Abstract:
Nitrogen plays a central role in the oceanic biogeochemical cycles. The marine nitrogen cycle is considered very complex and has hence attracted considerable research interest. The availability of nitrogen is an important factor controlling the primary productivity and species diversity of both aquatic and terrestrial ecosystems. Given that nitrogen in its elemental form is inert and, therefore, unavailable for biological use, various biologically-driven processes including nitrogen fixation, ammonification, nitrification, and denitrification are responsible for deciding the fate of nitrogen transformation and thus, its bioavailability. However, human activities linked to global climate change, such as the combustion of fossil fuels, the indiscriminate use of artificial fertilisers, and the consequent release of nitrogenous compounds through the runoff into aquatic systems, have tremendously altered the nitrogen cycle. The positive correlation between human population density and nitrogen fluxes has resulted in a 10-15 fold increase in nitrogen input in many coastal areas. This has catastrophic effects, such as eutrophication, harmful algal blooms, hypoxia/anoxia, and alterations in community structure and food web dynamics, and ultimately affects the structure and functioning of the coastal ecosystems. This chapter focuses on the response of primary producers to these changes and explores the concerns related to the altered nitrogen cycle, links with ocean acidification, and emerging paradigms.
