Sources of settling material: aggregation and zooplankton mediated fluxes in the Gulf of Riga

Abstract

The composition of sedimenting matter from the upper mixed layer was studied in relation to aggregation dynamics and food web structure in the stratified southern Gulf of Riga, Baltic Sea. Four stations were visited during three fortnight periods in spring (1995), midsummer (1994) and late summer (1993). In spring, diatoms constituted the major part of the sedimenting matter, and their abundance in the water column declined due to the sedimentation loss. A technique which preserves the structure of the sedimenting matter was applied; it revealed that neither diatoms nor other algae were sinking as aggregates. Occasionally both motile phytoplankton and heterotrophic organisms concentrated in the sediment traps, possibly due to vertical migration. In midsummer, and especially late summer, heterotrophic biomass was large relative to autotrophic biomass; however, total heterotrophic biomass had a maximum during midsummer. During the summer periods the sedimenting matter consisted of products from the metazoa (faecal pellets, eggs, cuticles) as well as of amorphous detritus, which mainly sedimented in the form of aggregates. Physical coagulation may have produced these aggregates, but turbulent shear, which was the major force making suspended particles collide, did not correlate with aggregate sedimentation. A disintegration factor related to high turbulence may also be of importance. The sedimentation of faecal particles and other detritus was low compared to the zooplankton biomass (0.6–8% day −1). Despite the eutrophic conditions of the Gulf of Riga there is a high recycling relative to the sedimentation. This may be related to the importance of auto- and heterotrophic picoplankton in the carbon flow, to the generally high heterotrophic biomass and to an efficient detritus decomposition.