Ingo Schewe

The Benthos of Arctic Seas and its Role for the Organic Carbon Cycle at the Seafloor

The exploration of the Arctic Ocean has a long and multi-national history, but still we are only beginning to understand the role of the Arctic Ocean in the global carbon cycle. This chapter addresses the contribution of the Arctic benthos to organic carbon utilization, modification and sequestration. We review findings in benthic ecology and biogeochemistry of the Arctic Ocean from the past 15 years. Several older reviews on the Arctic benthos are available with a different focus: Zenkevitch’s (1963) thorough volume covering the entire Eurasian Arctic benthos remains outstanding in its description of species composition, biomass and biogeographic aspects. Reviews by Curtis (1975), Dayton (1990), and Carey (1991) cover general aspects of the Arctic benthos as well as distribution patterns, productivity, and the feeding ecology of different species. Many recent studies have emphasized processes at the sediment-water boundary layer including respiration of sedimentinhabiting communities or single species (Piepenburg et al. 1995), calculation of carbon utilization and remineralization rates of benthic communities and their correlation to primary production (Boetius and Damm 1998), but also measuring biomass of benthic populations (Seiler 1999; Jorgensen et al. 1999; Deubel 2000). Recent discoveries of cold seeps north of the polar circle (Vogt et al. 1999) and of hydrothermal activity on the Gakkel Ridge (Thiede 2002) suggest a more diverse deep-sea benthos than previously known, but these on-going investigations cannot be reviewed herein.

Small-Sized Benthic Organisms of the Alpha Ridge, Central Arctic Ocean

Abundance, biomass and activity of the small-sized benthic organisms (bacteria to meiofauna, includingforaminifera) was studied in summer 1998 during the expedition ARK XIV/1a to the Amerasian area of theperennially ice covered central Arctic Ocean. With the help of two icebreakers, the German research vesselPOLARSTERN and the Russian nuclear-powered ARCTICA, it was possible to reach this remote, heavilyice-covered region in order to carry out the first benthic investigations. These focus on effects on the benthiccommunity of the expected low food availability under perennial ice coverage. Bacterial and meiofaunalabundances were determined by direct counting. Biomass determinations on bacteria and nematodes wereundertaken by size-imaging techniques. In addition biochemical analyses were carried out to estimate foodavailability (as sediment-bound chloroplastic pigments indicating phytodetritus) at the sea floor, the total microbialbiomass (TMB; i.e. the total amount of sediment inhabiting bacteria, flagellata, protozoa and small metazoa,estimated by phospholipid quantification) and the potential bacterial activity (turnover rates of ester-cleavingexoenzymes). Concentrations of chloroplastic pigment equivalents (CPE) in the main target area (Alpa-Ridge)ranged between 0.10 ± 0.02 and 0.17 ± 0.04 g/ml. A 2-3 times higher concentration was determined at astation on the Lomonosov Ridge crest (0.40 ± 0.15 g/ml). The standing stock of meiobenthic organisms(including foraminiferans) was extremely low and varied between 72 ± 17 individuals 10 cm-2 in the deepMakarov Basin (3,170 m) and 190 ± 56 individuals 10 cm-2 on the Alpha Ridge (1,470 m). Significantly highernumbers (U-test, p = 0.049) were found on the Lomonosov Ridge (297 ± 82 individuals 10 cm-2). Meiobenthicabundances from the area of investigations, were up to ten times lower than those reported from non-ice covereddeep-sea regions. However a significant water depth depending decrease of meiobenthic abundances was stilldetectable.A comparison of biomass data determined by volumetric measurements and biochemical methods showed thatabout 67% of the TMB are held by organisms of nanofauna size (2-32 m), approx. 32% belongs to bacteria.Only 0.5-1.5% of the TMB were held by metazoan meiofauna.

Activity and biomass of the small benthic biota under permanent ice-coverage in the central Arctic Ocean

Abstract Sediment samples collected during the expedition “Arctic Ocean `96” with the Swedish ice-breaker ODEN were investigated to estimate for the first time heterotrophic activity and total microbial biomass (size range from bacteria to small metazoans) from the perennially ice-covered central Arctic Ocean. Benthic activities and biomass were evaluated analysing a series of biogenic sediment compounds (i.e. bacterial exoenzymes, total adenylates, DNA, phospholipids, particulate proteins). In contrast to the very time-consuming sorting, enumeration and weight determination, analyses of biochemical sediment parameters may represent a useful method for obtaining rapid information on the ecological situation in a given benthic system. Bacterial cell numbers and biomass were estimated for comparison with biochemically determined biomass data, to evaluate the contribution of the bacterial biomass to the total microbial biomass. It appeared that bacterial biomass made up only 8–31% (average of all stations = 20%) of the total microbial biomass, suggesting a large fraction of other small infaunal organisms within the sediment samples (most probably fungi, yeasts, protozoans such as flagellates, ciliates or amoebae, as well as a fraction of small metazoans). Activity and biomass values determined within this study were generally extremely low, and often even slightly lower than those given for other deep oceanic regions, thus characterizing the seafloor of the central Arctic Ocean as a “benthic desert”. Nevertheless, some clear trends in the data could be found, e.g. generally sharply decreasing values within the sediment column, a vague tendency for declining values with increasing water depth of sampling stations, and also differences between various Arctic deep-sea regions.

Observations of floating anthropogenic litter in the Barents Sea and Fram Strait, Arctic

Although recent reports indicate that anthropogenic waste has made it to the remotest parts of our oceans, there is still only limited information about its spread, especially in polar seas. Here, we present litter densities recorded during ship- and helicopter-based observer surveys in the Barents Sea and Fram Strait (Arctic). Thirty-one items were recorded in total, 23 from helicopter and eight from research vessel transects. Litter quantities ranged between 0 and 0.216 items km−1 with a mean of 0.001 (±SEM 0.005) items km−1. All of the floating objects observed were plastic items. Litter densities were slightly higher in the Fram Strait (0.006 items km−1) compared with the Barents Sea (0.004 items km−1). More litter was recorded during helicopter-based surveys than during ship-based surveys (0.006 and 0.004 items km−1, respectively). When comparing with the few available data with the same unit (items km−1 transect), the densities found herein are slightly higher than those from Antarctica but substantially lower than those from temperate waters. However, since anthropogenic activities in the Fram Strait are expanding because of sea ice shrinkage, and since currents from the North Atlantic carry a continuous supply of litter to the north, this problem is likely to worsen in years to come unless serious mitigating actions are taken to reduce the amounts of litter entering the oceans.

Megafaunal assemblages from two shelf stations west of Svalbard

Abstract Megafauna plays an important role in benthic ecosystems and contributes significantly to benthic biomass in the Arctic. The distribution is mostly studied using towed cameras. Here, we compare the megafauna from two sites located at different distances from the Kongsfjord: one station at the entrance to the fjord, another on the outer shelf. Although they are only located 25 km apart and at comparable depth, there were significant differences in their species composition. While the inshore station was characterized by shrimps (2.57±2.18 ind. m−2) and brittlestars (3.21± 3.21 ind. m−2), the offshore site harboured even higher brittlestar densities (15.23±9.32 ind. m−2) and high numbers of the sea urchin Strongylocentrotus pallidus (1.23±1.09 ind. m−2). Phytodetrital concentrations of the upper sediment centimetres were significantly higher inshore compared with offshore. At a smaller scale, there were also differences in the composition of different transect sections. Several taxa were characterized by a patchy distribution along transects. We conclude that these differences were caused primarily by habitat characteristics. The seafloor inshore was characterized by glacial soft sediments, whereas the station offshore harboured large quantities of stones. Although the use of a new web-2.0-based tool, BIIGLE (http://www.BIIGLE.de), allowed us to analyse more images (∼90) than could have been achieved by hand, taxon area curves indicated that the number of images analysed was not sufficient to capture the species inventory fully. New automated image analysis tools would enable a rapid analysis of larger quantities of camera footage.

Effects of dropstone-induced habitat heterogeneity on Arctic deep-sea benthos with special reference to nematode communities

Abstract During an expedition to the deep-sea long-term observatory HAUSGARTEN in the eastern Fram Strait in summer 2003, the availability of a Remotely Operated Vehicle allowed a targeted sampling of surface sediments around a relatively large dropstone (0.9 m2) to determine suspected differences in community structure and dynamics of nematode assemblages in relation to the confined flow regime and patchy food availability in the immediate vicinity of the stone. The almost rectangular dropstone was about 150 cm in length, 60 cm in width, and up to 15 cm in height. Small-scale current measurements around the dropstone using a MAVS-3 acoustic current meter exhibited a rather complex pattern. A computational fluid dynamics simulation revealed areas of constantly flowing near-bottom currents as well as the generation of vortices in certain areas around the dropstone. Concentrations of biogenic compounds in the sediments surrounding the dropstone generally followed the complex flow pattern. The differences in physical and biochemical conditions around the dropstone were reflected in species composition and diversity, trophic structure and life-history traits of the nematode communities, and to a lesser extent in their total abundance and biomass.

Benthic Oxygen Uptake in the Arctic Ocean Margins - A Case Study at the Deep-Sea Observatory HAUSGARTEN (Fram Strait)

The past decades have seen remarkable changes in the Arctic, a hotspot for climate change. Nevertheless, impacts of such changes on the biogeochemical cycles and Arctic marine ecosystems are still largely unknown. During cruises to the deep-sea observatory HAUSGARTEN in July 2007 and 2008, we investigated the biogeochemical recycling of organic matter in Arctic margin sediments by performing shipboard measurements of oxygen profiles, bacterial activities and biogenic sediment compounds (pigment, protein, organic carbon, and phospholipid contents). Additional in situ oxygen profiles were performed at two sites. This study aims at characterizing benthic mineralization activity along local bathymetric and latitudinal transects. The spatial coverage of this study is unique since it focuses on the transition from shelf to Deep Ocean, and from close to the ice edge to more open waters. Biogeochemical recycling across the continental margin showed a classical bathymetric pattern with overall low fluxes except for the deepest station located in the Molloy Hole (5500 m), a seafloor depression acting as an organic matter depot center. A gradient in benthic mineralization rates arises along the latitudinal transect with clearly higher values at the southern stations (average diffusive oxygen uptake of 0.49 ± 0.18 mmol O2 m-2 d-1) compared to the northern sites (0.22 ± 0.09 mmol O2 m-2 d-1). The benthic mineralization activity at the HAUSGARTEN observatory thus increases southward and appears to reflect the amount of organic matter reaching the seafloor rather than its lability. Although organic matter content and potential bacterial activity clearly follow this gradient, sediment pigments and phospholipids exhibit no increase with latitude whereas satellite images of surface ocean chlorophyll a indicate local seasonal patterns of primary production. Our results suggest that predicted increases in primary production in the Arctic Ocean could induce a larger export of more refractory organic matter due to the longer production season and the extension of the ice-free zone.