Search

Regarding the lack of studies about lotic eutrophication, mainly for tropical rivers, thisresearch aimed to assess the density of phytoplanktonic community of two tropical riverslocated in Ribeira do Iguape Watershed (São Paulo state, Brazil), Canha and Pariquera-AçuRivers, based on sampling campaigns in January and April, 2007. These results werecompared to the mTSI (mean Trophic State Index) values. This index was originallydeveloped for lentic aquatic systems and its calculus depends on total phosphorous,orthophosphate and chlorophyll-a concentrations in water samples. Comparing mTSI valuesin all sampling stations with the phytoplankton quantitative results, it was possible to verifythat mTSI may overestimate the trophic state of lotic ecosystems, since the phosphorusspecies that integrate its calculus contributed to an improper augmentation on mTSI, whichwere incompatible with the real associated biological response. Thus, for April sampling,although mTSI values were higher, in general, phytoplankton density were lower (minimumof 83 indmL-1 for Canha River and of 66 indmL-1 for Pariquera-Açu River). In this case, thehigh total phosphorous concentrations by themselves were not enough to promoteproportional phytoplanktonic growth, which was probably limited by other factors, like watertemperature, suspended solids concentration, river discharge and zooplanktonic predation. InJanuary, on the other hand, mTSI values were lower, but total phytoplankton density washigher. For this sampling, the lower orthophosphate concentrations in water pointedphytoplankton assimilation of this nutrient.