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Phosphorus limitation of algae living in iron-rich, acidic lakes

Very acidic lakes (pH 2.3 to 3.2) have developed in abandoned open cast mines of eastern Germany. Recent research has shown that growth of Chlamydomonas acidophila inhabiting one of these lakes was likely limited by inorganic phosphorus (P). Although most measurements of soluble reactive phosphorus (SRP) concentrations do not identify P as the factor limiting growth (i.e. SRP > 0.1 µmol P l -1 ), the high iron concentrations (up to 10 mmol Fe l -1 ) can decrease the bio- available P fraction by complexation, or can lower P-uptake rates by adsorption processes. To test for P-limitation, 4 lakes (Lakes 107, 111, 113 and 117) were sampled 5 times in 2006 to measure SRP, particulate phosphate, particulate carbon, dissolved organic carbon, chlorophyll a concentration, algal abundance, maximum P uptake rate, and phosphatase activity; P-enrichment experiments were also conducted. All results suggest that algal growth was P limited, at least during some part of the year, even in the lake with the highest SRP concentration (Lake 107: 1 µmol P l -1 ). In P-enrichment experiments, C. acidophila always became the dominant species, suggesting that of all species present, this was the most P-limited. In contrast to expectations, the extent of algal P-limitation in these very acidic lakes could not be explained by SRP and iron concentrations.