On the basis of intensive collections of Microcrustacea (Cladocera, Copepoda, Anostraca, and Notostraca) from 11 sites in the Canadian arctic archipelago, patterns of species richness, distribution, and community composition were evaluated. Microcrustacean communities of pond habitats in the high arctic of Canada are much less diverse than communities at more mesic, subarctic sites. There is both a marked decline in species richness and a dramatic change in the composition of pond communities with increasing latitude and decreasing summer temperatures. Ponds in the low arctic have 3–4 times as many species as those in the high arctic and are dominated by calanoid copepods, a group largely absent from the high arctic. Sites with similar climatic conditions in Alaska have microcrustacean communities with higher species richness, a phenomenon correlated partially with the presence of glacial refuges throughout the Pleistocene. The failure of certain zooplankton taxa, especially calanoids, to colonize areas in the Canadian arctic remote from refuges in the 7000 yr since deglaciation suggests that their dispersal rates are low. Reestablishment of complete zooplankton communities in glaciated terrain with severe environmental conditions apparently requires long periods, although initial colonization by a few pioneer species may be rapid.

1. Earlier spring warming as predicted for climate change will alter combinations of water temperature and photoperiod that act as emergence cues for zooplankton resting stages. As a result, water temperature cue thresholds will be experienced at shorter photoperiods, a variable independent of weather variations. Also, light intensity, another potentially important cue for zooplankton emergence, could decrease in many lakes if symptoms of climate change resemble those of eutrophication. 2. We designed a laboratory experiment to test the effects of three factors, temperature (6, 9 and 12°C), photoperiod (13L : 11D and 16L : 8D) and light intensity (20 and 35 lE m )2 s )1 ) on hatchling abundance and timing of hatching of daphniids (Daphnia ambigua) and rotifers (Keratella spp. and Synchaeta pectinata) from resting eggs. Further, we investigated the implications of potential changes in hatching dynamics, following variations in hatching cues, on zooplankton spring population development using predator-prey simulation models. 3. For hatchling abundance and timing of hatching, photoperiod had a significant effect for D. ambigua but not rotifers. Daphnia ambigua hatchling abundance decreased by 50% when incubated at conditions mimicking early spring (12°C + 13-h photoperiod) compared to a later spring (12°C + 16-h photoperiod). Light intensity has a significant effect only for S. pectinata, producing greater hatchling abundance at lower light intensity. 4. Simulation models suggest that in contrast to a later spring, an early warming produces a shift in spring zooplankton community composition, from daphniid to rotifer dominance. These patterns are primarily driven by differential zooplankton emergence response with variations in temperature-photoperiod cues. 5. Overall, our laboratory experiments and simulation models suggest that lakes with strong dependence on the 'resting egg-bank', characteristic of many shallow northtemperate lakes or in years with low winter survivorship of adult zooplankton, may be most susceptible to climate change. Further, fewer large grazers such as daphniids with an earlier spring may result in less control of cyanobacterial blooms in eutrophic lakes.

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Ecological effects of blooms of filamentous green algae (FGA) were studied in an experimentally acidified Bake (pH 4.5) at the Experimental Lakes Area in northwestern Ontario, Canada. Blooms of FGA influenced the energy balance, chemical cycling, physical features, and biological conditions in the littoral zone. Photosynthetic capacity of the FGA, dominated by Zygogonium, was greater than that of acidified epilithon, the normally dominant littoral algal association, partially offsetting acidification-induced oligotrophication in the littoral zone. Intraand inter-annual variability of FGA growth was large, however, so that FGA were an unreliable energy source for the littoral food web. Nutrient uptake varied with the degree of FGA growth; e.g., FGA were occasionally the largest phosphorus pool in the epilirnnion. Nitrogen dynamics of FGA varied seasonally causing acidification in spring and summer, and alkalinization in fall. The blooms also affected epilimnetic cycling of dissolved inorganic carbon. Peak blooms prevented as much as 90% of the light from reaching the lake bottom. Local depletions of oxygen resulting from FGA decomposition also posed potential risks for animals using the HGA as habitat.RCsumC : Les effets Ccologiques de la prolifkration d'algues vertes (chlorophycCes filamenteuses) ont fait 190bjet d'Ctudes dans un lac expkrimentalement acidifiC (pH de 4,s) dans la region des lacs expCrimentaux du nord-ouest de 1'8ntario (Canada). Les prolifdrations d'algues vertes ont eu une incidence SUP le bilan knergktique, B e cycle chimique, les caractCristiques physiques et les conditions biologiques de B a zone Hittorale. La capacitC de photosynthkse des algues vertes, dsminCes par Zvgogonium, Ctait supCrieure h celle de 19Cpilithon acidifik, l'association algale qui dornine normalement dans la zone littorale, cornpensant en pai-tie 1901igotrophisation rksultant du processus d9acidification du littoral. Cependant, on a enregistre une grande variabilitk de la croissance des algues vertes au cours de l'annee et d9une annCe 2i l'autre, ce qui fait de ces algues une source d'knergie peu fiable pour le reseau trophique littoral. L'absorption des ClCrnents nutritifs varie avec le degrC de croissance des algues vertes; p. ex., les algues vertes csnstituaient parfois H a rCserve de phosphore la plus importante de lqCpilimnion. La dynamique de l'azote chez les algues vertes varie d'une saison B l'autre, entrainant une acidification au printemps et en CtC, et une alcalinisation en automwe. Les proliferations ont egalement nui au cycle du carbone inorganique dissous dans 1'Cpilimnion. Au cours des p&riodes de proliferation maximale, la proportion de I'kclairement qui we psuvait pas atteindre le fond du lac s9Clevait parfois h 98%. L'appauvrissement local en oxyg8ne resultant de la dCcomposition des aHgues vertes posait Cgalement des risques Cventuels pour les organismes utilisant les algues vertes comme habitat. [Traduit par Ia RCdaction]

The response of pelagic zooplankton to experimental fertilization was compared with the fossil record of Cladocera obtained from the annually laminated sediments of Lake 227, Experimental Lakes Area, northwestern Ontario. Constrained cluster analysis of both total fossil Cladocera and littoral chydorid communities clearly distinguished between pre- and post-eutrophication communities and further differentiated between years of high and low nitrogen:phosphorus fertilization ratios. However, there were few chydorid species extirpations resulting from the manipulation. Total chydorid accumulation rates and indices of species diversity, richness, and equitability were relatively constant over the last century and were not affected by fertilization. Among pelagic Cladocera, Bosmina longirostris abundance declined > 60% after initial fertilization. Although harsh chemical conditions (pH > 10) may have contributed to reduced abundance of pelagic Cladocera, Bosmina populations were also naturally variable prior to manipulation. Changes in Bosmina morphology (mucrone, antennule, and carapace length) and cladoceran size ratios (Daphnia/[Daphnia + Bosmina]) suggested that zooplanktivory by fish and invertebrates exercised important control of herbivore populations. Fossil Bosmina concentration (remains∙[g dry wt]−1 or remains∙[g organic matter]−1) were significantly correlated (r = 0.66, P < 0.01, 1970–1989) to standing crop in the water column (animals∙m−2). Fossil accumulation rate (remains∙cm−2∙yr−1) was not significantly correlated to Bosmina abundance, perhaps because of errors in determining bulk sediment accumulation rates.

Although most invertebrate predators are size-selective, two species of Hydra, H. oligactis and H. pseudoligactis, are not. A marked preference for Daphnia pulex over Simocephalus vetulus, similarly sized prey items, is observed in feeding trials. S. vetulus is virtually ignored and swims among the tentacles of Hydra, whereas D. pulex is rapidly attacked and captured. However, normal feeding responses are induced in the presence of homogenates of both Daphnia and Simocephalus. This suggests that the lack of response to live Simocephalus as prey items may be due to defense mechanisms evolved during the course of long-term coexistence in the shallow, weedy littoral zone of lakes and ponds. Such mechanisms could involve reduced activation of nematocysts, immunity to the toxin, or lack of penetration of nematocysts through the carapace of Simocephalus. The impact on the structure of the zooplankton community of this differential susceptibility to predation by Hydra is discussed.

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