ABSTRACT:The relationship between the trophic state of 39 reservoirs located in the semi-arid region of north-east Brazil and their planktonic structure was examined by a sampling survey during the 1998 drought (September to November). A total of 30 environmental and biological parameters, with special attention paid to phytoplankton, were studied once at each reservoir to obtain a synoptic view at a state scale (Pernambuco State). The application of the chlorophyll a-total phosphorus standard relationship for grades of eutrophication revealed a classification in which 90% of the sites were eutrophic-hypertrophic. Among the 202 taxa identified in the phytoplankton assemblages, the cyanobacterium Cylindrospermopsis (Wolsz.) Seenayya et Subba Raju represented more than 80% of the total phytoplankton density in 10 reservoirs and more than 50% in 17 reservoirs. The rarity of other phytoplanktonic groups in the case of the Cylindrospermopsis dominance is the most significant biological feature of these reservoirs. Although rotifers were more closely linked to hypereutrophic conditions than copepods or cladocerans, this dominant group did not seem to play an efficient role in cyanobacterial control. A statistical approach based on the analysis of cross tables (co-inertia analysis) allows us to describe species-environment relationships with a special focus on the factors governing the presence of Cylindrospermopsis. As determined by hierarchical analysis, a major group of studied reservoirs (70%) corresponded to deeper, warmer and hypertrophic reservoirs characterized by the combination of (1) low ammonia concentration, high temperature and pH values, and (2) high percentages of Cylindrospermopsis abundances. Lack of water renewal in 1998 linked to the 1997 El Niño consequences seems to be a major determinant for both trophic conditions and Cylindrospermopsis dominance in the reservoirs studied.KEY WORDS: Bacteria · Phytoplankton · Zooplankton · Cylindrospermopsis · El Niño event · Reservoirs · Co-inertia analysis Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 23: [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] 2000 Cyanobacteria are recognized as a critical problem worldwide and their blooms are typical phenomena in eutrophic lakes, often linked to external nutrient enrichment (Paerl 1988). The influence of local climatological factors (e.g. wind, light intensity, temperature) can accelerate these phenomena and modify the phytoplankton structure, as cyanobacteria are strongly driven by physical factors such as local weather conditions (e.g. Tóth & Padisák 1986, Fabbro & Duivenvoorden 1996. Other factors that potentially influence the biomass and composition of phytoplankton include a stable water column with a turbulence limitation defined as a high ratio between the mixing zone and the euphotic zone (Soranno 1997). In addition, cyanobacteria are inedible prey for many zooplankton organisms because of their large colony size, their toxic effects and low...

The species composition and the seasonal succession of phytoplankton were analyzed in a eutrophic drinking water reservoir located in the semi-arid region of northeast Brazil. Investigations were based on bimonthly or monthly sampling over 2 yr (1997 to 1998) conducted at 1 representative station with 2 sampled depths (0.5 and 5 m near the bottom). Limnological parameters (temperature, pH, dissolved oxygen, conductivity, light, dissolved inorganic nutrients) were simultaneously measured to determine the possible factors affecting the phytoplankton composition. We determined 30 taxa during the survey which were numerically dominated by the class Chlorophyceae. However, both in terms of abundance and biomass, Cyanobacteria dominated the phytoplankton community with Cylindrospermopsjs raciborslili (Wolsz.) Seenayya et Subba Raju. This species can represent biomass close to 96-100% of total phytoplankton biomass, with values reaching 70 mg 1-' (fresh weight) between April and November 1998. Over the survey, the filaments of C. raciborskii were coiled (average of 97 %) with a mean proportion of 12.3 % of terminal heterocytes. The species toxicity was determined from bioassay analysis and the presence of neurotoxins was revealed during the bloom. By March 1998, chlorophyll concentration reached 135 pg I-' at the surface level, inducing a sharp decrease of the euphotic zone depth. Favorable environmental conditions were observed for the bloom with high temperatures, high pH, low N/P ratio, and absence of efficient predators. The nutrient context seemed to play a role in the cyanobacterial bloom despite the absence of external nutrient supply. However, annual rain deficit and lack of water renewal in 1998 linked to the 1997 El Nifio consequences seem to be the major factors responsible for both hypereutrophic conditions and cyanobacterial blooms in the reservoir studied. Thus, global climate change can influence phytoplankton population dynamics in continental waters, as demonstrated frequently in oceanic ecosystems.

Species composition and seasonal succession of some planktonic components were studied through monthly samplings during 2 yr (1997 and 1998) in Ingazeira reservoir, northeast Brazil. Linked to the severe drought in this region (1997 El Niño event) was the dominance of the toxic filamentous cyanobacterium Cylindrospermopsis raciborskii in the phytoplankton in 1998 (96 to 100% of total phytoplankton biomass), with small proportions of heterocytes (12% of filaments). A great part of the variability of the particulate organic carbon (R 2 = 83.9%) was explained by changes in the C. raciborskii carbon biomass. A more significant change in bacterial communities was observed in the post-bloom phase when biomass increased due to the appearance of larger sizeclasses of cell volume. This bacterial size structure may be the consequence of a strong pressure by bacterivores. Among the zooplanktonic groups, rotifers were numerically more abundant throughout the survey, but microcrustaceans, especially the copepods, contributed the highest proportion of the biomass. Despite the low edibility of C. raciborskii (large trichomes; mean of 97 µm, n = 204), zooplankton diversity increased during and after the bloom (March to December 1998). Our data suggest that rotifers and copepods were able to cut up and shorten the filaments to edible size for other zooplankton species, especially the small-bodied herbivorous cladocerans. Thus, in the studied ecosystem, the heterotrophic micro-organism community appeared to be able to develop a strategy to cope with a dominant and relatively inedible algal food source. KEY WORDS: Bacteria · Phytoplankton · Zooplankton · Cylindrospermopsis raciborskii · Reservoir · BrazilResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 25: 215-227, 2001 competing algae that are more susceptible to grazing (Gragnani et al. 1999). Such selective grazing is often mentioned as an explanation for a shift to cyanobacterial dominance following the spring peak of zooplankton (Sarnelle 1993). Furthermore, the absence of large cladocerans such as Daphnia in tropical ecosystems facilitates the development of many cyanobacterial communities (Lazzaro 1997). The effect of toxins on different components of the microbial food web has been largely ignored in ecological studies (Christoffersen 1996, Sommaruga & Robarts 1997), but they may affect several processes such as bacterial grazing by protists (Paerl & Pinckney 1996). On the other hand, the cyanobacterial products can have a potentially important role in regulating bacterial development, as evidenced by the direct coupling existing between bacterioplankton abundance and cyanobacterial density (Wang & Priscu 1994, Worm & Sondergaard 1998.Although the filamentous cyanobacterium Cylindrospermopsis raciborskii can often dominate phytoplankton assemblages (Padisák 1997), only a few investigations concern its impact on the other pelagic components (Hawkins & Lampert 1989, Rothhaupt 1991, Branco & Senna 1996, Mayer et a...

This study describes, for the first time, the water chemistry and microbial diversity in Dziani Dzaha, a tropical crater lake located on Mayotte Island (Comoros archipelago, Western Indian Ocean). The lake water had a high level of dissolved matter and high alkalinity (10.6–14.5 g L-1 eq. CO32-, i.e. 160–220 mM compare to around 2–2.5 in seawater), with salinity up to 52 psu, 1.5 higher than seawater. Hierarchical clustering discriminated Dziani Dzaha water from other alkaline, saline lakes, highlighting its thalassohaline nature. The phytoplankton biomass was very high, with a total chlorophyll a concentration of 524 to 875 μg chl a L-1 depending on the survey, homogeneously distributed from surface to bottom (4 m). Throughout the whole water column the photosynthetic biomass was dominated (>97% of total biovolume) by the filamentous cyanobacteria Arthrospira sp. with a straight morphotype. In situ daily photosynthetic oxygen production ranged from 17.3 to 22.2 g O2 m-2 d-1, consistent with experimental production / irradiance measurements and modeling. Heterotrophic bacterioplankton was extremely abundant, with cell densities up to 1.5 108 cells mL-1 in the whole water column. Isolation and culture of 59 Eubacteria strains revealed the prevalence of alkaliphilic and halophilic organisms together with taxa unknown to date, based on 16S rRNA gene analysis. A single cloning-sequencing approach using archaeal 16S rDNA gene primers unveiled the presence of diverse extremophilic Euryarchaeota. The water chemistry of Dziani Dzaha Lake supports the hypothesis that it was derived from seawater and strongly modified by geological conditions and microbial activities that increased the alkalinity. Dziani Dzaha has a unique consortium of cyanobacteria, phytoplankton, heterotrophic Eubacteria and Archaea, with very few unicellular protozoa, that will deserve further deep analysis to unravel its uncommon diversity. A single taxon, belonging to the genus Arthrospira, was found responsible for almost all photosynthetic primary production.