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

Abstract

Seasonal variations in the oxygen uptake rate ($\dot V{\text{O}}_{\text{2}}$) and ammonia excretion rate ($\dot V{\text{NH}}_{\text{4}}^{\text{ + }}$) of the benthic deposit-feeding amphipod Monoporeia affinis vere studied in an open-sea population of the northern Baltic Sea during three consecutive years (1991 to 1993). In the early summer an increase in the weightspecific $\dot V{\text{O}}_{\text{2}}$ by 22% (“standard” 1-mg individual) was registered; an even more striking change was noted in the $\dot V{\text{NH}}_{\text{4}}^{\text{ + }}$, which showed an increase in the early summer of ca. six times the extremely low winter/spring valnes. The metabolic changes observed could not be attributed to temperature, which remained at steadystate throughout the year, nor could seasonal variations in the biochemical composition of the amphipods explain the results. Therefor, it is evident that quality-food abundance following the sedimentation of the spring phytoplankton bloom induces the observed early-summer elevations in $\dot V{\text{O}}_{\text{2}}$ and $\dot V{\text{NH}}_{\text{4}}^{\text{ + }}$. In turn, the marked decrease in the $\dot V{\text{NH}}_{\text{4}}^{\text{ + }}$ towards autumn reflects the gradual depletion of quality-food in the benthic environment. 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 was observed to be almost equal. Results also indicated a time-lag between sedimentation of the phytoplankton bloom and metabolic response in the amphipod population.