An investigation into the physiological effects of nitrogenous agricultural pollutants on a freshwater invertebrate, Pacifastacus leniusculus.

The nitrogenous compounds which primarily originate from farming practices are nitrate (NO3-), nitrite (NO2-) and ammonia (NH3). Concentrations of these compounds were determined In South Leicestershire streams flowing through agricultural land, one of which received a direct inflow of organic nitrogenous pollutants. These results provided environmentally realistic concentrations for use in lethal and sub-lethal laboratory experiments. Lethal toxicity concentrations of all three compounds were found to exceed levels found in the field. Nitrite was accumulated rapidly into the haemolymph of P. leniusculus to concentrations of up to thirty times the external medium and resulted in a significant depletion of chloride concentration. Elevated external chloride concentrations appear to reduce lethal and sub-lethal effects of nitrite. In P. leniusculus, as in fish, nitrite uptake appears to be associated with branchial Cl-/HCO3- exchange and the nitrite and chloride ions compete for sites of active uptake on the gill. In the un-ionised state (NH3). ammonia appears to diffuse rapidly across the gill epithelium and inhibits both the uptake of sodium from the surrounding medium and its reabsorption from the urine. In the ionised form (NH4+). ammonia also enters the animal, although at a slower rate, possibly via a reversed Na+/NH4+ exchange mechanism. This results in a small imbalance in haemolymph sodium concentrations and apparent changes in permeability. Both nitrite and ammonia appear to rapidly diffuse out of the animal when concentration gradients are reduced, although haemolymph electrolyte balance is not as rapidly redressed. At sub-lethal concentrations of nitrite and nitrate, 'scope for growth' experiments indicate that the overall fitness of the animal is affected. The effects of these nitrogenous compounds on P. leniusculus, reflects those potentially able to act on other freshwater invertebrates which regulate their internal ion balance through their gills.