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Abstract Gelatinous filter feeders (e.g., salps, doliolids, and pyrosomes) have high filtration rates and can feed at predator:prey size ratios exceeding 10,000:1, yet are seldom included in ecosystem or climate models. We investigated foodweb and trophic dynamics in the presence and absence of salp blooms using traditional productivity and grazing measurements combined with compound-specific isotopic analysis of amino acids estimation of trophic position during Lagrangian framework experiments in the Southern Ocean. Trophic positions of salps ranging 10–132 mm in size were 2.2 ± 0.3 (mean ± std) compared to 2.6 ± 0.4 for smaller (mostly crustacean) mesozooplankton. The mostly herbivorous salp trophic position was maintained despite biomass dominance of ~10-µm-sized primary producers. We show that potential energy flux to >10-cm organisms increases by approximately an order of magnitude when salps are abundant, even without substantial alteration to primary production. Comparison to a wider dataset from other marine regions shows that alterations to herbivore communities are a better predictor of ecosystem transfer efficiency than primary-producer dynamics. These results suggest that diverse consumer communities and intraguild predation complicate climate change predictions (e.g., trophic amplification) based on linear food chains. These compensatory foodweb dynamics should be included in models that forecast marine ecosystem responses to warming and reduced nutrient supply.
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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 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 >1.4-μm (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 >5.9-μm 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 × min((ESD⁄(θ×〖TL〗^γ ))^2/(0.16+(ESD⁄(θ×〖TL〗^γ )) ),1)× 〖Q_10〗^(((T-12°C))⁄10) where ESD is prey equivalent spherical diameter, TL is Salpa 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. We discuss the biogeochemical and food-web implications of competitive interactions between salps, krill, and protozoans.
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- PAR ID:
- 10226604
- Date Published:
- Journal Name:
- Limnology and Oceanography
- ISSN:
- 0024-3590
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/lno.11770
