Seasonal and spatial distribution of the microbenthic communities of the Rio de la Plata estuary (Argentina) and possible environmental controls

Relationships between the spatial distribution of oligochaetes (Annelida, Clitellata) and environmental variables in a temperate estuary system of South America (Río de la Plata, Argentina)

“…These occurrences might well be related because the two naidine species belong to the functional feeding group known as the gatherer-collectors (Barbour et al 1999), and the producers could be used by them as a food source. Consistent with the dynamics of this lotic system and with the logic of this putative relationship, the peak in abundance of these naidids was recorded some 15 km downstream from the site where Go´mez et al (2009) had cited the maximum cell density of those producers.…”

“…In the area studied, we found two gradients*one determined by anthropogenic influences related to pollution and eutrophication and the other related to turbidity and conductivity*varying continuously, but reciprocally, over the length of the river; but with the latter gradient being associated mainly with its tidal dynamic as an estuary (also cf. Go´mez et al 2009). From all the variables analysed, the most influential ones in determining the physical and chemical conditions that affected the spatial distribution of the oligochaetes were the turbidity, conductivity, dissolved oxygen, pH and nitrate levels.…”

“…The higher densities registered for these two species corresponded to those sites with a particularly high contribution of organic matter because of their proximity to sewage discharges (Sites 5 and 8). Go´mez et al (2009) had studied the community of producers (cyanophytes and diatoms) in the same area of the river and during the same seasons as in this work. What would appear to be cause-andeffect relevance is that the peak in the abundance in the two oligochaete species registered here occurred about 4Á5 months after the authors had recorded the peak in cell density of the producers.…”

“…In this way, organisms known to be tolerant (Barbour et al 1999)*such as N. variabilis, N. communis, L. hoffmeisteri and the Enchytraeidae*were abundant along the river coast, from the more polluted localities (such as Sites 4, 5 and 8) to those considered as reference locations (Sites 9, 10 and 11), although in the latter stations these indicator species were found to be accompanied by others such as C. diastrophus, P. frici and S. trivandrana. As pointed out by Go´mez et al (2009), descriptive investigations on the spatial and seasonal patterns with respect to the composition of environmentally informative species are useful and essential for ecologicalÁmanipu-lative studies. Our intention in characterising the fauna of oligochaetes and identifying their species-specific responses to environmental variables was to enable a more accurate understanding of those influences that determine their composition within this estuary.…”

“…This section of the estuary system corresponds to the geomorphologic unit known as Franja Costera Sur and is a freshwater zone (conductivity values B5000 mS/cm), though subjected to the influence of the Atlantic Ocean tides. The following littoral environments with different types of ecologic disturbances (after Go´mez et al 2009) were selected: Sites 1, 2 and 3, corresponding to highly urbanised areas with navigational and port activities as well as receiving domestic and industrial effluent discharges; Site 4, constituting a highly impacted area where industrialeffluent discharges predominate; Site 5 in close proximity to the sewage discharge of Buenos Aires city; Sites 6 and 7, protected and recreational areas, respectively, but influenced by poor water quality from human activities upstream; Site 8, located close to other major sewage discharges; and Sites 9, 10 and 11, consisting in areas only slightly impacted (thus considered here as 'reference sites')* where Site 11, for its part, is the closest to the maximum turbidity front. Site 4 was sampled only during the first two seasons because of coastal alterations stemming from urban development.…”