The contribution of fishes to phosphorus cycling in lakes /

Several hypotheses concerning the role of fishes in influencing nutrient dynamics in freshwater ecosystems were examined in Lake Memphremagog (Quebec-Vermont). Phosphorus excretion by fishes provides a negligible amount (< 0.33%) in comparison to the quantity taken up by the seston. The phosphorus pools present in the seston and fish community were approximately equal and both were twice the phosphorus content in the zooplankton. Part of this nutrient reservoir is released during the decomposition of fish carcasses. The potential is dependent upon post-spawning mortality rate, the fate of phosphorus in decomposing fish tissue, and the concentrations of allochthonous phosphorus entering the lake. Daily losses of epilimnetic phosphorus from the seston due to fish (up to 40%) and enhancement of nutrient excretion by zooplankton due to size-selective predation are important when open water fish populations are mainly planktivores. When benthic, detrital, or faculative consumers predominate, as in Lake Memphremagog, the effect of fishes on nutrient dynamics through foraging pressure is probably small