Phytoplanktonic associations : a tool to understanding dominance events in a tropical Brazilian reservoir

(Phytoplanktonic associations: a tool to understanding dominance events in a tropical Brazilian reservoir). The aim of this study was to characterize phytoplankton associations, as well as discuss controlling factors determining algal dominance in a eutrophic tropical reservoir, Mundaú, Pernambuco, Brazil. Water samples were collected during the dry period (January/2005) and the rainy period (June/2005). The samples were collected from both limnetic and littoral regions, and the phytoplankton assemblages identified from current literature after preservation in formaldehyde 4%. At the same time as sampling was done, in situ measurements of water temperature, transparency, dissolved oxygen, and pH were also taken. Total phosphorus, total nitrogen concentration and the Trophic State Index were subsequently determined in the laboratory. Phytoplankton density (ind. L-1) was estimated using an inverted Zeiss microscope. Grouping of the phytoplankton associations was carried out using the Reynolds phytosociological classification. During the dry period, reservoir water showed low dissolved oxygen concentrations, alkaline pH and was relatively turbid compared to the situation during the rainy season. Reservoir water is limited by nitrogen during both seasonal periods. The Trophic State Index is classified as determining eutrophic conditions. Phytoplankton was represented by 70 infrageneric taxa grouped in 16 functional associations, with the majority typical of eutrophic systems. This fact is supported by quantitative analysis, which shows the dominance of S associations comprising exclusively R-strategist cyanobacteria.


Introduction
Aquatic ecosystems show spatial and temporal variability promoted by high levels of uncertainty in relation to phytoplankton communities.This variability, a result of biotic and abiotic factors, causes constant reorganization in both relative abundance and composition of phytoplanktonic associations.
Knowledge of phytoplankton population dynamics is relevant because temporal and spatial fluctuations in composition and biomass may be efficient indicators of natural or anthropic alterations in the aquatic ecosystem.Besides, the short generation time of algae (hours or days) makes it possible to understand important processes such as ecological succession, and thus the community provides a useful model for a better understanding other types of communities (Harris 1986;Sommer 1989) and of ecosystems in general (Reynolds 1997).
Several recent studies have correlated phytoplankton composition with environmental factors in an attempt to propose models and patterns for lakes and reservoirs under distinct trophic states.This advance was motivated by a presupposition worked out by Hutchinson (1961), that phytoplankton community behavior is at variance with the Competitive Exclusion Principle, particularly in eutrophic systems.
This hypothesis, known as the Plankton Paradox (Hutchinson 1961), was revised by Reynolds (1988) who proposed a model for phytoplankton community based on survival strategies C, R and S, of algae groups which are clearly distinct but non-exclusive of each other, thus contradicting the ordinary r-K model of MacArthur & Wilson (1967).Reynolds et al. (2002) classified planktonic algae into 31 functional associations, using as a starting point the existing knowledge of species ecology, phylogenetic interactions, and seasonal variation caused by environmental change.In the broader context, phytoplankton associations and their survival strategies are good tools for characterizing and predicting the dynamics of aquatic ecosystems.
The current study aimed to characterize phytoplankton associations in order to determine the causes of algal dominance in drinking water reservoirs in northeastern Brazil.

Materials and methods
The Mundaú Reservoir lies within the coordinates 08°56'47" S and 36°29'33" W at an altitude of 716 m.This reservoir, intended for public water supply, was constructed with a capacity of 1,968,600 m 3 , nowadays receiving part of the urban drainage from Garanhuns, Pernambuco, Northeast Brazil (SHR 2000).Mundaú is a typical shallow tropical reservoir, and is considered to be a eutrophic system.
Phytoplankton samples were collected during the dry season (January 2005) and the rainy season (June 2005), from limnetic and littoral zones, using a plankton net with a 25 µm mesh size.Stations E 1 and E 2 are 9 and 2 meters deep, respectively, the latter being colonized by submerged macrophytes.The samples were preserved in formol 4% and taxonomic identification was done using an optical microscope (Zeiss/Axioskop), to species level or to the highest possible taxonomic resolution using relevant literature for each algal group (Round et al. 1990;Sant'Anna 1984;Komarék & Foot 1983;Komárek & Anagnostidis 1999;Komárek & Anagnostidis 2005).Density (ind.L -1 ) was obtained by Uthermöhl's (1958) method using an inverted microscope (Zeiss Axiovert).Concurrently with biotic variable sampling, in situ measurements of abiotic parameters such as water temperature and transparency, dissolved oxygen, and pH were determined.Total nitrogen (mg.NT.L -1 ) and total phosphorus (mg.PT.L -1 ) levels were determined by Valderrama's (1981) method.The Trophic State Index (TSI) was calculated according to Carlson (1977) using the mean values of total phosphorus.
The definition of the phytoplankton associations was based on the phytosociological classification of Reynolds et al. (2002), whilst survival strategies followed Kruk et al. (2002).Quantitative results were handled through descriptive statistics, by the analysis of data variation amplitude, dispersion around average values, as well as variation between stations and seasonal periods.The variance (ANOVA) was calculated for each sampled station with BioEstat 3.0 (Ayres et al. 2003), using 5% significance.

Results and discussion
Physical and chemical characteristics are shown in Table 1.In the dry period the reservoir had low oxygen, high pH, and relatively turbid water, when compared to the rainy season.In contrast in the rainy season the water was well oxygenated, with a pH ranging from near neutral to slightly acid, and with elevated turbidy, especially near the sediment surface.
Nutrient analysis revealed that there were no differences between sampling stations.However, differences were observed in the concentration of total nitrogen and phosphorus between the rainy and dry seasons.This situation points to possible pluvial transport and land runoff of nitrogeneous material into the reservoir, and to the possible impact of elevated water temperature on phosphorus cycling during the dry season (Tab.1).
A Trophic State Index of over 60 proves that the Mundaú reservoir was in a eutrophic state during the study period (Tab.1).
The reservoir phytoplankton net samples (Tab.3) included seven distinct cyanobacteria associations (Lo, Lm, M, S 1 , S 2 , Sn and Z), four chlorophyte associations (F, J, P and X 1 ), four for phytoflagellates (Lo, W 1 , W 2 and Y) and three diatom associations (B, D and P).
The majority of functional associations found in the Mundaú reservoir is typical of eutrophic systems or in those with a high trophic level (Tab.3), thus explaining the coexistence of several taxons.However, three functional groups (B, Lo and W 2 ) are more  closely related to less eutrophic systems, making them less competitive vis-à-vis the remaining groups.The J, Lm, M and Z associations do not tolerate a reduced photic layer, the F group is sensitive to high pH, while P organisms are not resistant to stratification.These factors could be contributing to the rarity of these taxons in the community.As revealed by both sampling periods, reduced water transparency was a hindrance factor for the C-strategists, which showed a preference for high nutrient rates as well as optimum light availability.The same unfavorable conditions were verified for the X 1 association.
Turning now to the other functional groups, we note that the organisms grouped in S 1 , S 2 and Sn are, generally speaking, non-palatable to zooplankton (Gragnani et al. 1999).In contrast, the organisms in W 1 e Y were the most affected, because they are extremely sensitive to zooplankton predation.Although the conditions were not unfavorable to those from the D association, they gain no advantage from zooplankton occurrence (Reynolds et al. 2002) and, therefore, algae belonging to the D association are likely to be present in the system only as subdominants.Finally, organisms from the Sn association would be favored over the remaining groups because they are better strategists in a high temperature system such as the Mundaú reservoir.
All groups other than the phytoflagellates exhibited statistical differences in density between the seasonal periods at both sampling stations (Tab.4), and dry season densities were always higher than those for the wet season (Fig. 2).Chlorophyta in E 2 was the only one with higher densities in the rainy season.Turbidity is one of the main factors regulating the development of zooplankton communities, with high turbidity thus contributing to the selection of smaller organisms (Pollard et al. 1998).This could be the reason for the greater development of zooplankton palatable algae in the rainy season, particularly those from the W 1 , X 1 and Y associations.
Regarding differences between sampling stations (Tab.5), the cyanobacteria always had higher densities in E 1 , independent of seasonal period (Fig. 2).Other algae groups had statistical differences between stations only in the dry season, with the highest densities occurring in E 2 .The littoral region represents a niche for the coexistence of other taxons, since it generally contains macrophytes that provide a substrate for the colonization of periphytic communities.Sládecková (1962) showed that the organisms in these hosting compartments, particularly in the periphyton, are able to find ideal conditions not only for protection against predators, but also because these compartments constitute a nutrient rich microcosm.Another important factor prevailing in the littoral region is its shallowness, which enables light to reach the sediment surface, while also maintaining good vertical circulation (Tundisi 1990).These factors contributed to the success of light-strategy dependent algae, such as the C-and S-strategists, as well as facilitating the resuspension of diatom frustules.Thus, the littoral region encompasses a spectrum of favorable conditions for the coexistence of Chlorophyta, Euglenophyta, Cryptophyta and Bacillariophyta, from the functional groups F, X 1 , W 1 , W 2 , Y and D, respectively.As far as taxa are concerned, Cylindrospermopsis raciborskii (Woloszinska) Seenayya et.Subba-Raju was dominant with a total density of organisms higher than 50% (Tab.3).This species belongs to the Sn association, thus corroborating qualitative inferences for system dominance events by phytoplankton association studies supported by the Reynolds theory.Other taxons were sub-dominated in the ecosystem with densities higher than 5% (Tab.3).Synedra rumpens Kützing belongs to the D association, and reached densities greater than 20%, particularly in the dry season and in E 2 .This taxon was favored by the shallowness of the littoral region, because it is common in shallow, turbid and eutrophic waters (Reynolds et al. 2002).
Two other cyanobacteria were prominent in the quantitative analysis: Geitlerinema amphibium (Agardh ex Gomont) Anagnostidis, belonging to the S 1 association, and Chroococcus turgidus Kützing, from the functional group Z.The former was abundant throughout the investigative period, with densities higher than 5% (Tab.3).The presence of the functional group S was appropriate to the system's conditions particularly since it tolerates low light regimes (Reynolds et al. 2002), an overriding factor in the community structure.Chroococcus turgidus Kützing, on the other hand, was present at higher density particularly in the rainy season in E 2 , where physical conditions probably played a more important role in controlling the structure of the zooplankton community, as discussed above.
Chlorella vulgaris Chodat was abundant in the rainy season.It belongs to the X 1 association, which is related to eutrophic and shallow systems (Reynolds et al. 2002).Happey-Wood (1988) considers it to be a zooplankton palatable algae, with a tendency to be dominant in turbulent systems.The mixing condition in the rainy season was the driving factor for the development of this association in the system.
Environmental conditions promote the dominance of S associations exclusively constituted by R-strategist cyanobacteria.Therefore, the use of functional associations to study phytoplankton associations was efficient at determining dominance events in the tropical reservoir under investigation.

Table 1 .
Abiotic variables in the Mundaú reservoir during the study period (average values and corresponding standard deviations).

Table 4 .
Results of ANOVA test of significance for the density of algae groups between seasonal periods in the Mundaú reservoir.

Table 5 .
Results from ANOVA test of significance for the density of algal groups between sampling stations in the Mundaú reservoir.