Decomposition of macroalgal blooms influences phosphorus release from the sediments and implications for coastal restoration in Swan Lake, Shandong, China

Abstract

Green tides (macroalgal blooms) have occurred worldwide frequently in coastal areas during the last few decades, which dramatically modify nutrient recycling in water column and the sediments. Rongcheng Swan Lake is a shallow coastal lagoon in eastern Shandong Peninsula, China, suffering from extensive macroalgal blooms dominated by Chaetomorpha spp. during the early summer and autumn. In the present study, the sediments collected from different lake regions and filamentous Chaetomorpha linum were incubated in the laboratory to investigate the influence of macroalgal decomposition on phosphorus (P) release from the sediments. In additon, some water quality parameters, including dissolved oxygen (DO), chemical oxygen demand (COD) and pH, were monitored during the 30-day experiment. Due to the Chaetomorpha decomposition, DO concentration and pH value in overlying water declined greatly. DO concentrations in sediment+30g algae+water treatment were very low (<0.15mgL−1), which remained anaerobic throughout the incubation. In addition, COD concentration in the treatments with algae increased dramatically and the maximum varied between 20.07 and 69.26mgL−1. In general, with the addition of greater amount of decomposing algae, a more visible change in each parameter was observed. During the decomposition process, SRP concentration in overlying water increased significantly, with the maximum of 1.51mgL−1. At the two studied regions, SRP concentrations in sediment+30g algae+water were significantly higher than those in sediment+water and algae+water treatments, respectively (p<0.05). The remarkable increase in P level in overlying water in sediment+algae+water treatment was mainly due to the P release from decomposing macroalgae during the initial 5 days of incubation and from the sediments at late stage, respectively. The decomposition of large qualities of Chaetomorpha promoted sediment P release obviously through changing environmental conditions, whose influence effect differed between the macroalgal dominated and non-dominated zone. In the macroalgal bloom zone, P release amount was high and the P released from sediments constituted the most part of P increase in overlying water. Our results indicate that the influence of Chaetomorpha blooms on sediment P release can not be neglected during the degradation phase in Swan Lake.