Early Spring Phytoplankton Dynamics in the Western Antarctic Peninsula

Abstract

The Palmer Long‐Term Ecological Research program has sampled waters of the western Antarctic Peninsula (wAP) annually each summer since 1990. However, information about the wAP prior to the peak of the phytoplankton bloom in January is sparse. Here we present results from a spring process cruise that sampled the wAP in the early stages of phytoplankton bloom development in 2014. Sea ice concentrations were high on the shelf relative to nonshelf waters, especially toward the south. Macronutrients were high and nonlimiting to phytoplankton growth in both shelf and nonshelf waters, while dissolved iron concentrations were high only on the shelf. Phytoplankton were in good physiological condition throughout the wAP, although biomass on the shelf was uniformly low, presumably because of heavy sea ice cover. In contrast, an early stage phytoplankton bloom was observed beneath variable sea ice cover just seaward of the shelf break. Chlorophyll a concentrations in the bloom reached 2 mg m−3 within a 100–150 km band between the SBACC and SACCF. The location of the bloom appeared to be controlled by a balance between enhanced vertical mixing at the position of the two fronts and increased stratification due to melting sea ice between them. Unlike summer, when diatoms overwhelmingly dominate the phytoplankton population of the wAP, the haptophyte Phaeocystis antarctica dominated in spring, although diatoms were common. These results suggest that factors controlling phytoplankton abundance and composition change seasonally and may differentially affect phytoplankton populations as environmental conditions within the wAP region continue to change.

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

“…During our spring cruise, the phytoplankton community in the WAP was largely dominated by P. antarctica (Arrigo et al, ; Selz et al, ), as has been previously observed (Annett et al, ), but is distinct from the typically reported diatom dominance during summer (Rozema et al, ; Trimborn et al, ). However, Prézelin et al () suggest that WAP species composition may be climatically forced rather than locally forced; hence, it is possible that our observation of P. antarctica dominance in the spring is not the norm.…”

“…We found no evidence that dFe concentration limited NPP in early spring (this study; Arrigo et al, ). Although many stations had dFe concentrations that hovered around the growth‐limiting level of 0.1 nM (Sedwick et al, ), we observed no evidence of dFe stress in phytoplankton (Arrigo et al, ).…”

“…Similarly, in the Amundsen Sea's Pine Island Polynya, Park et al (2017) found that light rather than dFe controls the magnitude of summer phytoplankton blooms. In both of these cases, dFe was sufficient so as to not limit phytoplankton growth: in the springtime WAP, this was because dFe resupplied during the winter had not yet been fully consumed (this study; Arrigo et al, 2017); in the summertime Pine Island Polynya, continual glacial melt provided sufficient dFe input so as to not limit growth (Park et al, 2017). Our results add further support to the conclusion that light is the dominant controlling factor for phytoplankton growth in the WAP in the spring, although there are regions where dFe limits growth in both the spring and summer (see, for example, the Amundsen Sea Polynya in Alderkamp et al (2015) and Park et al (2017)).…”

“…CHEMTAX identifies high‐level taxa, while species composition analyzed visually using FlowCam data (described in section ) enables identification often to the genus level. Photosynthetic accessory pigments (PSP; 19′‐butanoyloxyfucoxanthin, 19′‐hexanoyloxyfucoxanthin, fucoxanthin, peridinin, and prasinoxanthin), photoprotective pigments (PPP; diadinoxanthin (DD), diatoxanthin (DT), violaxanthin, zeaxanthin), and non‐photosynthetic carotenoids (NPC; zeaxanthin, DD, alloxanthin, β ‐carotene) were compared by normalizing to Chl a (Arrigo et al, ; Higgins et al, ; Van Leeuwe et al, ).…”

“…While annual and decadal shifts in coastal Antarctic ecosystems have been documented (Ducklow et al, 2007;Montes-Hugo et al, 2009; see review by Smith et al, 2012), connecting the changing physical and environmental conditions with observed ecosystem changes remains a challenge (Schofield et al, 2010). Although the WAP has been studied extensively through the Palmer Long-Term Ecological Research (PAL-LTER) program during the summer (Ducklow et al, 2007), we lack spring data in this region and hence a thorough understanding of the seasonal transition from winter to summer (Arrigo et al, 2017). This study seeks to fill this gap by characterizing controls on early season primary production in the WAP.…”