OceanEnvi, Eutrophication and Hypoxia
Coastal eutrophication is caused by excessive nutrient loading which stimulates phytoplankton blooms when physical, chemical, and biological conditions are favorable. It may lead to harmful algal blooms (HABs) and hypoxia (or “dead zones”, where dissolved oxygen is generally below 2 mg/L), both of which threaten the ecosystem. Coastal eutrophication has been a global environmental issue for decades, yet its persistence reflects the scientific and socio-economic complexities involved in alleviating the problem.
Eutrophication/hypoxia can cause ecosystem disruption (Curtis et al., 2015) by affecting phytoplankton stoichiometry and nutrient uptake kinetics, and consequently community structure and trophic efficiency. On the other hand, anthropogenically sourced nutrients rich in ammonia may inhibit phytoplankton nitrate uptake, reduce primary production, and alter significantly the phytoplankton community composition (e.g. Parker et al. 2012). Hypoxia reduces the species diversity of fish and benthic communities, and inhibits the respiration and growth of protozoan grazers, which causes a significant shift in microbial communities from grazing to parasitism-dominated energy and material pathways (Rocke and Liu, 2014). Eutrophication exhibited large space and time variations, because of the different sources and compositions of nutrients in the RES (Xu et al., 2012, Gan et al., 2014). Meanwhile, dissolved OM (DOM) originating from riverine and sewage discharge influences bacterial composition and activity (Xu et al., 2013), but the bacterial utilization of DOM and its role in marine carbon and nutrient cycling in the RES are largely unclear.