Phytoplankton dynamics in relation to hydrography, nutrients and zooplankton at the onset of sea ice formation in the eastern Weddell Sea (Antarctica)

Abstract

The quantitative and qualitative distribution of phytoplankton was investigated along five NorthSouth transects in the eastern Weddell Sea during the transition from late autumn to winter. Relationships with the regional hydrography, progressing sea ice coverage, nutrient distribution and zooplankton are discussed and compared with data from other seasons. To the north of the Antarctic Slope Front (ASF) a remnant temperature minimum layer was found above the primary pycnocline throughout summer. Surface waters had not entirely acquired typical winter characteristics. While temperature was already in the winter range, this was not the case for salinity. Highest biomass of phytoplankton, with the exception of the first transect, was found in the region adjoining the ASF to the north. Absolute chlorophyll a (Chl a) concentrations dropped from 0.35 to 0.19 lg l À1 . Nutrient pools exhibited a replenishing tendency. Ammonium concentrations were high (0.75-2 lmol l À1 ), indicating extensive heterotrophic activity. The phytoplankton in the ASF region was dominated by nanoflagellates, particularly Phaeocystis spp.. North of the ASF the abundance of diatoms increased, with Fragilariopsis spp., F. cylindrus and Thalassiosira spp. dominating. Community structure varied both due to hydrographical conditions and the advancing ice edge. The phytoplankton assemblage formed during late autumn were very similar to the ones found in early spring. A POC/PON ratio close to Redfield, decreasing POC concentration and a high phaeophytin/ Chl a ratio, as well as a high abundance of mesozooplankton indicated that a strong grazing pressure was exerted on the phytoplankton community. A comparison between primary production (PP) in the water column and the sea ice showed a shift of the major portion of PP into the ice during the period of investigation.

Whole season net community production in the Weddell Sea

“…4), hence non-diatoms may play a signiWcant role there as well. This spatial distribution appears to be in agreement with phytoplankton studies in the Weddell Sea (e.g., Krell et al 2005). An apparent paradox is that in the near-margin area where non-diatoms are signiWcant, the Si depletion is the highest of the entire transect (Fig.…”

Holocene productivity changes off Adélie Land (East Antarctica)

“…Other siliceous organisms present low abundances in core MD03‐2601, generally not in phase with PE. Third, other sources such as bacteria, broken diatoms and other phytoplankton, and fecal pellets can export BSi, C org and N org [ Krell et al , 2005]. These fractions are difficult to assess downcore.…”

Community structure of under-ice fauna in relation to winter sea-ice habitat properties from the Weddell Sea

“…The decrease in surface salinities below typical Winter Water values (\34.4) (Krell et al 2005) suggests melting had already started at the time of our sampling. Copepod abundances were low, while some other species, e.g.…”