Overwintering strategies in polar copepods : Physiological mechanisms and buoyancy regulation by ammonium

Abstract

Copepods play a vital role in marine pelagic food webs. They channel energy from primary production to higher trophic levels and, via the biological carbon pump, substantially affect biogeochemical cycles and carbon fluxes. The distinct seasonality of primary production is the most important factor influencing life-cycle adaptions of herbivorous copepods in polar ecosystems. Ontogenetic seasonal vertical migration (OVM) with a resting stage (diapause) at great depth is known as an adaption to escape food scarcity during winter. Diapause is characterised by reduced metabolic rates and the cessation of feeding. Therefore, diapausing copepods are presumably neutrally buoyant to avoid a depletion of their restricted recources by swimming movements. However, no experimental observations or density determinations had been conducted for Antarctic copepods so far and the mechanisms triggering the on- and offset of OVM and diapause, and regulating buoyancy, were still far from understood. Within the present study, novel hypotheses about the controlling factors for dormancy and OVM were established. The accumulation of ammonium (NH4 ) and the replacement of ions with a higher density is a known buoyancy regulation mechanism in several marine invertebrates. To keep ammonium in the less toxic ionised form, a low pH is required. Knowing that acidic pH conditions are a relevant factor inducing metabolic reduction, a low haemolymph pH might not only be a precondition for ammonium accumulation, but in addition trigger dormancy in copepods. Buoyancy observations of anaesthetised individuals revealed that diapausing Calanoides acutus was neutrally buoyant during austral winter, whereas actively overwintering Calanus propinquus was negatively buoyant. Diapausing and non-diapausing species differed sigificantly in their extracellular cation composition. In actively overwintering species, the cation composition of the haemolymph was similar to that of seawater. In diapausing copepods, severly elevated concentrations of up to 530 mmol L-1 NH4 and reduced contributions of the remaining cations Na , Ca2 and Mg2 were found. Moreover, elevated ammonium levels were accompanied by acidic conditions of pH ¤6. The replacement of cations with a higher molecular weight against low density ammonium decreases the overall density of the diapausing copepod and supports neutral buoyancy, while staying isoosmotic with the surrounding seawater. The present study contributes to a better understanding of the regulation mechanisms of dominant copepods' life cycles, in particular of the triggering and controlling factors of vertical migration and diapause. Since polar oceans are most strongly affected by climate change and global warming, knowledge about physiological and metabolic adaptations of polar species is of profound relevance for understanding and predicting effects on the whole polar marine ecosystem.