Relative nitrogen deficiency without occurrence of nitrogen fixing blue-green algae in a hypertrophic reservoir

Abstract

The N:P ratios (TN:TP, NO₃-N:TP and DIN:SRP) in the shallow, hypertrophic Marcali reservoir varied about 1, far below the physiological optimum for the planktonic algae. Moreover, N₂-fixing cyanobacteria were absent in the vegetation period. One possible explanation for this phenomenon could be the filtration of the stocked silver carp (Hypophthalmichthys molitrix Val.), a phytoplankton feeder, and bighead carp (Aristichthys nobilisRich.), an omnivorous plankton feeder population (600 kg ha^–1) which strongly reduces the large sized algae.

Enclosure and uptake experiments were carried out to confirm the relative N deficiency in the reservoir and, by excluding the fish, to check whether the reservoir water is suitable for the development of N₂-fixing cyanobacteria, or the planktivorous fish are responsible for the lack of blue-greens. The enclosures were free of fish. In one of them the NO₃-N enrichment resulted in a rapid growth of diatoms and cryptophytes, confirming the relative N limitation of the phytoplankton biomass. The results of ¹⁵N uptake experiments showed a high preference of algae for ammonium. Its actual uptake velocity (19.5 μg 1^–1 h^–1) was much higher than that for nitrate (1.0 μg1^_1 h^_1). But the relatively high value of the half-saturation constant (32 μg 1^_1) in the ammonium uptake experiment indicated only a moderate affinity of algae even to the preferred nitrogen source. The other enclosure was inoculated with N₂-fixing cyanobacteria. During the 29 days experiment they did not grow in the enclosure and were gradually eliminated from the water column. The results of enclosure experiments suggested that factors such as the relatively high abundance of inorganic N (30–50 μg 1^_1 ammonium and 90–140 μg 1^–1 nitrate, respectively) could be responsible for the absence of N₂-fixing cyanobacteria in the reservoir, rather than the size selective filtration of planktivorous fish.