Changes in phytoplankton community along a transversal section of the Lower Paraná floodplain, Argentina

UNREIN, F.

HYDROBIOLOGIA

Springer

Año: 2002 vol. 468 p. 123 - 134

0018-8158

Interactions between the main stream of large rivers and their floodplains are usually observed, determining gradual changes in environmental variables along a transversal axis. This paper studies the natural phytoplanktonic community as well as the main physical and chemical properties from a transversal section of the Lower Paraná River floodplain, encompassing the main channel, a lake and a swamp. In vitro nutrient bioassays for phytoplankton growth were performed in order to evaluate the importance of nutrients as potential limiting factors. A decrease in depth, pH, dissolved oxygen, suspended solids, DIN (dissolved inorganic N), particularly nitrates, and DIN:SRP (soluble reactive P) ratio, was observed from the main channel towards the swamp; as well as an increase in transparency, chlorophyll-a and phytoplankton density. Phytoplankton from the mainstream consisted mostly of typical potamoplanktonic species adapted to grow in turbid environments (mainly Aulacoseira granulata (Müll.) Sim. and its varieties), while in the swamp it was composed mainly of mixotrophic phytoflagellates (euglenoids and cryptomonads), facultative N-heterotrophic diatoms and small-size green algae. Values of both abiotic and biotic variables measured in the lake were always intermediate between the former ones, although closer to those of the river, an account of their permanent connection. Phytoplankton response to N-addition was more frequent in the swamp bioassays than in the other two sites. However, a great increase in algal density in the control flasks, suggests a plentiful source of N at the beginning of the experiments. Light scarcity is more likely to occur than N-limitation in both river and lake due to their low transparency and a high DIN concentration. Although N-limitation may be expected in the swamp, the growth in the controls in spite of the low initial  DIN concentration, suggests the existence of an N-supply other than DIN (perhaps organic N). Rather than indicate in situ N-limitation, present results seem therefore to suggest that phytoplankton of the swamp mainly composed of mixotrophic algae would be adapted to grow under low DIN concentrations. Possible reasons for the absence of N2-fixing Cyanobacteria in this environment are discussed.