canada)

ABSTRACT Many lakes in the Sudbury region of Ontario have been affected by acidic and metal inputs as a result of local smelting operations. Several initiatives are now underway to restore some of these systems. Long-term limnological data are rarely available, therefore lake managers must use proxy methods to set reasonable targets for lakes undergoing mitigation. Daisy Lake is presently an acidic (pH 4.7), metal-contaminated lake (Ni=370 μg L−1; Al=350 μg L−1), that has been selected for mitigation efforts. We used fossil pollen analysis to reconstruct past changes in terrestrial vegetation, and paleolimnological techniques (primarily diatoms and chrysophytes) to identify pre-disturbance conditions and to reconstruct past changes in water quality variables in Daisy Lake. Our data indicate that, if Daisy Lake and its watershed are limed to restore the lake to a state that approximates the pre-disturbance natural pH, the lake water pH has to be about 1.5 pH units higher than its present level.

Utility of scaled chrysophytes for inferring lakewater pH in northern New England lakes

Scaled chrysophytes in the surface sediments of 58 soft-water northern New England lakes were analyzed to assess their usefulness for inferring pH. The distributions of many taxa are correlated with lakewater pH and associated variables. Canonical correspondence analysis (CCA) and clustering grouped chrysophyte taxa according to their distributions along the pH gradient. For example, Chrysodidymus synuroideus, Mallomonas hindonii, and M. hamata commonly occur in acidic waters (pH<5.5), whereas M. caudata and M. pseudocoronata are common in circumneutral to alkaline waters. Of the five predictive models developed to infer pH, CCA based calibration had the lowest standard error (0.35 pH units). A CCA based predictive model was also developed to infer total alkalinity. The study provides strong evidence that, in the absence of past measured pH data, stratigraphic studies of sedimentary chrysophyte scales will provide accurate reconstructions of pH in northern New England lakes.

LAKE SEDIMENT CHRYSOPHYTE SCALES FROM THE NORTHEASTERN U.S.A. AND THEIR RELATIONSHIP TO ENVIRONMENTAL VARIABLES

Chrysophyte scale assemblages were analyzed in the surface sediments (0–1 cm) of 146 lakes sampled in the U.S. Environmental Protection Agency’s (EPA) Environmental Monitoring and Assessment Program–Surface Waters (EMAP‐SW) in the northeastern U.S.A. Chrysophyte data from the EMAP lakes were combined with a previous study of 71 Adirondack PIRLA (Paleoecological Investigation of Recent Lake Acidification) lakes and collectively analyzed to examine the indicator potential of scaled chrysophytes in the northeastern U.S.A. with respect to several environmental variables. Canonical correspondence analysis (CCA) was used to determine which environmental variables influenced the distributions of species. Forward selection and Monte Carlo permutation tests showed that 51% of the variance in the chrysophyte assemblages was related to pH. The other six significant variables (conductivity, chloride, total phosphorus [TP], elevation, lake depth, and watershed area) contributed an additional 31% of the total (82%) variance explained by the seven forward‐selected variables. Similar to previous studies, many taxa showed distinct distribution patterns with respect to pH. Partial and constrained CCAs indicated that, although all seven variables explained significant proportions of variation in the species data, a reliable inference model could be developed only for lake‐water pH. The strength of this model (R2= 0.78, RMSEboot= 0.47 of a pH unit) is comparable to a recently constructed diatom‐based model for the EMAP lakes. The use of both models in paleolimnological and biomonitoring studies would be advantageous because they would provide two independent lines of evidence of environmental change.