We saw in the previous chapter that except under very still conditions with virtually no wind or waves, there is always circulation of water in the upper layer. We also saw that this can be an advantage to the phytoplankton in so far as, by ensuring that they are not exposed to the intense light just below the surface for very long, they avoid photoinhibition. This circulation can, however, also be a disadvantage to the phytoplankton if, in the lower reaches of the mixed layer, the light intensity is too low for net photosynthesis to be achieved. As the depth of the mixed layer through which the phytoplankton is circulating increases, so the average light intensity to which the cells are exposed decreases, and consequently the total rate of photosynthesis by the whole phytoplankton population throughout the water column decreases. The rate of respiration of the whole phytoplankton population, on the other hand, will be essentially constant whatever the mixing depth. Thus, as was first pointed out by Braarud and Klem (1931), there exists a mixed-layer depth - the critical depth, zc - beyond which respiratory carbon loss by the whole population exceeds photosynthetic carbon gain, and so net phytoplankton growth cannot occur. Even when the critical depth is not exceeded, increases in mixing depth tend to reduce total photosynthesis.
The depth through which circulation can occur is limited either by the depth of the bottom or (from spring to autumn in temperate latitudes and throughout the year in tropical oceans) by the presence of a shallow thermocline. A thermocline (temperature gradient) gives rise to a pycno-cline (density gradient): density gradients within fluids are intrinsically stable, with a significant capacity to resist disruption. Thus, one answer to the question, where does aquatic photosynthesis take place is that it takes place best in shallow waters or in waters in which circulation is confined to a shallow layer by thermal stratification. The converse is that net photosynthesis occurs poorly, or not at all, in waters in which the phyto-plankton is circulated through great depths. For a series of Japanese lakes covering a wide range of depths, Sakamoto (1966) found a general tendency for deeper lakes to be less productive than shallow ones. Even when differences in nutrient supply were allowed for, the inhibiting effect of depth remained. Comparing two South African impoundments, of comparable optical and chemical character, Grobbelaar (1989) found the shallow Wuras Dam (zmax = 3.4 m) to be eight times as productive as the deep Hendrik Verwoerd Dam (zmax > 60 m). In the marine environment too, we may plausibly attribute the greater productivity of shallow coastal waters (relative to the deeper waters) in part (i.e. in addition to nutrient differences) to their lesser depth. Isolated shallow areas within the oceans such as the Faroebank (100 m depth) west of the Faroe Islands are also more productive than the surrounding oceanic waters.1296 In the turbid waters of the San Francisco Bay estuary, phytoplankton biomass is generally higher in the lateral shallows than in the channe1.248
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