Marine diatoms as indicators of modern changes in oceanographic conditions

Marine diatoms as indicators of modern environment change A substantial part of the ocean’s primary productivity is provided by diatoms (Tréguer et al., 1995). In general, they are the dominant primary producers in temperate and cold areas, and are very abundant in the recently upwelled waters of Eastern Boundary Currents and in diverging surface currents where nutrients are brought to the surface (Nelson et al., 1995, and references therein). On an annual basis, the relative contribution of diatoms to primary productivity is highly variable: Nelson et al. (1995) and Tréguer et al. (1995) proposed upper limits of 35% in oligotrophic areas and up to 75% in coastal upwelling areas and other nutrient-rich systems. Regardless of the area, the general trend is for an increase in the relative abundance of diatoms in the phytoplankton together with primary productivity (Ragueneau et al., 2000). As a general statement we may say that diatoms are the dominant primary producers in a number of oceanographic settings that offer both the required high-nutrient and turbulence conditions (e.g. coastal upwelling areas, equatorial divergences, ice-edges, river plumes; Ragueneau et al., 2000). In contrast, small, non-siliceous pico- and nanoplankton are of great importance to total productivity in oligotrophic regions (Tréguer et al., 1995, and references therein).