Productivity varies with depth in aquatic communities

Although the concentration of a limiting nutrient usually determines the productivity of aquatic communities on an areal basis, in any given water body there is also considerable variation with depth as a result of attenuation of light intensity. Figure 17.18a shows how GPP declines with depth. The depth at which GPP is just balanced by phytoplankton respiration, R, is known as the compensation point. Above this, NPP is positive. Light is absorbed by water molecules as well as by dissolved and particulate matter, and it declines exponentially with depth. Near the surface, light is superabundant, but at greater depths its supply is limited and light intensity ultimately determines the extent of the euphotic zone. Very close to the surface, particularly on sunny days, there may even be photoinhibition of photosynthesis. This seems to be due largely to radiation being absorbed by the photosynthetic pigments at such a rate that it cannot be used via the normal photosynthetic channels, and it overflows into destructive photo-oxidation reactions.

The more nutrient-rich a water body is, the shallower its euphotic zone is likely to be (Figure 17.18b). This is not really a paradox. Water bodies with higher nutrient concentrations usually possess a greater biomasses of phytoplankton that absorb light and reduce its availability at greater depth. (This is exactly analogous to the shading influence of the tree canopy in a forest, which may remove up to 98% of the radiant energy before it can reach the ground layer vegetation or, as we saw above, a stream bed.) Even quite shallow lakes, if sufficiently fertile, may be devoid of water weeds on the bottom because of shading by phytoplankton. The relationships shown in Figure 17.18a and b are derived from lakes but the pattern is qualitatively similar in ocean environments (Figure 17.19).

phytoplankton productivity varies with depth

Plankton With Depth

Figure 17.19 Examples of vertical chlorophyll profiles recorded in the ocean off the coast of Namibia. Example (a) is typical of locations associated with ocean upwelling: as cold upwelled water warms up, a surface phytoplankton bloom develops, reducing light penetration and thus productivity in deeper water. Example (b) illustrates how peak abundance can shift to deeper water as a surface bloom in an upwelling area depletes the nutrient concentrations there. The surface phytoplankton bloom in example (c) is less dramatic than in (a) (perhaps reflecting lower nutrient concentrations in the upwelling water); as a result, chlorophyll concentration remains relatively high to a greater depth. Examples (d) and (e) are for locations where nutrient concentrations are much lower. (After Silulwane et al., 2001.)

Figure 17.19 Examples of vertical chlorophyll profiles recorded in the ocean off the coast of Namibia. Example (a) is typical of locations associated with ocean upwelling: as cold upwelled water warms up, a surface phytoplankton bloom develops, reducing light penetration and thus productivity in deeper water. Example (b) illustrates how peak abundance can shift to deeper water as a surface bloom in an upwelling area depletes the nutrient concentrations there. The surface phytoplankton bloom in example (c) is less dramatic than in (a) (perhaps reflecting lower nutrient concentrations in the upwelling water); as a result, chlorophyll concentration remains relatively high to a greater depth. Examples (d) and (e) are for locations where nutrient concentrations are much lower. (After Silulwane et al., 2001.)

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