Sedimented cyanobacterial detritus as a source of nutrient for submerged macrophytes (Vallisneria spiralis and Elodea nuttallii): An isotope labeling experiment using 15N

A tracer experiment using the nitrogen isotope 15N investigated the uptake and incorporation of nitrogen from sedimented cyanobacterial detritus by two species of submerged macrophytes, the native Vallisneria spiralis and the exotic Elodea nuttallii, in Lake Taihu (China). The cyanobacterium Microcystis was labeled with 15Nammonium and dried to produce detritus, which was injected into vegetated sediments and traced to establish the fate of particulate nitrogenous material. Samples of the inoculated sediment and all parts of the plants growing therein were examined for excess 15N after 3, 5, 9, and 22 d. Microcystis-derived nitrogen was assimilated rapidly by the plants, then translocated from the roots to the shoots and stored in different parts of the plants, where it supports growth. The ability to utilize and retain nutrients originating from sedimented cyanobacterial detritus was significantly higher in V. spiralis than in E. nuttallii. Cyanobacterial scums such as the Microcystis blooms of Lake Taihu, which drift and deposit in the sediments of the littoral zones, are an important nutrient source for macrophytes, particularly in downwind areas