Taxonomical Composition and Functional Structure of Phytoplankton in Two Water Supply Reservoirs in Serbia - page 05

Reynolds (1984) proposed that the particular phytoplankton community existing in a lake at any given time could be predicted from the environmental conditions prevailing in the water column.

Thus, as a general pattern, the phytoplankton seasonal succession in the reservoir was described by Reynolds (1984, 1997, 1999a, b) for deep, eutrophic and thermally stratified temperate ecosystems, where the most noticeable changes in the phytoplankton community are associated to the development of the water column thermal structure. This general phytoplankton succession followed the sequence B(C) /Y →G → H1→ M → LM (Hoyer et al., 2009).

According to Reynolds (1984), the biomass recruited during the "spring outburst" of phytoplankton in the mixed columns of mesotrophic and mildly eutrophic temperate lakes is generally dominated by diatoms (B, Cgroups) and Y-group cryptomonads, gymnodinians or small peridinians. Diatoms species of group C were dominant during the mixing period, under light-harvesting and nutrient-enriched conditions in these reservoirs. As Reynolds (1984) indicated, as thermal stratification proceeds, the relative benevolence of the recently formed epilimnion offered to the fast-replicating, opportunist, neutrally buoyant chlorophytes species of groups G, F and J an opportunity to accelerate replication and recruitment, thus developing an ephemeral stage which was replaced by positively buoyant, dinitrogen-fixing cyanobacteria species of group H1 as thermal stability and nutrient depletion within the epilimnion proceed. Reynolds (1984) also indicated that it is known that the group H1 Cyanobacteria is very successive in the well-illuminated and nutrient-depleted epilimnion of thermally stratified lakes and reservoirs. Nitrogen deficiency may be cited as a selective factor operating in favor of the common occurrence of H1, H2 and, in warm-water locations, SN groups of nitrogen-fixers. Cyanobacteria of group M are adapted to elevated light intensity, low turbulence, and low nutrient requirements and they were also present in the summer epilimnion. Finally, during the late stratification period large motile dinoflagellates of group LM made advantage of their ability to perform substantial vertical migrations thus exploring both the well-insolated nutrient-deficient upper layers and the dark rich-in-nutrients hypolimnion. However, the deeper are the available resources, the more important is the light-harvesting ability and the less important is rapid motility. Late-summer mixing in temperate, mesotrophic and eutrophic lakes repeatedly selects for P-group diatoms and desmids, and in some instances, for T-group Tribonemaand Mougeotia and, especially, S1-group Planktothrixagardhii (Reynolds, 1984).

Thus, the phytoplankton seasonal succession in the reservoirs Garaši and Bukulja was generally consistent with that described by Reynolds. Our findings confirmed a general pattern of the phytoplankton seasonal succession with some exceptions. Hence, codon C was present with high percentage in total phytoplankton biomass in all studied seasons and in almost all depths in both reservoirs. This is, probably, due to the short-term exogenous external perturbations in the mixing or nutrient environment which may be a result of water management operations.

It is interesting that the codon LM was present in almost every season (except November) at least in one reservoir during this study. There are only a few data about codon LM presence in reservoirs (Naselli-Flores and Barone, 2003; 2005; Hoyer et al., 2009; Hu & Xiao, 2012). In the case of Garaši and Bukulja reservoirs Ceratium hirundinella and Microcystis aeruginosa are found together, but in almost each season which is rarely a situation. Ceratium hirundinella is found in mesotrophic to eutrophic systems. Higher concentration of nitrates, nitrites and phosphorous seems to favour the development of Ceratium hirundinella, while Microcystis aeruginosa is present when the concentration of nutrients is low (Van-Ginkel et al., 2001). The ability to migrate into nutrient-rich regions of the water column give Microcystis a competitive advantage over less motile algae during the time when algal populations in the epilimnion are dense and competition is high. Microcystis does, however, have to compete with other algae such as dinoflagellates, which are also able to migrate into the nutrient-rich hipolimnion, and the seasonal progression of many eutrophic lakes involves summer dominance by either Microcystis and Ceratium. The dominance of Microcystis or Ceratium in a particular season, appears to depend on which population develops first (Sigee, 2005). The origin of the Ceratium hirundinella development was found to have started during the cooler to temperate period (15-25 ºC), while the higher temperatures seem to suit the development of Microcystis (Van-Ginkel et al., 2001).

Usually, codon LO (Ceratium hirundinella) replaced codon M (Microcystis aeruginosa) after summer, so Ceratium hirundinella and Microcystis aeruginosa are rarely found together, which is not the situation in our case. That can be due to both sampling time and environmental conditions which were, obviously, suitable for both species.

These species can cause many problems when they occure in water supply reservoirs. The turbid clay particles discharged from the construction projects in the watershed are unfavorable for buoyant species such as Microcystis because the clay particles are especially effective at attaching to their mucilaginous covering. This would result in flocculation of the Cyanobacteria, thereby favoring chlorophytes. But, Ceratium hirundinella is not affected by the clay particles. Probably, the best way for solving the problem of Ceratium hirundinella and Microcystis aeruginosa presence is biomass harvesting. It is clear that reservoir managers can directly influence the phytoplankton functional structure and for that reason it is important to achieve an adequate limnologically based water quality management.

The trophic status of water is difficult to determine by using the species composition as a single criterium. Still by reviewing the most numerous species found in this study it may be concluded that these reservoirs represent mesotrophic to eutrophic systems. This is in accordance with the findings which have been publishing earlier (Karadžić et al., 2010).



This research was supported by the Ministry of Education, Science and Technological Development, Republic of Serbia, Project No. on 176020.