Biochemical components and energy content of the pelagic tunicate Salpa thompsoni measured across the Southern Ocean and forty years (1980–2020)

The pelagic tunicate Salpa thompsoni is a widespread cold-water metazoan and a major grazer of phyto- and microzooplankton in the Southern Ocean. Long-term time series and spatiotemporal models predict that salps will expand their distribution towards higher latitudes over the next decades with ramifications for all food web components, including higher tropic level predators. Salps are potentially less nutritious and energy-rich than co-occurring euphausiids. In a changing Southern Ocean ecosystem, predators such as baleen whales, seabirds, and planktivorous fish that historically relied on an energy-rich and numerous food source (euphausiids) may face an uncertain future. This, however, may differ by season too. Whether S. thompsoni are a less nutritious prey item than euphausiids across an annual cycle at circumpolar mid and high latitudes (51–70°S) has not been investigated. We utilised published and new body composition data, i.e., organic content (ash-free dry weight as percent of dry weight, DW), carbon content (carbon weight as percent of DW), and proximate biochemical composition (carbohydrate, lipid, and protein weight as percent of DW), collected over the past forty years (1980–2020). Energy content values were calculated based on these parameters using published conversion factors. We corrected for residual water (water remaining in tissue after drying) with a published conversion factor of 12.9 %. Samples (N = 303, sometimes comprising of several salps) were collected in four seasons using a variety of large plankton nets and midwater trawls between the surface and 3200 m (mostly less than 400 m). Each specimen was sized (oral-atrial or total length) and staged (blastozooid, oozooid). The carbon-to-nitrogen ratio (C/N value) was reported for most (77 %) of the samples. Samples were used for the determination of sometimes one or several body composition parameters: organic content (N = 151), carbon content (N = 220), and proximate biochemical composition (N = 70). The weight-specific energy content ranged between < 0.1 and 20.5 kJ g DW⁻¹.