Waterbird-Mediated Productivity of Two Soda Pans in the Carpathian Basin in Central Europe

The effect of aquatic birds on nutrient cycling and energy flow was investigated in two soda pans, one turbid and the other colored, with different physical and chemical characteristics. Primary plankton production and respiration were measured together with an estimation of waterbird carbon, nitrogen and phosphorus loading during 2014. Both pans were hypertrophic and showed net heterotrophy. The nutrient loading of the birds in the turbid pan was approximately five times higher (C: 758 kg/ha/year, N: 122 kg/ha/year, P: 20 kg/ha/year) than in the colored pan, with significant guanotrophication. Despite the high chlorophyll a concentrations (turbid: 752 mu g/i and colored: 369 mu g/I, on average), the annual surface-related planktonic production was relatively low (turbid: 64 mg C/m(2)/year and colored: 23 mg C/m(2)/year), by contrast, respiration was similar in the two pans (turbid: 75 C/m(2)/year and colored: 78 mg C/m(2)/year). Nutrient loading showed a significant positive correlation with total and soluble reactive phosphorus, chlorophyll a and gross planktonic production, supporting the conclusion that the waterbirds significantly affected primary production. By contrast, there was no significant correlation between the nutrient loading and planktonic respiration. The low production and respiration ratio (Pro/Res) in the colored pan was presumably caused by a high dissolved organic carbon concentration (polyhumic). A possible explanation for the difference of Pro/Res between the turbid and colored pans is variation in the decomposition of the bird excrement and surrounding macrophytes.