Seasonal changes of the gill epithelia of Arctic char (Salvelinus alpinus) from a high mountain lake

The gill structure of Arctic char from a low ionic (1-2 mu mol Cl(-)) high mountain lake (2344 m above sea level) showed distinct seasonal changes. From spring to early winter chloride cells (responsible for ion uptake) were numerous, and during summer gill epithelia were highly hypertrophic resulting in a thick blood-to-water barrier This gill morphology is typical for fish from soft water habitats and reflects osmoregulatory responses. In mid-winter, however, both chloride cell numbers and epithelia thickness were reduced and comparable to the situation in freshwater with high ionic contents. This pattern correlated with seasonal fluctuations of water chemistry. The most significant correlation was found with CO(2) which accumulated under the ice cover, reaching a tenfold oversatturation. The concentration of Ca, Na, and Cl varied only by a factor of two or less and the correlation with the epithelial thickness was much weaker.