Abstract
We investigated the dynamics of heterotrophic bacteria in the coastal western Antarctic Peninsula (WAP), using decadal (2002-2014) time series of two bacterial variables, bacterial production (BP) via 3H-leucine incorporation rates and bacterial biomass (BB) via bacterial abundance, collected at Palmer Antarctica Long Term Ecological Research (LTER) Station B (64.8°S, 64.1°W) over a full austral growing season (October-March). Strong seasonal and interannual variability in the degree of bacterial coupling with phytoplankton processes were observed with varying lags. On average, BP was only 4% of primary production (PP), consistent with low BP:PP ratios observed in polar waters. BP was more strongly correlated with chlorophyll (Chl), than with PP, implying that bacteria feed on DOC produced from a variety of trophic levels (e.g. zooplankton sloppy feeding and excretion) as well as directly on phytoplankton-derived DOC. The degree of bottom-up control on bacterial abundance was moderate and relatively consistent across entire growing seasons, suggesting that bacteria in the coastal WAP are under consistent DOC limitation. Temperature also influenced BP rates, though its effect was weaker than DOC. We established generalized linear models (GLMs) for monthly composites of BP and BB via stepwise regression to explore a set of physical and biogeochemical predictors. Physically, high BP and large BB were shaped by a stratified water-column, similar to forcing mechanisms favoring phytoplankton blooms, but high sea surface temperature (SST) also significantly promoted bacterial processes. High BP and large BB were influenced by high PP and bulk DOC concentrations. Based on these findings, we suggest an increasingly important role of marine heterotrophic bacteria in the coastal WAP food-web as climate change introduces a more favorable environmental setting for promoting BP, with increased DOC from retreating glaciers, a more stabilized upper water-column from ice-melt, and a baseline shift of water temperature due to more frequent delivery of warming Upper Circumpolar Deep Water (UCDW) onto the WAP shelf.
Highlights
Heterotrophic bacteria utilize and remineralize dissolved organic carbon (DOC) and organic nutrients, mobilize carbon for upper trophic levels via microzooplankton grazing, and affect carbon fluxes and cycling in the ocean (Azam et al, 1983; Ducklow, 1983)
To see if bacterial responses to physical and biogeochemical forcing factors differ outside of the DJF period, we examined bacterial production (BP) and bacterial biomass (BB) generalized linear models (GLMs) over entire phytoplankton growing seasons (October to March or ONDJFM)
Our seasonally-extended analysis of bacterial dynamics using a decadal (2002–2014) time series of BP and BB revealed a set of patterns which have not been identified previously due largely to the short-term, temporally limited focus of most previous studies
Summary
Heterotrophic bacteria utilize and remineralize dissolved organic carbon (DOC) and organic nutrients, mobilize carbon for upper trophic levels via microzooplankton grazing, and affect carbon fluxes and cycling in the ocean (Azam et al, 1983; Ducklow, 1983). Bacteria must rely on in situ DOC produced by phytoplankton (Baines and Pace, 1991; Nagata, 2000) or from other trophic levels and processes such as zooplankton grazing, sloppy feeding, excretion, and cell lysis (Strom et al, 1997). Bacterial coupling with phytoplankton has been demonstrated in the coastal WAP (Ducklow et al, 2012; Saba et al, 2014), consistent with other observations from Antarctic waters (Morán et al, 2001; Morán and Estrada, 2002). Bacterial coupling with phytoplankton has been demonstrated in the coastal WAP (Ducklow et al, 2012; Saba et al, 2014), consistent with other observations from Antarctic waters (Morán et al, 2001; Morán and Estrada, 2002). Saba et al (2014) demonstrated that bacterial blooms occurred in years of positive chlorophyll (Chl) anomalies. Ducklow et al (2012) showed that the degree of coupling between phytoplankton and bacterial properties was variable depending on the space and time scales analyzed
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