Freshwater shrimps of the genus Gammarus, for example, have a rate of oxygen uptake up to 65% higher than marine species, and at least 11% of the total metabolism of the freshwater species G.pulex is devoted to active ion transport to maintain concentrations (Sutcliffe, 1984). Acidification exacerbates the problem of ion exchange: not only does the low buffering capacity of acid water ensure reduced concentrations of ions such as sodium (Na+), but high concentrations of H+ ions compete with Na+ for active transport sites across gill membranes.