Pelagic microbial heterotrophy in response to a highly productive bloom of Phaeocystis antarctica in the Amundsen Sea Polynya, Antarctica

Abstract

Heterotrophic bacteria play a key role in marine carbon cycling, and understanding their activities in polar systems is important for considering climate change impacts there. One goal of the ASPIRE project was to examine the relationship between the phytoplankton bloom and bacterial heterotrophy in the Amundsen Sea Polynya (ASP). Bacterial abundance, production (BP), respiration, growth efficiency, and extracellular enzyme activity (EEA) were compared to nutrient and organic matter inventories, chlorophyll a (Chl a), viral and microzooplankton abundance, and net primary production (NPP). Bacterial production and respiration clearly responded (0.04-4.0 and 10-53 µg C L −1 d −1 , respectively) to the buildup of a massive Phaeocystis antarctica bloom (Chl a: 0.2-22 µg L −1 ), with highest rates observed in the central polynya where Chl a and particulate organic carbon (POC) were greatest. The highest BP rates exceeded those reported for the Ross Sea or any other Antarctic coastal system, yet the BP:NPP ratio (2.1-9.4%) was relatively low. Bacterial respiration was also high, and growth efficiency (2-27%; median = 10%) was similar to oligotrophic systems. Thus, the integrated bacterial carbon demand (0.8-2.8 g C m −2 d −1 ) was a high fraction (25-128%; median = 43%) of NPP during bloom development. During peak bloom, activity was particle-associated: BP and EEA correlated well with POC, and size fractionation experiments showed that the larger size fraction (> 3 µm) accounted for a majority (∼ 75%) of the BP. The community was psychrophilic, with a 5x reduction in BP when warmed to 20°C. In deeper waters, respiration remained relatively high, likely fueled by the significant downward particle flux in the region. A highly active, particle-associated, heterotrophic microbial community clearly responded to the extraordinary phytoplankton bloom in the ASP, likely limiting biological pump efficiency during the early season.

Fe availability drives phytoplankton photosynthesis rates during spring bloom in the Amundsen Sea Polynya, Antarctica

“…Bacterial productivity in the incubation experiments was similar to that reported in the high productivity stations in the central ASP, which was high compared to other Antarctic polynyas (see Williams et al, 2015, for a full description). In contrast to phytoplankton growth and photosynthesis, bacterial productivity did not respond to Fe addition, suggesting that bacteria are not Fe-limited during the early season in the ASP.…”

“…In addition, despite the increase in bacterial productivity over the four days of incubation, there was no secondary response of bacterial productivity to either the Fe-enhanced phytoplankton growth or photosynthesis rates. Bacterial productivity in surface waters of the ASP showed a positive relationship with phytoplankton biomass (Williams et al, 2015), suggesting bacterial productivity was coupled to phytoplankton biomass and productivity. Likely, the Fe effects on phytoplankton biomass, cellular composition, and photosynthesis rates in our experiments were not sufficient to yield a detectable response in bacterial productivity during the time scale of the incubations.…”

“…Bacterial productivity was estimated in the bioassay experiments from incorporation of 3 H-leucine into protein (Williams et al, 2015). Each of the incubation bottles served as a replicate in the measurements.…”

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

“…The major benefits of using such a tool over more traditional (mostly linear) models are: Regression trees allow for (1) non-linear relationships between input and output variables, (2) non-additive interactions between predictor and response variables and (3) correlations between different input variables (Breiman, 2001;Shi and Horvath, 2006). The relevant variables included were phytoplankton biomass, macronutrients, MLD, temperature, salinity, total irradiance and the relative abundance of the three most important phytoplankton groups, namely diatoms, haptophytes and cryptophytes (Behrenfeld and Falkowski, 1997;Arrigo et al, 1999;Dierssen and Smith, 2000;Vernet et al, 2008Vernet et al, , 2012Montes-Hugo et al, 2009;Piquet et al, 2011;Venables et al, 2013;Williams et al, 2016;Bown et al, 2017;Rozema et al, 2017a,b).…”

Particle flux on the continental shelf in the Amundsen Sea Polynya and Western Antarctic Peninsula

“…Net community production and export, were derived from changes in nutrient and carbon inventories . Bacterial production and community respiration rates, measured using 3 H-leucine incorporation and CO 2 production during 24-48 hour dark incubations, were obtained from Williams et al (2015).…”