Description: Zooplankton 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 spatial distribution in response to environmental variables (CTD raw data files from POLARSTERN cruise PS112, https://doi.org/10.1594/PANGAEA.895969) and the abundance of krill (Euphausia superba) and salps (Salpa thompsoni). Samples were taken using a Bongo net with a mesh size of 150 µm. The net was equipped with a flowmeter (HydroBios) to measure the filtered volume. On board, the net sample was sieved over a 2000 µm mesh in order to separate organisms 〉2000 µm. The smaller fraction (150 – 2000 µm) was homogenized in 200 mL 0.2 µm filtered seawater and equally split into 4 x 50 mL by using a Hensen-Stempel pipette. The mesozooplankton size range of 150 – 2000 µm was defined according to Atkinson et al. (2012). Two parts were then filtered on 47 mm GF/C Whatman filters (precombusted, acidified and weighed) for analysis of dry weight (DW), bulk carbon (C), nitrogen (N) and phosphorus (P) content, while the third part was preserved in 4 % formalin for abundance, biovolume and size structure analysis. The C/N filters were sealed in tin capsules and analyzed using a CHN analyzer (Thermo, Flash EA 1112). Prior to the analysis, filters for particulate phosphorus were combusted at 450 °C for 5 hours. Particulate organic phosphorus (POP) was measured photometrically as orthophosphate (PO4) by molybdate reaction after sulfuric acid and heat digestion at 90 °C, modified after (Grasshoff et al., 2009). Another filter containing the 4th part served as a back-up. Mesozooplankton bulk stoichiometry data are shown in dataset one. The zooplankton subsamples for taxonomic analysis were scanned using the ZooScan digital imaging system (Model Biotom, Hydroptic Inc., France), a water-proof scanner with a resolution of 2400 dpi (Gorsky et al., 2010; doi:10.1093/plankt/fbp124). Prior to scanning, the formalin preserved samples were rinsed and five samples were further subdivided with a Motoda splitter to reduce the number of organisms per scan and avoid overlapping in the scanning frame. The splits were then placed on the scanner and overlapping organisms were separated manually. Subsequently, the obtained scanning image was processed with ZooProcess, a macro of the image processing software ImageJ (Rasband, 2012) to allow automated processing and measurement of images. These single object images and their metadata were uploaded to the web-based application EcoTaxa (https://ecotaxa.obs-vlfr.fr/prj/2529). Manual validation of the results was required to ensure correct classification. The images were identified to the lowest taxonomic level possible. Prior to quantitative analysis of the obtained data, the image categories containing no zooplankton organisms such as “detritus”, “fiber”, “bubbles” etc. were removed. Abundance of zooplankton taxa was calculated based on the number of images per taxonomic category. Zooplankton organisms were identified to the lowest possible taxonomical level. Whenever identification to species level was not possible, the sample was identified to the next identifiable taxonomical category and assigned a putative species name. The abundance and biovolume data are shown in dataset two and three. The metadata of each image also contain the estimates for body size (body length: major axis of the best fitting ellipse; body width: minor axis) that were used to calculate the biovolume of each object. For the biovolume per size class, the biovolume (mm³/m³) was sorted in octave-scale size class intervals given as individual biovolume (mm3). The lowest limit of the first size class corresponded to the smallest detected ellipsoidal biovolume of 0.00025 mm³. Each size class was then doubled with respect to the previous one. Consequently, the resulting intervals were narrow for small body sizes and became progressively wider with increasing body size. The largest size class was determined by the largest individuals in each sample. As a result, the lower boundary of each size class equaled the interval width. The biovolume (mm³/m³) was then summed for each size class interval. The size distribution (mm³) with total biovolume (mm³/m³) per size bin is given in dataset four.

Description: 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).

Description: 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).

Description: 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.

Description: Planktonic primary consumers have been shown to strongly influence phytoplankton communities via top-down effects such as grazing and nutrient recycling. However, it remains unclear how changes in consumer richness may alter the stoichiometric constrains between producer and consumer assemblages. Here we test whether the stoichiometry of producer–consumer interactions is affected by the species richness of the consumer community (multispecies consumer assemblage vs single consumer species). Therefore, we fed a phytoplankton assemblage consisting of two flagellates and two diatom species reared under a 2 × 2 factorial combination of light and nitrogen supply to three planktonic consumer species in mono- and polycultures. As expected, phytoplankton biomass and C:nutrient ratios significantly increased with light intensity while nitrogen limitation resulted in reduced phytoplankton biomass and increasing phytoplankton C:N but lower N:P. Differences in phytoplankton stoichiometry were partly transferred to the consumer level, i.e., consumer C:N significantly increased with phytoplankton C:N. Consumer diversity significantly increased consumer biomass, resource use efficiency and nutrient uptake. In turn, consumer N:P ratios significantly decreased in consumer assemblages under high resource supply due to unequal changes in nutrient uptake. Consumer diversity further altered phytoplankton biomass, stoichiometry and species composition via increased consumption. Whether the effects of consumer diversity on phytoplankton and consumer performance were positive or negative strongly depended on the resource supply. In conclusion, the stoichiometric constraints of trophic interactions in multispecies assemblages cannot be predicted from monoculture traits alone, but consumer diversity effects are constrained by the resources supplied.

Description: Pronounced atmospheric and oceanic warming along the West Antarctic Peninsula (WAP) has resulted in abundance shifts in populations of Antarctic krill and Salpa thompsoni determined by changes in the timing of sea-ice advance, the duration of sea-ice cover and food availability. Krill and salps represent the most important macrozooplankton grazers at the WAP, but differ profoundly in their feeding biology, population dynamics and stoichiometry of excretion products with potential consequences for the relative availability of dissolved nitrogen and phosphorus. Alternation of the dissolved nutrient pool due to shifts in krill and salp densities have been hypothesized but never explicitly tested by using observational data. We therefore used the Palmer LTER dataset in order to investigate whether the dominance of either grazer is related with the observed dissolved nitrogen:phosphorus (N:P) ratios at the WAP. Across the whole sampling grid, the dominance of salps over krill was significantly correlated to higher concentrations of both N and P as well as a higher N:P ratios. Using actual long-term data, our study shows for the first time that changes in key grazer dominance may have consequences for the dynamics of dissolved nitrogen and phosphorus at the WAP.