The fate of cyanobacterial blooms in vegetated and unvegetated sediments of a shallow eutrophic lake: A stable isotope tracer study

An experiment using nitrogen stable isotope tracer (15N) was conducted to track the fate of nitrogen derived from cyanobacterial blooms and the effectiveness with which the seasonal blooms are retained by vegetated and unvegetated sediment in a large shallow eutrophic lake (Lake Taihu, China). 15N enriched Microcystis was injected into both unvegetated sediment and sediment occupied by common reed (Phragmites australis) in the littoral zone. Nutrient retention by the vegetated sediment was greater than by the unvegetated sediment, resulting in higher δ15N in the sediment nitrogen pool. The labeled Microcystis material was also distributed deeper into the vegetated sediment than the unvegetated sediment. A portion of the Microcystis-derived nitrogen was quickly assimilated, appearing first in the belowground biomass and subsequently in the aboveground biomass of the reed plants. The labeled nitrogen was found to support new growth as evidenced by 15N enrichment of new leaves. This study indicates that common reed beds in the littoral zone may play an important role in retention of sedimented planktonic materials.