Topdown and bottomup effects on periphyton

A large ecological literature explores the relative importance of top-down and bottom-up controls in food chains (Power 1992a). From a bottom-up perspective, higher productivity driven by nutrient enrichment potentially can propagate along food chains, increasing productivity of consumers and possibly lengthening food chains by supporting the addition of another trophic level. Top-down effects of predators typically reduce abundances of their prey, which benefits the next trophic level down the food chain. This pattern is widely observed when predators reduce grazers, leading to an increase of algae. When such effects extend over multiple trophic levels they are called trophic cascades, and because some species may be more strongly affected and others less so, the indirect ramifications can be complex.

Members of the periphyton are subject to a combination of top-down control through grazing and bottom-up control through nutrient availability. Both can dominate in a given ecosystem, and can act separately or synergistically. For instance, high grazing pressure can counteract nutrient limitation by reducing the thickness of the periphyton mat, thereby enhancing water circulation and nutrient uptake, and by recycling nutrients through excretion. In turn, nutrient enrichment may allow periphyton to accommodate higher grazing pressure. In some instances herbivory completely overrides any influence of nutrients. Grazing by the algivorous minnow Campostoma was sufficiently strong that algae were similarly reduced in fertilized and unfertilized stream pools (Stewart 1987). Grazing by snails in a headwater stream in Tennessee likewise reduced algae to a thin, grazer-resistant layer regardless of nutrient addition (Hill 1992). Several syntheses of the substantial literature on grazer-periphyton interactions agree that top-down regulation of stream periphyton is common, although almost all studies were conducted under benign environmental conditions (Feminella and Hawkins 1995, Steinman 1996). A meta-analysis1 of 85 studies that quantified the size of effects associated with grazer access and nutrient supply on periphyton biomass found large effects of both factors, indicating that top-down and bottom-up effects each can be strong (Hillebrand 2002). Grazer effects generally were stronger, which Hillebrand attributed to their immediacy, whereas nutrient stimulation of periphyton growth acts over a longer time period.

An interaction between grazing and nutrient supply would be expected if some algae are more vulnerable to grazing or more responsive to nutrients. By manipulating the density of the snail Gonoibasis in enclosures placed in pools of a Kentucky stream, McCormick and Stevenson (1991) showed that a diatom overstory was susceptible to grazing but not strongly affected by nutrients, whereas Stigeocloneum, an understo-ry alga that was more resistant to grazing, was highly responsive to nutrient enrichment only in the presence of grazing. Greater nutrient availability may have been the result of increased diffusion due to the removal of overstory algae or excretion by the snails. From simultaneous

1 A meta-analysis is a synthesis of prior studies that makes quantitative comparisons by treating individual studies as data points in subsequent statistical analysis. Direction and size of effect classified by experimental conditions and environmental variables typically are assessed.

manipulations of snail densities and N and P concentrations, Rosemond et al. (1993) demonstrated that nutrients had positive effects and herbivory had negative effects on algal biomass (chlorophyll a, ash-free dry mass, total algal biovolume) and primary productivity (area- and chlorophyll-specific). Grazing favored chloro-phytes and cyanobacteria, which were overgrown by diatoms when herbivores were removed, and N and P both appeared to be at limiting concentrations. Periphyton responded strongly to the combined influence of grazing and nutrients, indicating that both top-down and bottom-up controls were acting on the pe-riphyton (Figure 9 5). In addition, nutrient addition stimulated snail growth, implying that bottom-up nutrient enrichment could influence consumer trophic levels. The interactive effects of nutrients and grazers were also apparent in a tropical stream where the limiting effects of N and grazing fishes both were strong (Flecker et al. 2002). Grazers had greater overall effects

FIGURE 9.5 Ash-free dry mass of periphyton on tiles at week 7 of an experiment in stream-side channels at Walker Branch, Tennessee. Control = ambient stream-water; N, P, and N + P are nutrient additions; grazed channels included the snail Elimia clavaeformis at natural densities. Error bars are 1 standard error. (Reproduced from Rosemond et al. 1993.)

FIGURE 9.5 Ash-free dry mass of periphyton on tiles at week 7 of an experiment in stream-side channels at Walker Branch, Tennessee. Control = ambient stream-water; N, P, and N + P are nutrient additions; grazed channels included the snail Elimia clavaeformis at natural densities. Error bars are 1 standard error. (Reproduced from Rosemond et al. 1993.)

than nutrients on biomass and composition of the algae, but the periphyton response to N addition was more pronounced in the presence of grazers. Algal productivity appears to have been stimulated by grazing, possibly due to the dislodgement of sediments that otherwise inhibited rates of diffusion to the periphyton.

Environmental conditions can interact with nutrient supply and grazer abundance to determine the extent of top-down versus bottom-up control of periphyton. Light, nutrients, and grazing all exerted strong control over periphy-ton biomass and productivity in a woodland stream in Tennessee, but the relative importance of these factors shifted seasonally. Light was limiting to periphyton biomass in summer and autumn but not in spring, nutrients were more limiting in seasons in which light levels were higher, and herbivory was influential during all seasons (Rosemond et al. 2000).

In summary, evidence from laboratory and field experiments demonstrates that grazing not only reduces total periphyton biomass but also alters structural and functional characteristics of the assemblage. Direct consumption, physical disruption, and regeneration of nutrients can each be important. As we broaden our perspective to include all of the environmental variables that affect periphyton, including seasonal and episodic disturbance as well as the predators and parasites that can regulate grazer abundance, it becomes increasingly apparent that biological assemblages are complex entities subject to multiple, interacting controls.

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