Benthic primary production decreases internal phosphorus loading from lake sediments under light supplement

In aquatic ecosystems, light penetrating the sediment surface in shallow lakes may regulate the internal phosphorus (P) release through benthic primary production, which subsequently affects oxidation, pH levels, and alkaline phosphatase activity in the upper sediment. To study the effects of light exposure on the P dynamics at the sediment-water interface under eutrophic conditions, a two-month mesocosm experiment was conducted in twelve cement tanks (1000 L each). The tanks were equipped with Light-Emitting Diode (LED) lights, and surface sediments collected from eutrophic Lake Nanhu (China) were exposed to four different light intensities (0, 50, 100, 200 μmol m^-2 s^-1). The results revealed that: 1) Both the total phosphorus concentration and the phosphorus release flux from the sediment were lower in the light treatments (mean value, 0.59-0.71 mg L^-1 and 0.00-0.01 mg m^-2 d^-1, respectively) than in the control treatment (0.77 mg L^-1 and 0.01 mg m^-2 d^-1, respectively), indicating that light supplement could decrease the internal P release. 2) Benthic primary production promoted by light directly absorbed soluble reactive phosphorus and decreased the internal P release. The resulting improved production could also increase dissolved oxygen concentrations at the sediment-water interface, thus indirectly inhibiting internal P release. 3) The relative contributions of direct absorption and indirect inhibition on the internal P release ranged between 23% to 69% and 31% to 77% depending on the light intensity.