Tissue 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. From these tows, we collected 162 adult krill and 121 blastozooid salp individuals on board of which we analyzed 140 krill and 108 salp samples for their elemental content (total organic carbon, nitrogen and phosphorus). Size in mm (using graph paper, Seibert publisher) and stage of krill and salp individuals were determined on board before further processing. For size measurements of S. thompsoni, we used the oral atrial length. Total length of Antarctic krill was measured from the anterior margin of the eye to the tip of the telson, excluding the terminal spine. Prior to the elemental analysis, the digestive system of krill and salps was removed to avoid contamination of the tissue elemental composition by the elemental signature of the gut content. Salp individuals were immediately dissected on board after size measurements and frozen at -20 °C. Krill individuals were frozen on board and dissected later in the lab. Frozen krill and salp individuals were individually ground prior to the analyses using a pebble mill and homogenized with distilled water. Homogenization enabled us to take sub-samples of single individuals to analyze the total carbon, nitrogen, and phosphorus composition of the same specimen. For C/N analysis, 3 x 250 µl of the homogenate were transferred into pre-weighed tin capsules while the rest of the homogenized tissue was equally distributed into three pre-weighed glass tubes for phosphorus analysis. Tissue samples were dried at 70 °C for three weeks prior to analysis to minimize the effect of residual water bound in collagen. After drying, we measured the dry weight (DW) of all samples to obtain total individual DW for each specimen by using a high-resolution balance (Mettler Toledo, XP-26). The determination of dry weight in gelatinous zooplankton can be challenging, as DW may be overestimated due to residual water and salt. The DW measurements were therefore corrected for water content assuming that 13.5 % of the DW was residual water (Madin et al., 1981). his correction factor is based on previous calculations for residual water in salp body tissue. This conversion was only applied to our field data of S. thompsoni. The DW of E. superba was not corrected since crustacean zooplankton contains considerably less residual water in its tissue. Since the DW was not corrected for potential remaining salt the elemental content per DW of S. thompsoni should be considered as conservative. 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).