Salps are unique open-ocean animals that range in size from a few millimeters to greater than twenty centimeters, have a gelatinous (jelly-like) body, and can form long chains of many connected individuals (Figure 1). These oceanic organisms act as oceanic vacuum cleaners, having incredibly high feeding rates on algae (phytoplankton) and, unusual for consumers of their size, smaller bacteria-sized prey.  This rapid feeding and the salps? tendency to form dense blooms, allows them to move substantial amounts of prey carbon from the surface into the deep ocean (leading to carbon dioxide removal from the atmosphere).  However, salps are often considered a trophic dead-end, rather than a link, in the food web due to the assumption that they themselves are not consumed, since their gelatinous bodies are less nutritious than co-occurring crustacean prey.  Along with this, salp populations are hypothesized to be increasing due to climate change. 

Working with colleagues from New Zealand and the University of Hawaii, we have conducted the first whole ecosystem comparative food web analyses in similar regions with and without salp blooms.  Our research traced foodweb flows from nutrients through phytoplankton, protozoans, zooplankton, salps, and eventually into the deep sea.  This allows unprecedented understanding of the ecological and biogeochemical roles of these fascinating organisms.  Our findings include:

• By measuring sinking particle rates beneath water with and without salps, we determined that the presence or absence of salps has a greater impact on marine carbon sequestration through sinking particles than changes in the taxonomic composition or net primary production of phytoplankton communities (Figures 2 and 3).  This suggests that marine biogeochemists need to focus more attention on potential future changes in zooplankton food web structure in order to predict alterations in the biological pump and global carbon cycle.
• Using a suite of different measurement types including measurements of phytoplankton photosynthesis, zooplankton and salp grazing, and the trophic levels of different organisms in the ecosystem, we found that salp blooms increase the efficiency of trophic transfer because of their very high predator:prey size ratios (Figures 4 - 6).  This likely leads to higher biomass production of commercially valuable fish when salps are present than in their absence. 
• By comparing salp grazing rates to the grazing rates of heterotrophic protists, we found that salps and protists engage in intraguild predation and can have different competitive/predatory interactions depending on the stage of a salp bloom.  When salp communities are dominated by adult taxa, the salps may primarily act as predators of protistan grazers (including mixotrophs and obligate heterotrophs).  We hypothesize that their intense grazing pressure on protistan communities may enhance the net growth rates of picoplankton.  These picoplankton can then serve as abundant prey for the subsequent generations of young salps.
• All 7 species of salps preferentially retained nanoplankton (2 - 20 microns in diameter) relative to smaller picoplankton or larger microplankton.  This led to high predator:prey size ratios typically ranging from 1000:1 to 10,000:1.  For perspective, a 1000:1 predator:prey size ratio would be the equivalent of a lion feeding on a flea, while a 10,000:1 predator:prey size ratio would equate to a lion feeding on an amoeba (Figure 4).

These results are included in the 6 peer-reviewed manuscripts that have currently been published from this project, the one manuscript currently in review, and/or the four manuscripts that are in preparation for submission soon.  This project has also contributed to the education of three Ph.D. students.  18 datasets from this project are freely available through the BCO-DMO data archive: https://www.bco-dmo.org/project/754878

This project also supported the development of a week-long immersive high school class in biological oceanography (in partnership with the Illinois Math and Science Academy).  The course involved daily (~one-hour long) lectures about limitation in the pelagic ocean, phytoplankton production, and the biological carbon pump.  Following each lecture, the students were split into groups and given oceanographic datasets to analyze (including data from this project) to investigate the relationships driving primary production and export.  Groups then presented their results to each other.

Results from this project have been incorporated into three different courses taught at FSU (two graduate courses, and one undergraduate course).  The two graduate courses are plankton ecology and a biogeochemical modeling course   The undergraduate course was a newly-designed "flipped-classroom" course entitled "Applied mathematics for environmental and earth scientists".  Salp biology was introduced to these (non-oceanography) students through examples centered around using matrices to investigate changes in population structure and food webs, and included real problems based on our results from this project.  These topics were used to engage students with interesting oceanographic topics, while introducing them to key aspects of matrix algebra. 

Last Modified: 08/25/2023
Modified by: Michael R Stukel