Biased dry weight: The matter of residual water in seven pelagic tunicate species

Pelagic tunicates are cosmopolitan and, at times, highly abundant members of open-ocean ecosystems that are important for food web dynamics and biogeochemical cycling. Ecosystem models often use wet weights to parameterize biota. This may be erroneous for gelatinous plankton, such as pelagic tunicates, with high water content, compared to crustaceans. Furthermore, poorly quantified residual water should also be corrected when using dry weights for parameterization. This study aims to estimate the residual water content (as well as elemental and organic contents) for seven tunicate species. Tunicates (one pyrosome and six salps, N = 107), were collected during several research expeditions and analyzed for inter- and intraspecific variability. The H-surplus method was applied for the residual water content calculation. The residual water ranged from 1.5 to 20.6% (average ± SD: 8.9 ± 4.2%) of dry weight across species and did not differ significantly between taxonomic orders. Furthermore, the life cycle stage (blastozooid versus oozooid) and tissue type (tunic versus whole organism) were found to be insignificant in explaining residual water content patterns. Elemental and organic contents, however, were lower in tunics than in whole organisms, and in the salp Thetys vagina, stomachs had significantly higher elemental and organic contents compared to whole organisms. Effects of size and drying method (oven versus freeze-drying) significantly impacted the residual water content of pelagic tunicates. Accounting for residual water content in zooplankton is discussed from an ecological perspective and is strongly advised for future modeling studies.