Phosphate dynamics in a sub-tropical lake ecosystem

<p>Lake Ecosystem is a key component of biosphere that supports aquatic life and provide sink to the untreated effluent (domestic, industrial, and agricultural). Due to rapid industrialisation and changing climate, 30-40% of the lakes in the world are now eutrophic. The basic cause of eutrophication is the addition of nutrients (nitrogen and phosphate) into the lake system. Phosphate has been observed to be the limiting nutrient in 80% of the lakes and reservoirs in the world. Unlike other elements, phosphate does not escape from the system but changes from one form to the other depending upon the prevailing physico-chemical conditions. Chemical parameters like pH and redox potential are the major governing factors for phosphate fluxes. Sediments in the benthic zone serves as a sink as well as the source of phosphate for the photic zone. In the present study, a relationship between the physico-chemical properties of water and the fractions of phosphate in the sediments were studied. The study was conducted during three different seasons i.e. post-monsoon, winter, and summer to observe the seasonal variation. The pH, DO, ORP, and available phosphate in the water varied from 8.5, 14.7 mg/l, 39 mV, and 5.8 mg/l, 8.4, 3.5 mg/l, -64 mV, and 8.7 mg/l, and 7.8, 7.3 mg/l, 119 mV, and 10.5 mg/l during post-monsoon, winter, and summer, respectively. Phosphate in sediments was fractionated using SMT protocol. It was categorised under inorganic and organic phosphate classes, and the inorganic phosphate was further categorised as Non apatite inorganic phosphate (NAIP: Fe/Al bound) and Apatite inorganic phosphate (AIP: Ca bound).  The inorganic phosphate in the sediments was observed to be more than organic phosphate during post monsoon and summer, and at the same time the available phosphate in the overlying water was found low in concentration. The growth of phytoplankton is constrained by decreasing bio available phosphate in water. Concentration of NAIP was observed to vary with redox potential and concentration of AIP with pH. The study justified the hypothesis of direct relationship of sediment chemistry with bio availability of phosphate in water. Winter was found to be the extreme weather for phosphate fluxes. The findings point towards need of proper management such as chemical precipitation, sediment dredging etc. during this extreme weather conditions.</p>

Phosphate supply to the eutrophic lake Eichbaumsee (Germany): Sedimentary versus groundwater sources

<p>The eutrophic lake Eichbaumsee, a ~ 1 km long and 280m wide (maximum water depth 16m) dredging lake southeast of Hamburg (Germany), has been treated for water quality improvements using various techniques (i.e. aeration plants, removal of dissolved phosphate by aluminium phosphate precipitation and by Benthophos adsorption) during the past ~ 15 years. Despite these treatments no long-term improvement of the water quality was observed and the lake water phosphate content continued to increase by e.g. ~ 350 kg phosphate per year between March 2016 and February 2019. As no creeks or rivers drain into the lake and hydrological groundwater models do not suggest any major groundwater discharge into the lake, sources of phosphate (and other nutrients) are unknown.</p><p>We investigated the phosphate fluxes from sediment pore water and groundwater into the water body of the lake. Sediment pore water was extracted from sediment cores recovered by divers in August 2018 and February 2019. Diffusive phosphate fluxes from pore water were calculated based on phosphate gradients using first Fick`s law. Stable water isotopes (δ<sup>2</sup>H, δ<sup>18</sup>O) were measured in the lake water, sediment pore water, interstitial waters in the banks surrounding the lake, the Elbe river and in three groundwater wells close to lake. Stable isotope (δ<sup>2</sup>H, δ<sup>18</sup>O) water mass balance models were used to compute water inflow/outflow to/from the lake.</p><p>Our results revealed pore-water borne phosphate fluxes between – 0.07 mg/m²/d (i.e. slight phosphate uptake by the sediments) and 2.6 mg/m²/d (i.e. phosphate release to the lake). Assuming that the measured phosphate fluxes are temporarily and spatially representative for the whole lake, about 100 kg/a to 220 kg/a of phosphate is released from sediments. This amount is slightly lower than the observed phosphate increase of the lake water. Stable isotope signatures indicate a water exchange between the aquifer and the lake water. Based on stable isotope mass balances (δ<sup>2</sup>H, δ<sup>18</sup>O) we estimate an inflow of phosphate from the aquifer to the lake between 190 kg/a and 1400 kg/a. This inflow indicates that groundwater-born phosphate is as or even more important than phosphate supply via sediment pore-water. Our study suggests that groundwater may have an important impact on lake nutrient budgets.</p>