Assessing multimetric trophic state variability during an ENSO event in a large estuary (Río de la Plata, South America)

Abstract During the “El Nino”- Southern Oscillation (ENSO) event 2009–2010, trophic state uni (TSI’s) and multimetric (TRIX’s) indexes were determined in the north coast of the Rio de la Plata (RdlP). The relationships of such indexes with the hydrological, physical and chemical parameters were explored to identify climatic variability or anthropogenic effects on the eutrophication process. Eleven oceanographic sampling campaigns at 25 stations were conducted along the north coast of RdlP from 2009 to 2011. Temperature, salinity and oxygen saturation, water transparency (Secchi depth), chlorophyll (Chl a ) content and total nutrient concentrations (total nitrogen and phosphorous) were determined. Trophic indexes (TSI Chl a , TSI TP, TRIX NP-Voll. and TRIX NP-Mvdeo.) were quantified and the RdlP flow was measured. RdlP flow showed significant differences between ENSO phases, with higher values during “El Nino” (EN), negative and significant correlations with salinity, Secchi depth and positive with total nutrients. The trophic status indexes showed spatial and temporal variability and associations with flow, physical and chemical parameters. Trophic state indexes showed spatial and temporal heterogeneity and were strongly associated to environmental conditions (hydrological and seasonal oscillations) during the study period but also to the anthropogenic factors affecting the north coast of RdlP. The system has been classified as mesotrophic to hypertrophic, depending on the selected indicator. According to these results, we suggest the use of a multimetric trophic state indicator, i.e., the TRIX index, as explains better the causes and effects of the eutrophication processes. To implement the TRIX index, (TRIX NP-Mvdeo.) we suggest researchers to consider a large number of local records (Chl a , nutrients, oxygen) at seasonal scales. This index is adapted to the environmental conditions of the study zone and complemented with other community structure indexes, and should be used for eutrophication assessment programs in the north coast of RdlP.