Structure and dynamics of Baltic benthos communities, with particular reference to the relationship between macro- and meiofauna

The Baltic Sea is one of the largest brackish water areas in the world, and osmotic stress has severely reduced the number of species in its benthic macro-and meiofauna. This leads to an almost uniquety simple benthic ecosystem. - The benthic macrofauna shows a steep north to south increase in biomass, from mean values of about 1 gm-2 wet weight in the Bothnian Bay, to over 100gm-2 above the halocline in the northern Baltic proper, and even higher values in the southwestern Baltic. Meiofauna biomass is much less variable, and increases only from about 2 gm-2 wet weight to about 6 gm-2 along the same gradient. There is also a north-south gradient in phytoplankton primary production, with an increase by a factor of about 3 to 6. Very low salinities (2-3‰S) exclude filter-feeding bivalves from most of the Bothnian Bay, explaining in part the extremely low biomasses in the north. When filter-feeders are substracted, the benthos-feeding macrofauna biomass still increases more from north to south than the primary production, while the meiofauna biomass, on the other hand' increases less. Calculations are presented which indicate that the total resource use by benthos-feeding macro + meiofauna increases rounghly in proportion to the primary production. The observed shift in dominance from meiofauna to macrofauna is attributed to meiofauna being competitively superior in low food environments (Bothnian Bay), while in richer environments predation by macrofauna limits meiofauna populations (Bothnian Sea, Baltic proper). -Most of the Baltic shows strong salinity stratification, with little or no oxygen present below the primary halocline. This leads to an oxygen-dependent zonation of the fauna, with macrofauna disappearing at higher oxygen levels than some of the meiofauna. Nematodes in low numbers persist even in areas which have been anoxic for long periods. -The secondary productivity of the widely distributed Pontoporeia community is described, and it is concluded that the Pontoporeia populations are primarily food limited, and that the benthic system is intimately coupled to the pelagic system, and may respond to events in the plankton within weeks. -Attempts to estimate the energy flow through the benthos of the Askö-Landort area (N. Baltic proper) indicate that 40 -60gCm to fuel the benthos. - Finally, direct interactions between macro- and meiofauna are discussed. The evidence for control of meiofauna populations by macrofauna predation is suggestive, but not yet conclusive. Examples are also given of more positive interactions between macro- and meiofauna, and it is proposed that a proper balance between different size classes of benthos may be necessary for efficient remineralization