The answer is nitrogen. It is usually regarded as a growth-limiting factor for primary producers such as phytoplankton. It is usually taken up in the ‘fixed’ form of ammonium. It is converted from dissolved nitrogen gas in ocean surface to a ‘fixed’ form by particular microbes. Due to increasing global temperatures, as a result of global warming, the ocean temperatures have increased hence oceans hold less dissolved oxygen than they used to. The low oxygen concentration in the oceans induces the loss of nitrogen from oceans through increased activity of micro-organisms that are significant in denitrification. This rate becomes higher than that of nitrification that fixes nitrogen in the oceans.
