Phosphorus (P) release rates from bottom sediments are high (20.6 mg/m(2)/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. approximately 80% of 287.3 tons/yr of nitrate nitrogen (NO(3)-N) load is either eliminated by denitrification or gets entrapped for a short period in high macrophyte biomass of 3.2 kg/m(2) f.w., which eventually get decomposed and nitrogen (N) is released back. These processes result in low lake water NO(3)-N concentrations which potentially influence sediment phosphorus (P) release. Especially, nitrate nitrogen (NO(3)-N) <500 microg/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 (NH(4)-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.

Extensive watershed development has resulted in excessive total phosphorus (TP) loads to Dal Lake, a high altitude Himalayan lake known for its tourism and economic potential. External and internal TP loads of 5 and ~1 g m -2 yr -1 , respectively, were estimated for the lake. These loading rates are high in relation to the lake's critical tolerance range of 0.1-0.2 g m -2 yr -1 , and, over time, have resulted in severe eutrophication in view of extremely high macrophyte biomass (average = 3.2 kg m -2 -fresh weight) and bottom sediment enrichment (79 tons of TP reserves which contribute 88% of the annual TP budget). This study emphasizes the importance of external TP load reduction as a primary management objective to counteract internal TP loading and P storage within bottom sediments resulting from historic anthropogenic loads. IntroductionTotal phosphorus (TP) loading resulting from watershed development has long been recognized as an important factor affecting lake trophic status (VOLLENWEIDER, 1968;DILLON and KIRCHNER, 1975;CANFIELD, 1983). The effect of excessive TP loading to shallow lakes is especially pronounced as it can lead to high macrophyte production, which on senescence contributes significant amounts of nutrients to both sediments and overlying waters (NICHOLS and KEENEY, 1973;CARPENTER, 1980;CARIGNAN and KALFF, 1982). Lake enrichment resulting from the mobilization of TP from watersheds is often, over time, followed by internal TP loading from bottom sediments (AHLGREN et al., 1988;NÜRNBERG, 1984;NÜRNBERG and LAZERTE 2004;FRENCH and PETTICREW, 2007).Earlier studies have described TP loading to Dal Lake, a high-altitude Himalayan (India) lake (e.g. ENEX, 1978; KAUL, 1989, 1990;WANGANEO and WANGANEO, 1994). KAUL (1989, 1990) estimated a TP loading rate to the lake of 7.5 g m -2 yr -1 and reported high transparency (0.7-5.2 m Secchi depth). VASS and ZUTSHI (1983) reported low phytoplankton primary productivity (115 g C m -2 yr -1 ) in the lake, similarly, WANGA-NEO (1984) described Dal as low productive lake in terms of phytoplankton productivity (219 g C m -2 yr -1 ), a finding recently confirmed by SOLIM and WANGANEO (2007). It is believed that Dal Lake maintains high transparency and low phytoplankton biomass, despite having high TP loading rates, due to biogenic calcite precipitation induced by photosynthesis in dense macrophyte stands KAUL, 1989, 1990; SOLIM and WANGANEO, . While phytoplankton biomass in Dal Lake is generally low, unabated TP loading and seasonal macrophyte decomposition has resulted in benthic eutrophication such that average macrophyte biomass in the lake is about 3.2 kg m -2 fresh weight (f.w.) (SOLIM and WANGANEO, 2007).Several factors have contributed to the eutrophication of Dal Lake, including: (1) deforestation and urbanization of various sub-catchments, (2) entry of municipal sewage, (3) residential and commercial developments in riparian areas, (4) diffuse runoff from denuded and urbanized residential areas, and (5) recycling of high autochthonous buildup of...