Ecological role of pelagic appendicularians in sub-Antarctic coastal and oceanic waters of the Southwest Atlantic Ocean

Abstract

Appendicularians are recognized as a prominent mesozooplankton group in pelagic environments due to their abundance and their crucial role in food webs, as they bridge the gap between primary producers and higher trophic levels. Samples were collected during three spring oceanographic surveys conducted in 2014, 2015, and 2016, in the coastal area near Staten Island (SI) and the open sea Marine Protected Area Namuncurá-Burdwood Bank (BB). Surface currents and water propertiessuggest that BB is relatively isolated from the adjacent regions. We conducted a comparative analysis of density, biomass, and maturity stages of two appendicularians species, Oikopleura fusiformis and Fritillaria borealis, in these two areas with contrasting environmental characteristics. O. fusiformis dominated in both areas, while F. borealis was recorded mainly in the coastal. Higher densities of O. fusiformis were associated with higher chlorophyll-a concentrations in SI, as well as with higher density of pico-nanoplankton in the BB area. No correlation was found between the presence of appendicularians and environmental variables, such as temperature and salinity. However, in coastal areas with higher temperatures, smaller size classes were observed. Occasional occurrences of F. borealis on the BB are tentatively related to intrusions of offshorewaters into the bank’s plateau. Nano-picoeukaryotes and bacteria were more abundant in the oligotrophic open sea BB, which could explain the high abundance of appendicularians there. Baseline data of these species in protected areas and surrounding areas are crucial to inform stakeholders and guide responses to future regional and global environmental changes.