If one attempts to develop an entirely artificial medium for the culture of freshwater invertebrates, as has been done successfully for planktonic microcrustaceans, a long list of chemicals must be included (D'Agostino and Provasoli
1970). If one holds mayfly larvae in distilled water, 50% mortality is exceeded within a few days (Willoughby and Mappin 1988). Unquestionably, stream-dwelling organisms require water of some minimal ionic concentration. Unfortunately, there has been little effort to establish these requirements experimentally. The majority of the evidence linking the ionic content of water to the stream biota comes from surveys. Often such studies establish that stream-water of very low ionic concentration has a restricted flora and fauna, in both abundance and species richness. Studies may be reported in terms of water hardness, alkalinity, conductivity, or measurements of specific ions. Hynes (1970) describes a number of examples where the species of algae, mosses, and higher plants differ between soft versus hard waters. Among the invertebrates, it appears that mollusks, crustaceans, and leeches are more responsive to the range of ionic concentrations than are aquatic insects. The amphipod Gammarus apparently is common in streams of the English Lake District that have at least 3 mg L"1 calcium, and rare in streams of lower concentrations. A number of molluscan surveys have described the particular species that occupy soft waters, and reported a positive correlation between hardness and species richness. According to Russell-Hunter et al. (1967), roughly 5% of the molluscan species of a region will occur in extremely soft waters (<3mgL 1 calcium). Moderately soft waters (<10mg L"1) will support perhaps 40% of the species of a region, intermediate waters (10-25 mg L"1) will support up to 55%, and hard waters (>25 mg L"1) are needed to include all the molluscan species of an area. Shell growth of juvenile zebra mussels was strongly correlated with water buffer variables of calcium, alkalinity, and hardness, and corroborating field evidence suggests that this invasive species in North America will be unsuccessful at less than ~10 mg L"1 calcium and most successful at >25 mg L"1 (Hincks and Mackie 1997). Such a dependence on calcium availability is expected,
because aquatic mollusks derive a large fraction of their considerable calcium needs by absorption directly from the external medium. CaCO3 is necessary for shell deposition and growth, and calcium is important in general fluid and electrolyte balance.
Alkalinity is widely used as a surrogate for stream fertility and is thought to influence the productivity of fish populations via their food supply (Cooper and Scherer 1967). Compiling data on salmonid production and stream alkalinity from numerous studies in the United States, Kwak and Waters (1997) demonstrated a statistically significant but nonetheless imperfect relationship (Figure 4.6). However, their own data from 13 southeastern Minnesota streams did not show any dependence of salmonid production on stream alkalinity, which the authors suggest may be due to the modest range of alkalinity values among sites as well as the influence of additional variables including temperature and biological interactions.
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