Rates of Phytoplankton Primary Production

By varying light in a series of short-term incubations we can see the functional relationship between phytoplankton photosynthesis and irradiance (Fig. 9.6). The data from the Hudson, as with most other systems, fit a hyperbolic tangent function (Fig. 9.6). To compare to other sys tems, the photosynthetic parameters are normalized to the amount of biomass (expressed in terms of chlorophyll-«). Thus, Pmax is the biomass specific rate of photosynthesis at optimum light and ab is the biomass-specific initial slope in Figure 9.6. Ik is the value of light at which P^ is first reached. In very clear water systems photosynthesis can be inhibited at high light; in the turbid waters of the Hudson light inhibition is not an important consideration.

P^ in the tidal-freshwater Hudson is comparable to values found in other estuarine systems and in the lower Hudson as well (e.g., Cote and Platt, 1983; Shaw and Purdie, 2001). Ik is moderately high (and ab moderately low) compared to many systems. In chronically low light environments phytoplankton are often adapted to low light by having low Ik and high ab, that is, a high affinity for light. The Hudson phytoplankton are not strongly adapted to low light, but are able to grow as rapidly as phytoplankton in other systems at optimum light. In the well-mixed water column it is perhaps more reasonable to see the Hudson plankton as adapted to varying light, taking advantage of high light when they are mixed near the surface (see Cole etal., 1992).

Instantaneous photosynthesis is the product of algal biomass, the photosynthetic parameters and light. Light varies over depth according to light extinction and varies with season and time of day. Algal biomass and the photosynthetic parameters

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