Abundance, body composition and reproductive output of Gammarus minus (Crustacea: Amphipoda) in ten cold springs differing in pH and ionic content

SUMMARY 1. A survey of thirty‐two rheocrene springs in central Pennsylvania revealed that, Like Gammarus in lakes and streams, Gammarus minus is absent from springs with pH <6.0 and conductivity <25μS cm −1 (total range in pH = 4.6–7.7, and in conductivity = 14–411 μS cm −1 ). 2. In ten springs G. minus density was positively correlated with Ca 2+ and Mg 2+ hardness, but not with pH, unless three springs with either an exceptionally high velocity or extremely high densities of the potentially competing snail Fontigens nickliniana were omitted. 3. Adults were larger in the springs with few or no large predators than in those with more predators. In all ten springs, adult dry mass was unrelated to spring pH and ionic content, but brooding female dry mass covaried positively with Ca 2+ and Mg 2+ hardness in the five predator‐poor springs. 4. Body water, Na and Ca contents and body mass/length ratios varied independently of spring pH and ionic content. Water content was inversely correlated with fat content, but even when expressed as a percentage of fat‐free wet mass, it was unrelated to water chemistry. 5. In juveniles, males and non‐brooding females, fat content varied independently of spring pH and ionic content, but in brooding females it was correlated with alkalinity and Ca 2+ and Mg 2+ hardness. The cost of reproduction in brooding females may have been a factor here; they had significantly lower per cent fat than did non‐brooding females. Juvenile fat content did not differ significantly among spring populations, whereas adult fat content did. The per cent fat of brooding females covaried positively with body size among springs, and this was marginally true for non‐brooding females, as well. The residuals of brooding female per cent fat against dry mass were not related to water chemistry. 6. Brood size (number of embryos in a brood) and brood mass varied significantly among populations, but independently of spring pH and ionic content. Both covaried positively with maternal body size among springs. The residuals of these relationships were unrelated to water chemistry, as was the percentage of females brooding. 7. G. minus from a pH 6 spring survived better and lost less body mass in acidic soft water than did those from a pH 7.6 spring. However, although G. minus has apparently been able to adapt (or acclimate) to pH 6 water it has failed to adapt to more acidic waters. A physiological or structural constraint may be involved because this species has probably had ample opportunity to evolve resistance to dilute acidic water. This hypothesis is consistent with the threshold effect observed: above pH 6 G. minus shows very little evidence of osmotic or metabolic stress, but beiow pH 6 viable populations apparently cannot survive at all. However, the gradual linear decrease in population density of G. minus with decreasing alkalinity and Ca 2+ and Mg 2+ hardness suggests that other factors may also be involved (e.g. a decrease in food quality).