The effects of water chemistry on fish species distribution : a comparison between isolated and connected lakes in northern Sweden

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The correlation between water chemistry and fish species distribution was examined in forty small, coastal lakes in northern Sweden. Lakes were fished in summer and water chemistry sampled mainly in late winter. Twenty out of the forty lakes were isolated from other water bodies and twenty had a direct connection to the sea. The central question of the study was whether water chemical variables play a greater role in isolated lakes, since these cannot be recolonized if a species once has become extinct. As much as 48 % of the variation in fish species distribution in isolated lakes was explained by methane and acidity. Methane is an indicator of anoxia and not a controlling factor in itself. In connected lakes, lake area was the only significantly contributing factor, explaining 12 % of the variation. Different fish species were controlled in their distribution by different chemical variables. Methane indicated that the distribution of pike and perch was controlled by anoxia in isolated lakes. The distribution of the anoxia-tolerant crucian carp and sticklebacks was instead controlled by predation and acidity. In connected lakes, the possibility of migration decreased the effect of anoxia radically making pike and perch common, depressing the distribution of crucian carp. A third group o...