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

• Published in: Hydrobiologia 1997-01, Vol.342-343, p.55-55
• Main Authors: Présing, Mátyás, Balogh, Katalin V, Vörös, Lajos, Shafik, Hesham M
• Format: Article
• Language: English
• Subjects: Algae; Ammonium; Aquatic plants; Aristichthys nobilis; Freshwater; Hypophthalmichthys molitrix; Nitrogen; Nitrogen enrichment; Phytoplankton; Plankton; Reservoirs; Water column
• ISSN: 0018-8158; 1573-5117
• DOI: 10.1023/A:1017044702587

The N:P ratios (TN:TP, NO^sub 3^-N:TP and DIN:SRP) in theshallow,hypertrophic Marcali reservoir varied about 1, far below thephysiological optimum for the planktonic algae. Moreover,N^sub 2^-fixing cyanobacteria were absent in the vegetationperiod.One possible explanation for this phenomenon could be thefiltration of the stocked silver carp (Hypophthalmichthysmolitrix Val.), a phytoplankton feeder, and bighead carp(Aristichthys nobilis Rich.), an omnivorous plankton feederpopulation (600 kg ha^sup -1^) which strongly reduces the largesized algae.Enclosure and uptake experiments were carried out to confirmtherelative N deficiency in the reservoir and, by excluding thefish,to check whether the reservoir water is suitable for thedevelopment of N^sub 2^-fixing cyanobacteria, or theplanktivorousfish are responsible for the lack of blue-greens. Theenclosureswere free of fish. In one of them the NO^sub 3^-N enrichmentresulted in a rapid growth of diatoms and cryptophytes,confirmingthe relative N limitation of the phytoplankton biomass. Theresultsof ^sup 15^N uptake experiments showed a high preference ofalgaefor ammonium. Its actual uptake velocity (19.5 µg l^sup -1^h^sup -1^) was much higher than that for nitrate (1.0 µgl^sup -1^ h^sup -1^). But the relatively high value of thehalf-saturation constant (32 µg l^sup -1^) in the ammoniumuptake experiment indicated only a moderate affinity of algaeevento the preferred nitrogen source. The other enclosure wasinoculated with N^sub 2^-fixing cyanobacteria. During the 29daysexperiment they did not grow in the enclosure and weregraduallyeliminated from the water column. The results of enclosureexperiments suggested that factors such as the relatively highabundance of inorganic N (30-50 µg l^sup -1^ ammoniumand90-140 µg l^sup -1^ nitrate, respectively) could beresponsiblefor the absence of N^sub 2^-fixing cyanobacteria in thereservoir,rather than the size selective filtration of planktivorousfish.[PUBLICATION ABSTRACT]