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We combined 3 different approaches to determine the relative importance of microphytobenthos production as food for intertidal macrobenthic animals: (1) the natural abundance of stable-isotope ratios of carbon and nitrogen, (2) an in situ deliberate tracer addition of 13 C-bicarbonate, which was transferred through the benthic food chain after its incorporation by benthic algae, and (3) a dual labelling experiment in a flume, where pelagic and benthic algae were labelled with 15 N and 13 C, respectively. The results of the 3 approaches confirmed the high importance of microphytobenthos as a food source for (surface) deposit feeders. Despite the clearly demonstrated resuspension of benthic algae at high current velocities, suspension feeders appeared to depend almost exclusively on pelagic algae (and possibly detrital carbon) as a food source. Based on the results of the experiments, we determined an approximate degree of dependence on microphytobenthos for different species of intertidal macrobenthos. The macrobenthic biomass at 5 study locations, when weighted by these coefficients, correlated very well with measured productivity of the microphytobenthos.

Mussels Mytllus e d u l~s were sampled dunng the GEEP Workshop from 4 field sltes in Langesundflord, Norway, and from 4 experimental groups exposed to a range of water-accommodated diesel oil and copper concentrations in a mesocosm study. Measurements of physlolog~cal responsessuch as feeding rate, food absorption efficiency, respiration rate and excretion rate -were integrated by means of the energy balance equation, and performance was assessed in terms of 'scope for growth'. The results show a significant dechne in scope for growth, both along the pollution gradient in Langesundfjord and with increasing exposure to copper and diesel oil in the mesocosm expenment Feedlng rate was the pnmary component of the energy budget that accounted for the decline in scope for growth with lncreaslng pollution.

Seasonal cycles in the rates of oxygen consumption, feeding, absorption efficiency and ammonia-nitrogen excretion in two populations of Mytilus edulis were measured in the field under ambient conditions and related to body size, the gametogenic cycle, the concentration of suspended particulate matter in the water and temperature. Relationships between the various physiological variables are also considered and protein and energy budgets estimated. Both the "scope for growth" and the "relative maintenance cost" were seasonally variable, demonstrating a minimum capacity for growth in the winter and a maximum capacity in the summer. In one population subjected to abnormally high temperatures in the winter the scope for growth was negative for four or five months between January and May. These population differences are discussed and the potential for using physiological integrations in intra-specific comparisons of fitness is identified.