POPULATION-DYNAMICS, RESPONSE TO SEDIMENTATION AND ROLE IN BENTHIC METABOLISM OF THE AMPHIPOD MONOPOREIA-AFFINIS IN AN OPEN-SEA AREA OF THENORTHERN BALTIC SEA

0Population dynamics of the benthic, deposit-feeding amphipod Monopore ia affinis was studied over 3 spring bloom periods (1991 to 1993) at a deep (125 m), northern Baltic (Bothnian Sea) open-sea station. Simult aneously recorded sedimentation data (1991 and 1993) were used to inve stigate pelagic-benthic coupling that is believed to regulate the grow th of the amphipod population. Growth of individuals and population bi omass both showed a distinct response to the sedimentation of the spri ng bloom after, approximately, a 4 wk time lag. Thus, the dynamics of the amphipod population seems to be tightly coupled to the autochthono us food source. Using parallel metabolic and body composition data, th e instantaneous and annual requirements of carbon and nitrogen (produc tion, respiration and nitrogen excretion) of the population were calcu lated. The results showed that the role of the amphipod population in benthic mineralization of organic matter is probably highly significan t in the study area. Moreover, the results implied that local primary production and the measured sedimentation rates at the location cannot provide the benthos with sufficient food. This suggests that resuspen sion and the lateral transport of organic particulates from adjacent s ea areas play a critical role in supporting the amphipod population in the study area. In addition to strong seasonal variability in growth of the amphipods, marked interannual variability was observed in the o ther population parameters measured. Biomass, density and annual produ ction rates of the population all-showed a marked decreasing trend dur ing the study period. Marked interannual variation in the recruitment success (survival of the offspring) of the different annual cohorts al so seemed to show density- and/or nutrition-dependency. This study als o corroborates previously recorded long-term fluctuations (ca 7 yr cyc le) in amphipod populations in the Bothnian Sea and reveals the possib le factors causing these fluctuations.