Organic Sediment Impacts on Seagrasses and their Associations with Benthic Fauna
Crowley, Sean Matthew
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Human-population growth and rapid rates of urbanization have led to eutrophic conditions in estuaries throughout the world. Effects often include severe algal blooms, decreased dissolved oxygen levels, and the accumulation of fine-grained organic-rich sediments. One frequent negative consequence of eutrophication is the decline of seagrasses, a vital component of coastal ecosystems. Seagrasses provide shelter from predation, serve as nursery habitat, stabilize sediments, act as a sink for excess carbon, produce oxygen, and are a food source for many herbivores. Seagrasses are also sensitive to environmental changes, such as declining water quality and increased sedimentation rates. This study examines the impacts of fine-grained organic sediments on seagrasses, including their associations with benthic fauna. A unique benthic community was found associated with high seagrass cover, relative to intermediate, low, and absent seagrass coverage (p=0.001). Reduced abundances of three key species, especially the clam Mulinia lateralis(p=0.029), in high seagrass cover likely helps drive the differences. In addition, total benthic community fauna abundances and species richness were lower (p=0.035 and p<0.001, respectively) in high seagrass cover compared to low. Seagrasses were not found in sediments with water content exceeding 45%, silt/clay content exceeding 15%, or organic content exceeding 4%. Low to intermediate seagrass coverage can harbor an abundant and rich benthic community relative to higher coverage, but the disappearance of seagrass altogether is detrimental to that community. Seagrasses that are patchy and sparse because of polluted organic sediments or poor water quality may still host relatively rich populations, which can be source stock for recolonizing restored or improved benthic habitat.