Processes in the benthic system above and below the halocline was studied and the aim was to elucidate how the transformation of energy from the pelagic system effected the benthic system.

The flow of food (organic matter) from the photic zone is the major source of energy for the benthic fauna. An excess of organic matter can lead to oxygen deficiency. In the Kattegat a steady decline in oxygen levels over an increasing area culminated in 1988 with oxygen concentrations down to 0.2 ml/l, the death of large numbers of Norway lobster (Nephrops norvegicus) and extensive negative effects on soft-bottom macro-benthos.

The influence of benthos on the pelagic system was studied in the shallow part of Laholm Bay SE Kattegat, above the halocline. It was found that Cerastoderma edule and Mya arenaria at that location, filtered the equivalent of the entire water column down to 10 m over a period of 6 days and that their filtration rate amounted to only half of their potential capacity. Even studies of a mussel culture in the northern part of the Swedish west coast showed that the particle retention in the centre of a mussel culture was 15-50 % at low current speeds (1-3 cm/s). The size of the mussel culture could be nearly doubled without significant growth depression.

Energetics and some life-history parameters were studied on Mytilus edulis, C. edule and M. arenaria. In the case of M. edulis it was shown that the energy-cost for ingestion activity is of minor importance in relation to the total energy budget. Quantity and quality of food, rather than temperature were major limiting factors for mussel growth during winter and spring at the Swedish west coast. The production (secondary) in a mussel culture was of the same order as production (primary) of wheat in an intensive cultivation. Results from a study of repeated spawning of M. edulis give a reproductive output approximately 3 times higher than in earlier published measurements. Growth in a mussel culture during the spring provided an increase in biomass of 127 %. The autumn increase amounted to 225 %. This increase in biomass corresponds proportionately to the amount of seston available at each season.

This investigation indicates that suspension feeders have an important role as regulating, linking organisms between the pelagic and benthic systems.