Changes in nutrient concentrations can provide information on changes in the physical and biological processes that affect the carbon cycle and could potentially be used as indicators for large-scale changes in marine biology. However, only a few studies reported decadal changes in inorganic nutrient concentrations. In the North Pacific, the concentration of nitrate plus nitrite (N) and phosphate decreased at the surface (Freeland et al., 1997; Watanabe et al., 2005) and increased below the surface (Emerson et al., 2001; Ono et al., 2001; Keller et al., 2002) in the past two decades. Nutrient changes were observed in the deep ocean of all basins but no clear pattern emerges from available observations. Pahlow and Riebesell (2000) found changes in the ratio of nutrients in the North Pacific and Atlantic Oceans, and no significant changes in the South Pacific. In the North Pacific, Keller et al. (2002) observed a decrease in N associated with the increase in O2 between 1970 and 1990 at 1,050 m, opposite to the results of Pahlow and Riebesell’s longer study. Using the same data set extended to the world, large regional changes in nutrient ratios were observed (Li and Peng, 2002) but no consistent basin-scale patterns. Uncertainties in deep ocean nutrient observations may be responsible for the lack of coherence in the nutrient changes. Sources of inaccuracy include the limited number of observations and the lack of compatibility between measurements from different laboratories at different times.
In some cases, the observed trends in nutrients can be explained by either a change in thermocline ventilation or a change in biological activity (Pahlow and Riebesell, 2000; Emerson et al., 2001), but in other cases are mostly consistent with a reduction in thermocline ventilation (Freeland et al., 1997; Ono et al., 2001; Watanabe et al., 2005). Thus, all of the reported trends are consistent with a physical explanation of the observed changes, although changes in biological activity cannot be ruled out.
The concentration of surface nutrients can also be influenced by surface mixing, as a reduction in mixing leads to a decreased concentration of surface nutrients. The observed changes in surface temperature and salinity (see Sections 5.2.3 and 5.3) are indicative of changes in the surface mixing (see Section 18.104.22.168). In most of the Pacific Ocean, surface warming and freshening act in the same direction and contribute to reduced mixing (Figures 5.2 and 5.5), consistent with regional observations (Freeland et al., 1997; Watanabe et al., 2005). In the Atlantic and Indian Oceans, temperature and salinity trends generally act in opposite directions and changes in mixing have not been quantified regionally.