Is the untreated sewage discharge affecting the zooplankton community?: a case study in a turbid estuary (Bahía Blanca, Argentina)

DUTTO, M. S.; LOPÉZ ABATE, M. C.; BIANCALANA, F.; BERASATEGUI, A. A.; HOFFMEYER, M. S.

Pucon, Chile

Simposio; 5th International Zooplankton Production Symposium; 2011

Is the untreated sewage discharge affecting the zooplankton community?:a case study in a turbid estuary (Bahía Blanca, Argentina)M. SofíaDutto, M. CelesteLópez Abbate, FlorenciaBiancalana, AnabelaA. Berasateguiand Mónica S. HoffmeyerInstituto Argentino de Oceanografía (CONICET-UNS). La Carrindanga Km 7.5B8000FWB,Bahía Blanca, Argentina. E-mail: msdutto@criba.edu.arThe Bahía Blanca Estuary receivesthe untreated sewage from a growing urbanpopulation. To study whether the contribution of allochthonous organic matter affects themesozooplankton community, surface tows were made in the channel which receives thesewage input(Canal Vieja, CV) and in a control site(Bahía del Medio, BM). Samplingswere made during summer and winter 2009. As expected, the disturbedsite (CVpresented significantly higher values of nutrients (except silicates), ammonium andtemperature and lower ones of pH, dissolved oxygen and salinity. Phaeopigments werealways higher in BM. Althoughboth sites presented similar zooplankton compositionpatterns, dominance was different. The native copepod Acartia tonsa dominated duringsummer in BM showing there its maximum population value (9,610 ind.m-3) coinciding withhistorical data. In CV, A. tonsa was codominat with Neohelicegranulata zoea crabs whichpresented  its maximum  abundance  (1,383  ind.m-3).  Observations  on  themicroheterotrophic community determined the existence of the typical assemblages andhigher abundances in BM,and peculiar maximum abundances of aloricate ciliatesespecially Lohmmanniella sp,in CV. The high turbidity coupledwiththe contributions othe vast extensions of surrounding salt marshes makethat this well mixed systempresents an elevated basal eutrophication level. Therefore, zooplankton would beadaptedto high organic matter content. Nevertheless, certain differences in planktonic variablesbetween the sites could be registered. This might be the first biological signs of anallochthonous eutrophication process. The geomorphology and circulationpatternsof thisestuaryare probably minimizing the effect of theuntreated sewage discharge.