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Abstract We investigated competition betweenSalpa thompsoniand 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 μm 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 > 1.4μm (high end of picoplankton size, 0.6–2 μm cells) and also obtained substantial nutrition from smaller bacteria‐sized cells. Larger salps could only feed efficiently on > 5.9μm 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:urn:x-wiley:00243590:media:lno11770:lno11770-math-0001where ESD is prey equivalent spherical diameter (µm), TL isS. thompsonitotal length,φ = 5.6 × 10⁻³ ± 3.6 × 10⁻⁴,ψ = 2.1 ± 0.13,θ = 0.58 ± 0.08, andγ = 0.46 ± 0.03 and clearance rate is L d^‐1salp^‐1. We discuss the biogeochemical and food‐web implications of competitive interactions among salps, krill, and protozoans. 

Abstract Polar marine ecosystems are particularly vulnerable to the effects of climate change. Warming temperatures, freshening seawater, and disruption to sea‐ice formation potentially all have cascading effects on food webs. New approaches are needed to better understand spatiotemporal interactions among biogeochemical processes at the base of Southern Ocean food webs. In marine systems, isoscapes (models of the spatial variation in the stable isotopic composition) of carbon and nitrogen have proven useful in identifying spatial variation in a range of biogeochemical processes, such as nutrient utilization by phytoplankton. Isoscapes provide a baseline for interpreting stable isotope compositions of higher trophic level animals in movement, migration, and diet research. Here, we produce carbon and nitrogen isoscapes across the entire Southern Ocean (>40°S) using surface particulate organic matter isotope data, collected over the past 50 years. We use Integrated Nested Laplace Approximation‐based approaches to predict mean annual isoscapes and four seasonal isoscapes using a suite of environmental data as predictor variables. Clear spatial gradients in δ¹³C and δ¹⁵N values were predicted across the Southern Ocean, consistent with previous statistical and mechanistic views of isotopic variability in this region. We identify strong seasonal variability in both carbon and nitrogen isoscapes, with key implications for the use of static or annual average isoscape baselines in animal studies attempting to document seasonal migratory or foraging behaviors.