Chapter 2 - Sources, types, and effects of nutrients (N and P) in coastal sediments

Abstract

The global cycling of nutrients in coastal waters has been changed profoundly due to natural and anthropogenic reasons. Global nutrient input is increasing and expected to increase with demand for food production and fuel. Concomitantly, this has led to an increase in nutrient transport from land to sea resulting in environmental deterioration as well as changes to biogeochemical processes. Human-driven changes in nutrient cycling are not uniform around the world due to differences in economic development. Also, the greatest changes are concentrated in the areas of high population density and intensive agricultural production. Sources of nutrients in coastal ecosystem are due to combustion and food production activities. While sewage and industrial discharges are point sources, agricultural runoff and atmospheric deposition are nonpoint sources. Extend of nutrient feeding into coastal ecosystems depends on the population density as well as relative size of the catchments. The primary source of inorganic nutrients in coastal ecosystems is catchments, whereas that of organic matter is catchments and tidal wetlands. Nutrient enrichment in aquatic ecosystems leads to eutrophication that further promotes changes in the structure of ecological communities directly by altering nutrient composition and indirectly through oxygen depletion through decomposition of the high biomass produced. The most common and obvious effect of eutrophication is depletion of dissolved oxygen leading to hypoxia and anoxia in the water column and in the sediment. Eutrophication causes increased sedimentation of organic matter to the coastal sediments and in systems of recurring hypoxia, more organic matter is available for remineralization by microbial communities. The effect of nutrient enrichment–induced eutrophication results in increased supply of organic matter that stimulates microbial populations and processes in the sediment, thus elevating the microbial loop to a more prominent trophic pathway. As oxygen gets depleted and the system moves from oxic to hypoxic conditions, aerobic microorganisms are replaced by facultative anaerobes such as denitrifying, manganese-reducing, and iron-reducing bacteria that utilize nitrate, manganese, and iron hydroxides as the terminal electron acceptors. Further from hypoxic to anoxic conditions, sulfate reducers and methanogens predominate under obligate or strict anaerobic conditions. This cascadal sequence is governed by thermodynamic yield, reaction kinetics, and the physiology of the microorganisms involved. This chapter analyzes and compares the various sources of nutrients (N and P) in global coastal ecosystems and its effect on eutrophication, which is the major consequence of nutrient enrichment. Further, eutrophication-induced sediment hypoxia and resultant change in microbial community structure are also discussed.