Ecological impact of land-derived anthropogenic nutrients and organic matter on tropical estuarine and coastal systems of Hainan, China

Abstract

Human development of the coastal zone causes cumulative effects on the nutrient and organic matter (OM) transport to coastal waters, especially by increased inputs, modified transport and altered biological processing. These effects are among the primary reasons for increasing degradation and losses of valuable coastal habitats, such as seagrasses and coral reefs. Generally, fertilizers from agriculture are thought to be the main source for nutrient enrichment in aquatic ecosystems. This may be different in the Southeast Asian region, where pond aquaculture for the production of shrimp and fish has expanded rapidly during the last decades at the expense of natural wetlands. The major objective of this thesis was to study on the example of NE Hainan, tropical China, how land-based activities alter nutrient and OM fluxes to tropical coastal waters, and how these, in turn, affect the health and ecological functions of coastal habitats. Biogeochemical, ecological and stable isotope methods, as well as experiments with bioindicators were used to determine the sources, pathways and fate of nutrients and OM in the Wenchang/Wenjiao Estuary (WWE) and nearshore coastal waters. Furthermore, the state of the seagrass meadows in three back-reef areas was determined. Samples and measurements were obtained in riverine, estuarine and coastal waters and sediments, as well as from potential land-based sources, including agriculture fields and aquaculture ponds and drainage channels, during four field campaigns between 2006 and 2009, one of that in the wake of a typhoon. Results indicate that effluents from aquaculture ponds lead to high nutrient and OM inputs to the WWE and coastal waters, in particular dissolved ammonium and dissolved organic nitrogen. Those stimulate phytoplankton growth, causing eutrophication of estuarine and nearshore coastal waters. While effluents released into the WWE are removed from the water column to large extents due to efficient phytoplankton uptake in the estuarine lagoon during low to moderate precipitation conditions, aquaculture ponds releasing their effluents directly into coastal waters cause continuous nutrient enrichment in usually oligotrophic back-reef areas, especially close to shore. Additionally, precipitation-induced freshwater pulses, which were strongest after typhoon events, are responsible for export of nutrients and OM from the agriculture-dominated hinterland of the WWE into coastal waters. These export pulses add to water quality deterioration in the region with the highest frequency of tropical storms worldwide and, are likely aggravated due to the loss of nutrient-reducing and sediment-retaining mangroves in the course of land use change. Stable nitrogen isotope values (delta 15N) of the ammonium in pond effluents reported for the first time were extremely high (~17 per mill). Consequently, elevated delta 15N values in TSM and seagrass leaves (5-12 per mill) and a significant delta 15N increase from <7 up to 14 per mill over time in the phytoplankton bioassays specified pond effluents as predominant nutrient source affecting the entire back-reef areas that were adjacent to aquaculture ponds over a distance of >2.5 km from the shore. A much lower seagrass species abundance, shoot density and total biomass and higher sulphide intrusion indicated by stable sulfur isotopes in seagrass leaves and higher epiphyte loads at the pond-affected sites compared to the almost aquaculture-free control site indicate adverse growth conditions due to reduced photosynthesis and sulphide poisoning. Our study indicates that still increasing pond aquaculture has to be taken into account for global estimates of land-derived nutrient inputs into the ocean and as a direct threat to coastal ecosystem health.