SIZE SPECTRUM DYNAMICS AND THE PLANKTONIC ECOSYSTEM OF LOCH-LINNHE

Zooplankton data from 12 surveys in Loch Linnhe, west Scotland, conduc ted at monthly intervals have been used to assess some aspects of ecos ystem size spectrum theory. On each survey sampling was carried out by towed deployments of an Optical PIankton Counter (OPC), together with lower resolution collections by conventional sampling nets. Compariso ns between biomass analyses of the net-caught material, and the in sit u OPC data showed acceptable agreement over the year. Monthly variatio ns in net import/export and in situ production of particle size classe s in each basin of the sea loch between successive surveys were estima ted from the OPC and conservative tracer (salinity) data. The results showed that, despite the semi-enclosed nature of the system, exchange with the open sea was an important component of annual population dyna mics. Import of zooplankton was especially significant during the spri ng period. Over the year as a whole, the loch was a net importer of sm all zooplankton and a net exporter of large size classes. Changes in t he biomass of particle size classes, not accounted for by advection an d mixing, were interpreted as being due to growth and mortality in the loch. The ratio of mortality:growth showed a consistent seasonal patt ern of changes with size. Net biomass growth occurred only during the summer months, with net loss of biomass due to mortality during the wi nter. The results allow a quantitative illustration of the principles of the match-mismatch hypothesis, whereby species cohorts which coinci de in time and size with periods of high net biomass growth are those which generate high recruitment. (C) 1995 International Council for th e Exploration of the Sea