Factors influencing release of phosphorus from sediments in a high productive polymictic lake system

Phosphorus (P) release rates from bottom sediments are high (20.6 mg/m²/day) in Dal Lake (India), a polymictic hyper-eutrophic lake. These gross release rates occur over a period of 72 days during summer only. Likewise, a net internal load of 11.3 tons was obtained from mass balance estimates. Significant proportion i.e. ∼ 80% of 287.3 tons/yr of nitrate nitrogen (NO₃-N) load is either eliminated by denitrification or gets entrapped for a short period in high macrophyte biomass of 3.2 kg/m² f.w., which eventually get decomposed and nitrogen (N) is released back. These processes result in low lake water NO₃-N concentrations which potentially influence sediment phosphorus (P) release. Especially, nitrate nitrogen (NO₃-N) <500 μg/L in the lake waters were associated with high P concentrations. Phosphorus was also observed to increase significantly in relation to temperature and pH, and it seems likely that release of phosphorus and ammonical nitrogen (NH₄-N) depend on decomposition of rich reserves of organic matter (893 tons d.w. in superficial 10-cm bottom sediment layer). Lake P concentrations were significantly predicted by a multivariate regression model developed for the lake. This study describes significance of various lake water variables in relation to P-release from bottom sediments.