July Auii Sept

Fig. 11.11 Seasonal variation of the primary production rate of a mixed macrophyte community (predominantly Chara vulgaris, Potamogeton Richardsonii and Myriophyllum alterniflorum) in West Blue Lake, Manit., Canada (after Love and Robinson, 1977).

In our consideration of where and when aquatic photosynthesis takes place we have noted that some ecosystems are much more productive per unit area than are others. We shall now look more closely at the basis of this variation in productivity, particularly with respect to the role of light, and shall also examine some of the published data on the primary production rates actually achieved by aquatic ecosystems.

Comparisons of phytoplankton productivity in different waters are best carried out in terms of the total phytoplankton production per unit area, i.e. the integral, or areal, photosynthetic rate (PA) for the whole water column beneath 1 m2 of surface. This can be determined, as we saw earlier, by measuring the depth profile of photosynthesis per unit volume at a given location, and summing through the euphotic zone. This is, however, a very expensive and time-consuming way of determining productivity, and is not feasible if it is wished to map the distribution of productivity within some short time frame, over substantial regions of the ocean. A variety of attempts have therefore been made over the years to find ways of estimating integral photosynthetic rates from smaller amounts of data or more easily measured parameters.388,1157,1334,1420 In recent times this has been given new impetus by the perceived need to

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