Microzooplankton and phytoplankton of Ross Sea polynya areas and potential linkage among functional traits

The Ross Sea is characterized by a series of subsystems with different characteristics making it an extremely productive area from a biological viewpoint. To understand whether species composition and functional traits of the plankton community can be used as biological tracers of different subsystems, we have analyzed the composition of phytoplankton and microzooplankton, and their potential relationships, in two different polynyas of the Ross Sea during the austral summer 2017. Sampling activities were carried out near Terra Nova Bay, between Cape Washington and the northern shore of the Drygalski Ice Tongue, and in the South-Central Ross Sea. We investigated the phytoplankton and microzooplankton communities’ structure using the phytoplankton body size classes and the tintinnids lorica oral diameter as functional traits, speculating on the relationship between the two plankton communities and their use as biological indicators in a changing Southern Ocean. Our data showed significant differences in terms of plankton taxonomic composition and related functional traits between the two areas, suggesting the existence of distinct ecological dynamics despite the similar total carbon content. In Terra Nova Bay, heterotrophic dinoflagellates were the most abundant microzooplankton organisms, in association with a large amounts of phytoplankton biomass mainly represented by diatoms and size classes nano- and micro-phytoplankton. Tintinnids with large lorica oral diameter were abundant in Central Ross Sea, where phytoplankton was dominated by Phaeocystis antarctica and by the micro size class. Among microzooplankton, Protoperidinium defectum, P. applanatum and P. incertum were the most abundant dinoflagellates species, while Codonellopsis gaussi, C. gaussi forma cylindroconica, Laackmanniella prolongata and Cymatocylis drygalskii were the most abundant tintinnids species. The phytoplankton community was dominated by diatoms Pseudo-nitzschia subcurvata, Fragilariopsis cylindrus, F. curta and by the haptophyte P. antarctica. The overall picture emerging from our data indicates that beyond physical and chemical features defining distinct sectors of the Ross Sea, both species composition and functional traits of phytoplankton and microzooplankton represent a valid monitoring tools in such a complex system, especially in light of the ongoing global warming and its amplified effects on Antarctic trophic chains.