ECOPHYSIOLOGY OF THE BENTHIC AMPHIPOD MONOPOREIA-AFFINIS IN AN OPEN-SEA AREA OF THE NORTHERN BALTIC SEA - SEASONAL-VARIATIONS IN OXYGEN-CONSUMPTION AND AMMONIA EXCRETION

Seasonal variations in the oxygen uptake rate (VO2) and ammonia excret ion rate (VNH4+) of the benthic deposit-feeding amphipod Monoporeia af finis were studied in an open-sea population of the northern Baltic Se a during three consecutive years (1991 to 1993). In the early summer a n increase in the weight-specific VO2 by 22% (''standard'' 1-mg indivi dual) was registered; an even more striking change was noted in the VN H4+ which showed an increase in the early summer of ca. six times the extremely low winter/spring values. The metabolic changes observed cou ld not be attributed to temperature, which remained at steady-state th roughout the year, nor could seasonal variations in the biochemical co mposition of the amphipods explain the results. Therefore, it is evide nt that quality-food abundance following the sedimentation of the spri ng phytoplankton bloom induces the observed early-summer elevations in VO2 and VNH4+. In turn, the marked decrease in the VNH+4 towards autu mn reflects the gradual depletion of quality-food in the benthic envir onment. Atomic O:N ratios indicated that, at the sampling location, M. affinis uses lipid almost exclusively for metabolic energy throughout the year, except in early summer when the use of lipid and protein wa s observed to be almost equal. Results also indicated a time-lag betwe en sedimentation of the phytoplankton bloom and metabolic response in the amphipod population.