The impact of anthropogenic activities on nutrient dynamics in the tropical Wenchanghe and Wenjiaohe Estuary and Lagoon system in East Hainan, China

Biogeochemical observations were carried out in the Wenchanghe and Wenjiaohe Estuary, Bamen Bay and Gaolong Bay during 2006-2009 to understand the nutrient dynamics of these areas and their relationship with the sustainability of the ecosystems in the coastal areas of Eastern Hainan Island and its adjacent South China Sea. Nutrients in river/estuary waters, groundwater, aquaculture effluents and rainwater samples were analyzed using spectrophotometry. Nutrient levels in the tropical Wenchanghe and Wenjiaohe show a wide range of variation depending on the system, nutrient element and season. These two rivers are enriched with DIN and depleted in PO₄^3- with the DIN:PO₄^3- ratios varied from 60 to 411. In the rivers, TDP was mainly composed of DOP, representing ~65%. DON accounted for 40% of TDN in the Wenchanghe and 76% of that in the Wenjiaohe. Dissolved silicate levels in the Wenjiaohe and Wenchanghe were lower than average levels in tropical systems.Nutrients in the Wenchanghe and Wenjiaohe Estuary behave either conservatively or non-conservatively depending on the element being considered and the season. Based on observations of nutrients in various aquatic environments, a simple steady-state mass-balance box model was employed to assess nutrient budgets in the estuary system. Nutrients in the studied system were mostly from riverine input, groundwater discharge and aquaculture effluents. The nutrients exported in the studied system are largely confined to the immediate estuaries. The typhoon-induced runoff of terrestrial rainwater can not only increase nutrient inputs to the coastal ecosystem but can also result in nutrient imbalance, affecting phytoplankton production and composition.