Abstract
Weddell Sea hydrography and circulation is driven by influx of Circumpolar Deep Water (CDW) from the Antarctic Circumpolar Current (ACC) at its eastern margin. Entrainment and upwelling of this high-nutrient, oxygen-depleted water mass within the Weddell Gyre also supports the mesopelagic ecosystem within the gyre and the rich benthic community along the Antarctic shelf. We used Conductivity-Temperature-Depth Satellite Relay Data Loggers (CTD-SRDLs) to examine the importance of hydrographic variability, ice cover and season on the movements and diving behavior of southern elephant seals in the eastern Weddell Sea region during their overwinter feeding trips from Bouvetøya. We developed a model describing diving depth as a function of local time of day to account for diel variation in diving behavior. Seals feeding in pelagic ice-free waters during the summer months displayed clear diel variation, with daytime dives reaching 500-1500 m and night-time targeting of the subsurface temperature and salinity maxima characteristic of CDW around 150–300 meters. This pattern was especially clear in the Weddell Cold and Warm Regimes within the gyre, occurred in the ACC, but was absent at the Dronning Maud Land shelf region where seals fed benthically. Diel variation was almost absent in pelagic feeding areas covered by winter sea ice, where seals targeted deep layers around 500–700 meters. Thus, elephant seals appear to switch between feeding strategies when moving between oceanic regimes or in response to seasonal environmental conditions. While they are on the shelf, they exploit the locally-rich benthic ecosystem, while diel patterns in pelagic waters in summer are probably a response to strong vertical migration patterns within the copepod-based pelagic food web. Behavioral flexibility that permits such switching between different feeding strategies may have important consequences regarding the potential for southern elephant seals to adapt to variability or systematic changes in their environment resulting from climate change.
Highlights
The Weddell Sea plays an important role in the Southern Ocean circulation system
Following the general largescale circulation patterns [3,4] and broad ecological regions described for the Weddell Sea [52], we divided the study area into four broad oceanic regimes (Fig. 1B): the Antarctic Circumpolar Current Regime (ACCR); the Weddell Cold Regime (WCR); the Weddell Warm Regime (WWR); and the Dronning Maud Land Shelf Regime (SR)
Two adult females travelled directly southwards until they reached the frontal region between the Weddell Cold Regime (WCR) and Weddell Warm Regime (WWR), where they remained for several weeks before they too continued to the shelf
Summary
The Weddell Sea plays an important role in the Southern Ocean circulation system. In addition to being one of the most important regions of Antarctic Bottom Water formation [1] and a globally significant contributor to natural CO2 sequestration [2], the Weddell Sea supports rich and diverse ecosystems. They have substantially higher total CO2 compared to the source waters to the southeast [2], implying local enrichment due to high biological activity This local biological activity results from Ekman pumping within the cyclonic Weddell Gyre which causes upwelling of the high-nutrient, low-O2 WDW towards the surface, where it stimulates biological primary production and subsequent grazing by zooplankton. This productivity is further increased during ice retreat because of released nutrients and exposure of the water column to light, leading to substantial spring phytoplankton blooms. The CDW appears to play a crucial role for physical as well as biological processes within the Weddell Gyre, and water mass distribution and mixing processes are likely to influence the abundance and the horizontal and vertical distribution of organisms in these high-Antarctic marine ecosystems substantially
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