Nutrient concentrations in the surface ocean surface are influenced by human impacts on coastal runoff and on atmospheric deposition, and by changing nutrient supply from the ocean’s interior into the mixed layer (for instance due to increased stratification). Changing nutrient distributions might influence the magnitude and variability of the ocean’s biological carbon pump.
Globally, the manufacture of nitrogen fertilizers has continued to increase (Galloway et al., 2008) accompanied by increasing eutrophication of coastal waters (Diaz and Rosenberg, 2008; Seitzinger et al., 2010; Kim et al., 2011), which amplifies the drawdown of CO 2 (Borges and Gypens, 2010; Provoost et al., 2010). In addition, atmospheric deposition of anthropogenic fixed nitrogen may now account for up to about 3% of oceanic new production, and this nutrient source is projected to increase (Duce et al., 2008).
Satellite observations of chlorophyll reveal that oligotrophic provinces in four of the world’s major oceans expanded at average rates of 0.8 to 4.3% yr –1 from 1998 to 2006 (Polovina et al., 2008; Irwin and Oliver, 2009), consistent with a reduction in nutrient availability owing to increases in stratification. Model and observational studies suggest interannual and multi-decadal fluctuations in nutrients are coupled with variability of mode water and the NAO in the Atlantic Ocean (Cianca et al., 2007; Pérez et al., 2010), climate modes of variability in the Pacific Ocean (Wong et al., 2007; Di Lorenzo et al., 2009), and variability of subtropical gyre circulation in the Indian Ocean (Álvarez et al., 2011). However, there are no published studies quantifying long- term trends in ocean nutrient concentrations.