Effects of temperature and ultraviolet radiation on diel vertical migration of freshwater crustacean zooplankton

In transparent, thermally stratified lakes, ultraviolet radiation (UV) and temperature are among the primary factors influencing diel vertical migration (DVM) of zooplankton. It is not well known how behavioral responses will vary across a wide range of depths with steep gradients in UV, photosynthetically active radiation (PAR), and temperature. Our objective was to determine the relative importance of UV and temperature on DVM of Daphnia catawba and Leptodiaptomus minutus in a high UV, thermally stratified lake. Using UV-transparent (+UV) and UV-opaque (–UV) columns suspended within the mixed layer and metalimnion, we found that both species were generally shallower in the –UV than in the +UV columns. Daphnia catawba responded negatively to UV, even below the 1% depth for 320 nm, whereas L. minutus responded to UV only within the mixed layer and upper metalimnion. Daphnia catawba did not migrate in the –UV in the mixed layer, but migrated upwards in the –UV in the deep part of the metalimnion, indicating a temperature-induced rather than phototactic response. At night, both species migrated upwards in the metalimnion and were evenly distributed in the mixed layer. These results indicate that in transparent lakes, UV may constrain some zooplankton to cooler, suboptimal temperatures, which may compromise fitness.

Evaluation of factors related to increased zooplankton biomass and altered species composition following impoundment of a Newfoundland reservoir

Zooplankton biomass and species composition were monitored in Cat Arm Lake and Reservoir on Newfoundland's Great Northern Peninsula from 1983 to 1993. Zooplankton biomass increased approximately 19-fold in the oligotrophic reservoir following impoundment in 1984, relative to biomass in the preexisting lake. Microcrustaceans Cyclops scutifer and Daphnia dubia, either rare or absent from Cat Arm Lake prior to impoundment, were consistently measurable components of the zooplankton community by 1993. Similar changes elsewhere have been attributed to both increased water retention time and enhanced phytoplankton biomass, factors whose effects are usually interdependent. In Cat Arm, there were no increases in either phytoplankton biomass or primary productivity during the first 3 years of impoundment, and natality of the dominant cladoceran, Daphnia catawba, did not increase. Summer water retention time increased from preimpoundment levels of 4 days in 1983 to 338 days in 1993. Zooplankton biomass was significantly correlated with water retention time (Spearman's rs = 0.72, p = 0.04) and showed no significant correlation with phytoplankton biomass, primary productivity, nutrient concentrations, pH, colour, or epilimnetic temperature. Changes in the zooplankton community in this subarctic system can thus be attributed most directly to a decrease in losses due to washout.

A taxonomic reevaluation of North American Daphnia (Crustacea: Cladocera). III. The D. catawba complex

Despite the importance of Daphnia in freshwater zooplankton assemblages, species boundaries in the genus are unclear. This study verifies the taxonomic validity of D. catawba by establishing its genetic divergence from other species of Daphnia that occur in eastern North America. In addition, it reveals the presence of a second, closely allied species, D. minnehaha, which had previously been placed in synonomy with D. pulex. Daphnia catawba and D. minnehaha share a preference for acidic habitats and are restricted to the deciduous and boreal forest regions of the eastern portion of the continent, where D. catawba is restricted to lakes, while D. minnehaha occurs in ponds. Both species reproduce by cyclic parthenogenesis and, based on the extent of their allozyme differentiation, last had a common ancestor more than 7 million years ago. Populations of D. minnehaha fall into two genetic clades; those from the Great Lakes watershed are morphologically divergent and have much lower levels of genotypic diversity than those from eastern Canada and the New England states.

Biomanipulation of Lake 221 in the Experimental Lakes Area (ELA): Effects on Phytoplankton and Nutrients

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.

Effects of Neutralization and Addition of Brook Trout (Salvelinus fontinalis) on the Limnetic Zooplankton Communities of Two Acidic

The zooplankton communities in two acidified lakes in the Adirondack region of New York changed considerably following liming and the reintroduction of brook trout (Salvelinus fontinalis). Most rotifer taxa were greatly reduced in numbers within a week following base addition. Keratella taurocephala declined by orders of magnitude in both lakes. Rotifer species that replaced K. taurocephala were far less abundant. The crustacean communities were also affected by liming. Diaptomus minutus, the dominant in both lakes, declined following base addition. Longer-term population responses appeared to be related to the dynamics of the individual populations, and changes in predation pressure. Four additional crustacean species became prominent in the lakes after liming: the caldocerans Bosmina longirostris and Daphnia catawba in Cranberry Pond, and D. catawba and the cyclopoid copepod Cyclops scutifer in Woods Lake. Over the longer-term liming and the introduction of brook trout tended to have an overall positive effect on the zooplankton communities in the two lakes.

Vertical Distribution of Zooplankton in an Experimentally Acidified Lake Containing a Metalimnetic Phytoplankton Peak

A large biomass of phytoplankton occurred as a narrow band in the metalimnion of a small Canadian Shield lake which had been undergoing experimental acidification. This community represented the maximum concentration of algal biomass in the lake and was composed of small phytoplankton species. The diurnal distribution of crustacean zooplankton was determined in relation to this potential food source. Cyclopoid nauplii, Cyclops bicuspidatus thomasi, and Daphnia catawba remained diurnally associated with the peak while other species were uniformly distributed throughout the water column. Those species which remained diurnally associated with the peak were exposed to the greatest food concentration in the lake and were also exposed to lower acid conditions than those in the upper waters. A deep-dwelling phytoplankton community may offer a refuge for some zooplankton species in acid lakes by providing a large food supply under less acidic conditions. The importance of sampling acid lakes through the entire water column is supported by these observations.

Seminatural Rearing of Atlantic Salmon (Salmo salar) in Newfoundland

Atlantic salmon fry (Salmo salar) obtained from an artificial spawning channel and a deep-substrate incubator, were reared in mesh "troughs" in an artificial rearing channel and in floating lake cages. Fry placed in the lake cages were able to complete their first feeding phase on a diet of natural lake zooplankton but growth performance was improved when artifical diets were used to supplement natural food. Parr fed selectively on Daphnia catawba and mature Epischura lacustris. Parr reared in the mesh troughs were hand-fed a commercial diet. The 90-d average release weight ranged from 1.8 to 3.1 g for parr from the mesh troughs and from 0.9 to 3.1 g for parr from the lake cages. Growth of parr in these experiments resulted in an average weight advantage of approximately 2.3–4.5 times over parr captured from natural riverine habitat at the end of the experiment. Overall swim-up fry to fall-fingerling survival from these experiments was 52% in 1983 and 76% in 1984. Survival and growth varied inversely with density in the mesh trough experiments.

Toxicity of aluminum and hydrogen ions to Daphnia catawba, Holopedium gibberum, Chaoborus punctipennis, and Chironomus anthrocinus from Mirror Lake, New Hampshire

The effect of aluminum on the survival of Daphnia catawba, Holopedium gibberum, Chaoborus punctipennis, and Chironomus anthrocinus was determined. Experiments were conducted in soft water adjusted to pH 6.5 (original), 5.0, 4.5, 4.0, and 3.5, at three concentrations of aluminum (0.02, 0.32, 1.02 mg/L) which cover the range typical of soft-water lakes exposed to acidic precipitation in eastern North America. Our results indicate that D. catawba was the most acid sensitive of the four species tested. It died rapidly below pH 5.0, whereas the others could survive at pHs as low as 4.0 (H. gibberum) and 3.5 (C. punctipennis and C. anthrocinus). None of the invertebrates were particularly sensitive to aluminum. Mortality, attributable to aluminum, occurred only at pH 6.5 for D. catawba at the highest aluminum concentration tested (1.02 mg/L). Most of the aluminum in this treatment was present as a solid-phase aluminum hydroxide complex. Based on our results, these species should be able to tolerate aluminum concentrations in excess of those which now occur in recently acidified oligotrophic lakes. If they are eliminated from acidic environments it is likely to be due to competition for food and (or) prédation pressure rather than direct aluminum toxicity.