Positive responses of a seagrass ecosystem to experimental nutrient enrichment

Nutrient enrichment of coastal waters is widely recognized as a major driver of seagrass decline. Under conditions where seagrasses are nutrient-limited, however, moderately elevated nutrient loads can enhance seagrass biomass and increase above- and below-ground consumers that support higher order predators. To improve understanding of bottom-up processes in seagrass ecosystems, we conducted a manipulative field experiment to simultaneously evaluate the responses of primary producers (seagrass and epiphytes) and the epiphyte- and the sediment-based components of seagrass food webs to moderate and high levels of waterborne nutrients. Fifteen 7 m² sites in Zostera muelleri meadows were assigned randomly to control, moderate or high nutrient treatments and were enriched with 0, 1800 g and 3600 g respectively of slow-release fertilizer in above-ground dispensers. The experiment ran for 9 mo (August 2006 to April 2007) and the fertilizer was replaced every 2 mo to ensure continuous enrichment. The biomass of primary producers (seagrasses Z. muelleri, Halophila ovalis and associated epiphytes) and the abundance of predators in the epiphyte- and the sediment-based components of the food web were greater in nutrient-enriched treatments than in controls. Epiphyte grazers, deposit feeders/detritivores, suspension feeders and benthic grazers did not respond significantly to the nutrient enrichment. In general, responses to nutrient enrichment were similar for medium and high nutrient treatments except that the biomass and surface area of seagrass was greater in high enrichment sites. These results demonstrate that Z. muelleri-dominated seagrass meadows in oligotrophic systems may be resilient to greater nutrient loads. Effective conservation strategies for Z. muelleri meadows should continue to consider interactions among nutrient enrichment and other key anthropogenic stressors, particularly non-nutrient pollutants in runoff and sewage discharge.