Some environmental factors influencing phytoplankton in the Southern Ocean around South Georgia

Data on phytoplankton and zooplankton biomass, and physical and chemical variables, are combined with a published multivariate description of diatom species composition to interpret variation within an area around South Georgia surveyed during an austral summer. Large-scale species distributions could be equated to the different water masses which reflected the interaction of the Antarctic Circumpolar Current with the island and the Scotia Ridge. Small-scale factors were found to act at an interstation scale and imposed local variation on the biogeographic pattern. Nutrient depletion could be related to phytoplankton biomass but no single inorganic nutrient of those measured (NO 3 −N, PO 4 −P and silica) could be identified as important. The ratio Si:P appeared to be more important as an ecological factor. The impact of grazing by krill and other zooplankton could only be resolved as differences in phytoplankton biomass and phaeopigment content. Diatom species composition showed a relation to local krill abundance very different from that suggested by published studies, but could be explained as the effect of earlier grazing outside the study area. The effects of vertical mixing could not account for interstation differences as pycnocline depth was uniformly greater than euphotic depth, and vertical stability very low. Some comparison was made with data collected in 1926–31 by the Discovery Investigations. Significant differences in the distribution of certain taxa such as Chaetoceros criophilum and C. socialis were traced to major differences in hydrology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46983/1/300_2004_Article_BF00443379.pd

Meso- and macrozooplankton communities in the Weddell Sea, Antarctica

The present paper describes composition and abundance of meso- and macrozooplankton in the epipelagic zone of the Weddell Sea and gives a systematic review of encountered species regarding results of earlier expeditions. Material was sampled from 6 February to 10 March 1983 from RV Polarstern with a RMT 1+8 m (320 and 4500 μm mesh size). In agreement with topography and water mass distribution three distinct communities were defined, clearly separated by cluster analysis: The Southern Shelf Community has lowest abundances (approx. 9000 ind./1000 m3). Euphausia crystallorophias and Metridia gerlachei are predominating. Compared with the low overall abundance the number of regularly occurring species is high (55) due to many neritic forms. Herbivores and omnivores are dominating (58% and 35%). The North-eastern Shelf Community has highest abundances (about 31 000 ind./1000 m3). It is predominated by copepodites I–III of Calanus propinquus and Calanoides acutus (61%). The faunal composition is characterized by both oceanic and neritic species (64). Fine-filter feeders are prevailing (65%). The Oceanic Community has a mean abundance of approximately 23 000 ind./1000 m3, consisting of 61 species. Dominances are not as pronounced as in the shelf communities. Apart from abundant species like Calanus propinquus, Calanoides acutus, Metridia gerlachei, Oithona spp. and Oncaea spp. many typical inhabitants of the Eastwind Drift are encountered. All feeding types have about the same importance in the Oceanic Community

Krill motion in the Southern Ocean: quantifying in situ krill movement behaviors and distributions during the late austral autumn and spring

Krill movement behaviors and vertical distributions were measured in spring and autumn using a profiling stereo-camera and environmental sensor system to quantify seasonal changes in the role of krill in Southern Ocean food webs. Krill were observed in May–June 2013 and December 2014 in 3 bays in the Western Antarctic Peninsula. Krill abundances and movement behaviors were determined from in situ image sequences collected for up to 10 min throughout the water column, up to 625 m deep; 3,345 individual krill tracks were collected. Seasonal changes in individual krill behaviors coincided with seasonal shifts in krill vertical distributions. During late spring, net upward swimming direction (0.9 ± 2.1° from horizontal) and vertical velocity (0.3 ± 0.2 body lengths [BL] s−1) resulted in shallower maximum abundances of krill within the water column proximate to near-surface phytoplankton distributions. During late autumn, krill swimming patterns tended downward, including swimming direction (−5.2 ± 0.8° from horizontal) and vertical velocity (−0.1 ± 0.0 BL s−1), leading to deeper distributions proximate to the benthos. Individual krill motility was greater in spring than autumn, as evidenced by an increase in swimming speeds (5.4 ± 0.2 vs. 2.8 ± 0.0 BL s−1) and turning rates (120 ± 5 vs. 107 ± 2° s−1). Remarkably, krill in autumn were capable of swimming as quickly as krill in spring. These results suggest seasonal shifts in krill movement behaviors have direct ramifications for krill distributions, proximity to food sources, and impacts on biogeochemical cycling in coastal Antarctic waters