The role of phosphorus release by roach [Rutilus rutilus (L.)] in the water quality changes of a biomanipulated lake

1. Following fish removal, the water quality in biomanipulated lakes often improves concomitant with decreased phosphorus (P) levels. Because the decrease in P concentrations derives most probably either directly or indirectly from fish, which are the main target of biomanipulation, this study examined the P release of 0+, 1+ and 2+ roach [Rutilus rutilus (L.)] and changes in the P release during summer in a shallow eutrophic lake in Finland. 2. The P release was separated into P derived from benthic and littoral food items and into recycled P derived from feeding on zooplankton, to estimate the contribution of net P additions to the water column by the fish to the increase in P concentrations of the lake water (75-110 mg P m(-3)) in summer 1991-96. . Individual P release of roach by both egestion and excretion was estimated with a bioenergetics model. The size of the roach population was estimated with a depletion method and the proportions of different age groups from catch samples, using a programme separating mixtures of normal distributions. The sensitivity of the release estimates to variation in the growth data was estimated with the jackknife technique. 4. The biomass-specific P release by 0+ roach (0.36-0.54 mg P g(-1) day(-1)) was higher than that by older roach (0.07-0.16 mg P g(-1) day(-1)) throughout the summer. The P release by the whole roach population deriving from benthic and littoral food items (0.7-2.7 mg m(-3) during July to August, representing a net addition to the water column) was 5-19 times lower in 1991-96 than the recycled P release deriving from zooplankton (8.9-25.7 mg m(-3)), and too low to explain the increase in the P concentration of the lake water during the summer. Because the biomass-specific P release and roach diet composition vary with fish age, it is important to consider the age structure of fish populations to obtain correct estimates of P release and net additions to the water column. 5. The removal of roach by fishing diminished the roach stock greatly, but the fish-mediated P release to the water column changed little. This effect was because of the high compensation capacity of the roach population, leading to high recruitment of young fish with higher biomass-specific P release rates. 6. External loading is very low during summer months and therefore it cannot explain the increase in the P concentration of water during that time. Internal loading from the sediment might be as high as 10.2 mg P m(-2) day(-1), i.e. 50 times higher than the maximum net P addition by the total roach population.