WaterSubject

The significance of nutrient release by fish for plankton communities is uncertain; many authors have suggested that nutrient release by fish is important whereas others suggest it is a small Some of this difference in opinion arises from whether nutrients released by fish are compared to external loading or internal cycling. While nutrient regeneration by fish may be large relative to external sources of nutrients to lake, fish are an internal source of nutrients and internal cycling of nutrients (e.g. planktonic regeneration) in lakes is large relative to external loading.Some recent studies have explored the role of fish as benthic-pelagic couplers, transporting and releasing benthic nutrients into the pelagic zone. This translocation of nutrients has been considered a source of ‘new’ nutrients for the plankton community.

Benthic organisms comprise a significant proportion of the diet of many littoral zone fish species, particularly in small lakes. However, fish undergo ontogenetic, biomass-dependent and seasonal diet shifts due to prey availability. When planktivorous fish feed and release nutrients in the pelagic zone, they only recycle nutrients in the water column and do not provide new nutrients to the plankton community. More importantly, since there is usually no thermal barrier to horizontal movement of nutrients regenerated by littoral organisms, it is not clear that the involvement of motile organisms such as fish is required for nutrients to reach the plankton; water currents can bring nutrients into the pelagic zone and vice versa. Fish feeding on profundal benthos could also bring new nutrients into the epilimnion, but this is a much less likely migration and the net effect of fish movements between these zones could be to move nutrients out of the euphotic zone.

Nutrient release rates have been estimated through both bioenergetic models and direct measurements. However, parameters of consumption and growth, as well as the nutrient and energy content of both predator and prey, must be known for bioenergetic modelling. These parameters are not known for many species, and must often be inferred from other closely related species. Regression models of nutrient release have been developed for single species from direct measurements. These empirical relationships have been applied to fish populations to estimate nutrient release over multiple years. However, these relationships only capture the variability in release rates of a single species, and may not be applicable to other systems. Therefore, the development of an empirical relationship of nutrient release for multiple species would be useful for the rapid estimation of the total nutrients released by all fish in a given waterbody.

Fish may contain a substantial amount of P in their biomass which is unavailable to primary producers. found up to 75% of total phosphorus (TP) in the pelagic zone of Lake Wingra (Wisconsin) was in fish biomass. In addition, they estimated that 30–35% of the annual input of P into Lake Wingra was sequestered into fish biomass. estimated that the P content of pelagic fish in Lake Memphremagog was comparable to that of the seston, and much greater than that of the zooplankton. estimated that P sequestered by young-of-the-year yellow perch in Lake Memphremagog (Vermont/Quebec) was similar to P losses from algal sedimentation. estimated that 40% of the total fish P may remain immobilized in bone and scales which, upon death of the fish, may be incorporated into the sediment and lost permanently from the water column. Such studies suggest that fish may be more important as sinks rather than sources of nutrients.

Our study had five objectives. (1) To determine the amount of phosphorus (TP) and nitrogen (ammonium-N) released by the five dominant fish species in Mouse and Ranger Lakes (Ontario) during the summers of 1993–95. (2) To develop empirical relationships of nutrient release of TP and N for multiple species that could be used for the rapid estimation of fish nutrient release in other systems. (3) To compare the release rates of fish to planktonic regeneration rates of P in both lakes. (4) To characterize the stoichiometry of direct P and N release from the fish assemblage. (5) To determine the quantity of nutrients (P and N) bound in fish and plankton, and to compare the P turnover rates of both.