Abstract

The patchy distribution of the phytoplankton determines biases in the quantitative and distribution results. Relative abundance (RA) represents more appropriately species distribution, especially when the specific densities are low, and provides an insight of species abundance and temporal variations. RA is a semi-quantitative estimator of the abundance for a given species based on net samples and expressed in a nominal scale. It is defined for an ample geographic area and considers interannual variability. RA is referred to a species in particular and not to its relationship to the other species assemblage. RA in comparison to density estimations for four harmful species (Alexandrium catenella, Dinophysis acuminata, D. acuta, Protoceratium reticulatum) and a potentially noxious species (A. ostenfeldii) from southern Chile are presented. Data were collected between 41° to 55° S along 151 fixed sampling sites from May 2006 to May 2010. There is not a straight forward relationship between RA and density although both are statistically correlated. Results have practical meanings for harmful species in their boundary areas of distribution. RA for A. catenella is correlated to PSP toxicity. Also RA for the other species, excluding A. ostenfeldii, is significantly correlated to DSP mouse bioassay results.