Salps
Salps together with doliolids, pyrosomes, and appendicularians belong to the globally distributed group of pelagic tunicates (Urochordata). Together with their sessile relatives, the ascidians, they all share their feeding mechanism relying on an incurrent and excurrent siphon. Internal filtering nets allow the tunicates to highly efficiently capture even small suspended particles (<0.5 µm). Furthermore, tunicates produce tunicin, animal cellulose, but possess only low lipid and protein contents, thus have a low energetic value for potential predators.
Salps have characteristic life cycles, alternating between sexual and asexual generations. Asexual reproduction together with the highest growth rates known for metazoans, favor at times mass aggregations, the so called “blooms”. These “jelly” blooms can have severe consequences for e.g. fisheries and tourisms, when nearing shores and beaches. On the other hand, tunicates are in the lime light of genetic and developmental science as they are closely related to vertebrates and help to decipher our evolutionary past.
In Antarctic waters, salps are among the major zooplankton grazers and reach a size of several centimeters. Especially at the end of summer and in early fall, massive salp blooms of the salp Salpa thompsoni have been observed along the Polar Frontal Zone and in the Antarctic Peninsula region in some years. Such salp blooms graze down primary production, and generally imply weak populations of crustacean zooplankton like the Antarctic krill (Euphausia superba). In turn, in years of reduced krill abundance, the reproductive output and population sizes of predators like penguins or seals, e.g. in the South Georgia region, are severely reduced. Recent investigations indicate that in Antarctic waters, S. thompsoni show a southward extension and a biomass increase in the last decades. These changes might be related to global warming trends and might in the long-term negatively impact the abundance of krill. Dramatic consequences for the Antarctic food web and fishery are suspected.
One of the major interests in our working group is thus to better understand distribution and life cycles traits of the Antarctic salps in regard to the marked seasonality of salp occurrence and their relation to environmental conditions like water temperature and food regimes. Generally, it has been assumed that salps suffer directly from high phytoplankton concentration, e.g. near the ice-edge and in neritic areas, as high particles loads can clog their feeding mechanism and thus restrict them to oligotrophic areas. However, we could show in field studies that salps are not directly harmed by phytoplankton blooms and that salps do occur also in high phytoplankton concentrations although not with high abundances. Features governing the mutual exclusion of crustaceans and gelatinous salps are thus not fully understood.
Furthermore, our working group focuses on the salp’s impact on the biological carbon pump as they short cut the transfer from small, submicron particles to large metazoan biomass and large fecal pellets. On a global scale, salps thus have a significant influence both as grazers and by channeling primary production from surface waters to the deep oceans. Also in Antarctic waters, salps might substantially contribute to downward flux of organic carbon.
Research topics on Salps in Antarctic waters
We search to qualify and quantify the - potentially increasing - impact of salps in the Antarctic food web and carbon cycle. Hereby, we aim to
…study the size- and generation dependent distribution of salps in the field to understand salp life history traits, their seasonality and their relation to specific environmental factors like temperature or food regimes
….relate different food scenarios encountered in different seasons and different Antarctic regions with salp feeding capacities (e.g. digestion efficiency) and thus ultimately with the salp’s carbon export potential via fecal pellets
….compare directly ecophysiological capacities of salps at varying life cycle stages, e.g. their temperature stress tolerance with that of krill (Euphausia superba)