Alteration of nutrient cycles and algal production resulting from fish introductions into mountain lakes

The introduction of salmonid fishes into naturally fishless lakes represent s one of the most prevalent environmental modifications of aquatic ecosyste ms in western North America. Introduced fish may alter lake nutrient cycles and primary production, but the magnitude and variation of these effects h ave not been fully explored. We used bioenergetics modeling to estimate the contributions of stocked trout to phosphorus (P) cycles across a wide rang e of fish densities in lakes of the Sierra Nevada, California. We also asse ssed the larger effects of fish-induced changes in phosphorus cycling on pr imary production using paleolimnological analyses from lakes in the souther n Canadian Rockies. Our analyses showed that total P recycling by fish was independent of fish density but positively related to fish biomass in the S ierra Nevada. In lakes with fish populations maintained by continued stocki ng, fish recycled P at over twice the rate of those in lakes where introduc ed fish populations are maintained by natural reproduction and stocking has been discontinued. We estimate that P regeneration by introduced fishes is approximately equivalent to atmospheric P deposition to these lakes. Paleo limnological analyses indicated that algal production increased substantial ly following trout introductions to Rocky Mountain lakes and was maintained for the duration of fish presence. The results of our modeling and paleoli mnological analyses indicate that introduced trout fundamentally alter nutr ient cycles and stimulate primary production by accessing benthic P sources that are not normally available to pelagic communities in oligotrophic mou ntain lakes. These effects pose a difficult challenge for managers charged with balancing the demand for recreational fisheries with the need to maint ain natural ecosystem processes.