ESTIMATES OF PHOSPHORUS AND NITROGEN CYCLING BY FISH USING A BIOENERGETICS APPROACH

A modelling approach based on bioenergetics was used to estimate the role of fish in nutrient recycling. Phosphorus excretion (P(or)) and nitrogen excretion by young-of-the-year (YOY) and older yellow perch (Perca flavescens) were estimated for a limnetic system: Lake Memphremagog. This model successfully predicted P(or) by comparison with previous estimates for yellow perch. YOY fish contributed more to limnetic nutrient cycling through excretion than older age-classes, and YOY fish could also serve as an important P sink relative to algal sinking losses. Volumetric P(or) by YOY yellow perch reached a maximum of 0.3 mug P.L-1.d-1; N excretion peaked at 10 mug N.L-1.d-1. The ratio of excreted N:P varied fourfold during the summer. P(or) estimates were also sensitive to changes in fish P content, for which few values have been reported for early life stages.