Microzooplankton grazing and phytoplankton growth in salp-dominated subantarctic waters in austral summer 2018: results from seawater dilution experiments omitting salps, competitors and predators of microzooplankton

The fate of phytoplankton in {\it Salpa thompsoni}-dominated open ocean subantarctic waters to the south of the Chatham Rise was explored in Oct-Nov 2018 using seawater dilution experiments to quantify the grazing impact of microzooplankton consumers. Grazer dilution experiments were conducted daily at 6 depths (with no salps in the bottles) and incubated in situ during Lagrangian experiments in water parcels with different stages of a salp bloom, or in waters with no salp bloom. At the start and end of each 24 h incubation, unpreserved samples were analyzed by flow cytometry. Euphotic zone integrated production of {\it Synechococcus} (SYN) ranged from 106–306 mg C/m2/d at salp stations, and 319–471 mg C/m2/d at no salp stations. Despite lower production at salp stations, SYN showed no consistent pattern of net production (production minus micrograzer mortality) relative to salp presence. Picoeukaryotic phytoplankton (PICO, $<$2$\mu$m) production was higher at salp relative to no salp stations (71–240 vs. 26-38 mg C/m2/d, respectively). PICO net production followed the same pattern. This suggests that consumers of PICO decreased at salp-containing stations, likely due to their consumption by salps. Results of separate shipboard experiments showed the smallest phytoplankton consumed by {\it S. thompsoni} are nano-phytoplankton (NANO, 2-20 $\mu$m). NANO production was similar at salp-containing and salp-absent stations (71–287 vs. 19-200 mg C/m2/d, respectively). However, NANO showed higher net production at salp stations when diatoms dominated phytoplankton biomass, perhaps due to preferential grazing of diatoms over NANO by salps. NANO net production was lower where diatoms were a small fraction of the total biomass, with or without salps present. Overall, the main impact of salp grazing appears to be on larger cells.