Nitrogen-phosphorus relationship in high mountain lakes: effects of the size of catchment basins

We analyzed the changes in epilimnetic total nitrogen (TN), total phosphoru s (TP), dissolved inorganic nitrogen (DIN), and soluble reactive phosphorus (SRP) in 31 small high-mountain lakes in the Sierra Nevada (Spain) during an annual cycle, just after the spring thaw, and in the middle of the growi ng season. Chlorophyll a, TN, and TP increased, whereas the TN:TP ratio fel l substantially between the two periods, reaching values generally between 25 and 10 (by weight). On the contrary, DIN, SRP, and DIN:SRP ratios were s imilar for both periods in each lake. DIN:SRP ratios generally ranged from 5 to 20 (by weight). This ratio was low in the lakes with small catchment a reas and increased progressively with catchment basin size. A regression an alysis for the smallest catchments showed that chlorophyll a concentrations were not accounted for by variability in TP concentration. The results obt ained are discussed in relation to the influence of episodes of Saharan dus t, rich in P, reaching the Southern Mediterranean area.