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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.

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.

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.

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 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.

Filamentous green algae, predominantly Mougeotia and Zygogonium, bloom frequently in the littoral zones of acidified lakes. Growth characteristics of Zygogonium-dominated filamentous green algae were studied for 4 yr in an experimentally acidified (pH 4.5) lake at the Experimental Lakes Area of northwestern Ontario. They were present in low abundance as periphyton (algal associations attached to surfaces) during spring, and as blooms of metaphyton (benthic algae unconstrained by surfaces) beginning in midsummer and reaching a maximum in early fall. Metaphytic filamentous green algae displayed high photosynthetic capacity in summer despite the oligotrophic nature of the acidified lake. The major factors controlling photosynthetic rates of Zygogonium were similar to those controlling Mougeotia, and included algal crowding, irradiance, dissolved inorganic carbon, and water temperature. Rates of photosynthesis were negatively dependent upon algal crowding, so that highest rates were associated with minimum algal crowding. Light requirements for photosynthesis were higher than those of the epilithon, which were dominant prior to acidification. The dependence of photosynthesis on ambient concentrations of dissolved inorganic carbon was partly regulated by water temperature. Anthropogenically caused releases from growth limitations (e.g., increased availability of limiting nutrients, increased water temperature, and extension of the growing season) may cause proliferation of filamentous green algae in the future.

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.

A significant relationship between seasonal N2-fixation rate and heterocyst abundance has been found in Lake 227 and can be modelled to estimate areal rates of N2-fixation based on the known historical phytoplankton records. Experimental imposition of low nitrogen (N): phosphorus (P) loading ratios stimulated N,-fixation. The N:P molar ratio of the internal pool of nutrients oscillates around a mean of 30:1, despite experimental loading ratios that vary from 13:1 to 0. Maintenance of this internal nutrient mass ratio requires preferential regeneration of N through time at all loading rates. Heterocystous, cyanobacteria blooms are most predictable during imposition of a very low external loading N:P ratio and high P loading relative to natural sources. Knowledge of the internal loading N:P ratio and relative contribution of internal and external loadings are required for predicting blooms at intermediate external N:P loading rates. The internal N:P ratio has fluctuated within narrow limits over 25 yr of nearly constant P loading because of adjustments in N2-fixation, N sedimentation, and denitrification, which have counterbalanced changing experimental N loading regimes.

Continuous artificial additions of nitrogen (N) and phosphorus (P) to Lake 227 at a loading ratio of 11:1 (atom: atom; 5:1 by mass) from 1975 to 1989, combined with natural inputs, resulted in variable mean annual total N and total P concentrations and changes in phytoplankton species composition. Summer phytoplankton occasionally shifted from N2-fixing cyanobacteria to non-N2-fixing forms. In 1990, additions of N fertilizer were stopped in order to test the hypothesis that low N:P loading rates cause selection for heterocystous cyanobacteria and stimulate N2-fixation. Five years of phytoplankton nutrient-status measurements, together with estimates of N2-fixation before and after lowering the N:P loading ratio, verified the importance of this ratio in producing blooms of N2-fixing cyanobacteria. Nitrogen input via biological N2fixation increased, and summer dominance by heterocystous cyanobacteria returned as a consequence of lowering the N:P ratio of external inputs. Seasonally, N return from the epilimnetic sediments increased after a period of high planktonic N2-fixation rates. This regenerated N reduced the abundance of heterocystous cyanobacteria and lowered late summer N2-fixation rates. Over several years, preferential regeneration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loading ratio.

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.

We studied the periphyton in two small lakes acidified experimentally with either sulfuric or nitric acid. For comparison, we monitored periphyton in one previously acidified (sulfuric) lake and in two neutral lakes. Net photosynthesis declined in epilithic periphyton in the middle littoral zone (1–2 m) of both recently acidified basins in response to reduced concentrations of dissolved inorganic carbon. Dark respiration and taxonomic composition remained largely unchanged in this zone. Attached filamentous algae proliferated early [Formula: see text] in the upper littoral zone (+and better adapted to obtain inorganic carbon at low concentrations, in combination with reduced grazing.

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.