A diver establishes an experimental treatment to exclude grazers from a nutrient-enriched plot of seagrass meadow.
Do grazers and wave energy offset the effects of nutrient pollution on seagrasses?
Eutrophication is one of the most serious human impacts on seagrass ecosystems. The commonly accepted pathway of impact is for nutrients to promote epiphytic algal growth, resulting in shading, and possibly death, of seagrasses. Until recently, it was generally accepted that light and nutrients were the main determinant of epiphytic algal biomass on seagrass leaves. However, recent research suggests that other factors may be at least as important.
This research project examined whether grazers and wave exposure could affect the impact of nutrient enrichment in seagrass ecosystems. Field and laboratory experiments have been undertaken which demonstrate the strong, negative effects of leaf movement and grazing on epiphyte biomass. Nutrient enrichment always increased epiphyte biomass, but if grazers were present, the increase was 43% less than if they were absent, and at exposed sites the effect of nutrients was only 50% of that at sheltered sites. The results are clearly indicating that: 1) grazers and wave energy can dramatically offset much of the negative effect of nutrient pollution; and 2) not all seagrasses are equally susceptible to nutrient pollution, with those in high hydrodynamic energy sites or with healthy grazer assemblages likely to be more resilient.
The project is a collaboration between ECU and the CSIRO Wealth from Ocean Flagship and was funded by the CSIRO Flagship Collaboration Fund and the European Union Marie Curie Outgoing Fellowships Scheme. The Commonwealth Dept of Defence (Navy, Garden Is., WA) has provided logistical support.