Wintertime controls on summer stratification and productivity at the western Antarctic Peninsula

Abstract

We report results collected year-round since 1998 in northern Marguerite Bay, just inside the Antarctic Circle. The magnitude of the spring phytoplankton bloom is much reduced following winters with reduced sea-ice cover. In years with little winter sea-ice the exposed sea surface leads to deep mixed layers in winter, and reduced watercolumn stratification the following spring. Summer mixed-layer depths are similar, however, so the change is not in overall light availability but toward a less stable water column with greater vertical mixing and increased variability in the light conditions experienced by phytoplankton. Macronutrient concentrations are replete at all times, but the increased vertical mixing likely reduces iron availability. The timing of bloom initiation is similar between heavy and light ice years, occurring soon after light returns in early spring, at a mixed-layer averaged light level of , 1 mol photon m 22 d 21 . Ongoing regional climate change in the WAP area, and notably the ongoing loss of winter sea-ice, is likely to drive a downward trend in the magnitude of phytoplankton blooms in this region of the Antarctic Peninsula.

Feedbacks between ice cover, ocean stratification, and heat content in Ryder Bay, western Antarctic Peninsula

“…As has been discussed previously [Venables et al, 2013], the variation in ice cover in Ryder Bay strongly influences winter MLD (Figure 3a). This is as expected due to reduced sea ice, especially fast ice, exposing the sea surface to increased mechanical mixing from wind stress.…”

“…Water column sampling has been carried out at Rothera Research Station (Figure 1) since 1998 [Venables et al, 2013], as part of the Rothera oceanographic and biological time series (RaTS). The primary RaTS site (67.570 S, 68.225 W) is 520 m deep ( Figure 1) and located at the deepest point of Ryder Bay, about 4 km from the base.…”

“…This study reports on 15 years of continuous oceanographic sampling from the British Antarctic Survey base at Rothera, just inside the Antarctic Circle on the western Antarctic Peninsula Meredith et al, 2010;Venables et al, 2013]. We present year-round water column data and analyze the effects of changing sea ice cover through the course of the time series.…”

“…At the Antarctic Peninsula, the ice declines significantly influence the ecosystem [Ducklow et al, 2007] with the associated changes in hydrography leading to sensitive effects between ice and phytoplankton along the Antarctic margins [Venables et al, 2013;Williams et al, 2008]. Linked to the ice loss, atmospheric winter temperatures are increasing at 1 C per decade [Turner et al, 2005].…”

Changing distributions of sea ice melt and meteoric water west of the Antarctic Peninsula

“…Each of the contributors to the freshwater budget of the Southern Ocean (sea ice, glacial melt from Antarctica, iceberg calving and subsequent melt, precipitation) has the capacity to influence marine ecology and biodiversity, not only by shaping the physical environment, but also by influencing habitat suitability and the biogeochemistry of the marine system (Boyd and Ellwood, 2010;Ducklow et al, 2012;Raiswell, 2011;Venables et al, 2013).…”