Interactive effects of hypoxia and dissolved nutrients on the physiology and biochemistry of the freshwater crayfish Paranephrops zealandicus

A key contributing factor to the decline of the at-risk freshwater crayfish Paranephrops zealandicus is the presence of hypoxia associated with eutrophication. However, there is little information regarding the effects that dissolved nutrients have on the physiological and biochemical mechanisms that this and other freshwater crayfish use to cope with lowered dissolved oxygen. In the present study, P. zealandicus were exposed to an abrupt hypoxia (1.3 kPa) in the absence or presence of either ammonia (30 mg L -1 total ammonia; 0.04 mg L -1 as un-ionised ammonia) or nitrite (20 mg L -1) over a 6-h exposure using semi-closed box respirometry. Respiratory (oxygen consumption, haemolymph oxygen partial pressure), cardiac (heart rate) and biochemical (haemolymph acid-base status, glucose and lactate; tissue glucose and lactate) end points were measured. In hypoxia alone, haemolymph oxygen partial pressure, glucose and pH fell, whereas haemolymph lactate increased. The only significant effects of nutrients on hypoxic responses were a further decline in haemolymph oxygen partial pressure and an enhanced hypoglycaemia in the presence of nitrite. These effects may be linked to a nitrite-mediated improvement in tissue perfusion. Overall, these data indicate little additional effect of the presence of nutrients on the responses of P. zealandicus to hypoxia.