SALINITY-TEMPERATURE TOLERANCE AND OSMOREGULATION IN EURYTEMORA-AFFINIS (POPPE) (COPEPODA, CALANOIDA) IN RELATION TO ITS DISTRIBUTION IN THE ZOOPLANKTON OF THE UPPER REACHES OF THE FORTH ESTUARY

Eurytemora affinis (Poppe) was the dominant copepod in summer and winter surveys of the littoral zooplankton of the upper Forth estuary, Scotland. Maximum numbers were observed in the 0–10%. segment, reaching 500000· m−3 in summer and consistently exceeding main channel abundances reported from other studies. In winter, E. affinis was less abundant, and shared dominance with the rotifer Synchaeta, which occasionally reached numbers of 311000·m−3. Tidal movements and seasonal changes in freshwater flow were both observed to effect longitudinal displacement of plankton populations.

Laboratory tests of salinity-temperature tolerance in Eurytemora showed optimal conditions to be 3–10%. at low temperatures, broadly matching distribution patterns in the estuary. Salinity acclimation tests and haemolymph concentration measurements showed that copepods could adapt to changing conditions within 12 h (i.e. the duration of a tidal cycle). Acclimation extended tolerance limits in the direction of the acclimation treatment, with survival being enhanced by gradual rather than abrupt changes in salinity.

Investigation of haemolymph Δ°C in a range of test salinities revealed a pattern of hyper/hypo-osmoregulation, with concentrations hyperosmotic to the external medium below 15%. and evidence of hypo-osmoregulation at salinities >20%.. Minimum haemolymph concentrations were found to be equivalent to ≈6%. sea water (Δ°C = 0.4) when held in fresh water.

No evidence of salinity-associated respiratory distress was found in respiration experiments. The oxygen consumption values determined (5–7.5 μl O2·mg dry wt−1·h−1) at optimal salinities and temperatures were similar to reported values for copepods of comparable size.