in small forest streams, light and nutrients interact to determine productivity
Streams flowing through deciduous forests undergo marked transitions in primary production by algae on the stream bed during the growing season as conditions shift from light-replete early in spring to severely light-limited when leaves develop on the overhanging trees. In a stream in Tennessee, leaf emergence reduced PAR reaching the stream bed from more than 1000 to less than 30 |lmol m-2 s-1 (Hill et al., 2001). The reduction in PAR was paralleled by an equally dramatic fall in stream GPP (Figure 17.14). This is despite a large increase in photosynthetic efficiency from less than 0.3 to 2%; the higher efficiencies arose both because existing taxa acclimated physiologically to low irradiances and because more efficient taxa became dominant later in the season. Intriguingly, as PAR levels fell, the concentration of both nitrate (Figure 17.14a) and phosphate rose. It seems that nutrients limited primary production when PAR was abundant early in spring, with uptake by the algae reducing the concentration in the water at this time. When light became limiting, however, the reduction in algal productivity meant that less of the available nutrients were removed from the supply in the flowing water.
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