Lessons learned: The effects of nutrient enrichment on the support of nekton by seagrass and salt marsh ecosystems

Abstract

Coastal ecosystems such as eelgrass beds and salt marshes have always been valued for their high productivity and rich bounty of fish and shellfish. High plant productivity, complex physical structure, and suitable environmental characteristics combine to create areas of high production of important recreational and commercial species. If we are to successfully manage and restore these ecosystems, it is important to understand the mechanisms by which support of nekton may be affected by nutrient enrichment. A review of the literature suggests that there are some similarities and differences in the effects of nutrient enrichment on the support of nekton by seagrass and salt marsh ecosystems. Nutrient enrichment may compromise the ability of these habitats to support fish and invertebrates before the habitat itself is gone. In both ecosystems, alteration of characteristics within the ecosystem (for example, stem density in seagrass and food webs in marshes) affect the support of nekton, even though the basic ecosystem is still clearly extant. Because of differences in natural ecosystem characteristics, loss of ecosystem function does not occur through the same mechanisms. In seagrass systems, physical structure is usually lost first, followed by alteration of food webs and finally changes in dissolved oxygen. In salt marsh systems, loss of dissolved oxygen may occur early in the process, followed by food web alterations and eventually changes in the physical structure may occur. For both seagrass and salt marsh ecosystems, the mechanisms suggested to operate at the ecosystem-level are often based on relatively small-scale plot experiments that have been conducted in only a few locations. A better understanding of how these ecosystems function across broad geographic regions will be needed to ensure functioning coastal ecosystems.