The role of copepods in cryo-pelago-benthic coupling in the Weddell Sea, Antarctica
This thesis aimed at investigating the importance of copepods for cryo-pelagic coupling and improving the knowledge on the contribution of copepods to pelago-benthic coupling. Sampling, measurements and experiments were conducted during the two expeditions ANT XXI/2 and ANT XXII/2 with RV “Polarstern” to the shelf of the eastern Weddell Sea in late spring 2003, and to the continental slope of the western Weddell Sea in late spring 2004/2005, respectively. In the western Weddell Sea, the population dynamics and the spatial and temporal variability of (1) the metazoan fauna in the surface and sub-ice layers of a drifting ice floe, and (2) the copepod communities within the ice proper of the drifting floe and of pack ice on a transect from the ice edge to the ice drift station were studied. In the sea ice proper, the harpacticoids Drescheriella spp., mainly naupliar stages, were by far the most abundant species throughout the study (72 - 87 %). Drescheriella spp. and the calanoid Stephos longipes were present in all layers of the ice, whereas the occurrence of the other copepod species was restricted to the lowermost ice layer. The distribution of all species was very patchy and varied greatly between the sampling sites. The metazoan fauna within the sea ice surface layer was dominated by Drescheriella spp. and S. longipes with maximum abundances of 3830 and 1293 ind. L-1, respectively. The populations were mainly comprised of adults and early naupliar stages indicating reproduction of these species within the sea ice surface layer. The copepod abundances were generally higher at the edge of the floe than in the inner part. Drescheriella spp. and S. longipes also occurred regularly in the sub-ice water layer (nauplii, copepodids and adults in Drescheriella spp. and mainly nauplii and adults in S. longipes), however, the dominant sympagic copepod species in this habitat was Ectinosoma sp. with a maximum abundance of 599 ind. m-3. Feeding experiments were conducted with Drescheriella spp. females and copepodids (C) V and S. longipes adults, C I - V and nauplii VI, and sea ice protist communities as food. In both species high ingestion rates were measured, and no evidence for satiation feeding was found even at the highest chlorophyll a (Chl a) concentrations (up to 76.86 µg L-1). Food selection by Drescheriella spp. and S. longipes was related to the size of the protists with small-sized species such as Fragilariopsis cylindrus and F. curta being preferentially ingested. At some sampling sites the estimated grazing impact of Drescheriella spp. and S. longipes on infiltration layer communities was extremely high reaching a maximum population grazing rate of 313.8 % of the ice algae stock per day. The amount and composition of vertical particle flux at depths of 10 m and 70 m under the drifting ice floe were determined during a period of 30 days in order to investigate the influence of sea ice related biological processes on the flux. The total mass flux was dominated by diatoms, faecal material, and aggregates, and ranged from 95.28 to 197.67 mg m-2 d-1 at 10 m depth and from 51.54 to 55.34 mg m-2 d-1 at 70 m depth. A strong increase with time of the flux of chlorophyll equivalents, biogenic silica, and faecal material was recorded during the observation period, coincident with an increase in the concentration of Chl a in the bottom ice layer above the trap array. No copepod faecal pellets were found within the sinking faecal material, which was dominated by krill faecal strings and contained large amounts of diatom frustule debris, as well as intact diatom frustules, mainly of the species F. curta and F. cylindrus. Low POC/PON and biogenic silica/POC ratios of the sinking particulate matter suggest that the material collected in the traps was relatively fresh. The population dynamics of dominant calanoid copepods in the water column under the drifting ice floe in the western Weddell Sea, and on the shelf of the eastern Weddell Sea were studied during periods of about one month and three weeks, respectively. The goal was to contribute to the understanding of the importance of these copepods for carbon cycling and vertical particle flux in the pelagial. Considerable amounts of copepods were present in all investigated depth layers (down to 1000 m depth in the western Weddell Sea, and the whole water column on the eastern Weddell Sea shelf with a maximum sampling depth of 464 m). At both sites the calanoid copepod communities were characterised by the dominance of only a few species with Microcalanus pygmaeus being most numerous. This species contributed on average 70 % (western Weddell Sea) and 66 % (eastern Weddell Sea) of all calanoid copepods. Further dominant species at both sites were Calanoides acutus and Metridia gerlachei. Interestingly, Ctenocalanus citer comprised on average 13.4 % of the calanoid copepods on the eastern Weddell Sea shelf, while it occurred only in very low numbers in the western Weddell Sea. The sympagic S. longipes was not abundant in the water column of the western Weddell Sea. A similar situation was observed on the eastern Weddell Sea shelf for most of the study period, however, after a strong storm S. longipes contributed a relatively large amount (8.8 %) of the calanoid copepods present on the last sampling date. The S. longipes population was then strongly dominated by C I (53 %) that had probably been released from the sea ice into the under ice water layer due to ice break-up and ice melt. Although copepods were abundant in the water column, the vertical particle flux close to the sea floor on the eastern Weddell Sea shelf did not contain any copepod faecal pellets, similar to the situation observed under the ice floe in the western Weddell Sea. The results indicate that sympagic copepods are main mediators of carbon cycling and nutrient regeneration in Antarctic sea ice, thus contributing considerably to the productivity of sea ice communities. A considerable amount of copepod faecal matter produced within the ice may be transported into the under-ice water layer and provide an important food source for pelagic grazers. In the pelagial major amounts of copepod faecal pellets do not sink to depth and are probably rapidly degraded and recycled. The direct contribution of copepods to the vertical flux of particulate organic matter in the water column is thus relatively small. However, large amounts of faecal pellets produced by sympagic and pelagic copepods might be transported to the seafloor within marine snow and serve as important food source for the benthos. Accordingly, the contribution of copepods to flux of particulate organic matter from sea ice into the water column and down to the seafloor may facilitate the coupling between the surface and the deep water layers in the Southern Ocean. Furthermore, copepods may contribute to pelago-benthic coupling due to vertical migration and as prey both for omnivorous and carnivorous organisms in deep water layers, and for benthic organisms.