GEOGRAPHICAL VARIABILITY IN THE BIOENERGETIC CHARACTERISTICS OF MONOPOREIA PONTOPOREIA SPP POPULATIONS FROM THE NORTHERN BALTIC SEA, AND THEIR POTENTIAL CONTRIBUTION TO BENTHIC NITROGEN MINERALIZATION

The physiological condition, determined as the ammonia excretion rate (V NH4+), total lipid level and lipid class composition, of two deposi t-feeding benthic amphipods, Monoporeia ( = Pontoporeia) affinis and P ontoporeia femorata, was studied from 12 open-sea stations in the nort hern Baltic Sea between 24 May and 11 June 1993. The M. affinis popula tions can be geographically grouped according to their physiological c ondition: (1) eastern Gulf of Finland, with moderate lipid level (mean 24.4% of dry wt) and high V NH4+ (45.2 mu mol NH4+ g(-1) dry wt d(-1) );(2) Bothnian Sea, with high lipid level(34.5%) and low V NH4+ (24.6 mu mol NH4+ g(-1) dry wt d(-1)); and (3) Bothnian Bay, with low lipid level (15.2%) and high V NH4+ (44.3 mu mol N H-4(+) g(-1) dry wt d(-1) ). A similar pattern could be observed also in the level of triacylgly cerols and the neutral-to-polar lipid ratio. P. femorata, the dominati ng species in the western Gulf of Finland, showed variable ''station-s pecific'' excretion rates (22.3 to 43.0 mu mol NH4+ g(-1) dry wt d(-1) ) and lipid levels (23.4 to 30.4%). The spatial variability in the wei ght-specific V NH4+ of M. affinis could not be explained by the differ ences in the size of individuals, lipid level or lipid class compositi on; this emphasizes the significance of the effects of spatially diffe ring nutritional conditions, which manifest themselves as different mo des of metabolic energy production and different intensities of energy storage. In addition, the potential contribution of the amphipod popu lations to benthic nitrogen mineralization was estimated; in May to Ju ne, the NH, release of different populations ranged from 12 to 237 mu mol NH4+ m(-2) d(-1). In general, populations with high abundance and/ or biomass release the greatest amounts of NH4+ but the values are mod ified by the physiological condition of the individuals.