Cycling of organic matter and trace metals in nearshore coastal ecosystem sediments

Abstract

Nearshore coastal systems provide habitat and food for millions of organisms worldwide. While it has been argued that nutrients are readily transferred between coastal ecosystems, this is not always the case. To study the cycling of organic matter and trace metals, we analyzed the lipid and trace metal content of seagrass meadows, mangrove forests, and coral reefs surface sediments. Preserved in surface sediment are the nutrients that were not readily transferred outside of the ecosystem, creating a record of allochthonous materials. Lipids within the surface sediment can be a food source or biological markers to determine the origin of organic matter, making it a vital tool when studying organic matter cycling. While metals can be detrimental, many metals are essential for an ecosystem to function properly and can be stored within surface sediments. We found that surface sediments can act as a catchment of trace metals such as Fe, Mn, Pb, and Zn and mangroves forests sequestering more metals than any other ecosystem type. Previously research has indicated that a strong correlation exists between trace metals and organic matter. However, here no correlation was found between trace metals and organic matter content for mangrove forests, seagrass meadows, and coral reefs; suggesting that the two cycles are mostly independent. The surface sediment surrounding mangrove forests acted as a net sink of organic matter and trace metals, while organic matter was quickly cycled through seagrass meadows and coral reefs. Endemic organic matter from each ecosystem was likely rapidly degraded and remineralized bacteria before being exported out of the system instead of being preserved in its original form. Existing research suggests a consistent correlation between organic matter and trace metals. However, the findings of this study suggest no uniform relationship between organic matter and trace metals across near shore coastal ecosystems. We instead suggest individually assessing ecosystem and study areas to determine correlations between organic matter and trace metals.