MrnsHar-r-, J. S. 1978. Population dynamics of Daphnia galeata ntendolae as modified by chronic cadmium stress. J. Fish. Res. Board Can. 35:461-469.Forty populati ons of Daphnia galeata mendotae were maintained for 22 wk (or until they became extinct) in a medium consisting of filtered Lake Michigan water to which Chlamydomonas reinhardi and different concentrations of a mixture of labelled (10ecd) plus stable CdCI: were added 24 h before use. The medium was changed daily Monday through Fridays. Weekly determinations were made of total numbers, average individual dry weight, mean brood size, egg-female ratio, aborted eggs and embryos, and average whole-body r0ecd content. Chronic cadmium stress reduced the populations' average numbers and biomass, while it increased the populations' turnover rates, temporal variability, probability of extinction, and proportion of ovigerous females. For individuals there was increased prenatal mortality, reduced life expeclancy, increased average weight, and increased brood size. Due to compensatory increases in the pioportion of ovigerous females and mean brood size, the cadmium sensitivity of population attiibutes was considerably less than that of the most sensitive individual attribute, prenatal mortality. A linear relationship between population carrying capacity (relative mean numbers) and added cadmium concentration indicated that the effective concentration for a 50fo reduction of carrying capacity (EC50) is 7 .1 pg CdlL and that for a 7(l reduction (ECl) is 0.15pgCd/L. An increase of 0.15pgCd/L in Lake Michigan itself, barring synergistic interaction with other factors in the lake, would probably not have a detectable effect on the population dynamics of D. galeata mendotae. Mrnsnnlr-, J. S. 1978. Population dynamics of Dapluria galeata mendorae as modified by chronic cadmium stress. J. Fish. Res. Board Can. 35:461-469. Nous avons maintenu 40 populations de Daphnia galeata mendotae durant 22 sem (ou jusqu'd extinction) dans un milieu constitud d'eau filtrde du lac Michigan ir laquelle on avait ajoutl,24 h avant emploi, Chlamydomonas reinhardi et diffdrentes concentrations d'un m6lange de CdClz stable et doCdClr marqu6 (meCd). Nous avons renouveld quotidiennement le milieu de culture du lundi au vendredi. Nous avons d6termin6 chaque semaine les nombres totaux d'individus, le poids sec individuel moyen, la taille moyenne de la prog6niture, le rapport

No abstract

The surplus production model is a simple and easily applied model for the assessment of environmental impacts on exploited fish populations. However, its application requires several assumptions about the relationships between the degree of impact and the population response that are dijficult to test using field data. To examine these assumptions, the model is applied to exploited laboratory populations of Daphnia pulex exposed to chronic radiation stress. The model describes the observed relationship between equilibrium population size and the level of radiation exposure. It also describes the relationship between yield and population size.

Structural and functional responses of plankton communities to cadmium stress were studied during 1977 in Lake Michigan using small-volume (8 L) completely sealed enclosures, and in Canada's Experimental Lakes Area (ELA) Lake 223 using large-volume (1.5 × 105 L) open-surface enclosures. In Lake Michigan, reductions of the average abundance of micro-crustaceans by cadmium were significantly greater in "light" or shallow epilimnetic incubations than they were in "dark" or deep epilimnetic incubations. Measurements of dissolved oxygen indicated that this interaction with light (depth) was an indirect effect due to a reduction of photosynthesis and primary production. Zooplankton density and species diversity were not significantly affected within 21 d by cadmium concentrations [Formula: see text] and [Formula: see text] Cd/L, respectively, whereas final dissolved oxygen concentration and percentage similarity (PS) of the crustacean zooplankton community were significantly reduced by [Formula: see text] Cd/L. In the ELA Lake 223 experiment, the reducing effect of cadmium on zooplankton density increased up to 31 d after Cd enrichment and then decreased, probably due to decreasing Cd concentrations in the water. Values of PS on day 24 for the ELA enclosures enriched with 1 and 3 μg Cd/L were within the 95% confidence limits for individual values predicted from a regression of PS on cadmium for the 21-d Lake Michigan experiments.Key words: plankton communities, zooplankton, phytoplankton, cadmium stress, Lake Michigan, Canadian Shield lakes

Twenty—five self—regulating populations of Daphnia pulex were exposed to different levels of external 60Co gamma radiation for 18.5 hr per day for 55 weeks. Dose rates ranged from 0 to 516 R/day. Although the population at the 3 highest dose rates became extinct, the remaining 22 populations established quasi—equilibrium densities which decreased with increasing dose rate. The maximum tolerable dose rate was 436 R/day. Average turnover rates increased with dose rate. An approximate inverse relationship between mean population size and mean turnover rate suggested that net production might be unaffected by the radiation. Estimates of yield to decomposers indicated that net production was, indeed, unaffected. Radiation stress became a limiting factor for population size and turnover rate at much lower dose rates than those needed to limit net production. Within the entire range of indefinitely tolerable dose rates, the population consequences of radiation stress stem almost entirely from effects on individual fertility and survival, whereas individual metabolism is apparently unaffected. Increased individual growth is an indirect effect, due to reduced fecundity at a given food supply per individual or increased food consumption at a given food supply per population.