Impact of Local Iron Enrichment on the Small Benthic Biota in the deep Arctic Ocean
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Impact of Local Iron Enrichment on the Small Benthic Biota in the deep Arctic Ocean The study assesses the impact of local iron enrichment on the small benthic biota (bacteria, meiofauna) together with environmental parameters indicating the input of food at the deep seafloor. To evaluate the hypothesis that abundance, distribution, and diversity of the small benthic biota varies in relation to a local input of structural steel at the seabed, we analyzed sediment samples and the associated infauna along a short transect with increasing distance to an iron source, i.e., corroding steel weights (plates) of a free-falling observational platform (bottom-lander), lying on the seafloor for approximately seven years. Iron-enriched surface sediments in the vicinity of the bottom-weight left in summer 2008 after a short-term deployment of a bottom-lander in 2433 m water depth at the LTER (Long-Term Ecological Research) observation HAUSGARTEN in eastern parts of the Fram Strait were sampled on 28th July 2015 using push-corer (PC) handled by the Remotely Operated Vehicle (ROV) QUEST 4000 (MARUM Center for Marine Environmental Sciences, Germany) during Dive 369 from board RV Polarstern. During sampling in 2015, the plates were largely corroded. Surface sediments around the plates had an orange-red color with a gradient of decreasing color intensity with increasing distance from the source, i.e., the bottom weight. A total of eight push-corer samples (PC1-8) were taken at approx. regular distances (on average every 18 cm) along a short transect (about 1.5 m) crossing the iron gradient. Push-corers PC1-4 retrieved sediment from heavily impacted sediments, while samples taken from push-corers PC5-8 were visually indistinguishable from background sediments in the wider area. After recovery of the ROV, sediment cores (8 cm in diameter, and 20-25 cm in height) were sub-sampled using plastic syringes with cut-off anterior ends for meiofauna and nematode communities as well as for environmental parameters. The position specified in the data sets (longitude / latitude) refers to the position of the ROV. The uploaded datasets contain meiofauna (major taxonomic groups) abundances (ind. / 10 cm²) (Meiofauna.xlsx), for the nematodes (genera) abundance (ind. / 10 cm²) and biomass (µg dry weight / 10 cm²) are given (Nematoda.xlsx). Data on environmental parameters are given for FE content (iron content in the sediments), H2O (water content in sediments as parameter for porosity), AFDW (ash free dry weight of sediment as parameter for total organic matter content in sediments), Chloroplastic pigments (chlorophyll a) and its degradation products (phaeopigments) as parameter for the input from phytodetritus sedimentation. The bulk of pigments, defined as chloroplastic pigment equivalents, CPE (Environmental parameters.xlsx). All data are given for the top 5 sediment centimeters (sediment cores were sectioned in 1-cm layers down to 5 cm sediment depth).
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Meiofauna and Nematodes One 20-ml syringe (2 cm in diameter) from each core was taken for meiofauna analyses. Sediment cores were sectioned in 1-cm layers down to 5 cm sediment depth. After staining in Rose Bengal for 24 h, single sediment layers were passed through a set of sieves with various mesh sizes (1000, 500, 250, 125, 63, and 32 µm) to facilitate further processing (Pfannkuche and Thiel 1988). All organisms were enumerated under a low-power stereo microscope. Organisms were identified to major taxonomic groups. Meiofaunal densities were standardized to individuals per 10 cm2. All nematodes found in the samples were transferred to anhydrous glycerol and mounted on permanent slides (De Grisse 1969) and identified to genus level (Schmidt-Rhaesa 2014). Nematode biomasses were estimated from digital microscope images of individual nematodes using the cellP software (Olympus®). The length (excluding filiform tails) and width of each nematode specimen was measured. Nematode wet weight (WW) was calculated following Andrassy (1956): WW(µg) = (LxW²)/Cf, with L [µm] = nematode’s length, W [µm] nematode’s width at the widest point and Cf = conversion factor that equals 1.6 x 106. The taxonomic identification and measurements of the nematode specimens was done using light microscopy (Nomarski optic). Sediment Parameters One 5-ml syringe (Ø 1 cm) from each core was analyzed for iron concentrations; subsample from push-corer PC1 was lost during analyses. The bulk metal composition of the sediments was determined after total acid digestion (see Nöthen and Kasten 2011, Volz et al. 2019). Approximately 75 mg of freeze dried and powdered sediment was dissolved in a mixture of 65% distilled HNO3 (3 ml), 32% distilled HCl (2 ml) and 40% HF of suprapur® grade (0.5 ml) by use of a CEM Mars Xpress microwave system. The acids were fumed off and the residue was re-dissolved in 5 ml 1M HNO3. The solution was topped up with 1M HNO3 to a volume of 50 ml and analyzed on an iCAP 7400 ICP-OES instrument. Yttrium was applied as an internal standard to correct for physical differences of samples. Sediment samples taken from each push-corer with 20-ml syringes were split vertically into equal parts. One half was analyzed for water content as proxy for the porosity of the sediments and to determine organic matter content. The second half was used to determine chloroplastic pigments, indicating food/energy availability from phytodetritus sedimentation. Water contents (H2O) were determined by measuring the weight loss of wet sediment samples dried for 48 h at 60°C. Total organic matter content of the sediments was estimated as ash-free dry-weight (AFDW) after combustion of sediment samples for 2 h at 500°C. Chloroplastic pigments (chlorophyll a [CHL] and its degradation products, phaeopigments) were extracted in 90% acetone and measured with a TURNER fluorometer (Shuman and Lorenzen 1975). The bulk of pigments, defined as chloroplastic pigment equivalents, CPE (Thiel 1978).