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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.

Glacial runoff is predicted to increase in many parts of the Arctic with climate change, yet little is known about the biogeochemical impacts of meltwaters on downstream freshwater ecosystems. Here we document the contemporary limnology of the rapidly changing glacierized watershed of the world’s largest High Arctic lake (Lake Hazen), where warming since 2007 has increased delivery of glacial meltwaters to the lake by up to 10-times. Annually, glacial meltwaters accounted for 62–98% of dissolved nutrient inputs to the lake, depending on the chemical species and year. Lake Hazen was a strong sink for NO3−-NO2−, NH4+ and DOC, but a source of DIC to its outflow the Ruggles River. Most nutrients entering Lake Hazen were, however, particle-bound and directly transported well below the photic zone via dense turbidity currents, thus reinforcing ultraoligotrophy in the lake rather than overcoming it. For the first time, we apply the land-to-ocean aquatic continuum framework in a large glacierized Arctic watershed, and provide a detailed and holistic description of the physical, chemical and biological limnology of the rapidly changing Lake Hazen watershed. Our findings highlight the sensitivity of freshwater ecosystems to the changing cryosphere, with implications for future water quality and productivity at high latitudes.

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.

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.

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P-rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north-western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N-fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P. 2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH4, –NH4) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment. 3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH4 had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients. 4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH4). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures. 5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.

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.

Fossil pigment analyses and 19 year-long historical records were used to quantify whole-lake algal response to changes in optical and chemical properties following experimental acidification of Lake 302 with H 2 SO 4 (south basin, 302S; 1981-1989) or HNO 3 (north basin, 302N; 1982-1986) and HC (1987-1989). Undisturbed sediments were collected by freeze-coring, sectioned in approximately annual intervals, and analyzed for fossil carotenoids, chlorophylls, and derivatives by high performance liquid chromatography. Concentrations of fucoxanthin (diatoms, chrysophytes, some dinoflagellates) were correlated with algal standing crop (r 2 = 0.67, P < 0.05; 1978-1989) and increased 6-fold following acidification of Lake 302S with H 2 SO 4 from pH 6.6 to 5.0, consistent with observed reductions in dissolved organic carbon (DOC) from 7 to 4.5 mg liter 1 , improved water clarity, and increased biomass of deep-water chrysophytes. However, fucoxanthin concentrations declined to baseline values in sediments from 1988 to 1990, concomitant with severe acidification to pH 4.5, continued DOC loss (

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.