ARTICLE IN PRESS Deep-Sea Research I 54 (2007) 109–129 www.elsevier.com/locate/dsr Spatial and temporal variations in deep-sea meiofauna assemblages in the Marginal Ice Zone of the Arctic Ocean Eveline Hostea,Ã, Sandra Vanhovea, Ingo Scheweb, Thomas Soltwedelb, Ann Vanreusela aMarine Biology Section, University of Gent, Krijgslaan 281-S8, B-9000 Gent, Belgium bAlfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany Received 12 January 2006; received in revised form 13 September 2006; accepted 19 September 2006 Available online 16 November 2006 Abstract In order to understand the response of the deep-sea meiobenthos to a highly varying, ice-edge-related input of phytodetritus, we investigated the abundance and composition of the meiobenthos at the arctic long-term deep-sea station HAUSGARTEN (791N, 41E) along a bathymetric transect (1200–5500 m water depth) over 5 consecutive years (from 2000 to 2004) in relation to changes in environmental conditions. Results showed high sediment-bound pigment concentrations (chlorophyll a and degradation products) ranging from 4.5 to 41.6 mg/cm3, and coinciding high meiobenthic densities ranging from 14973to 34097525 ind/10 cm2. Nematodes dominated the metazoan meiofaunal communities at every depth and time (85–99% of total meiofauna abundance), followed by harpacticoid copepods (0–4.6% of total meiofauna abundance). The expected pattern of gradually decreasing meiobenthic densities with increasing water depth was not confirmed. Instead, the bathymetric transect could be subdivided into a shallow area with equally high nematode and copepod densities from 1000 to 2000 m water depth (means: 22597157 Nematoda/10 cm2,and5074 Copepoda/10 cm2), and a deeper area from 3000 to 5500 m water depth with similar low nematode and copepod densities (means: 595752 Nematoda/10 cm2,and1172 Copepoda/10 cm2). Depth-related investigations on the meiobenthos at the HAUSGARTEN site showed a significant correlation between meiobenthos densities, microbial exo-enzymatic activity (esterase turnover) and phytodetrital food availability (chlorophyll a and phaeophytines). In time-series investigations, our data showed inter-annual variations in meiofauna abundance. However, no consistent relationship between nematode and copepod densities, and measures for organic matter input were found. r 2006 Elsevier Ltd. All rights reserved. Keywords: Arctic; Greenland Sea; Deep water; Benthos; Meiofauna; Abundances 1. Introduction however, are some of the most dynamic areas in the world’s oceans with large seasonal, inter-annual and Polar oceans are extreme environments with low spatial fluctuations in ice-cover and high ice-related temperature and seasonal light and food limitation, primary production (Falk-Petersen et al., 2000). which exert major influences on global climate and This variability is a critical factor, which structures ocean systems. The Marginal Ice Zones (MIZs), the arctic marine ecosystem. The spring phytoplankton bloom follows the ÃCorresponding author. Tel.: +32 09264 85 23. receding ice edge as it melts (Sakshaug and Skjoldal, E-mail address: [email protected] (E. Hoste). 1989) and intensive blooms occur in leads as the 0967-0637/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2006.09.007 ARTICLE IN PRESS 110 E. Hoste et al. / Deep-Sea Research I 54 (2007) 109–129 MIZ opens up (Schewe and Soltwedel, 2003). The environmental variables on meiobenthos densities development of such blooms requires 2–3 weeks of in time and with water depth. open water, a relatively stable ice cover during This study addresses the following questions. Are winter, and stratification of the water column deep-sea meiofauna densities along the productive (Falk-Petersen et al., 2000; Engelsen et al., 2002). MIZ higher than in other polar deep-sea regions? One to 2 months prior to the pelagic production, ice Do meiofauna densities and vertical depth profiles algal production is initiated (Falk-Petersen et al., in the sediment change along a bathymetric 2000). In ice-free areas of the MIZ to the north- transect? Do meiofauna densities and vertical depth west of Svalbard, primary production rates of profiles change over time? Are these changes 18–20 mg cmÀ2 hÀ1 were measured, while at the correlated with organic matter input or other ice-edge, production rates even increased up to environmental variables? Are influences of time- 37 mg cmÀ2 hÀ1 (Heimdal, 1983). related changes in environmental variables on Several studies in the deep sea indicated a rapid meiofauna densities comparable to influences of downward transport of fresh phytodetritus and depth-related changes in environmental variables? fecal pellets (Billett et al., 1983; Graf, 1989) and The goal of this study was to gain a better possibly a rapid processing of this material by the understanding of the relation between benthos and deep-sea benthos, which is sustained by this organic environmental variables possibly related to ice matter from the euphotic zone (Moodley et al., conditions. 2002; Witte et al., 2003). The flux of organic matter to the deep seafloor, however, is highly variable in 2. Material and methods time and space. At high latitudes, inter-annual variations in ice coverage determine the start and 2.1. Sampling site intensity of the phytoplankton bloom (Sakshaug and Skjoldal, 1989). As the presence and persistence The long-term deep-sea observatory HAUS- of life at the ocean floor can be seen as a response to GARTEN is situated in Fram Strait, west of organic matter input (Thiel, 1975; Gooday and Svalbard at 79 1N(Soltwedel et al., 2005). The Turley, 1990; Grebmeier and Barry, 1991; Gooday, majority of the sampling sites in this area form a 2002), the variability in organic matter fluxes to the bathymetric transect of nine stations from the upper seafloor is bound to have an influence on the slope of the Svalbard Margin (1200 m) to Molloy benthos. Hole (75500 m), the deepest depression recorded in To investigate the impact of large-scale environ- the Arctic Ocean (Myhre and Thiede, 1995)(Fig. 1). mental changes in the transition zone between the The sampling sites between 1200 and 2500 m water North Atlantic and the central Arctic Ocean, and to depth are located on a gentle slope while stations determine the factors controlling deep-sea biodiver- between 3000 and 5000 m are located on a steep sity, the German Alfred Wegener Institute for Polar slope (up to 401 inclination between 4000 and and Marine Research (AWI) established the deep- 5000 m) towards Molloy Hole (Fig. 1)(Soltwedel sea, long-term observatory HAUSGARTEN, re- et al., 2005). presenting the first, and by now only, open-ocean, Hydrographic conditions in the HAUSGARTEN long-term station in a polar region (Soltwedel et al., area are characterized by the inflow of relatively 2005). In this part of the HAUSGARTEN research warm and nutrient-rich Atlantic Water into the project, the emphasis is on the impact of changing central Arctic Ocean (Manley, 1995). Circulation environmental variables on the metazoan meio- patterns in the Fram Strait result in a variable sea- benthos. ice cover, with permanent ice-covered areas in the Food quality and quantity reaching the deep- west, permanent ice-free areas in the southeast, and seafloor decreases with increasing water depth seasonally varying conditions in central and north- (Billett et al., 1983; Falk-Petersen et al., 2000; eastern parts, where the HAUSGARTEN area is Engelsen et al., 2002; Schewe and Soltwedel, 2003). located (Soltwedel et al., 2005). As food availability is thought to be the most important structuring factor for meiobenthos com- 2.2. Sampling strategy munities, the unique combination of a time series along a bathymetric transect at the summer MIZ Samples were obtained during cruises ARK-XVI allows us to analyze the impact of changing to ARK-XX of the German ice-breaker R.V. ARTICLE IN PRESS E. Hoste et al. / Deep-Sea Research I 54 (2007) 109–129 111 Fig. 1. Map of the Greenland Sea with Sea minimum (2004) and maximum (2003). Ice concentration in Juli (http://nsidc.org/sotc/ sea_ice.html), a detail of the sampling transect and detail of the bathymetric transect. Polarstern, in the summer months of 2000–04. A Bengal, counted and identified up to higher taxon multiple-corer (MUC) was used to collect sediment level. For technical and logistical reasons, meiofau- cores with virtually undisturbed surfaces (Gage and na samples are missing for 2000, 5000 and 5500 in Tyler, 1991). For meiofaunal analysis, 3 samples 2001, for 5500 m in 2000 and 2002, and for 5000 m from different cores of the same MUC haul were in the years 2003 and 2004. taken by means of a modified plastic syringe Samples for biogenic sediment compounds (in- (3.14 cm2 cross-sectional area) and subdivided into dicators for organic matter input, sediment-bound 1 cm slices down to 5 cm sediment depth in order to biomass and microbial activity) were also taken study the vertical distribution of the meiofauna in with modified syringes (1.17 and 3.14 cm2 cross- the sediment. After elutriation with the Ludox sectional area) and analyzed at 1-cm-intervals down centrifugation method (Heip et al., 1985) all to 5 cm sediment depth. Sediment composition, metazoan organisms passing a 1 mm sieve and determined using a Coulter Counter LS 100TM, retained on a 32 mm sieve were stained with Rose was only analyzed for the 2001 samples, and are ARTICLE IN PRESS 112 E. Hoste et al. / Deep-Sea Research I 54 (2007) 109–129 lacking for 2000 and 5000 m water depth. Also, in All statistical analyses were performed on the 2001, data on organic matter input are lacking for original meiofauna densities per 3.6 cm2. Formal the station at 4000 m water depth. significance tests for differences in taxon community Concentrations of chloroplastic pigments (chlor- structure between the depths and years were carried ophyll a [Chl a] and its degradation products ¼ out using the one-way ANOSIM tests (Clarke, chloroplasic pigment equivalents [CPE]; Thiel, 1993), performed on the Bray–Curtis similarity 1978) in sediments were studied to estimate the indices.