Plankton and environmental conditions in the inner area of bahía Blanca estuary, Argentina, during one annual cycle

Bahía Blanca estuary, located south-east of the Province of Buenos Aires (approximately 39° SL and 62° WL), Argentina, has a complex dynamics and a high biological productivity that results from its particular geological, hydrological and biological conditions. Furthermore, natural processes co-exist with others of anthropic origin mainly in its inner area.

In the present study, an updated evaluation of phytoplankton and environmental variables was carried out in four sites of the inner area of Bahía Blanca estuary from June, 1996, to April, 1997. Phytoplankton composition, numerical abundance and total biomass were determined. Phytoplankton spatial and seasonal variations were also analyzed in relation to the fluctuation of the following physical and chemical variables: temperature, salinity, transparency, nutrients (nitrates, nitrites, silicates and phosphates), chlorophyll a and phaeopigments, and particulate organic matter. This evaluation was carried out on a bimonth basis.

Fifty six (56) taxa were identified, 75% of which corresponded to Bacillariophyceae group – mainly central diatoms – and the rest to Cyanophyceae, Xantophyceae, Dinophyceae and other phytoflagellates. Typical phytoplankton bloom in the study area began in June and declined towards October. The association of Thalassiosira curviseriata, Thalassiosira anguste-lineata, Thalassiosira pacifica, Thalassiosira rotula and Chaetoceros spp. characterized this bloom. The absence of grazing pressure from herbivorous zooplankton and the previous high availability of nutrients provoked this algal bloom. The maximum value of cholorophyll a (20 ug l-1) coincided with that of phytoplankton density equivalent to 1,310,488 cel l-1. These values correlated to temperatures of the order of approximately 8°C and to a minimum concentration of nutrients resulting from high algal consumption. At this moment, a significant increase of heterotrophic bacteriae and particulate organic matter was registered. The appearance of Chaetoceros spp resting spores announced bloom decline. Finally, the increase of zooplankton grazing pressure and temperature values above 10 °C jointly with the previous low concentration of nutrients stimulated bloom disappearance in September-October. A small pulse generated by Thassiosira cuviseriata, Thalassionema nitzschioides, Cyclotella spp. and Paralia sulcata was registered in February. This phytoplankton pulse which was generated by a recovery of nutrients and a decrease of herbivorous grazing pressure, corresponded to the highest temperatures in summer.

Annual variation ranges of environmental variables were the following: temperature (4.4 – 22.5 °C), salinity (36, 34 – 6.4 ppt), nitrates (0.4 – 26.75 ug at l-1), nitrites (0.09 – 11.51 ug at l-1), phosphates (0.3 – 3.79 ug at l-1), silicates (33.91 – 155.09 ug at l-1), and transparency (0.3 – 1.3 m).

Phytoplankton abundance and biomass values observed in this research were lower than those of previous studies although our results with respect to the latter, show no significant differences in the composition and spatial and seasonal variation