N2-fixing cyanobacteria: Why they do not become dominant in shallow hypertrophic lakes

Summary Phytoplankton species shifts and succession phenomenona in lakes of increasing trophic state were considered, using the basic information on the growth kinetics of the species involved. One of the most obvious signs of advanced eutrophication is the dominance of cyanobacteria (blue-green algae). Striking examples are the shallow, hypertrophic Dutch lakes The Veluwerandmeren (e.g., Wolderwijd and Veluwemeer), whereOscillatoria agardhii, a non-N₂-fixing cyanobacterium, has become dominant over the green algae, diatoms and N₂-fixing cyanobacteria (BERGER, 1975).We have studied the natural population ofO.agardhii during the growing season, by using physiological indicators, and could adduce that the natural population was successively growing under phosphorus, light, or nitrogen limitation (ZEVENBOOM and MUR, 1978a,b; ZEVENBOOMet al., 1982). One might expect that during the period of nitrogen limitation the N₂-fixing speciesAphanizomenon flos-aquae would be favoured and would be able to outgrow the nitrogen-limitedO.agardhii. However, in these lakes,A. flos-aquae was present only in few numbers and a succession fromO. agardhii toA. flos-aquae did not occur. Although field observations may give some indication, they cannot give decisive answers to the question which factor is triggering the observed species shifts and species dominance in natural waters. Such answers can only be obtained from growth kinetic and physiological data of the species involved. In our opinion, the most important factor to consider is the availability of light energy, which decreases with increasing eutrophication.The hypothesis was proposed by Mur and coworkers (MURet al., 1978) that in hypertrophic lakes the prevailing light conditions (low light irradiance) are more favourable forO.agardhii, since this species has a much lower requirement of light energy for growth than green algae as a consequence of its lower specific maintenance rate constant, _e (VAN LIERE, 1979; GONS, 1977). Competition experiments, performed withO. agardhii andScenedesmus protuberans under lightlimiting conditions, confirmed the hypothesis (MURet al., 1978), Continuous culture experiments withA. flos-aquae showed that also this species had a higher energy requirement thanO. agardhii (ZEVENBOOM, 1980). The differences were not found in the value of _e, but in the growth efficiency. The higher energy requirement ofA.flos-aquae was expected, since energy is needed for heterocyst production and N₂ fixation. Under light-limiting conditions and nutrient sufficiency (including nitrogen-nitrate) it can thus be expected that the N₂-fixer will be outcompeted by the non-N₂-fixing cyanobacterium. This was indeed observed (ZEVENBOOM et al., 1981).We further investigated the competitive interactions betweenA.flos-aquae, O. agardhii andS. protuberans under different sets of irradiance values and nitrate concentrations. We used the growth kinetic data of the species involved, which were obtained by means of continuous culture experiments (GONS, 1977; VAN LIERE. 1979; VAN LIERE and MUR, 1979; GONS and MUR, 1980; ZEVENBOOM and MUR, 1980; ZEVENBOOMet al., 1980; ZEVENBOOMet al., 1981). The competing species could be placed along the gradients of light irradiance values and nitrate concentrations, their positions being defined by their energy requirements and half-saturation constants for nitrate-limited growth, respectively. Distinct niches for the three species were found with respect to light and nitrate. Under conditions of low irradiance values and low (realistic) nitrate concentrations, nitrogen-limitedO.agardhii was able to outgrowA. flos-aquae andS. protuberans as a consequence of its low energy requirement and its high affinity for nitrate. The growth rates of the last two species were restricted by the limited availability of light. However, at high irradiance values,O.agardhii was inhibited in its growth rate and therefore failed to outgrow the other two species. The competition was then restricted to nitrogen-limitedS.protuberans and light-limitedA.flos-aquae; the latter could dominate at low nitrate concentrations. The results of competition experiments withO.agardhii andA.flos-aquae under different sets of irradiance values and nitrate concentrations agreed well with the niche-model described above (Zevenboom, unpubl. results).In conclusion, kinetic data of growth, obtained with continuous culture experiments, can provide basic information to explain species shifts and dominance in lakes with increasing eutrophication. Nitrogen-limiting conditions favour N₂-fixing cyanobacteria only when sufficient light is available for their growth (in less hypertrophic waters). The trophic state is thus of major importance and decisive with regard to which species will dominate.