Factors associated with interannual and intraannual variation in nutrient limitation of phytoplankton growth in Castle Lake, California

In a 3-year study of Castle Lake, California, potential nutrient (N,P) limitation of phytoplankton growth occurred rapidly (within 1–4 d of ice-out). Both N and P acted as potential limiting factors to phytoplankton growth in short-term (4–5 d) bioassays. Phytoplankton responded strongly to single additions of N or P in 1990 and 1992 but weakly so in 1991. This difference was associated with low inorganic N concentrations during spring 1991. In 1990 and 1991, variation of the primary limiting element correlated with the N:P ratio of the zooplankton community; phytoplankton tended to be N limited when the zooplankton was dominated by species with high N:P ratios (Diaptomus novamexicanus and Diacyclops thomasi: N:P ratios, by mass = 10.6–12.5) but limited by P when low N:P taxa (Daphnia rosea, N:P = 4.7) dominated. However, N vs. P limitation and zooplankton elemental data for 1992 did not fit the 1990–1991 pattern and there was no correlation for the 3-year data set. A field experiment demonstrated that the inorganic N:P ratio (NH4/SRP) increased dramatically with elevated Daphnia grazing but declined significantly with increased Diaptomus; this supported the 1990–1991 correlation between phytoplankton N/P limitation status and zooplankton community elemental ratio.

A Reappraisal of Phenotypic Variation in Daphnia galeata mendotae: The Role of Interspecific Hybridization

Seasonal, within-lake, and among-lake variation in helmet size and shape has been reported for Daphnia galeata mendotae. The genetic component of this variation was assessed by an allozyme and morphological reanalysis of nine previously studied sites. Genetic associations with morphological variation were found, but all were attributed to D. galeata mendotae × Daphnia rosea hybridization or to taxonomic confusion with D. rosea. The existence of hybrids accounted for spatial polymorphism both within and among lakes. In addition, hybrids contributed to seasonal variation as they increased in abundance from summer to winter. Nevertheless, cyclomorphosis in D. galeata mendotae s.s was real and due largely to phenotypic plasticity. Local hybridization between D. galeata mendotae and D. rosea was widespread from the midwestern states to Vermont. The distributions and habitat associations of D. galeata mendotae and D. rosea indicated that ecological segregation at the lake level acts as an important reproductive isolating mechanism. Reinterpretation of previous studies, however, suggests that disturbance followed by one or a few hybridization events often leads to hybrid domination of a lake.

Comparative Population Dynamics of Daphnia rosea and Holopedium gibberum in Four Oligotrophic Lakes

Seasonal abundance patterns of Daphnia rosea and Holopedium gibberum showed great variability over the period 1974–83 in four coastal montane lakes of British Columbia. Though the lakes differed considerably in size, depth, and history of experimental disturbance (fish introductions, fertilization, plankton harvesting}, these differences apparently had much less influence on seasonal abundance patterns than did interannual variation in environmental factors shared by all the lakes. Spring rates of population increase differed strongly among years and were positively correlated between the two species. The timing and magnitude of summer population maxima also differed significantly among years, but were not correlated between the species. Adult mortality rates tended to increase through each season for both species, but showed no clear correlations either between the species or between years. Annual differences in growth and mortality rates and peak abundances were not associated with any obvious differences in environmental factors (insolation, rainfall, water temperature). There was no evidence of direct competition between the species, in terms of negative correlations in abundance. This is surprising in view of their similar life histories and feeding ecologies.

The effects of interspecific competition on Diaptomus leptopus in a montane lake: an experimental field study

Despite high densities in an upstream lake, the calanoid copepod Diaptomus leptopus is extremely scarce in oligotrophic montane Eunice Lake, British Columbia. Field experiments tested the hypothesis that competition from zooplankton resident in Eunice Lake prevents immigrant D. leptopus from colonizing.Experiments in 1979 exposed a standard density of D. leptopus to all Eunice Lake zooplankton species at lake densities (control) and at reduced densities (low density), and to all Eunice Lake species except one of Daphnia rosea (Daphnia removal), Diaptomus kenai (kenai removal), or Diaptomus tyrrelli (tyrrelli removal). Improved D. leptopus performance in noncontrol treatments was evidence for competition in controls. Performance measures included density, survival, and reproduction. Similar and poor performances of D. leptopus in control, Daphnia-removal, and kenai-removal treatments plus high concentrations of D. leptopus nauplii in the tyrrelli-removal treatment suggested that only D. tyrrelli competed with D. leptopus. However, similarity between tyrrelli-removal and low-density treatments in D. leptopus naupliar concentrations confounded competition from D. tyrrelli with diffuse competition from several Eunice Lake species combined. Experiments in 1980, designed to separate these effects, provided no evidence of interspecific interactions. Cool 1980 temperatures presumably reduced competition for algal foods by lowering zooplankton respiration rates.

Developmental Responses of Juvenile Daphnia rosea to Experimental Alteration of Temperature and Natural Seston Concentration

A need for accurate estimates of juvenile development rates of Daphnia rosea from oligotrophic lakes prompted this investigation into the combined effects of temperature and natural seston concentrations on growth of neonates. Significant effects of seston concentration and temperature and especially their interaction were detected upon survival, number of prereproductive instars, growth increment per instar, and duration of development. Low summertime seston concentrations that were sufficient for good survival and growth at a low rearing temperature (12 °C) generally produced high mortality, slow development, and reduced biomass increments at higher temperature (21 °C). Further, poor growth and survival under low seston – high temperature conditions were exaggerated in August compared with May, suggesting possible seasonal changes in natural seston quality and/or juvenile daphnid physiology. In contrast, higher food levels typical of springtime conditions produced excellent survival and growth regardless of the test temperature employed or season examined. Food limitation of daphnid development was thus dependent upon both temperature and food concentration within the range of conditions commonly observed in oligotrophic lakes. Extrapolation of development rates measured under only a limited range of food–temperature conditions or taken from literature values should be done cautiously.Key words: Daphnia, development, juvenile, seston, food, temperature, growth, survival, Cladocera, oligotrophic

An experimental test of Dodson's hypothesis that Ambystoma (a salamander) and Chaoborus (a phantom midge) have complementary feeding niches

The removal from an alpine pond of eggs of the salamander Ambystoma gracile had a large impact on the zooplankton community. The pond was dominated by the large Daphnia rosea, while a nearby pond, from which salamander eggs were not removed, was dominated by the small Diaphanosoma brachyurum. Chaoborus trivittatus crop content analyses revealed that in the experimental pond, food intake of second and third instars was low. In the fall, fourth instars were scarce and their dry weight was 50% lower than those in the previous year or those of the control pond.When larval Ambystoma is abundant, it preys heavily upon the cladocerans Holopedium gibberum and Daphnia, which are too large for Chaoborus to eat. Whereas Holopedium appears not to be much affected by the intensity of Ambystoma predation, the reduction in density of Daphnia leads to increased density of Diaphanosoma, possibly because of reduced competitive interaction. Diaphanosoma is the main food source, when available, of second and third instar Chaoborus. In this indirect way, Chaoborus depends on vertebrate predation to reach successfully its fourth instar. This lends support to Dodson's hypothesis of complementary feeding niches.

Population dynamics of cladocerans in Sunfish Lake, Ontario

Investigation of populations of Daphnia rosea, D. retrocurva, and Diaphanosoma leuchtenbergianum in Sunfish Lake, southern Ontario, during 1969 indicated that, in the absence of predation, food supply was the major factor controlling population size and influencing population parameters. Generally with each species, a population peak was preceded by a rise in fecundity, an increase in birth rate, and a fall in death rate. As the carrying capacity of the environment in terms of food was surpassed the population numbers declined, fecundity dropped off, and the death rate rose. A strong negative correlation between fecundity and death rate was observed.