Interactions in Elemental Cycles

Why is the distribution of most chemicals not uniform within the ocean (Fig. 9.1)? In the absence of sources and sinks the effect of ocean circulation and mixing would be to smooth the distribution of chemicals. An important reason for nonuniformity is the increase in concentration that occurs in regions of evaporation, and the reduction in concentration that occurs in regions of rainfall or river input at the ocean atmosphere interface. Evaporation, precipitation, and river input lead to concentration ranges usually well below 10%, as evidenced in the oceanic variability of salinity. However, as Fig. 9.1 shows, many chemicals have concentration ranges much greater than this. The concentration of inorganic nitrogen, for example, is more than five times smaller near the surface than in the deep ocean. Dissolved inorganic carbon shows a surface depletion that is also substantially larger than the 10% variation that could be induced by freshwater fluxes. The primary mechanism that drives such large concentration gradients is the use of dissolved inorganic chemicals by living organisms for formation of organic matter