Physical forcing of phytoplankton community structure and primary production in continental shelf waters of the Western Antarctic Peninsula

Abstract

Analyses of a multidisciplinary data set, collected in continental shelf waters of the Western Antarctic Peninsula (WAP) during austral summer of January 1993, identified a previously unrecognized forcing mechanism that sets up a physical and chemical structure that supports and assures site-specific diatom-dominated communities and enhanced biological production . This forcing is active when the southern boundary of the Antarctic Circumpolar Current (ACC) flows along the shelf edge, thereby facilitating onshelf bottom intrusions of nutrient-rich Upper Circumpolar Deep Water (UCDW), which then is upwelled or mixed into the upper water column. At times or locations where UCDW is not introduced to the upper water column, diatoms no longer dominate phytoplankton assemblages over the mid-to outer WAP continental shelf. This analysis extends the area and seasons studied through similar analyses of multidisciplinary data sets collected on four additional cruises to the WAP that cover all seasons. Results show that onshelf intrusions of UCDW: (1) occur in other regions of the WAP continental shelf; (2) are episodic; (3) are forced by nonseasonal physical processes; and (4) produce areas of diatom-dominated phytoplankton assemblages and enhanced primary production. At times, multiple intrusions are observed on the WAP continental shelf, and each event may be in a different stage. Further, the occurrence of an intrusion event in one area does not necessarily imply that similar events are ongoing in other areas along the WAP shelf. The UCDW bottom intrusions originate along the outer shelf but they can extend into the inner shelf region because the deep troughs that transect the WAP shelf provide connections between the inner and outer shelf. The boundary between the intruded water and the shelf water is variable in location because of the episodic nature of the onshelf intrusions, and is moved farther inshore as an event occurs. These observations show clearly that the phytoplankton community structure on the WAP shelf is determined by physical forcing and that primary production is likely to be considerably greater than previously believed. Moreover, variability in this physical forcing, such as may occur via climate change, can potentially affect the overall biological production of the WAP continental shelf system.

Ocean circulation and dynamics on the west Antarctic Peninsula continental shelf

“…• S, along the eastern side of Marguerite Trough. Warm water at these two sites was also reported in previous work examining data collected during hydrographic surveys conducted in 1993 and 1994, which also identified two other intrusion sites to the north of the SO GLOBEC study region (Prézelin et al, 2000;Prézelin et al, 2004).…”

“…Hydrographic data from a summer cruise conducted in 1993 suggested that CDW intruded in four specific locations along the shelf, two of which are within the SO GLOBEC study region (Prézelin et al, 2000;Prézelin et al, 2004). Remnants of CDW were found along Marguerite Trough during the SO GLOBEC cruises, which also suggested that this bathymetric feature, connecting the shelfbreak to Marguerite Bay, provides a path for CDW intrusions to move across the shelf .…”

Influence of oceanographic features on spatial and interannual variability of phytoplankton in the Bransfield Strait, Antarctica

“…Finally, a phytoplankton community co-dominated primarily by very small diatoms and other unidentified flagellates are associated with low Chl-a (Garibotti et al, 2005;Moline and Prézelin, 1996). There are numerous sources in the literature on phytoplankton dynamics in some wAP systems, principally near Marguerite Bay (Prézelin et al, 2004 and references therein).…”

Diversity of the diatom genus Fragilariopsis in the Argentine Sea and Antarctic waters: morphology, distribution and abundance

“…Productive areas, such as the Western Antarctic Peninsula (Walsh et al 2001;Garibotti et al 2003;Prézelin et al 2004), the Weddell Sea (Fryxell and Kendrick 1988), Prydz Bay (Kopczyńska et al 1995), the Bellingshausen Sea (Barlow et al 1998;van Leeuwe et al 1998) and the Ross Sea (Arrigo et al 1999), tend to be rich in diatoms (Kopczyńska et al 2007;Kozlowski 2008). Productive waters of the Argentine Sea (Southwestern South Atlantic) are also rich in diatoms (e.g., Carreto et al 1981;Akselman 1996;Almandoz et al 2007).…”