Factors determining the nutritive status and production of zooplankton in a humic lake

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Abstract
Zooplankton egg production and nutritional status as related to environmental factors were studied in a humic lake. Both parameters exhibited a pronounced spring peak, a mid-summer depression and a second, smaller autumnal peak. The overall macrozooplankton community apparently suffered from severe food limitation, particularly with regard to food quality, during the study period. This limitation was confirmed with length/carbon regressions of well-fed and starved laboratory individuals of Daphnia. Body carbon in the lake animals corresponded to that of laboratory-raised, starved individuals. Regression analysis indicated that bacteria, particulate organic carbon and particulate nitrogen best explained changes in egg production, while algal biomass was of negligible importance for all species. For Daphnia, size-specific carbon content (length/carbon regressions) and C/N ratios were computed for all sampling dates. a and b from the equation W=aLb were positively correlated (P=0.001). a ranged over 1.17–4.44 and was positively correlated with size-specific carbon content (PP=0.01). When a mean time lag of 12 days was introduced, a was positively correlated with egg production (P=0.04.6). b ranged over 2.05–3.54 and was inversely correlated to the same parameters. Both simple and multiple regression analysis confirmed that bacteria, particulate organic carbon and particulate nitrogen gave the major positive contribution to variance in size-specific C content, a and C/N ratio in Daphnia, whereas total zooplankton biomass was the main negative contributor to the same parameters.