[Research on the influence of mixing layer depth on algal growth]

Frequent blooms have been observed in tributary bays of the Three Gorges Reservoir since its initial filling. In order to identify the primary factor controlling water blooms, some experiments were conducted based on the critical depth theory. In the winter of early 2012, the enclosures on Xiangxi River were built to investigate the mechanism of mixing layer depth (Z(mix)) on algal growth. The length of the enclosures under water was changed in gradient, and the water of each enclosures was injected from Xiangxi River, in which the algal concentration was low and the zooplanktons were filtered out, and then the water was mixed evenly. The results showed that the algae grew quickly in the enclosures with Z(eu)/Z(mix) (the favorable ratio of euphotic depth to mixing depth) > 1, and the maximum density of chlorophyll-a (ChI-a) reached 90 mg x m(-3), otherwise the algae grew slowly. The critical depth theory was appropriate for the experiment, and there was a critical value of Z(eu)/Z(mix). Water bloom will not break out if the Z(eu)/Z(mix) is below the critical value. Based on the critical depth theory, it was found that there was a negative correlation between algal net primary productivity and Z(mix), the lower the Z(mix), the greater the net primary productivity.