Long-term impact of experimental harvesting on tropical seagrass meadows in Gazi Bay, Kenya

Seagrass meadows provide a wealth of ecosystem services, among which the provision of habitat for many taxonomic groups and a significant contribution to sediment carbon storage. Along the African coast research on seagrasses is still limited. This study examined the long term perturbation impact (monthly removal of leaves during 1.5 years) of a harvested on four plots (3x2m) in a seagrass meadow in Gazi Bay, Kenya and compared these with four control plots. Seagrass growth was determined through leaf puncturing techniques and leaf counts. Macrofauna was sampled through dropsamples and cores. Soil organic carbon was measured through Loss on Ignition. Hydrodynamics was estimated through weight loss of plaster cubes. Bioturbation activity was estimated via direct entrapment of sediment expelled by burrowing shrimp.

Six months after the end of the perturbation, the average shoot density in harvested plot (19 +/-12 (SE) shoots.m2) is much lower than in the control plot (548 +/-207 (SE) shoots.m2). The organic carbon content in soils was significantly impacted by treatment down to 15cm (p<0.01) but not by depth (p>0.05). The hydrodynamics was significantly higher at the surface (p-value = 0.03) in harvested plots compared to control plots. Treatment significantly influenced fauna abundance and taxon richness in dropsamples (p<0.01 for both variables) with lower values in harvested plots. Same result was observed for infauna cores (p=0.01 in both case), in which treatment also has a significant effect on biomass (pvalue is equal to 0.003) with lower values for harvested plots. Big mounds present in intertidal area are assumed to be built by callianassids. They possess a high sediment turnover rate of 945 +/-685 (SE) g.d-1 dry weight per mound of expelled sediment. This report discusses the sampling methodology used, in particular for the burrowing shrimp experiments as these techniques are new for the region. The results are clearly displaying a loss of productivity and resilience of coastal ecosystems when seagrass meadows have undergone a long term perturbation. With nowadays seagrass meadows cover loss rate, coastal ecosystem services will significantly decrease. In a world where 38% of the population live on those services, this loss is unacceptable. There is an urgent need to globally improve the management of seagrass meadows.