Nutrient transfer between the seagrass Zostera marina and its epiphytes

Abstract

THE leaves of seagrasses often harbour dense populations of macroscopic and microscopic algal epiphytes. In many cases seagrasses grow in waters that are extremely low in dissolved inorganic nutrients yet they have high epiphyte standing stocks. These field observations led to direct experiments on the transfer of nitrogen and carbon from the dissolved nutrient pool in the interstitial waters of the sediments into the root system of eelgrass, Zostera marina, and through the plant to the algae on its leaves. Previous studies^1,2 on seagrasses have shown that the high productivity of seagrass meadows is largely maintained by the nutrient pool of the sediments. Since the plant system leaks phosphorus and, presumably, other nutrients, it is possible that the production of the leaf epiphytes is indirectly sustained by the nutrients in the sediments. This mechanism has been suggested by several workers for marine and freshwater macrophytes^3–7 but definitive experiments are lacking. Here we report the results of experiments designed to test this hypothesis.