Abstract
Surface sediment and water samples were collected from Daihai Lake to study the biogeochemical characteristics of nitrogen and phosphorus, to estimate the loads of these nutrients, and to assess their effects on water quality. The contents and spatial distributions of total phosphorus (TP), total nitrogen (TN), and different nitrogen forms in sediments were analyzed. The results showed that concentrations of TN and TP in surface sediments ranged from 0.27 to 1.78 g/kg and from 558.31 to 891.29 mg/kg, respectively. Ratios of C : N ranged between 8.2 and 12.1, which indicated that nitrogen accumulated came mainly from terrestrial source. Ratios of N : P in all sampling sites were below 10, which indicated that N was the limiting nutrient for algal growth in this lake. Effects of environment factors on the release of nitrogen and phosphorus in lake sediments were also determined; high pH values could encourage the release of nitrogen and phosphorus. Modified Carlson's trophic state index (TSIM) and comprehensive trophic state index (TSIC) were applied to ascertain the trophic classification of the studied lake, and the values of TSIM and TSIC ranged from 53.72 to 70.61 and from 47.73 to 53.67, respectively, which indicated that the Daihai Lake was in the stage of hypereutropher.
Highlights
Along with the rapid economic development and population growth, eutrophication caused by excessive inputs of phosphorus (P) and nitrogen (N) has become one of the most common impairments of surface waters in China and presents an ongoing threat to the vitality of freshwater ecosystems, where it often manifests as harmful algal blooms that prevent sunlight from reaching underwater plants and lead to lower oxygen concentrations [1,2,3,4]
Dissolved oxygen concentrations ranged from 2.81 to 4.83 mg/L, and dissolved oxygen (DO) and temperature were inversely correlated in all sampling sites
Measured DO concentration was lower in high temperature site, and the depletion of oxygen level in these sites could possibly be the influence of biological activities and reduce oxygen gas solubility supported by the warmer temperature
Summary
Along with the rapid economic development and population growth, eutrophication caused by excessive inputs of phosphorus (P) and nitrogen (N) has become one of the most common impairments of surface waters in China and presents an ongoing threat to the vitality of freshwater ecosystems, where it often manifests as harmful algal blooms that prevent sunlight from reaching underwater plants and lead to lower oxygen concentrations [1,2,3,4]. Nutrients are introduced into lakes and reservoirs from external sources, such as sewage discharges, agricultural wastewater, and diffuse runoff from agricultural land [5]. Over time, these nutrients could build up in the sediment and create the potential for an internal load that can be recycled back into the water column under different environmental conditions [6]. There is an internal source of nutrients, and it may be important as dissolved oxygen can be very low in summer [7], which will help release nutrient from sediment to water column and promote algal growth. Such complex exchange processes of nutrients across the sediment-water interface are of considerable importance in understanding the impacts of the chemical composition and the trophic level of aquatic
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