Population dynamics, response to sedimentation and role in benthic metabolism of the amphipod Monoporeia affinis in an open-sea area of the northern Baltic Sea

Abstract

Population dynamics of the benthic, deposit-feeding amphipod Monoporeia affinis was studied over 3 spring bloom periods (1991 to 1993) at a deep (125 m), northern Baltic (Bothnian Sea) open-sea station. Simultaneously recorded sedimentation data (1991 and 1993) were used to investigate pelagic-benthic coupling that is believed to regulate the growth of the amphipod population. Growth of individuals and population biomass both showed a distinct response to the sedimentation of the spring bloom after, approximately, a 4 wk time lag. Thus, the dynamics of the amphipod population seems to be tightly coupled to the autochthonous food source. Using parallel metabolic and body composition data, the instantaneous and annual requirements of carbon and nitrogen (production, respiration and nitrogen excretion) of the population were calculated. The results showed that the role of the amphipod population in benthic mineralization of organic matter is probably highly significant 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 resuspension and the lateral transport of organic particulates from adjacent sea 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 other population parameters measured. Biomass, density and annual production rates of the population all showed a marked decreasing trend during the study period. Marked interannual variation in the recruitment success (survival of the offspring) of the different annual cohorts also seemed to show density- and/or nutrition-dependency. This study also corroborates previously recorded long-term fluctuations (ca 7 yr cycle) in amphipod populations in the Bothnian Sea and reveals the possible factors causing these fluctuations.