Abstract
Abstract Antarctic seas, and particularly the Amundsen Sea Polynya, are some of the most productive oceanic regions on Earth. Ice-algal production during austral spring is followed by open-water pelagic production later in the season. Although ice-free growth accounts for a greater percentage of the annual net primary production, ice algae provide an important source of nutrients to organisms throughout the water column and benthos in areas and seasons when open-water production is insignificant. The objectives of this study were to assess the bioavailability of dissolved organic matter (DOM), sourced from ice algae or the chlorophyll maximum (chl max), to marine bacterioplankton and to determine the fate of carbon within these different DOM pools, including loss to respiration, incorporation into bacterial biomass and retention within the DOM pool itself. Nutrient concentrations and bacterial abundance, production, and cell volume were monitored during a 7-day bioassay study involving four treatments conducted shipboard in the Amundsen Sea Polynya, Antarctica. The greatest response in bacterial abundance and activity was observed when ice-algal meltwater was supplied to aphotic zone bacterioplankton collected from 170-m depth. However, bacterial growth efficiency was higher (24%) when chl max water was supplied to the same aphotic zone bacterial community compared to the bacterial growth efficiency of the ice-algal treatment (15%). Approximately 15% of dissolved organic carbon (DOC) from the ice-algal source and 18% from the chl max was consumed by aphotic bacterial communities over the relatively short, one-week incubation. In contrast, 65% of the dissolved organic nitrogen (DON) added as an integral part of the ice-algal DOM was consumed, but none of the DON supplied with chl max water was labile. This study underscores the importance of considering DOM sources when investigating or predicting changes in carbon and nitrogen cycling within the Amundsen Sea.
Highlights
Antarctic shelves are extremely productive systems that play a crucial role in carbon and nitrogen cycles (Sarmiento et al, 2004; Vancoppenolle et al, 2013)
The experiment consisted of four dissolved organic matter (DOM)-containing bioassay treatments run in duplicate: ice-algal meltwater inoculated with aphotic zone bacterioplankton, chl max water inoculated with aphotic zone bacterioplankton, aphotic zone water inoculated with chl max bacterioplankton, and aphotic zone water inoculated with aphotic zone bacterioplankton
A 200-m depth profile of the Amundsen Sea Polynya site showed elevated photosynthetically active radiation (PAR), temperature, fluorescence and oxygen concentrations at the surface that generally decreased with depth (Figure 1)
Summary
Antarctic shelves are extremely productive systems that play a crucial role in carbon and nitrogen cycles (Sarmiento et al, 2004; Vancoppenolle et al, 2013). High concentrations of nitrate and increasing inputs of iron from surrounding melting glaciers make the Amundsen Sea a highly productive sink for carbon dioxide (Alderkamp et al, 2012; Yager et al, 2012) and a source of dissolved organic matter (DOM; Williams et al, 2015). In Antarctic seas, phytoplankton and ice algae are the dominant sources of DOM (Knox, 2007) that fuel heterotrophic bacterial growth and respiration. Ice-algal production is made available to planktonic consumers during ice melt, which can release high concentrations of organic matter into the water column (Fischer et al, 1988; Knox, 1990; Thomas and Dieckmann, 2002). Location, timing and intensity of primary production in the Antarctic appear to be intrinsically linked
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