The impact of seagrass canopy removal on infaunal macrobenthic communities

Supervisor: Mark Huxham (Napier University, Edinburgh, & Kenya Marine and Fisheries Research Institute)
Seagrasses are marine angiosperms occurring along the coast of all continents, except Antarctica and are known for their high productivity. They play an important role as ecosystem engineers, contribute significantly to nutrient cycling within coastal areas and cover around 20% of the global carbon stored in marine sediments. They provide shelter and food supply for a large variety of (juvenile) fish, crustaceans, mollusks and polychaetes. Nevertheless seagrasses are globally threatened due to anthropogenic activities.

Within this study the relationship between seagrass meadows and infaunal macrobenthic communities is analysed within Gazi Bay, Kenya. The East African coast is known for its large seagrass diversity (12 species) while local research on this ecosystem is still limited. Seagrass systems provide a habitat for several commercially important fish species, making its health important for coastal communities who depend on fisheries for their daily income. The objective of this study is to assess differences in infaunal macrobenthic richness, abundance and community structure among sediment from control and harvested seagrass meadows and among sediment from the species Thalassia hemprichii and Enhalus acoroides.

An intertidal mixed meadow with the dominant seagrass species T. hemprichii and E. acoroides is selected for sampling. Eight monospecific plots of 3×2 meters are created (4 per species) and two plots per species are randomly selected and aboveground seagrass is removed (harvested). Right before harvesting (T=1) and around one month after harvesting (T=2) 15 cm deep cores are randomly taken for fauna and sediment. Above and belowground seagrass biomass, infaunal taxon counts (family level), abundance, biomass and volume are measured. Also several abiotic parameters are measured as the current speed (through the dissolving rate of plaster cubes) at different depths, the grainsize distribution and the organic matter content of the sediment.

Data analysis is performed through a nested ANOVA in Minitab 17 and a cluster analysis, MDS plot and ANOSIM test in PRIMER 6.1.9. Harvesting of aboveground seagrass biomass leads to a significant decline in infaunal macrobenthic richness (R2adj = 91.33%, p = .03, one-tailed) and abundance (R2adj = 82.99%, p = .042). Also there is a significant difference in community structure (Global R = 0.427, p = 2.9%) as a result of treatment. Small crustaceans typically inhabit control plots while harvested plots are associated with larger infaunal organisms as polychaetes. No significant differences were found in richness (R2adj = 15.84%, p = .473) or abundance (R2adj = 17.77%, p = .463) of infaunal macrobenthic communities or in aboveground (R2adj = 77.09%, p = .197) or belowground (R2adj = 0.00%, p = .571) seagrass biomass as a result of seagrass species. Nevertheless there appears to be a tendency for higher aboveground biomass, taxon counts and fauna abundance in plots with T. hemprichii compared to E. acoroides.

Grainsize analysis showed dominance of medium sand within all the plots and no differences in percentage organic matter between treatments (R2adj = 34.35%, p = .352). The use of plaster cubes for measurement of the current speed within seagrass meadows proved a simple and usefull technique. Close to the sediment there is significantly less weight loss (R2adj = 77.48%, p = .001) from plaster cubes associated with seagrass meadows compared with control plots.

To the author’s knowledge this is the first time that the effect of seagrass canopy on infaunal macrobenthic communities has been tested through the removal of aboveground seagrass biomass. The clear results illustrate the close relationship between seagrass meadows and associated infauna. Also they emphasize the need for proper management of this valuable ecosystem in Eastern Africa. As anthropogenic impact is likely to increase in the near future and local fisheries depend on healthy seagrass meadows for their catch.