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

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 m22 d21. 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. Within the Southern Hemisphere, the region that is changing most rapidly is the western Antarctic Peninsula (WAP), which has experienced one of the strongest warmings of anywhere in the world (King 1994; Vaughan et al. 2003). The annual mean atmospheric warming at the WAP is 3.7 6 1.6uC century-1 unweighted, or 3.4uC century-1 when weighted by length of record (Vaughan et al. 2003). This warming is most pronounced in austral autumn and winter (Turner et al. 2005). Although the full mechanisms controlling the warming have not yet been elucidated, it has been linked to atmospheric circulation changes. Warm winters at the WAP are associated with more cyclonic circulation and increased warm air advection. Increased precipitation at the WAP in recent decades (Thomas et al. 2008) is consistent with the hypothesis that the warming trend is accompanied by a shift toward more cyclonic conditions. Increased temperatures and shifts in the wind pattern have affected the length of the sea-ice season along the WAP. Sea-ice duration has declined significantly to the west of the Antarctic Peninsula. Changes in duration show a strong trend toward a later advance in autumn and a weaker trend toward an earlier retreat in spring (Stammerjohn et al. 2008). Superimposed on this trend are anomalous years of more extreme conditions that are linked to variations in coupled modes of climate variability, such as the Southern Annular Mode (SAM) and the El Nino–Southern Oscillation (ENSO) phenomenon (Meredith et al. 2004; Massom et al. 2006). In addition to reduced sea-ice extent, 80% of glaciers on the Antarctic Peninsula are retreating, and retreat rates are accelerating (Cook et al. 2005). This retreat includes the Sheldon Glacier that flows into Ryder Bay at the northern edge of Marguerite Bay (Fig. 1; Peck et al. 2010). The ocean also affects regional warming via the on-shelf flow of Circumpolar Deep Water (CDW) from the Antarctic Circumpolar Current, which lies immediately adjacent to the continental shelf break (Klinck 1998) at the WAP. The CDW water mass is warmer than surface water, being initially < 2uC before modification by mixing with cooler shelf water. Because of strengthening westerly winds associated with an increasing trend in the SAM, it is likely that the on-shelf flow of warm CDW has increased in recent decades, with an associated rise in heat flux (Martinson et al. 2008). The effects of reducing sea-ice cover on local hydrography and productivity have been studied at various locations in the Arctic and Antarctic (Clarke et al. 2007; Tremblay et al. 2008). The effect on productivity can be both positive and negative, depending on the local conditions and the date of ice breakup. A shift to open water from permanent ice cover removes the shading effect of ice and can lead to a significant increase in productivity (Arrigo et al. 2008; Montes-Hugo et al. 2009). If, however, the presence of ice early in the growth season increases stratification, then a reduction in productivity can result (Vernet et al. 2008; Montes-Hugo et al. 2009). Changes in salinity, from changes in sea ice or glacial melt, can also affect phytoplankton community composition (Moline et al. 2004). The relationship between sea-ice dynamics and biological productivity is thus complex. Changes in ice cover also have implications for higher trophic levels (Ducklow et al. 2007; Schofield et al. 2010). Zooplankton, especially krill, are dependent on sea ice (Atkinson et al. 2004) and highly sensitive to sea-ice changes (Wiedenmann et al. 2009). However, consequences of altered sea-ice conditions on zooplankton populations are extremely hard to predict with any accuracy. This potential for change along the Antarctic Peninsula highlights the importance of long-term studies of the phytoplankton community (Schloss et al. 2012) and the export * Corresponding author: [email protected] Limnol. Oceanogr., 58(3), 2013, 1035–1047 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.3.1035