Ecophysiological effects of temporary acidification on crucian carp, Carassius carassius (L.): a case history of a forest pond in eastern Finland

During a dry summer, the pH of a small (0.13 ha), natural forest pond in eastern Finland decreased from > 6.0 to 4.0, returning to normal the following winter. Experimental lowering of the pH, using a strong acid, in a laboratory water-sediment system and in pond water revealed qualitatively similar changes to those in the naturally acidified pond: decreases in water colour, dissolved organic material and total Fe, and increases in total Ca, Mg and Al concentrations. The effects of acidification, together with the simultaneously increased bioavailability of aluminium, were assessed on crucian carp, the only fish species present, by comparison with data from a connected pond that retained its normal pH (> 6.0). The growth in the length of the fish did not differ significantly between the ponds, and the liver glycogen stores were actually slightly larger in the acidic pond. Yet, finds of dead fish after 4 months of acidification, suggested high mortality. In comparison with the carp in the reference pond, the crucian carp in the acidified pond suffered from ionoregulatory imbalance and chronic stress, indicated by decreased plasma chloride, and increased cortisol and glucose concentrations, respectively. In addition, their blood haematocrit and haemoglobin concentration were elevated and the relative size of their livers had increased.