Functional Responses Associated with Growth Form in Stream Algae

The relationship of growth form to phosphorus uptake, carbon fixation, and susceptibility to grazing was tested in stream algae. We hypothesized that algal species with high surface area: volume (SA:V) ratios would have higher biomass-specific rates of nutrient uptake and photosynthesis than species with low SA:V ratios. We also predicted that algal species with gelatinous or prostrate growth forms would have low susceptibility to grazing. Physiological measurements were made using ten taxa comprising five different algal growth forms commonly found in flowing waters (unbranched filamentous; branched filamentous; gelatinous, branched filamentous; gelatinous, colonial; and prostrate). Significant positive relationships were obtained between SA:V and phosphorus uptake, and SA:V and carbon fixation at high light (200 μmol quanta m-2 s-1), but only after two outlying taxa were deleted from the analysis. A significant regression was obtained between SA:V and carbon fixation at low light (20 μmol quanta m-2 s-1) when all ten taxa were included. Herbivore preference for different growth forms was assessed in a short-term experiment which compared the amount of algae in snail feces vs. that on substrates. The ranking for least preferred to most preferred growth form was: gelatinous (colonial) = unbranched filamentous (Oedogonium) < prostrate < unbranched filamentous (Fragilaria). The overall results are consistent with the hypothesis that the functional attributes of lotic algae are related to their growth form. However, other factors beside growth form also can influence the functional performance and, by inference, the distribution of lotic algae in nature.

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