Size spectrum dynamics and the planktonic ecosystem of Loch Linnhe

Zooplankton data from 12 surveys in Loch Linnhe, west Scotland, conducted at monthly intervals have been used to assess some aspects of ecosystem size spectrum theory. On each survey sampling was carried out by towed deployments of an Optical Plankton Counter (OPC), together with lower resolution collections by conventional sampling nets. Comparisons between biomass analyses of the net-caught material, and the in situ OPC data showed acceptable agreement over the year. Monthly variations in net import/export and in situ production of particle size classes in each basin of the sea loch between successive surveys were estimated 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 dynamics. Import of zooplankton was especially significant during the spring period. Over the year as a whole, the loch was a net importer of small zooplankton and a net exporter of large size classes. Changes in the biomass of particle size classes, not accounted for by advection and mixing, were interpreted as being due to growth and mortality in the loch. The ratio of mortality: growth showed a consistent seasonal pattern of changes with size. Net biomass growth occurred only during the summer months, with net loss of biomass due to mortality during the winter. The results allow a quantitative illustration of the principles of the match–mismatch hypothesis, whereby species cohorts which coincide in time and size with periods of high net biomass growth are those which generate high recruitment.