Assessing the trophic pathways that dominate planktonic food webs: an approach based on simple ecological ratios

It was recently proposed that there is, in the pelagic environment, a continuum of trophic pathways ranging from the herbivorous food web to the multivorous food web, the microbial food web and the microbial loop. It was also suggested that combining specific ecological ratios could provide a way of assessing the dominance of planktonic food webs by specific trophic pathways. Three large data sets, collected in different but adjacent marine systems (the Scotian Shelf, off eastern Canada; the Gulf of St Lawrence; and nearshore waters of the Baie des Chaleurs, in the northwestern Gulf), are used to compute two ratios: small-sized (5 μm) particulate phytoplankton production, and phaeopigments in small- to phaeopigments in large-sized particles. By combining the two ecological ratios it is possible to delineate successfully, in each studied system, coherent periods during the year; to interpret the combined ratios in terms of dominance by specific trophic pathways; and to show that the interpretation is consistent with additional information collected in the three systems.