Phytoplankton community of an urban river during a flood event (Salado, Argentina)

GARCÍA DE EMILIANI, M.O.; DEVERCELLI, M.

Ilhéus, Brasil

Congreso; X Congreso Brasileiro de Limnologia; 2005

Sociedad Brasilera de Limnología

Abstract The Salado is a saline, eutrophic and turbid tributary river of the Middle Paraná River system, affected by human activities. The phytoplankton and environmental factors of a site, located at the center of the channel (near to the mouth of the river, between Santa Fe and Santo Tomé cities), were monthly studied during a period of positive hydrological anomalies (February-December 1998), induced by El Niño-Southern Oscillation (ENSO). Phytoplankton (abundance and composition) and physico-chemical data (water level, temperature, pH, conductivity, water transparency, dissolved oxygen, and nutrient concentrations) were analyzed, in comparison with those obtained at different water level during a normal hydrological cycle (July 1993-March 1994). On average, from the flood to the normal period, hydrometric level (7.1 and 3.8, daily measurements), pH, conductivity, dissolved oxygen and phosphates decreased significantly, whereas temperature and nitrates were similar. During the ENSO, phytoplankton density and chlorophyll-a maintained low values (mean: 578 ind. ml-1 and 2.47 mg m-3, respectively), and they were inversely correlated with water level and conductivity (p<0.01). The classes Cryptophyceae (mean relative contribution: 42%) and Chlorophyceae (35%) were dominant and sub-dominant, respectively. During normal conditions, phytoplankton density and chlorophyll-a had significant higher mean values (6773 ind. ml-1 and 15 mg m-3); Chlorophyceae was dominant (57%), while Cryptophyceae, Cyanophyceae, and Bacillariophyceae (12%, 10% and 9%, respectively) shared the subdominance. Chlorophyceae and Bacillariophyceae had inverse correlations with water level and conductivity (p<0.01). During the major inundation (Feb.-May 1998), as well as in the following 4 months of decreasing water level, the phytoplankton community showed the most important changes, while at the end of the period (Sept.-Dic.1998) phytoplankton density and structure were similar to those found during normal period. Temporal variations of phytoplankton during the flood event mainly depended on Cryptomonas and Rhodomonas minuta. Chloroccocales (especially, Monoraphidium and Scenedesmus) predominated over Volvocales. Contrarily, Chlamydomonas and Pyramimonas minima were the main contributors to algal density at lower water levels (inversely correlated with water level, p<0.01), followed by Chloroccocales (mainly, Oocystis and Monoraphidium). Centric diatoms (principally Cyclotella) dominated over pennates in both periods. Skeletonema cf. subsalsum, Fragilaria construens, Aulacoseira granulata and A. distans were registered during the ENSO, probably due to the inflow of the Paraná River water, while Thalassiosira weisflogii and Chaetoceros muelleri were observed at lower water stages. The PCA, based on the relative density of selected algal groups for the whole data set, reinforce the mentioned structural change according to hydrological conditions. The results revealed an important impact of the ENSO on the phytoplankton community and water quality. In general, the changes responded to water dilution and, in lower magnitude, to the inflow of water from the Paraná River and inundated areas of the Santa Fe city. As consequence of the dilution, phytoplankton density was noticeably lower during the flood period, but the same algal species remained in the river, with the exception of a few taxa. The structural change was important, mainly due to Cryptophyceae, which are better adapted to growth under conditions of greater flow heterogeneity. The sporadic occurrence of potentially harmful Cyanophyceae during warm and low water reflected the recruitment of algal population from storage zone. To conclude, it is possible to support the view that the extreme flood acts as a severe disturbance on the phytoplankton of the Salado River, and its typical characteristics are progressively recovered at the end of the study period, when the water level return to normal fluctuations.