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m-2 a-1 (in more mesotrophic systems and shelf waters able to support some 50-500 mg producer Cm-3 through depths between 15 and 30m; i.e. ~15gC m-2), and to 200-500gCm-2 a-1 among those relatively eutrophic systems sustaining the fixation of enriched lakes and upwellings, capable of supporting perhaps 1500-5000 mg C m-3 through depths of only 3-10 m i.e. ~15gCm-2.

This series is rather imprecise but it serves to illustrate the fact that although the supportive capacity of pelagic habitats probably varies over 3 or 4 orders of magnitude, the annual production that is achieved varies over rather less than 2 (maximum annual fixation rates of up to 800-900 g C m-2 are possible in some shallow lakes and estuaries). This is mainly because the exploitation of high supportive capacity results in a diminished euphotic depth. There is thus an asymptotic area-specific maximum capacity and, on many occasions, a diminishing productivity, sensu production rate over biomass (see also Mar-galef, 1997).

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