Size‐specific grazing and competitive interactions between large salps and protistan grazers

We investigated competition between Salpa thompsoni and protistan grazers during Lagrangian experiments near the Subtropical Front in the southwest Pacific sector of the Southern Ocean. Over a month, the salp community shifted from dominance by large (> 100 mm) oozooids and small (< 20 mm) blastozooids to large (~ 60 mm) blastozooids. Phytoplankton biomass was consistently dominated by nano‐ and microphytoplankton (> 2  μ m cells). Using bead‐calibrated flow‐cytometry light scatter to estimate phytoplankton size, we quantified size‐specific salp and protistan zooplankton grazing pressure. Salps were able to feed at a > 10,000 : 1 predator : prey size (linear‐dimension) ratio. Small blastozooids efficiently retained cells > 1.4 μ m (high end of picoplankton size, 0.6–2  μ m cells) and also obtained substantial nutrition from smaller bacteria‐sized cells. Larger salps could only feed efficiently on > 5.9 μ m cells and were largely incapable of feeding on picoplankton. Due to the high biomass of nano‐ and microphytoplankton, however, all salps derived most of their (phytoplankton‐based) nutrition from these larger autotrophs. Phagotrophic protists were the dominant competitors for these prey items and consumed approximately 50% of the biomass of all phytoplankton size classes each day. Using a Bayesian statistical framework, we developed an allometric‐scaling equation for salp clearance rates as a function of salp and prey size: Clearance ESD = φ ∙ TL ψ × minESD θ × TL γ2 0.16 + ESD θ × TL γ1 × Q 10T − 12 ° C / 10where ESD is prey equivalent spherical diameter (µm), TL is S. thompsoni total length, φ  = 5.6 × 10 −3  ± 3.6 × 10 −4 , ψ  = 2.1 ± 0.13, θ  = 0.58 ± 0.08, and γ  = 0.46 ± 0.03 and clearance rate is L d ‐1 salp ‐1 . We discuss the biogeochemical and food‐web implications of competitive interactions among salps, krill, and protozoans.