The abundances, population dynamics and production of the rotifer community of Lough Neagh were examined for a three year period. Keratella cochlearis was the most abundant species accounting for over 40% of biomass followed by Polyarthra dolichoptera and Notholca acuminata. The mean standing crop for the rotiferan zooplankton increased in successive years (41, 51, 75 mg dwt m-2) as did production (1037, 1322, 1417 mg dwt m-2 y-'). The seasonal pattern of biomass expression and production varies markedly in different years. Instantaneous birth rates tend to be lower but more consistent for the more abundant species, instantaneous death rates show periods of negative mortality indicating an inadequacy of the model employed but explicable as hatching of resting eggs. K. cochlearis as the most successful species is explained as its perennial appearance and adaptation to the low annual temperature cycle found in the lough. The population succession and the annual occurrence of species differs in each year.
Respiration rates of the resting eggs of Leptodora kindti and Bythotrephes longimanus were measured in the laboratory at temperatures between 2.4 and 6.2° C. Results are used to explore the relationships between the duration of dormancy and egg size in the natural habitat.
A single set of population data for Daphnia hyalina was used to estimate production using four methods on a daily and annual basis . Estimates varied from 12 .94 to 50 .60 g dry weight m-2 y-1 . The differences obtained are attributed to faults in the method such as the ignoring of post embryonic development times and poor estimates of basic variables used for the calculations . The latter include poor estimations of specific parts of the population census, particularly egg estimates, and choice of values used for weight estimates and duration times of stages . The normally used birth rate model probably underestimates production whereas the growth increment method probably overestimates production .
Measurements of metabolic rates of natural populations of zooplankton were made using a closed bottle technique in situ. Results showed a marked seasonality inexplicable in terms of simple temperature functions. For this cladoceran dominated zooplankton, seasonal variations in metabolic response are attributed to a degree of temperature acclimation, to changes in the size structure and species composition of the populations and seasonal variations in food sources.
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