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1. Ubiquitous filter feeders shape open ocean microbial community structure and function

   https://doi.org/10.1093/pnasnexus/pgae091  

   Thompson, Anne W; Nyerges, Györgyi; Lamberson, Kylee M; Sutherland, Kelly R (February 2024, PNAS Nexus)

   Bahar, Ivet (Ed.)

   Abstract The mechanism of mortality plays a large role in how microorganisms in the open ocean contribute to global energy and nutrient cycling. Salps are ubiquitous pelagic tunicates that are a well-known mortality source for large phototrophic microorganisms in coastal and high-latitude systems, but their impact on the immense populations of smaller prokaryotes in the tropical and subtropical open ocean gyres is not well quantified. We used robustly quantitative techniques to measure salp clearance and enrichment of specific microbial functional groups in the North Pacific Subtropical Gyre, one of the largest ecosystems on Earth. We discovered that salps are a previously unknown predator of the globally abundant nitrogen fixer Crocosphaera; thus, salps restrain new nitrogen delivery to the marine ecosystem. We show that the ocean's two numerically dominant cells, Prochlorococcus and SAR11, are not consumed by salps, which offers a new explanation for the dominance of small cells in open ocean systems. We also identified a double bonus for Prochlorococcus, wherein it not only escapes salp predation but the salps also remove one of its major mixotrophic predators, the prymnesiophyte Chrysochromulina. When we modeled the interaction between salp mesh and particles, we found that cell size alone could not account for these prey selection patterns. Instead, the results suggest that alternative mechanisms, such as surface property, shape, nutritional quality, or even prey behavior, determine which microbial cells are consumed by salps. Together, these results identify salps as a major factor in shaping the structure, function, and ecology of open ocean microbial communities. 

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

   https://doi.org/10.1002/lno.11770  

   Stukel, Michael R.; Décima, Moira; Selph, Karen E.; Gutiérrez‐Rodríguez, Andres (May 2021, Limnology and Oceanography)

   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. 

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3. Salp blooms drive strong increases in passive carbon export in the Southern Ocean

   https://doi.org/10.1038/s41467-022-35204-6  

   Décima, Moira; Stukel, Michael R.; Nodder, Scott D.; Gutiérrez-Rodríguez, Andrés; Selph, Karen E.; dos Santos, Adriana Lopes; Safi, Karl; Kelly, Thomas B.; Deans, Fenella; Morales, Sergio E.; et al (December 2023, Nature Communications)

   Abstract The Southern Ocean contributes substantially to the global biological carbon pump (BCP). Salps in the Southern Ocean, in particular Salpa thompsoni , are important grazers that produce large, fast-sinking fecal pellets. Here, we quantify the salp bloom impacts on microbial dynamics and the BCP, by contrasting locations differing in salp bloom presence/absence. Salp blooms coincide with phytoplankton dominated by diatoms or prymnesiophytes, depending on water mass characteristics. Their grazing is comparable to microzooplankton during their early bloom, resulting in a decrease of ~1/3 of primary production, and negative phytoplankton rates of change are associated with all salp locations. Particle export in salp waters is always higher, ranging 2- to 8- fold (average 5-fold), compared to non-salp locations, exporting up to 46% of primary production out of the euphotic zone. BCP efficiency increases from 5 to 28% in salp areas, which is among the highest recorded in the global ocean. 

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4. Arctic lagoon and nearshore food webs: Relative contributions of terrestrial organic matter, phytoplankton, and phytobenthos vary with consumer foraging dynamics

   https://doi.org/https://doi.org/10.1016/j.ecss.2021.107388  

   McMahon, Kelton W; Ambrose, William G.; Reynolds, Melinda J.; Johnson, Beverly; Whiting, A.; Clough, Lisa M. (May 2021, Estuarine coastal and shelf science)

   null (Ed.)

   Characterizing energy flow and trophic linkages is fundamental to understanding the functioning and resilience of Arctic ecosystems under increasing pressure from climate change and anthropogenic exploitation. We used carbon and nitrogen stable isotopes to examine trophic dynamics and the relative contribution of terrestrial organic matter, water column phytoplankton, and phytobenthos (benthic micro- and macro-autotrophs as well as sea ice algae) to the food webs supporting 45 macroconsumers in three Arctic coastal lagoon ecosystems (Krusenstern, Sisualik, Akulaaq) and the adjacent Kotzebue Sound with varying degrees of connectivity in Cape Krusenstern National Monument, Alaska. A two-source (water column particulate organic matter and benthic sediment organic matter), two-isotope trophic dynamics model informed by a Bayesian isotope mixing model revealed that the Lagoon-Kotzebue Sound coastal ecosystem supported consumers along a trophic position continuum from primary consumers, including amphipods, copepods, and clams to trophic level five predators, such as seastars, piscivorous fishes, seals, and seabirds. The relative contribution of the three primary producer end members, terrestrial organic matter (41 ± 21%), phytoplankton (25 ± 21%), and phytobenthos (34 ± 23%) varied as a function of: 1) consumer foraging ecology and 2) consumer location. Suspension feeders received most of their carbon from food webs based on phytoplankton (49 ± 11%) and terrestrial organic matter (23 ± 5%), whereas herbivores and detritivores received the majority of their carbon from phytobenthos-based food webs, 58 ± 10% and 60 ± 8%, respectively. Omnivores and predators showed more even distributions of resource reliance and greater overall variance among species. Within the invertebrates, the importance of terrestrial organic matter decreased and phytobenthos increased with increasing trophic position. The importance of terrestrial organic matter contribution increased with lagoon proximity to major rivers inputs and isolation from Kotzebue Sound. Several taxa with cultural and subsistence food importance to local communities showed significant reliance (30–90% of baseline carbon) on food chains linked to fresh terrestrial organic matter. Our study indicates that terrestrial-marine linkages are important to the function of Arctic coastal lagoon ecosystems and artisanal fisheries. These linkages are likely to strengthen in the future with regional changes in erosion and runoff associated with climate change and anthropogenic disturbance. 

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5. Disturbance-Induced Trophic Niche Shifts In Ground Beetles (Coleoptera: Carabidae) In Restored Grasslands

   https://doi.org/10.1093/ee/nvab065  

   Rahman, Azeem U; Jones, Holly P; Hosler, Sheryl C; Geddes, Seth; Nelson, Melissa; Barber, Nicholas A (July 2021, Environmental Entomology)

   Blubaugh, Carmen (Ed.)

   Abstract Ecosystem restoration is a critical component of land management, countering the loss of native biodiversity. Restoration efforts are enhanced by reintroducing naturally occurring ecosystem processes, including disturbances that may impact species characteristics such as niche position or niche size. In grasslands, grazing and fire affect plant diversity and habitat complexity, which potentially influence insect dietary behaviors and thus their contributions to functions like seed and arthropod predation. Using carbon and nitrogen stable isotopes, we characterized variation in the dietary niche of six ground beetle species (Coleoptera: Carabidae) in response to grazing by reintroduced bison and prescribed fire disturbances in twenty tallgrass prairies. Management disturbances did not affect activity density for most beetle species and mean trophic position was mostly unaffected. However, five of six species exhibited increased trophic niche area and breadth with disturbances, indicating a switch to a more generalist diet that incorporated a wider range of food items. The combination of bison and fire impacts may increase vegetation patchiness and heterogeneity, driving these diet changes. Morphological traits and microhabitat preferences might mediate response to disturbances and the resulting heterogeneity. Combining prescribed fire and grazing, which increases plant diversity and vegetation structural diversity, may help beetle communities establish over time and support the ecological functions to which these insects contribute. 

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