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Reservoir creation often leads to a “trophic upsurge” of nutrients that may affect communities living near impoundments. We determined the duration of the nutrient upsurge and associated changes in plankton in a study of a new reservoir (Lake 979; L979) in northwestern Ontario that included 2 years pre-impoundment, 16 years of impoundment, and 2 years postimpoundment. Secondarily, we determined allochthonous versus autochthonous carbon (C) use by zooplankton with reservoir development. For the first 6–14 years of impoundment, mean concentrations of total phosphorus (TP), total nitrogen (TN), and dissolved organic carbon increased 3.6, 2.4, and 1.7 times above pre-impoundment levels, respectively, decreasing thereafter. L979 shifted from a net sink to a source of TP and TN for the first 6 years of impoundment. Mean annual biomasses of bacteria, phytoplankton, and zooplankton increased 4.6, 17, and 32.6 times above pre-impoundment levels, respectively, with associated changes in community composition. Bacteria and phytoplankton returned to pre-impoundment levels within 4 and 10 years, respectively, but zooplankton densities remained elevated even after 18 years. Changes in δ13C suggested that postimpoundment zooplankton biomass was not supported predominantly by allochthonous C.

A whole-ecosystem experiment in Lake 227 (L227) at the Experimental Lakes Area, ongoing since 1969, examined the roles of carbon (C), nitrogen (N), and phosphorus (P) in controlling eutrophication. During 2011, we conducted a series of sub-experiments and more intensive monitoring to improve estimates of N fixation and its ability to meet algal growth demands in the decades following the cessation of artificial N loading, while maintaining long-term high artificial P loading. Stoichiometric nutrient ratios indicated both moderate N and P limitation of the phytoplankton during spring, preceding a shift in phytoplankton community structure toward dominance by N fixing cyanobacteria. During bloom development, and for the remainder of the stratified period, stoichiometric nutrient ratios indicated moderate to strong P limitation. N fixation rates, corrected using 15N2 methods, increased 2× after 1990, when N loading ceased. Ambient dissolved inorganic nitrogen prior to the bloom represented less than 3% of N demands of the phytoplankton. N fixation accounted for between 69–86% of total N loading to the epilimnion during the period of rapid bloom development, and 72–86% of total N loading during the May–October period. Phytoplankton biomass did not decline in L227 during the 40 years since artificial N loading was reduced, or the nearly 25 years since artificial N loads ceased entirely (1990–2013), and remained approximately 20× higher than four nearby reference lakes. These results suggest that despite constraints on biological N fixation, it retains a large capacity to offset potential N loading reductions in freshwaters.

A recent rise in the relative abundance of Discostella species (D. stelligera and D. pseudostelligera) has been well documented from sedimentary diatom assemblages across the Northern Hemisphere. This unprecedented change over the last ~150 years has been linked to rises in atmospheric temperatures, changes in ice cover, and/or increases in thermal stability, among other factors. The bi-weekly monitoring data from two boreal lakes at the Experimental Lakes Area (ELA) in northwestern Ontario were analyzed across seasons (spring, summer, and fall) and decades (1970s-2000s). We found that Discostella species are primarily spring/early summer bloomers (i.e., late April to June) in these lakes and changes in concentrations of Discostella over time were most pronounced in the spring or early summer months. Increases in Discostella abundance over time may be linked to earlier ice-off and a longer period of spring turnover, resulting from increased winter and spring temperatures. It is also possible that a trophic mismatch between the spring diatom bloom and zooplankton is occurring, thus reducing diatom loss rates, and resulting in greater overall abundance. Moreover, the spring dominance of Discostella in our study lakes occurred at a time of the year when nutrient concentrations were at their highest seasonally, suggesting that these taxa are neither limited directly by nutrients, nor responding to enhanced stratification during the summer months in these lakes.

Nutrients released by cage fish farms to the aquatic environment are an issue of concern since these can result in deleterious environmental changes. In the present study, a mass balance model was used to calculate nutrient loadings from an experimental rainbow trout cage farm located in a freshwater, oligotrophic lake. Detection of these nutrient loadings using water quality monitoring was then investigated. The loading of total solids of faecal and feed origin (TS), solid phosphorus (SP) and nitrogen (SN), and dissolved P (DP) and N (DN) wastes from the farmed fish were estimated over two production cycles. Waste outputs were estimated using the Fish-PrFEQ feed requirement and waste output model using measured inputs including diet composition, nutrient digestibility, intake and retention by the fish, and water temperature. Nutrient loading predictions were compared with measured nutrient concentrations of lake water. In 2003, TS, SN and SP and DN and DP waste outputs were 236.0, 12.8, 5.3, 41.3, and 3.4 kg tonne − 1 of fish produced, respectively. In 2004, the TS, SN and SP and DN and DP waste outputs were 220.0, 12.2, 5.3, 38.0, and 3.4 kg tonne − 1 of fish produced, respectively. Over 60% of the P waste output from the cage was predicted to be solid for both years while over 65% of the total N waste from the cage farm was predicted to be excreted as ammonia. Concentrations of ammonia and of dissolved and particulate phosphorus were not reflective of waste loading of cage origin, suggesting efficient removal through uptake by biota and/or in the case of ammonia by nitrification. Fish-PrFEQ model is a valuable cage management tool that allows realistic estimation of waste outputs for cage farms and can be used to examine effect of management and feeding practices on waste outputs. However, the model is of limited use as a lake management tool, as it does not consider effects and fate of wastes released by fish. Similarly, reliance on periodic water quality monitoring at stations near cage farms may not be protective of the environment, as our results demonstrate that rapid diffusion, uptake, transformation and removal of nutrients resulted in water quality measures that were relatively insensitive to cage loading. Combining the Fish-PrFEQ model with a consideration of assimilative capacity of the system in addition to the monitoring of chemical and biological variables of the lake is recommended for environmental impact assessment of cage culture operations.

There is hope that increased development of aquaculture will help meet future global needs for protein. The growth of the freshwater aquaculture industry in Canada, however, has been hampered by insufficient information on environmental impacts. We examined the effects of an experimental cage aquaculture operation for rainbow trout ( Oncorhynchus mykiss ) on planktonic Crustacea and rotifers using 6 years of precage and 7 years of postcage data. Following the initiation of aquaculture, total crustacean and rotifer biomass, egg production, and depth distributions did not change significantly when compared with data from three nearby unimpacted lakes. We found statistically detectable increases in densities of Bosmina cf. longirostris and Diacyclops thomasi and an overall change in crustacean zooplankton community structure using correspondence analysis. The response of zooplankton was less than we expected because comparatively large changes were observed in phytoplankton biomass, hypolimnetic O2, and densities of invertebrate ( Mysis diluviana ) and fish predators. Our study emphasizes the need to monitor multiple variables when assessing the ecosystem impacts of potential stressors such as aquaculture.

A whole-lake experiment to examine the impacts of aquaculture on a freshwater ecosystem was conducted at the Experimental Lakes Area in northwestern Ontario, Canada. From 2003 to 2006, a 10 tonne fish capacity aquaculture cage stocked with rainbow trout ( Oncorhynchus mykiss ) was operated in Lake 375 and the impact of excess nutrients on the algal and bacteria communities was examined. The experiment was designed as a nutrient loading experiment with fish food and fish excretion the source of nutrients. Total N and P concentrations increased over the 4 years (15× and 4×, respectively). Phytoplankton biomass increased 4× annually following the start of aquaculture operation in 2003. The most dramatic responses occurred during spring and fall mixing, with blooms of chrysophytes and dinoflagellates increasing biomass by up to 12×. Bacteria biomass and densities were unaffected except for increases in late fall. Periphyton biomass was relatively unaffected except for an increase in biomass in the fourth year. The combination of a long water residence time in the lake coupled with an extremely high fish stocking density in Lake 375 resulted in an immediate impact on water quality. The results suggest that the impacts of aquaculture are accumulative and continual stocking will further impact water quality.

Chemical recovery is hypothesized to promote rapid recovery of benthic algal communities in formerly acidified lakes; this expectation needs modification. We evaluated the hypothesis in a small lake (L302S) in the boreal forest of northwestern Ontario, Canada, during a decade of pH recovery following a prior decade of experimental acidification from pH 6.7 to 4.5. To account for regional changes during the study, we also studied a nearby reference lake (L239). Taxonomic changes in the epilithon (biofilm on rock surfaces) included persistently lower cyanobacterial biomass following its acidification-related decline and increases in both diatoms and greens. Epilithic metabolic recovery was incomplete because the acidification-induced increase in respiration continued, although the prior decline in photosynthesis was reversed. Unexpectedly, blooms of metaphytic filamentous green algae occurred at higher pH during recovery than during acidification. Although several community attributes recovered fully, recovery of many aggregate functional and taxonomic properties lagged improvements in pH. Divergence was greater at the taxonomic than at the functional level. Despite pH recovery, potential causes of incomplete algal recovery include incomplete chemical recovery and the persistent absence of functionally important biota. Our findings counter the assumption that ecological recovery mirrors the pathway of damage caused by a human stressor.

Twenty lakes recovering from a century of atmospheric acid deposition over Northeastern Ontario were resurveyed for phytoplankton following a 20-year period and were compared with a 23-year study of an experimentally acidified lake, L302S (Experimental Lakes Area, ON, Canada). Phytoplankton species significantly tracked abiotic changes during both acidification and chemical recovery in all lakes based on concordance testing. However, ordination analyses showed that many phytoplankton communities had not returned to their preacidification state. Significant explanatory variables of taxonomic responses were pH, dissolved organic carbon (DOC), and inorganic nutrients (N, P), based on canonical correspondence analysis (CCA). Increases in DOC and pH influenced a significant taxonomic shift from acid-tolerant dinoflagellates to a diverse assemblage of cyanobacteria, chlorophytes, and diatoms. Declining nitrogen levels defined a secondary environmental gradient, which was characterized by a decrease in filamentous green algal abundance. L302S remained remote in ordination space from the more chronically and heavily polluted lakes in Northeastern Ontario, indicating that experimental acidification provided a conservative estimate of the true damage to atmospherically polluted lakes. However, L302S did increasingly resemble lakes in Northeastern Ontario, suggesting that experimental acidification simulated the impacts of moderate levels of atmospheric pollution. Our findings demonstrate the importance of ecological history in understanding the responses by boreal lake ecosystems to environmental change.

The number of combinations of anthropogenic stressors affecting global change is increasing; however, few studies have empirically tested for their interactive effects on ecosystems. Most importantly, interactions among ecological stressors generate nonadditive effects that cannot be easily predicted based on single-stressor studies. Here, we corroborate findings from an in situ mesocosm experiment with evidence from a whole-ecosystem manipulation to demonstrate for the first time that interactions between climate and acidification determine their cumulative impact on the food-web structure of coldwater lakes. Interactions among warming, drought, and acidification, rather than the sum of their individual effects, best explained significant changes in planktonic consumer and producer biomass over a 23-year period. Further, these stressors interactively exerted significant synergistic and antagonistic effects on consumers and producers, respectively. The observed prevalence of long- and short-term ecological surprises involving the cumulative impacts of multiple anthropogenic stressors highlights the high degree of uncertainty surrounding current forecasts of the consequences of global change.

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m−2 d−1 before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.

We experimentally warmed a series of shallow enclosures by 4.58C and measured responses of the epilithon (biofilm on rocky surfaces) and invertebrates. Maximum rates of net photosynthesis increased by 28‐115% and rates of dark respiration increased by 29‐103% as a result of warming. Long-term analyses using data from unmanipulated Lake 239 corroborated these findings, showing that rates of light-saturated photosynthesis and dark respiration were positively correlated with water temperature. Warming effects differed between communities (on natural and tile substrates, as well as well-developed and early successional communities). Warming consistently led to increased bacterial cell densities, but increases in total algal biovolume and diatom biovolume were seen only in an early successional tile community. Effects on the composition of the invertebrate community (studied only on well-developed tile biofilms) were small. We observed warming-related increases in carbon accrual within one community, and late in the experiment observed a change in carbon : phosphorus ratios of another community, possibly indicative of a degradation of food quality. Our study suggests that climate warming effects on epilithic community composition are likely to be heterogeneous and difficult to predict; however, the agreement between long-term and experimental results suggests that increased temperatures will increase metabolic rates of the epilithon.

Water-surface elevation in lake 226 (L226) of the Experimental Lakes Area in northwestern Ontario, Canada, was lowered experimentally by 2–3 m during each of three successive winters, and increased naturally but incompletely during the ensuing summers. Our objective was to compare the responses of the littoral and pelagic plant communities to this physical disturbance. Water-chemistry changes were muted, and neither nitrogen nor phosphorus concentration changed. Phytoplankton biomass, species assemblages, productivity, and nutrient status were largely unaffected except for small changes in species diversity and relative abundance of cyanobacteria and cryptophytes. Despite possible transient changes in functional and structural properties, the principal disruption for benthic algae was loss of colonizable surfaces. Floating-leaved and submersed macrophytes (hydrophytes) responded initially with large decreases in biomass and cover. The subsequent response of hydrophytes to drawdown varied: relative frequency of isoetids such as Eriocaulon septangulare decreased, while that of pondweeds such as Potamogeton spirillus increased. The trophic impacts of declining lake levels, whether due to hydroelectric reservoir manipulations or climate change, are likely to be much greater in the littoral zone than in the pelagic zone if major nutrients are unaltered.

It has been widely speculated that controls of SO2 emissions would stimulate recovery of acidified freshwater lakes in Canada, the United States and Europe. Phytoplankton communities from 22 lakes near Killarney Park Ontario, covering a pH range from 4.5-7.7, were studied from 1998-2000 and compared to data from experimentally acidified (pH decreased 6.7 to 4.5) and recovered (pH increased to 6.0) Lake 302 South at the Experimental Lakes Area (ELA), northwestern Ontario to assess recovery from acidification. Based on historical data, pH levels have rebounded to above 6.0 in several lakes in the Killarney area that were previously acidified to pH 5.0-5.5. Phytoplankton biomass was not correlated to pH, but there was a highly significant relationship between species richness and pH. Recovery trajectories were observed in a subset of 6 lakes, combining species diversity data from the present study with historical data. Correspondence analysis indicated that several of the lakes that experienced increased pH have shifted towards phytoplankton assemblages typical of circumneutral environments.

Ecosystem resistance to the impacts of diverse human insults depends on the replacement of sensitive species by ones more tolerant of the stressor. Here we present evidence from a whole-lake acidification experiment (Lake 302S, Experimental Lakes Area, Canada) that resistance and species compensation decline with increasing trophic level. Diverse and fast-growing algal and rotifer assemblages with high dispersal potentials showed significant compensatory species dynamics, resulting in the maintenance of total biomass despite 30%–80% declines in species richness. Canonical correspondence analysis showed that significant compensatory algal and rotifer dynamics were best explained by differential species tolerances of acidified chemical conditions coupled with release from resource limitation and predation. However, less diverse cladoceran, copepod, and fish assemblages showed significant declines in total biomass and weak species compensation with loss of species during acidification. In comparison, algal and zooplankton species dynamics remained relatively synchronized in a nearby unperturbed reference lake (Lake 239) during the experiment. As a result, Lake 302S showed limited ecosystem resistance to anthropogenic acidification. Therefore, we hypothesize that lost species will increase the susceptibility of acidified lakes to the adverse impacts of other environmental stressors (for example, climate warming, stratospheric ozone depletion, invasive species). Consequently, the ecosystem stability of boreal lakes is expected to decline as global change proceeds.

We have attempted to quantify the roles of physical, chemical, and biological interactions structuring the attached algal and invertebrate communities in eight boreal lakes at the Experimental Lakes Area, northwestern Ontario, varying in dissolved organic carbon (DOC) concentrations (3‐9 mg L 21 ). Attached benthic communities on rocky, south-facing shores were sampled at 0.1, 0.3, 0.7, and 1.5 m depths. Multivariate redundancy analysis explained 58% of the variance in algal communities and 75% of the variance in invertebrate communities. Algal composition was determined most by photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) fluxes, whereas the composition of associated invertebrates was driven by a combination of exposure to solar radiation and food quality and quantity. Communities on rock surfaces exposed to high solar fluxes in these lakes had very high concentrations of a photoprotective scytonemin-like pigment (SLP-A), and were dominated by filamentous green algae. Conversely, deeper, more shielded communities were dominated by diatoms and had low concentrations of SLP-A. This suggests that previous paleolimnological evidence of SLP-A increases might reflect changes in littoral attached algal communities rather than planktonic ones. Decreases in DOC concentrations as a result of climate warming will result in increased penetration of solar radiation in boreal lakes. As a result, in the event of decreased precipitation, we predict that the dominance of UVR-resistant epilithic algae in littoral zones will occur in deeper waters than before, with coincident reductions in densities of many invertebrate taxa in algal biofilms increasingly exposed to high solar fluxes.

The conclusion from our earlier paper, that decreasing nitrogen (N) inputs to lakes does not substantially reduce symptoms of eutrophication (Schindler et al. 2008), was recently challenged by Scott and McCarthy (2010). They reanalyzed graphs from our paper showing data from Lake 227 at the Experimental Lakes Area (ELA) in Ontario, which was artificially eutrophied with N and phosphorus (P) from 1969 through 1989 and with P alone from 1990 until the present. They interpreted declining trends in total nitrogen (TN), chlorophyll a (Chl a), and phytoplankton biomass to mean that the lake became less eutrophic after N inputs were terminated in 1990. They then concluded that ‘‘the degree of eutrophication can be controlled by managing N inputs concurrently with P.’’ We now have four more years of data for Lake 227, for a total of 41 yr at a constant P loading rate. Schindler et al. (2008) review the loading history of Lake 227, while Findlay et al. (1994) give full N and P budgets from 1970 to 1992. All samples were collected and analyzed using consistent methods (Stainton et al. 1977; Findlay et al. 1994; Schindler et al. 2008). Statistics reported below were calculated using SYSTAT 12 (SYSTAT 2007). We have used a threshold for significance of 0.05; and, to remain consistent with Scott and McCarthy (2010), we have not corrected probabilities for multiple comparisons. Reanalysis of our results including the more recent data clearly refutes the conclusion of Scott and McCarthy that the lake has become less eutrophic as the result of decreased inputs of N. There has been no significant declining trend in either Chl a (r 5 20.26, p 5 0.27, n 5 20) or phytoplankton biomass (r 5 20.22, p 5 0.36, n 5 20) since we ceased adding N to the lake in 1990 (Fig. 1a,b). Separate analyses of trends after 1996, which Scott and McCarthy used to make many of their arguments, are not different from those for 1990–2009 (Chl a: r 5 20.16, p 5 0.61; phytoplankton biomass: r 5 20.35, p 5 0.14; n 5 13). In fact, there is no significant trend in the annual average Chl a or biomass over the 41 yr since fertilization with P began, regardless of the N : P ratio used in loading (Chl a: r 5 20.05, p 5 0.77; biomass: r 5 20.23, p 5 0.14). We note that in fig. 1 of Schindler et al. (2008), the 1975 data point for Chl a was repeated for 1976 and all subsequent points were offset in error by 1 yr. This error does not affect the major conclusions of Schindler et al. (2008) and only marginally affects the correlations undertaken by Scott and McCarthy (2010). The biomass of Cyanobacteria (not shown) also showed no significant trend over time (r 5 20.38, p 5 0.10, n 5 20 after 1990; r 5 0.08, p 5 0.60, n 5 41 for the entire data set). However, the abundance of heterocysts has increased significantly (r 5 0.59, p , 0.0005, n 5 36), particularly after reduction of N loading in 1990 (Fig. 1c). Using the regression equation of Finday et al. (1994), which is based on studies of Lake 227 by Hendzel et al. (1994), this increase in heterocyst abundance indicates that N fixation has increased considerably since 1990 (Fig. 1d; r 5 0.61, p 5 0.005, n 5 20). The rate of increase in heterocysts roughly doubled after 1997, indicating that N fixation is still increasing. This observation reinforces our earlier arguments (Schindler et al. 1977, 1987) that the responses of lakes to changes in nutrient inputs result from slow changes in species and biogeochemical processes, requiring several years to fully play out. Successful strategies to control eutrophication must account for these rates of change. The negative trend in TN concentrations after 1990 identified by Scott and McCarthy continues to be significant with our longer data set (1990–2009: r 5 20.74, p , 0.0005, n 5 20) (Fig. 2a). Over the same time period, there is no significant change in total phosphorus (TP) (r 5 20.18, p 5 0.26, n 5 20) or TN : TP (r 5 20.32, p 5 0.16, n 5 20) (Fig. 2c,d). Most of the decrease in TN is in total dissolved nitrogen (TDN) (r 5 20.70, p , 0.0005, n 5 20; Fig. 2b) and there are also significant declines in the ratios of dissolved inorganic nitrogen : total dissolved phosphorus (DIN : TDP; r 5 20.50, p 5 0.02, n 5 20) and TDN : TDP (r 5 20.45; p 5 0.05, n 5 20) (not shown). Clearly, reduction in artificial N loading has resulted in lower N concentrations in Lake 227. N limitation for many species of phytoplankton may also have increased because of declines in DIN, which includes the forms of N that are most available to many phytoplankton species. However, N-fixing cyanobacteria increased after the cessation of artificial N loading (Schindler et al. 2008), clearly demonstrating their ability to offset declines in DIN, and overall algal abundance has remained proportional to annual P loading, which has been constant since 1969. To be effective, eutrophication management must reduce excessive phytoplankton abundance. The Lake 227 data show that reductions of N in the absence of reductions in P will shift the competitive advantage to N-fixing cyanobac* Corresponding author: michael.paterson@dfo-mpo.gc.ca

We examined long-term phytoplankton data records for four oligotrophic boreal lakes situated in the Experimental Lakes Area (ELA), western Ontario, for responses to climatic change. ELA experienced a cyclical wet-dry-wet pattern from 1968 to 1998, with the early 1970s and 1990s having above-average precipitation and the 1980s being a period of drought with a 2° C increase in air temperature. During this drought, the length of ice-free season, duration of stratification, depth of the euphotic zone, and light extinction increased while precipitation and nutrient inputs to the lakes decreased. Phytoplankton assemblages of four study lakes were temporally coherent. During the drought, phytoplankton biomass and the number of phytoplankton species increased despite decreased nutrient inputs. There was a noticeable shift in species composition to greater abundances of dinoflagellates and large chrysophytes-mixotrophic species capable of cycling through the deeper, lower light, high-nutrient waters, presumably to consume bacteria as an alternative to autotrophic production. These species have slow turnover times; therefore, suspended nutrients were held in the water column for a longer period of time. Phytoplankton photosynthesis was less responsive.

Stratospheric ozone depletion, climate change, and acidification will increase the exposure of aquatic eco systems to ultraviolet radiation (UVR; 280–400 nm). The objective of this study was to determine the ecological effects of ambient UVR exposure on epilithon (algal communities attached to rocky substrata) relative to an artificially reduced UVR environment. UVR exposure was altered in the littoral zone of a boreal lake by selectively filtering segments of the solar spectrum with large acrylic sheets. This 130-day study took place at the Experimental Lakes Area, northwestern Ontario, in 1998. Epilithon were monitored for changes in productivity, biomass, pigment, and nutrient and taxonomic composition. The absence of UVR increased epilithic photosynthetic rates 37–46% above epilithon exposed to ambient UVR, increased cellular carbon and nitrogen content, and consequently lowered C:P. UVR effects on algal metabolism were dependent on seasonal trends. Epilithic respiration rates and chlorophyll a concentrations were not significantly different among treatments. A reduction in UV induced taxonomic shifts in epilithon, with diatoms increasingly favoured under low UV conditions. Species richness and diversity were not affected. Overall, the results of this study, considered together with previous research, suggest that the epilithic community is sensitive to changes in the UVR environment of aquatic ecosystems.

Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Addition of northern pike eliminated populations of planktivorous minnows by the third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodied cladoceran Daphnia pulicaria occurred. Accompanying this increase in Daphnia abundance, zooplankton community N:P declined, seston concentration and C:P ratio decreased, and dissolved N and P pools increased. During peak abundance, zooplankton biomass comprised a significant proportion of total epilimnetic phosphorus (greater than 30%). During the period of increased Daphnia abundance, concentrations of dissolved inorganic nitrogen (TIN) increased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water column during 1996. Finally, previously dominant N-fixing cyanobacteria were absent during 1996. Our results show that strong effects of food-web structure can occur in eutrophic lakes and that stoichiometric mechanisms play a potentially important role in generating these effects.

1. Lake 302S in the Experimental Lakes Area of Canada was acidified from pH 6.7 (1981) to 5.1 (1986). The pH was further reduced to 4.5 in 1987 and held at that level until 1991. From 1992 to 1995, the pH was allowed to increase to a target value of 5.8. 2. The response of the phytoplankton community to decreasing pH from 6.0 to 5.1 was similar to that observed in another experimentally acidified lake (223) and in other atmospherically acidified lakes. Acidification affected species diversity of both the phytoplankton and epilithon. Phytoplankton diversity was positively correlated with pH. Epilithic algal diversity was more variable and did not correlate with pH. 3. Phytoplankton biomass was enhanced by acidification as the assemblage shifted from a dominance of chrysophytes to large dinoflagellates (Gymnodinium sp. and Peridinium inconspicuum). Epilithon biomass was unaffected, but dominance shifted from filamentous cyanophytes (Lyngbya) to acidophilic diatoms (Tabellaria quadriseptata and Anomoeonis brachysira). 4. The only taxon to be similarly affected in both the phytoplankton and epilithon was the cyanobacteria, being significantly reduced below pH 5.1. During early recovery (pH 5.5–5.8), cyanobacteria increased and species present prior to acidification recolonized both habitats. 5. In the early stages of recovery, planktonic and benthic assemblages remained more similar to acidified than natural assemblages, but more profound change began at pH > 5.5.

Changes in plankton community structure were examined in an experimental reservoir (Lake 979) before and after impoundment. The role of allochthonous organic matter in planktonic food webs is unclear, and reservoir creation can be viewed as an extreme manipulation of terrestrial organic matter inputs. After impoundment of Lake 979, concentrations of phosphorus, nitrogen, and dissolved organic carbon increased as a result of decomposition of flooded terrestrial organic matter. In the first year of impoundment, mean bacterial biomass increased 10 times , and individual bacterial cell volumes increased 2 times over pre-flooding averages. Phytoplankton production and biomass decreased to approximately 25% of pre-flooding levels. Zooplankton biomass and production by Cladocera increased 10 times , and zooplankton community composition changed from dominance by small-sized Bosmina longirostris to dominance by large Daphnia rosea. In the first year of impoundment, production by Cladocera usually exceeded phytoplankton14C productivity, suggesting that the main pathway of carbon flow to secondary producers shifted from an autochthonous to an allochthonous base derived from flooded terrestrial vegetation. In the second year of flooding, bacterial biomass decreased and phytoplankton biomass was higher than in the two previous years of study.

Experimental additions of cadmium (Cd) raised concentrations of Cd in the epilimnion of Lake 382 from background levels (0.0016 μg·L n1 ) to averages as high as 0.185 μg·L n1 for 150 days from 1987 to 1993. The effects on the algal community were assessed on the basis of 9 years of premanipulation data and 7 years of experimental data on phytoplankton population, phytoplankton photosynthesis, and epilimnetic chemistry. In addition, trends in the Lake 382 data were compared with those from two reference lakes over the full 16-year period. Cd additions to Lake 382 had no significant effects on the phytoplankton community. Phytoplankton biomass was relatively stable, and small changes were related to nutrient inputs. Species diversity and community size structure were not affected. Species described in the literature as being sensitive to Cd did not respond significantly in Lake 382. Cd additions did not significantly affect phytoplankton photosynthesis. Changes in photosynthesis were related to nutrient inputs and phytoplankton biomass. Resume : Des apports expOrimentaux de cadmium (Cd) ont augmentO la concentration de ce mOtal dans liOpilimnion du lac 382 jusqui‡ des moyennes atteignant 0,185 μg·L n1 pour 150 journOes de la pOriode 1987n1993, la concentration naturelle Otant de 0,0016 μg·L n1 . Nous avons OvaluO les effets de cette augmentation sur les communautOs dialgues, en utilisant 9 annOes de donnOes antOrieures au traitement et 7 annOes de donnOes recueillies pendant le traitement; ces donnOes portaient sur la population de phytoplancton, la photosynthse phytoplanctonique et la chimie de lieau Opilimnique. Nous avons Ogalement comparO les tendances de ces donnOes ‡ celles caractOrisant deux lacs de rOfOrence pour la mŒme pOriode de 16 annOes. Les apports de Cd dans le lac 382 niont eu aucun effet significatif sur la communautO de phytoplancton. La biomasse phytoplanctonique est demeurOe relativement stable, les lOgers changements Otant reliOs aux apports de nutriments. La diversitO en espces ainsi que la rOpartition des classes de taille dans la communautO niont pas OtO touchOes. Les espces mentionnOes dans les publications comme Otant sensibles au Cd niont pas rOagi de faAon significative dans le cas du lac 382. De mŒme, les apports de Cd niont pas modifiO de manire significative la photosynthse phytoplanctonique; les changements observOs ‡ ce titre Otaient reliOs aux apports de nutriments et ‡ la biomasse phytoplanctonique. (Traduit par la ROdaction)

L'acidification experimentale de pH 6,7 a pH 5,1 du lac 223 a cause chez la communaute phytoplanctonique des effets saisissants semblables a ceux observes dans les systemes subissant des effets d'origine anthropique. Il y avait diminution de la diversite des especes, accroissement de la biomasse et deplacement de la dominance des chrysophytes vers les dinoflagelles. L'accroissement du pH de 5,0 a 5,8 n'entrainait qu'une tres faible reponse de recuperation. La biomasse et la composition restaient inchangees, mais la diversite des especes augmentaient au fur et a mesure qu'augmentait le pH. L'augmentation du pH au-dessus de 6 entrainait une forte recuperation de la communaute phytoplanctonique du lac 223. La diversite des especes continuait a augmenter au fur et a mesure qu'augmentait le pH. La biomasse des chrysophytes augmentait, mais la biomasse totale commencait a diminuer jusqu'aux niveaux pre-acidification en raison de la diminution d'abondance des cyanobacteries, des dinoflagelles, des chlorophytes et des cryptophytes. La structure de la taille de la communaute phytoplanctonique commencait a ressembler a celle des communautes des lacs de reference.

Dans la region des lacs experimentaux, au nord-ouest de l'Ontario, on a examine les effets de l'acidification des lacs sur les associations d'algues dans la zone littorale. La photosynthese nette du periphyton a diminue, en partie a cause d'une plus grande limitation du carbone, dans le cadre d'une experience d'acidification du lac 302S, auquel on a ajoute de l'acide sulfurique, faisant passer le pH de 6,7 a 4,5. Le rapport entre la respiration et la photosynthese a augmente dans l'epilithon (periphyton se trouvant sur des surfaces rocheuses) parallelement a la diminution du pH, phenomene imputable surtout a l'acroissement rapide de la respiration. Les modifications survenues dans le lac 302N, qui a ete acidifie au moyen d'acide nitrique, etaient analogues a celles du lac 302S. Des changements majeurs sont egalement survenus dans la composition des algues de l'epilithon, en particulier lorsque le pH est tombe a moins de 5. L'abondance de chlorophycees filamenteuses ou les algues vertes s'est accrue lorsque le pH est passe a moins de 6. Les algues vertes sont devenues du metaphyton (apres leur detachement du substratum) et ont prolifere a un pH de 5,5 et moins, Zygogonium etant predominant a un pH de 4,5. Les algues vertes filamenteuses du metaphyton couvraient parfois presque la totalite du fond de la zone littorale pendant les periodes de proliferation. Contrairement a l'epilithon, ces algues ne presentent pas une augmentation du rapport entre la respiration et la photosynthese parallelement a la baisse du pH. Les modifications fonctionnelles observees dans les associations algales du littoral, les plus sensibles qui soient connues dans les ecosystemes aquatiques, qui sont attribuables a l'acidification deviennent plus evidentes a mesure que l'acidification progresse.

Lake 221 was dominated by omnivorous perch (Perca flavescens); the addition of northern pike (Esox lucius) resulted in trophic changes that eventually affected the phytoplankton community. Two years following the introduction of pike, perch were greatly reduced. Subsequently, Chaoborus abundance increased, rotifer and cladoceran abundance and biomass decreased, and the large-bodied cladoceran Daphnia catawba increased. The phytoplankton community shifted from chlorophyte to cyanophyte codominance with dinoflagellates. Phytoplankton biomass and phosphorus (P) increased because of nutrient recycling and excretion by pike and zooplankton. In years three and four, algal biomass and the ratio of suspended to dissolved P decreased because a larger portion of dissolved P was bound in an increased bacterial population. Phytoplankton cell size and production decreased, but the production:biomass ratio increased. In year five, chrysophytes dominated as phytoplankton biomass and production increased and bacterial abundance declined. Phytoplankton responses were primarily an indirect result of the introduction of piscivorous fish, which altered internal nutrient recycling.

Aphanizomenon schindleri sp.nov., a new nostocacean, bluegreen, cyanoprokaryote (cyanobacterium), is described from artificially eutrophied Canadian Shield lakes in the Experimental Lakes Area, northwestern Ontario. It is unique to low conductivity Canadian Shield waters to which nutrients have been added, and it became the dominant bloom species after years of phosphorus (P) and nitrogen (N) loading followed by N limitation (N:P ≤ 5:1 by mass) (Findlay et al. 1994. Can. J. Fish. Aquat. Sci. 51: 2254–2266). The new Aphanizomenon species appears to be most closely related to A. gracile but also resembles species of Anabaena with straight filaments. It differs from other species of Aphanizomenon in cell dimensions, length and width of akinetes and heterocytes, and in akinete and end cell shape.

We examined the diatom and chrysophyte assemblages preserved in Lake 227 sediments prior to, during, and following the annual additions of phosphorus (P) and nitrogen (N) (from 1969–1989). Premanipulation (1957–1968) diatom and chrysophyte assemblages were dominated by taxa characteristic of acidic, unproductive lake conditions. In 1969, the assemblages immediately shifted in response to increased nutrient availability and/or increased pH values in the epilimnion. Large increases in the relative abundance of Synedra PIRLA 2 and S. rumpens var. familiaris, Mallomonas crassisquama, M. doignonii var. tenuicostis, and cysts 1, 15, 29, 233, and 120 largely replaced the prefertilization assemblages. From 1970–1990, the diatom assemblage was dominated by alkaliphilic or pH-indifferent taxa. A second chrysophyte assemblage shift, which coincided with a lowering of the N:P addition ratio in 1975, was characterized by decreases in M. crassisquama and cysts 1, 15, 29, 233, and 120, and corresponding increases in Synura sphagnicola Korshikov scales and cysts 62 and 33. The stratigraphical analyses and numerical rate-of-change analyses clearly showed that diatom and chrysophyte species composition, particularly chrysophyte cysts, closely tracked short-term changes in lakewater chemistry. The amount of variance in the stratigraphic assemblages explained by lake-water chemical changes was high (68–90%), indicating the close algae–chemistry relationships in Lake 227 from 1969–1989.

Fossil pigments from annually laminated sediments were calibrated with coeval phytoplankton data (1970–1989) from experimentally eutrophied Lake 227 in the Experimental Lakes Area, Ontario. Concentrations of ubiquitous pigments (β-carotene, pheophytin a) were correlated to total algal biomass standing crop (r = 0.56–0.65; P −1) from green algae and filamentous cyanobacteria (myxoxanthophyll) increased 4- to 10-fold in response to eutrophication of Lake 227. N2-fixing cyanobacteria (recorded as aphanizophyll) replaced chlorophytes after the nitrogen additions decreased threefold in 1975. In contrast, accumulation rates of pigments (nmol pigment∙rrr−2∙yr−1) were rarely correlated with algal standing crop or production and were less satisfactory than fossil concentrations for the purpose of detecting changes in phytoplankton community composition.

Two predators of zooplankton, fish (yellow perch) and larval insects (Chaoborus), were manipulated within in situ mesocosms to assess their indirect effects on phytoplankton. The two predators caused similar reductions in zooplankton size and biomass, and as a consequence, zooplankton grazing rates on phytoplankton. However, phytoplankton increased relative to predator—free controls only in enclosures with fish and not in enclosures with Chaoborus, suggesting that fish have effects on phytoplankton independent of their effects on zooplankton grazing rates. Estimation of phosphorus excretion rates of zooplankton and fish suggests that fish render P more available to phytoplankton in two ways: (1) by directly excreting and egesting P, and (2) by increasing rates of P excretion by the zooplankton community. The latter effect resulted because small zooplankton have higher biomass—specific excretion rates than large zooplankton. The importance of nutrient regeneration is further supported by the response of phytoplankton community structure. Phytoplankton taxa exhibiting enhanced biomass in the presence of fish were those with high P requirements, including green and blue—green algae and dinoflagellates; other phytoplankton groups did not respond to manipulations. These results suggest that the mechanisms underlying the tropic cascade are complex and that predators affect phytoplankton by altering nutrient recycling rates as well as zooplankton grazing rates.

Algal assemblages can be highly responsive to environmental changes in recovering acidified lakes. We compared epilithic algal assemblages in boreal lakes during chemical recovery from atmospheric (Killarney Park, Ontario) and experimental (Lake 302S, Experimental Lakes Area, Ontario) acidification to assess the impact of spatial and temporal scale of severe acidification on taxonomic resilience (i.e. recovery rate). Resilience was measured as the distance traveled by lakes in ordination space during pH recovery based on canonical correspondence analysis. Resilience was relatively negligible in the Killarney lakes, suggesting that eight years of experimental acidification in Lake 302S had less impact on biological recovery than did decades of regional acidification. Increases in dissolved organic carbon, dissolved inorganic carbon, and calcium best explained temporal variance of epilithic species abundances in the recovering acidified lakes. In Lake 302S, contrasting trajectories of taxonomic resilience and resistance, i.e. displacement from reference conditions following a perturbation, indicated that ecological factors affecting epilithon differed at corresponding pH levels during recovery and acidification. Our findings reveal that modeling of ecosystem recovery from severe acidification must account for the spatial and temporal scale of the perturbation, and biological delay responses that result in differences between recovery and acidification trajectories.

The response of zooplankton to sodium selenite at concentrations of 1,10, and 100 μg Se∙L−1was studied using large (100 m3) enclosures in mercury-contaminated Clay Lake, Ontario (50°03′N, 90°30′W). Crustacean community development in treated enclosures exhibited no obvious acute or chronic effects from the range of selenium concentrations applied. Seasonal changes in zooplankton composition, abundance, and reproduction were similar among controls and the 1 and 10μg Se∙L−1enclosures. Differences in species dynamics observed in the 100 μg Se∙L−1enclosure and in a sediment control enclosure were attributable to biotic interactions rather than selenium. Predation appeared to be the dominant factor governing zooplankton community structure in all enclosures. Differences in the number of planktivorous fish among enclosures led to variations in the abundance of predatory Leptodora kindtii, which in turn affected abundances of the small cladoceran Bosmina longirostris.

Experimental acidification of a small lake from an original pH value of 6.8 to 5.0 over an 8-year period caused a number of dramatic changes in the lake's food web. Changes in phytoplankton species, cessation of fish reproduction, disappearance of the benthic crustaceans, and appearance of filamentous algae in the littoral zone were consistent with deductions from synoptic surveys of lakes in regions of high acid deposition. Contrary to what had been expected from synoptic surveys, acidification of Lake 223 did not cause decreases in primary production, rates of decomposition, or nutrient concentrations. Key organisms in the food web leading to lake trout, including Mysis relicta and Pimephales promelas, were eliminated from the lake at pH values as high as 5.8, an indication that irreversible stresses on aquatic ecosystems occur earlier in the acidification process than was heretofore believed. These changes are caused by hydrogen ion alone, and not by the secondary effect of aluminum toxicity. Since no species of fish reproduced at pH values below 5.4, the lake would become fishless within about a decade on the basis of the natural mortalities of the most long-lived species.