Application of the lognormal equation to assess phytoplankton community structural changes induced by marine eutrophication

A methodological approach was developed for the quantification of the structural changes of phytoplankton communities induced by marine eutrophication. The lognormal equation assigning species abundance to doubling intervals (octaves) of individuals formed the basis of the proposed methodology and the field validation process was based on phytoplankton enumeration and classification data characteristic of eutrophic, mesotrophic and oligotrophic waters. Five octave sets with different sizes were tested for goodness of fit against field data and the set with the smallest size of doubling intervals was selected for further consideration. The application of the lognormal equation was evaluated statistically with field data and it was considered satisfactory at the 87% level. The changes in the shape of the lognormal equation induced by eutrophication were expressed by three characteristic parameters of the equation: the number of the modal octave, the number of species in the modal octave, and the shaping factor. Significant differences were observed for the three parameters among eutrophic, mesotrophic, and oligotrophic waters; the number of the modal octave was high in eutrophic and mesotrophic waters, the number of species in the modal octave has shown a trend of low values under mesotrophic conditions and the shaping factor has shown a considerable increase from eutrophic to oligotrophic waters. Handling editor: L. Naselli-Flores