High sulfide production induced by algae decomposition and its potential stimulation to phosphorus mobility in sediment.

This study is devoted to addressing the effects of algae blooms on sulfur cycle and the consequent phosphorus mobility in the sediments of freshwater lake ecosystems. A mesocosm experiment was conducted to investigate these effects through monitoring the dynamics of sulfur (S), iron (Fe) and phosphorus (P) in water and sediments, and their diffusion fluxes at the sediment-water interface (SWI). In addition, the abundance of sulfate-reducing bacteria (SRB) in the water column was also detected. The addition of the algae lead to an increase of SRB, a drastic decline of sulfate and a significant increase of total dissolved sulfide (ΣS2-, the peak value of near 3.0 mmol/L on day 6) in the water column. These results suggest the sulfate reduction was dramatically promoted during algae decomposition. Indeed the ΣS2- was 2 to 3 times of SO42- initial concentration, and higher ΣS2- was produced with higher algal biomass. Moreover, the diffusive flux of ΣS2- at the SWI was negative, indicating that diffusion of ΣS2- from water column toward sediment was occurring. These results indicated that algae decomposition might also be another important source of ΣS2- (termed "algae-derived ΣS2-") in addition to sulfate reduction. The increase of Fe(II) in surface sediment pore-water was slightly delayed compared to the ΣS2- generation in the water column, which illustrated that Fe oxyhydroxides in sediments were transformed into Fe(II) through chemical reduction of ΣS2-. Concomitantly, the vertical distribution of PO43- in high amounts algae group suggested that desorption and release of iron oxides-bound PO43- occurred in sediments. Collectively, algae bloom can boost the lake eutrophication not only through direct release of nutrients but also through the high production of ΣS2-and indirect promotion of phosphorus mobility in sediment.