How generally predictable are the effects of stress on freshwater ecosystems? We chose to test this question by focusing on Chilean ecosystems and their potential stressors. We attempted to develop a series of predictions for these Southern Hemisphere ecosystems.
The Lake Region in southern Chile, South America, contains large (50 to 870 km2) and deep (up to 350 m maximum depth) lakes. The basins around most of these lakes have some deforested areas that have been used extensively for agriculture, human settlements (including very rapidly growing cities), tourism development and, very recently, salmon farming. Human perturbation in the surroundings of these lakes accounts for as much as 60% of vegetation changes in the natural forested basin area during the past 200 years. Despite this, the lakes are still largely oligotrophic (Campos 1984; Campos et at. 1988). They have very low nutrient content, low chlorophyll values, generally low primary productivity and extremely high water clarity (Table 12.1). Owing to the extreme water clarity, benthic macrophytes and periphyton account for a large proportion of the carbon fixed. A large, rich bcnthic fauna has developed with abundant crayfish and freshwater crabs.
Species richness, particularly for zooplankton and fish, is lower than expected for the lakes' area and latitude (Soto and Zuniga 1991). The zooplankton of Chilean oligotrophic lakes is small, usually dominated by calanoid copepods and small eladocerans (Soto and Zuniga 1991). Such animals may have minimal potential to control phytoplankton even if they were released from control by zooplanktivores. Daphnia, which exerts major control over phytoplankton in many lakes (Carpenter and Kitchcll 1993), is historically absent from this region.
In most of the oligotrophic lakes, fish biomass is dominated by zooplank-tivorous silversides (Basilichthys spp.) and whitebait (Gataxias spp.), as well as benthivorous fish such as the native trout Percichthys truchu and introduced trout, which also feed mostly on zoobenthos (crayfish and freshwater crabs) and only occasionally on fish. Greater piscivory is observed in less oligotrophic lakes (Arenas 1978), but no species is predominantly piscivorous.
Two important changes have the potential to a fleet these lakes: (1) shifts in land use causing eutrophication; (2) the introduction of exotic species such as salmonids, with less predictable effects (Soto 1996). Can we make predictions about the impact of these changes, generalizing mainly from research and knowledge from the Northern Hemisphere?
12.3.2 Potential shifts in Chilean lakes
There are both independent and interactive features of the two possible changes that we have considered as having the potential to influence the
Chilean Lakes. Land-use changes can increase the loading of nutrients to the lakes. Consequences could include adverse effects on water quality such as decreased water clarity, increased chlorophyll levels, and increased anoxic conditions (Likens 1972; Vollcnweider and Kerekes 1980). Specific predictions on the changes to be expected in water quality will depend upon projected levels of nutrient increases and the natural occurrence of those nutrients occurring in the lakes.
Changes in nutrient loading could also influence the likelihood of successful species invasions. Early salmon introductions were not successful because the lakes were too oligotrophic (Stockner ¡981). Attempted introductions of several salmon species in the south of Chile look place at the beginning of the century, but only rainbow trout (Oncorhynchus my kiss) and brown trout (Salmo trutta fario) were successful. Higher nutrient loading rates could increase eutrophication and improve the success rate of invading or introduced species. Changes in nutrient loading could also accomplish the types of shifts in lake food webs that have been discussed previously.
Salmon culturing has become an important activity in these lakes. It can increase the chances of the introduction of an exotic species by an accidental escape or a purposeful release. Also, culturing practices can themselves lead to substantial lake nutrient inputs.
How can the potential effects of nutrient inputs and species introductions on biodiversity and ecosystem processes in these lakes be summarized? In the present trophic web (Figure 12.4a), pelagic piscivorous fish are essentially lacking, while trout are using the zoobenthos as a primary resource. Considering present productivity and chlorophyll values (Figure 12.4a), it is possible that invading salmonids would not succecd immediately as pelagic piscivores but rather as benthos feeders. Thus, strong competition with native trout would be expected. Increased nutrient inputs and productivity would reduce water clarity and increase the amount of carbon fixed in the water column. Shifting of primary production to the water column, together with the increased predation pressure, would potentially reduce the large zoobenthos (crabs and crayfish). Competition among benthivorous fish would increase. At the same time, zooplanktivorous fish production might increase and potentially support piscivorous fish. The piscivores would probably consume some benthos, possibly depleting benthic production. These changes would lead to a food web where salmon and perhaps also trout become piscivorous (Figure 12.4b and c). This trophic web and a less oligotrophic environment would favor colonization by larger zooplankton, possibly Daphnia. This scenario would convert Chilean ecosystems to resemble typical temperate lakes of the Northern Hemisphere (Soto and Zuniga 1991). This prediction is consistent with the outcome of salmon invasions in other lakes. Both species diversity and the diversity of ecosystem types will diminish. On the other hand, the establishment of pisci-
PERCICHTHYS TRUCHA (nalive "trout")
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