Gut carbon, nitrogen and phosphorus composition of Euphausia superba and Salpa thompsoni from the northern Antarctic Peninsula

Samples were collected between 03/26/2018 and 04/27/2018 around the northern tip of the Antarctic Peninsula (63° 0' 1.843'' S, 60° 0' 16.901''W) onboard the RV Polarstern during the PS112 campaign in order to identify the elemental composition and stoichiometry of the Antarctic krill (Euphausia superba) and salps (Salpa thompsoni). The sampling stations were situated in the survey grid of the Antarctic Marine Living Resources Program (AMLR). At the time of sampling, the study area was ice free and ambient Chlorophyll a levels were comparable to previous studies in this time frame (e.g. Schofield et al., 2017). The phytoplankton community was dominated by dinoflagellates, diatoms and prymnesiophytes, which is typical for autumn (March–May) around the NAP (Pauli et al., 2021). Krill and salps were sampled by using the 1.8 m² Isaacs-Kidd Midwater Trawl Net (IKMT) equipped with a 505 μm mesh which is suitable to collect both salps and krill in good condition. The net was towed obliquely to 170 m, or 20 m from the bottom, at a speed of 2 kts. Size in mm (using graph paper, Seibert publisher) and stage of krill and salp individuals were determined on board before further processing. Prior to the elemental analysis, the digestive system of krill and salps was dissected and stored at -20 °C to determine the elemental signature of the gut content. Salp individuals were immediately dissected on board, krill individuals were frozen on board and dissected later in the lab. Measurements of gut elemental composition were done on 50 and 16 samples for krill and salps, respectively. Prior to the analyses each sample was homogenized with 500 µl distilled water. For the carbon/nitrogen (C/N) samples, 250 µl of the homogenate were transferred into pre-weighed tin capsules while the rest of the homogenized tissue was transferred into pre-weighed glass tubes for phosphorus analysis. Samples were dried at 70 °C for three weeks prior to analysis. After drying, we measured the dry weight (DW) of all samples by using a high-resolution balance (Mettler Toledo, XP-26). After drying, all C/N samples were sealed and analyzed using a CHN analyzer (Thermo, Flash EA 1112). The phosphorus samples were combusted at 450 °C for 5 hours and the ash-free dry weight (AFDW) was measured. Particulate organic phosphorus (POP) was measured photometrically by molybdate reaction after sulfuric acid and heat digestion at 90 °C, modified after (Grasshoff et al., 2009).