The role of ion regulation in the control of the distribution of Gammarus tigrinus (Sexton) in salt-polluted rivers

Fluctuating salinities at different sites on the German salt-polluted river s Werra and Weser were compared with extracellular ion levels of specimens of Gammarus tigrinus (Sexton; Amphipoda, Crustacea), collected at the same sites. G. tigrinus regulated haemolymph concentrations of inorganic anions (Cl-, SO42-, PO43-) and cations (Na+, K+, Mg2+, Ca2+) during fluctuations o f salt pollution in the upper Weser. This capacity to regulate varying leve ls of salt pollution in the upper Weser, correlated well with the distribut ion of the brackish amphipods in this river ecosystem. G. tigrinus tolerate d periods of Na+ and Cl- stress (> 380 mmol l(-1)) without compensating the se maxima by regulating extracellular Na+ and Cl-. However, during such bur sts of Na+ and Cl- stress in Werra and Weser, the ability to regulate extra cellular [K+] at river water K+ stress of greater than or equal to 6.0 mmol l(-1) may explain why this brackish species has been more successful in th ese rivers than its competitors like Gammarus pulex. The present investigat ion demonstrates that the water salinity affects the [NO3-] in the haemolym ph of G. tigrinus. With increasing hypo-osmotic stress the animals accumula te increasing amounts of NO3-. A simultaneous increase in stream water [NO3 -] causes an additional accumulation of NO3- in the haemolymph. The high ex tent of accumulation indicates that active ion transport systems may be inv olved. The accumulation of NO3- in the haemolymph has low physiological con sequences to G. tigrinus, but when hypo-osmotically stressed under anoxic c onditions, nitrite formed by the reduction of nitrate may have an adverse a ffect on the metabolism of G, tigrinus.