ALBERT

Notes: SUMMARY 1. Some predatory zooplankton, such as the large cladoceran Bythotrephes longimanus (Leydig), have a large compound eye, hence vision may play an important role in prey detection and encounter. Our objectives were to quantify, under different light regimes, the reaction distance of Bythotrephes to an assortment of zooplankton prey and to model encounter rate with prey from Harp Lake, Ontario. Reaction distance to prey increased at higher light intensity.2. Results from the encounter model show that small, slow-moving prey faced the greatest risk from Bythotrephes and most encounters occurred in the upper 10 m of the water column throughout the 24-h period. The model was highly sensitive to ambient light. Encounter rate and prey risk were two to three orders of magnitude more sensitive to swimming velocity of the predator than to that of the prey.

Notes: 1. Following the 1993 invasion of Harp Lake by Bythotrephes longimanus a number of small-bodied zooplankton declined dramatically compared to pre-invasion densities, and some larger species increased. 
2. To test whether these changes were caused by Bythotrephes, we measured its consumption of zooplankton in 1995 and compared this to production by the prey species with which it overlapped spatially and temporally. 
3. On a seasonal basis Bythotrephes consumption was 199 mg dry mass (DM) m−2, or 25% of zooplankton production which was 783 mg DW m−2. However, for some species, such as the small copepod Tropocyclops extensus, consumption greatly exceeded production—this was one of the species noted to decline in Harp Lake following the appearance of Bythotrephes. By contrast, for the larger cladocerans Daphnia galeata mendotae and Holopedium gibberum, consumption was much less than production—these species were observed to increase following the invasion. 
4. Our results thus support the hypothesis that zooplankton changes in Harp Lake were caused by Bythotrephes. We speculate that lower quality prey remaining in Harp Lake may lead to reduced densities of Bythotrephes, or impose strong selective forces that lead to new adaptations by this predator.

Notes: Summary Densities of the cladoceran, Holopedium gibberum, were manipulated in 18 enclosures containing juvenile (age 0+) yellow perch (Perca flavescens) and mean-lake densities of other zooplankton. In enclosures, where nearlake densities of all zooplankton species including Holopedium were maintained, young-of-the-year perch grew significantly heavier and longer than in experimental enclosures where Holopedium was excluded. Holopedium comprised between 15–45% of the diet (wet weight) of perch in the first 2 weeks of July in the control treatment (Holopedium at or near ambient lake densities) and only 3–7% of total biomass ingested in the experimental treatment (Holopedium density selectively reduced). Predation on Holopedium decreased dramatically after the 2nd week of July in the control treatment after which Chaoborus, chironomids, and Sida became dominant prey items (by weight) of juvenile perch. These findings suggest that growth and survivorship of age 0+ perch in Precambrian Shield lakes may be coupled to Holopedium abundance. Thus, utilization of Holopedium by young-of-the-year yellow perch may affect recruitment of this species since overwintering survivorship, range of accessible prey sizes or species, and vulnerability of juvenile perch to predation by larger fish depend on body size, which is reduced when Holopedium is excluded from the diet.

Notes: Summary We used an automated technique for the observation and quantification of zooplankton swimming behavior to study the behavioral responses of two congeneric, herbivorous, freshwater copepod prey to a copepod predator (Limnocalanus macrurus). One prey, Diaptomus sicilis, often co-occurs with Limnocalanus, while previous studies indicated that the zoogeographic distribution of the second prey, Diaptomus oregonensis, was independent of the predator. We found that in the presence of Limnocalanus, D. sicilis swims more slowly and with less hopping and jumping than D. oregonensis. Diaptomus sicilis is also attacked and consumed by the predator Limnocalanus macrurus less frequently than D. oregonensis. We suggest that the faster, “noisier” swimming of D. oregonensis increases its vulnerability to Limnocalanus. The behavioral defenses to both prey are induced by the presence of the predator, and may represent two different anti-predator strategies, “crypsis” and “avoidance” for D. sicilis and D. oregonensis respectively. In a zoogeographical analysis D. oregonensis occurs at densities below D. sicilis in lakes where Limnocalanus is at elevated abundances, while in “low-predator” lakes the opposite is true. This distribution pattern supports our experimental results, and suggests that D. sicilis is adapted to survive with Limnocalanus, while D. oregonensis is not.

Notes: ABSTRACT Nonindigenous species may exert strong effects on ecosystem structure and function. The zebra mussel (Dreissena polymorpha) has been attributed with profound changes in invaded ecosystems across eastern North America. We explored vertical profiles of water flow velocity and chlorophyll a concentration in western Lake Erie, over rocky substrates encrusted with Dreissena, to assess the extent to which mussels influence coupling between benthic and pelagic regions of the lake. Flow velocity was always low at surveyed sites (less than or equal to 2.9 cm s-1) and declined in direct proximity to the lakebed. Mean chlorophyll a concentration was also low (less than 5μg L-1) at all sites and depths. Chlorophyll a concentration was positively correlated with distance above lakebed and was lowest (0.3μg L-1) directly adjacent to the lakebed. Spatial patterns of zooplankton grazers could not explain observed vertical gradients in chlorophyll concentration. Hydrodynamic modeling revealed that filtering effects of Dreissena in a nonstratified, shallow basin depend mainly on upstream chlorophyll concentration, intensity of turbulent diffusion, feeding efficiency of the mussel colony, and the distance downstream from the leading edge of the mussel colony. In contrast to widespread perceptions that molluscs reduce phytoplankton concentration only adjacent to the lakebed, modeling scenarios indicated that depletion occurs throughout the water column. Depletion was, however, inversely proportional to distance above the lakebed. Simulation results are consistent with field-based observations made in shallow water habitats populated by large Dreissena populations in the Great Lakes and elsewhere. Results from this study indicate that zebra mussels strongly enhance coupling between pelagic and benthic regions in shallow lakes. Enhanced coupling between these regions explains, in part, high population densities of Dreissena and of many benthic invertebrates in ecosystems invaded by zebra mussels.

Notes: Abstract Patterns in the size distribution and taxonomic composition of phytoplankton and zooplankton communities for 1974 in Georgian Bay and the North Channel are described. The Diatomeae predominate the phytoplankton in both areas. Copepods, particularly Calanoida, comprise the greatest fraction of the zooplankton biomass. Normalized plankton biomass spectra for both ecosystems are typical of those found in Lake Superior and offshore Lake Huron. The plankton communities of Georgian Bay and the North Channel are thus similar to the most oligotrophic of the Laurentian Great Lakes.

Notes: Abstract Spatial and seasonal patterns in phytoplankton and zooplankton communities of Lake St. Clair from June through September, 1984 are described. Phytoplankton biomass averages 586 µg l-1 with the Diatomae and Chrysophyceae predominating. Zooplankton biomass averages 663 µg l- with small bosminid Cladocera being the most abundant organisms. Lake St. Clair zooplankton biomass is second only to that of Lake Erie amongst the St. Lawrence Great Lakes. Biomass size spectra are typical in structure for mesotrophic lakes but low explained variance in the annual normalized spectrum is indicative of a perturbed system. Since 1972/1973 there appears to have been a slight decrease in zooplankton abundance in the lake accompanied by a shift from dominance of rotifers to dominance of cladocerans. We hypothesize that high flushing rate and seasonal variability coupled with contaminant loadings have resulted in a plankton community reduced in taxonomic diversity and dominated by small-bodied species.

Notes: Abstract Nonmetric multidimensional scaling analysis was used to graphically summarize and analyze seasonal changes in the structure of limnetic zooplankton communities in Blelham Tarn, English Lake District and two large experimental enclosures (tubes A and B) therein. Species abundances in weekly samples taken from June–December, 1976 from the three sites were ordinated. Paths through species space describing temporal changes in community structure indicated that the tube B community oscillated around some average composition and was dominated by small zooplankters whereas those from tube A and the tarn changed quite distinctly through the season and had higher predator densities. Examination of other characteristics of the sites indicated that the observed differences in seasonal cycles could be related to variations in the intensity of predation, the quality of food for herbivorous zooplankters and the dynamics of nutrient input. The importance of considering time variation in lake properties, particularly in assessing lake responses to stress and in typing lakes for experimentation or management, is emphasized.

Notes: Abstract Samples of microplankton and larger nanoplankton (5 to 200 μm) are preserved with a combination of Lugol's solution and DaFano's fixative. Organisms are then settled on a gelatin-coated slide, dried and embedded in 40 percent glycerin. Counting and sizing is performed under a microscope using a drawing tube, which facilitates measuring the organisms with a microcomputer-interfaced caliper. An interactive computer program, written in BASIC, allows for estimating the volumes of cells in up to 40 shape/species categories. The program then saves data on a disk, retrieves them, and calculates the results either for individual species (abundance, biomass, and mean cell volume) or as a pooled size spectrum of all organisms measured.

Notes: Abstract Field-measured grazing rates (ml/animal/d) of cladocerans (mostly daphniids) and diaptomids were assembled from various published studies and plotted as a function of corresponding phytoplankton concentration (µg l−1 f.w.). Filtering rates of both zooplankton groups initially increased with seston concentration until maximal grazing rates were observed at approximately 4 × 102 and 1 × 102 µg l−1 for cladocerans and copepods, respectively; at higher algal concentrations, filtering rates of both declined as a function of food concentration. The shape of these curves are most consistent with Holling's (1966) Type 3 functional response. We found little support for the Type 3 functional response in published laboratory studies of Daphnia; most investigators report either a Type 1 or Type 2 response. The one study in which the Type 3 response was observed involved experiments where animals were acclimated at low food concentrations for 24 h, whereas those studies associated with response Types 1 or 2 had acclimation periods of only 1 to 3 h. We therefore assembled relevant data from the literature to examine the effect of acclimation period on the feeding rates of Daphnia at low food concentrations. In the absence of any acclimation, animals filtered at extremely low rates. After 2 h of acclimation, however, filtering rates increased 4 to 5-fold but declined again with longer durations; after 〉 70 h of pre-conditioning, filtering rates were almost as low as they had been with no acclimation. We also found little support for the Type 3 functional response in published studies of copepods. The only study associated with a Type 3 response involved a marine copepod that had been subjected to a starvation period of 48 h; however, an analysis of the effects of acclimation period did not yield conclusive evidence that filtering rates of freshwater copepods (Diaptomus and Eudiaptomus) decrease significantly with acclimation duration. The low filtering rates associated with long acclimation periods in laboratory experiments appears to be a direct result of animals becoming emaciated from prolonged exposure to low food concentrations, a situation which renders them incapable of high filtering rates. This may explain the Type 3 functional response for field cladocerans, since zooplankton in food-limiting situations are constantly exposed to low food concentrations, and would therefore have low body carbon and consequently less energy to filter-feed. We cannot, however, use this to explain the Type 3 response for field diaptomids, since copepods in the laboratory did not appear to lose body carbon even after 72 h of feeding at very low food levels, and there was inconclusive evidence that either Diaptomus or Eudiaptomus decrease their filtering rates with acclimation period. Although Incipient Limiting Concentrations (ILC) for Daphnia ranged from 1 to 8.5 × 103 µg 1−1, more than half of these fell between 1 and 3 × 103 µg l−1, bracketing the value of 2.7 × 102 µg l−1 for field cladocerans. There was, however, a great deal of variation in reported maximum ingestion rates (MIR), maximum filtering rates (MFR) and ILC values for Daphnia magna. ILC values from the few laboratory studies of freshwater copepods ranged between 0.5 to 2.8 × 103 µg 1−1, and was higher than the ILC value of approximately 0.2 × 103 µg l−1 calculated for field populations of D. minutus. Generally, there was considerable agreement among laboratory studies regarding the shape of grazing-rate and ingestion-rate curves when data were converted to similar units and presented on standardized scales.