E

Figure 12.1 Shifts in fish communities along the primary productivity gradient for (a) European lakes and (b) North American lakes, (a) is based on data in Hartmann and Nümann (1977) and Persson et al. (1991), and (b) is adapted from Oglesby et al. (1987) (from Persson 1993)

copepod zooplankton communities to very low biomasses and small forms (Persson 1991). The species is an omnivore and can be sustained by alternative food when animal food resource levels are low. This flexibility further increases its capacity to suppress zooplankton densities. Experimental studies have also demonstrated roach's ability to translocate nutrients within the water column and from the sediments (Andersson el al. 1988). Finally, by depressing zooplankton densities and translocating nutrients, roach have secondary effects on interactions between phytoplankton and submerged macrophytes, and hence have the capacity to affect physical features of systems.

Figure 12.2 (a) The proportion of piscivores of total fish biomass in relation to phosphorous loading. 9 , benthic total piscivores; —O—> bgnthic piscivorous perch; ---■---, pelagic total piscivores; ---□---, pelagic piscivorous perch, (b) The proportion of total fish biomass as piscivorous perch and chlorophyll (mg_l. an index of phytoplankton biomass) in relation to phosphorous loading. —□—, benthic piscivorous perch; —■—, pelagic piscivorous perch; —□—, phytoplankton biomass

Phosphorus loading (g/m2 year)

Figure 12.2 (a) The proportion of piscivores of total fish biomass in relation to phosphorous loading. 9 , benthic total piscivores; —O—> bgnthic piscivorous perch; ---■---, pelagic total piscivores; ---□---, pelagic piscivorous perch, (b) The proportion of total fish biomass as piscivorous perch and chlorophyll (mg_l. an index of phytoplankton biomass) in relation to phosphorous loading. —□—, benthic piscivorous perch; —■—, pelagic piscivorous perch; —□—, phytoplankton biomass

Perch are potentially piscivorous. In comparison with many other piscivorous species, perch use a wide range of resources over their ontogeny (Persson 1988). Young perch feed on zooplankton. As they become larger, macroinvertebrates are preferred prey, and the largest perch turn to piscivory. Over their ontogeny, prey length may increase a thousand-fold.

Perch's dominant role in fish communities is illustrated by data from many systems (Figure J2,2a).

The numerical dominance of roach (and cyprinids in general) in highly productive lakes is attributed to a competitive asymmetry, where roach as efficient zooplanktivores outcompete juvenile perch (Persson 1988). In addition, roach's relatively high capacity to assimilate bluegreen algae, abundant in highly productive lakes, and its ability to forage under turbid (low light) conditions intensifies the limitations set by roach on perch (Persson 1991, 1994). In other situations, perch prédation aiïects the numbers and size structures of roach populations if a substantial number of perch individuals become large. Thus, although competitive asymmetry explains the dominance of cyprinids in highly productive lakes, a prédation asymmetry favoring perch over roach appears to explain the numerical dominance of percids in moderately productive systems.

Shifts along the productivity gradient in relative strengths of the competitive and prédation asymmetries have ramifications for total lake trophic structure and dynamics. The proportion of piscivores in fish biomass, and hence the capacity of piscivores to control planktivores, changes in relation to the strengths and asymmetries of competition and prédation. The proportion of piscivores is generally higher in systems dominated by percid species than in systems dominated by either salmonid or cyprinid species (Persson et al. 1991, 1994) (Figure 12.2a). Comparative studies of Swedish lakes show that the presence/absence of pelagic piscivores (i.e. piscivorous perch) has major impact on planktivore biomass, zooplankton biomass, and phyto-plankton production and biomass, (Persson et al. 1992). The data on changes in the proportion of piscivores along the productivity gradient demonstrate two interesting patterns (Figure 12.2b). First, it appears that piscivores (secondary carnivores) have a capacity to suppress changes in phytoplankton biomass between phosphorous loading rates of 0.03 and 0.3 g P per (m2 year) (Persson et al. 1992). Second, a threshold around 0.3 g P per (nr year) is indicated by a sharp decrease in relative piscivore biomass and an increase in phytoplankton biomass with increasing phosphorous loading (Figure 12.2b).

While fish community structure, food web structure and dynamics, and primary productivity are correlated, habitat heterogeneity is also associated with trophic dynamics, Habitat heterogeneity varies along the productivity gradient, and peaks in moderately productive system (where piscivore biomass also peaks) because submerged vegetation generally has its maximum development in such systems (Sand-Jensen 1979). In highly productive systems, the hypolimnion also often becomes anaerobic, which further decreases the available habitat. Thus, a major negative consequence of cultural eutrophication is habitat degradation, which affects both water clarity and the fish community of lakes.

Hydrology, disturbance and river food webs As in lakes, the identities and linkages of strong interactors in rivers can change in different environmental contexts. For example, altered hydrologic regimes, during drought or following impoundment, can change impacts of fish in river food webs, and alter the length of functional food chains in these webs. Functional food chains depict linkages of consumers to resources whose populations or abundances they potentially regulate. This is an obvious oversimplification of food webs, in which complexities like omnivory can obscure abstractions like trophic levels. In a variety of lake, marine and river systems, however, experimental manipulations have revealed chains of strong interactions that link predators through herbivores to plants (Estes and Palmisano 1974; Rstes et al. 1978: Carpenter et al. 1985; Power et al. 1985; Carpenter 1988; Power 1990a). Thus, this abstraction serves as a useful key for unraveling impacts of environmental change in multi-trophic level communities.

Food chain length has enormous practical implications. Rivers with no functional trophic levels will convey excessive nutrients to ground water or downstream water bodies. This problem has occurred in the arid, overgrazed watersheds around Phoenix. USA, where nitrates derived from air pollution have caused water wells to be shut down. As Arizona State University ecologist Stuart Fisher has commented, residents end up drinking nitrates from their own automobile exhaust because watersheds are too degraded to convert nitrate into aquatic or riparian vegetation (Koppes 1990). With one trophic level (plants), nutrients would be better retained, but excessive algal blooms could clog channels. A second trophic level (grazers) could control algae, but if uncheckcd could produced pestiferous insect emergences. In rivers with three-, four- or five-level food chains, these basal trophic levels are increasingly controlled by fish, large fish and wildlife. The more complex system processes excess nutrients and converts them to potentially useful fish and wildlife, a clear case of ecosystem services created by complexity.

There is presently no secure theory for predicting the length of functionally important food chains in natural ecosystcms. The two most studied hypotheses predict that (1) food chains should lengthen with environmental productivity or the metabolic efficiency of consumers, and (2) food chains should shorten with increased environmental disturbance (Pimm 1982; Jenkins et al. 1992). While these two hypotheses have been considered as alternatives, it is obvious that disturbance and productivity regimes might interact to influence food chain length. Surveys and experiments in northern California rivers suggest that, in contrast to previous predictions, the length of functionally important food chains probably decreases initially, but then increases, with disturbance. Mechanisms for this response involve familiar life-history tradeoffs between resilience following physical disturbance and resistance to predators for early versus late successional species at lower trophic levels. Here we use "disturbance" in the narrow, ecological sense of

Diatoms, Nostoc

Figure 12.3 (a) A partial depiction of the Eel River food web, showing strong and weak links following scouring winter floods (with no late spring flooding). The dominating food chain linking predators to plants is four levels long. Top predators are large fish, which suppress small predators like damselfly nymphs, releasing tuft-weaving midges from predation, so that these can suppress algae. Thick arrows in this figure designate consumer impacts that are strong because of the population density and/or per capita impacts of the consumer taxon. Thin arrows designate weak interactions: becausc of limited densities or per capita effects, removal of these consumers would not produce conspicuous changes in the populations of their prey

Diatoms, Nostoc

Flood Year

Figure 12.3 (a) A partial depiction of the Eel River food web, showing strong and weak links following scouring winter floods (with no late spring flooding). The dominating food chain linking predators to plants is four levels long. Top predators are large fish, which suppress small predators like damselfly nymphs, releasing tuft-weaving midges from predation, so that these can suppress algae. Thick arrows in this figure designate consumer impacts that are strong because of the population density and/or per capita impacts of the consumer taxon. Thin arrows designate weak interactions: becausc of limited densities or per capita effects, removal of these consumers would not produce conspicuous changes in the populations of their prey ail event that removes organisms, thereby creating empty habitat or freed resources. Disturbances are not necessarily harmful, and indeed may be essential to ecosystem function and services.

Northern California, like other regions with Mediterranean climates, typically experiences highly seasonal rainfall that causes winter-flood, summer-drought hydrographs in its rivers. Food-web structure and its effects on ecosystem processes depend on whether scouring floods occurred in winter.

In years with scouring winter floods, the food web has three especially strong links (Figure 12.3a), Fish predation has cffects that cascade to algae and affect primary production of the system (Power 1990a,b,' 1992a,b). California roach (Hesperoleucas symmetricus, an omnivore) and juvenile steelhead (Oncorhynchus mykiss, a carnivore) suppress a guild of small predators (the fry of roach and stickleback, (Gasterosteus aculeatus) and large invertebrate predators (primarily damselfly nymphs, Ar chiles tes califor-nica). These small predators, but not the larger fish, are capable of suppressing tuft-weaving midges, dominated by Pseudnchirommus (Power el at. 1992), which in turn can suppress algae.

Scouring winter floods may be absent in drought years, or may be permanently eliminated by water projects that stabilize flow artificially. In the absence of winter floods, the food web has only one strong link (Figure 12,3b). The grazer guild becomes dominated by sessile (e.g. the aquatic moth Peirophitia) or heavily armored (e.g. the large caddis fly Dicosmoecus) taxa that are relatively invulnerable to predators (Power 1992a). In the absence of winter floods, fish become functionally irrelevant to the ecosystem and algae are suppressed by armored grazers (Power 1994; Power et at. 1994).

These year-to-year contrasts in river food webs indicate that functional food chain length and the trophic positions of particular taxa arc not fixed, but change in response to temporal and spatial (Power et at. 1985; Power 1992b) variation in the environment. It is also evident, however, that the functional significance of predators like steelhead in suppressing lower trophic levels, in this case their primary consumer prey, wanes in the prolonged absence of seasonal benthic disturbances by scouring floods. In both flood years, steelhead had strong effects that cascaded down to algae (from the 4th and 3rd trophic levels in 1989 and 1933, respectively). In this sense, lack of disturbance shortens the functional food chains in these river communities.

Figure 12.3(b) Eel River food web during drought, with one or more years elapsed since floods scoured the bed. The same biota are present, but the identities of strong interactors have switched from fish to armored and sessile grazers (Dicosmoecus and Petrophila, respectively). Impact of these predator-resistant grazers is enhanced by increases in density that follow winters free of scour-induced mortality. In rivers after prolonged absence of flood disturbance, these late successional grazers can sequester much of the available primary productivity without passing it up the food chain to fish. Consequently, functionally dominant food chains shorten to two levels

Drought Year

Figure 12.3(b) Eel River food web during drought, with one or more years elapsed since floods scoured the bed. The same biota are present, but the identities of strong interactors have switched from fish to armored and sessile grazers (Dicosmoecus and Petrophila, respectively). Impact of these predator-resistant grazers is enhanced by increases in density that follow winters free of scour-induced mortality. In rivers after prolonged absence of flood disturbance, these late successional grazers can sequester much of the available primary productivity without passing it up the food chain to fish. Consequently, functionally dominant food chains shorten to two levels

Table 12.1 Mean, and standard deviation for ccrtain limnological variables of 12 Chilean temperate lake from the rainforest region 39-43°S (Campos 1984; Campos et a/. 1988; Soto 1996)

Mean (SD)

Area (m)

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