Comparison of nitrate tolerance between different populations of the common frog, Rana temporaria

Euthrophication-associated changes in the physical and biological environme nt of lakes and ponds are potentially a source of major stress for many aqu atic organisms. In Scandinavia, the nitrate concentrations in lakes and pon ds decrease towards north due to a naturally lower productivity of the habi tats, but also due to lower supplementation of anthropogenic nitrogen. A ch ronic experiment using ecologically relevant concentrations of sodium nitra te (0-5000 mu gl(-1)) was used to test whether common frog (Rana temporaria L.) larvae from northern parts of Scandinavia are less well adapted to cop e with high nitrate concentrations than those from the southern parts. Slig ht, but significant differences in nitrate tolerance, as measured in terms of growth rate and size at metamorphosis, between the two regions were foun d. High concentrations of nitrate reduced the growth rates and metamorphic size in north, but not in south. However, there was no clear-cut impact of high nitrate concentrations on developmental rate or on mortality until met amorphosis. The general lack of large effects of nitrate treatment on the r esponse variables suggests that nitrates per se do not pose any significant threat to the development of R. temporaria tadpoles under a natural range of concentrations. This was confirmed in an acute test where results sugges t that ammonia and nitrite, compounds seldom found in high concentrations i n Fermoscandian lakes, are possibly responsible for the larger negative eff ects of 'nitrate' observed in previous studies of amphibians. (C) 2001 Else vier Science B.V. All rights reserved.