Abstract
The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica between 2002 and 2005 to acquire a more complete understanding of the mechanisms and rates of carbon export from the euphotic layer. Aggregate abundance was determined by photographic techniques, and water column parameters (temperature, salinity, fluorescence, transmissometry) were assessed from CTD profiles. During the first three years the numbers of aggregates increased seasonally, being much more abundant within the upper 200 m in late summer than in early summer from 50 to 100 m (12.5 L–1 in early summer vs. 42.9 L–1 in late summer). In Year 4 aggregate numbers were substantially greater than in other years, and average aggregate abundance was maximal in early rather than late summer (177 vs. 84.5 L–1), which we attributed to the maximum biomass and aggregate formation being reached earlier than in other years. The contribution of aggregate particulate organic carbon to the total particulate carbon pool was estimated to be 20%. Ghost colonies, collapsed colonies of the haptophyte Phaeocystis antarctica, were observed during late summer in Year 4, with maximum numbers in the upper 100 m of ca. 40 L–1. Aggregate abundance, particulate organic carbon and ghost colonies all decreased exponentially with depth, and the rate of ghost colony disappearance suggested that their contribution to sedimentary input was small at the time of sampling. Bottom nepheloid layers were commonly observed in late summer in both transmissometer and aggregate data. Late summer nepheloid layers had fluorescent material within them, suggesting that the particles were likely generated during the same growing season. Longer studies encompassing the entire production season would be useful in further elucidating the role of these aggregates in the carbon cycle of these regions.
Highlights
The Ross Sea is the largest area of elevated pigment concentrations and primary productivity in the Antarctic, and is characterized by intense phytoplankton blooms during austral spring and summer (Smith et al, 2014)
The blooms of P. antarctica are notable for the significant amounts of organic matter that are partitioned into its mucoid envelope (Mathot et al, 2000), and these blooms reduce the dissolved iron to limiting concentrations by mid-November (Sedwick et al, 2011)
The early summer cruises were always characterized by large surface chlorophyll and particulate organic carbon concentrations and relatively shallow mixed layers (Table 2; Smith et al, 2011a); by February the biomass levels had decreased in most years, and mixed layers had begun to deepen (Table 2)
Summary
The Ross Sea is the largest area of elevated pigment concentrations and primary productivity in the Antarctic, and is characterized by intense phytoplankton blooms during austral spring and summer (Smith et al, 2014). These blooms are critical contributors to biogeochemical cycles and food web dynamics of the region (Arrigo et al, 2008; Smith et al, 2014). In most years the colonial haptophyte Phaeocystis antarctica blooms in austral spring, followed by a diatom growth and accumulation in summer (Arrigo et al, 1999; Smith et al, 2000, 2011a) Both blooms result in extensive concentrations of biogenic material in the upper water column (e.g., biogenic silica and particulate organic carbon concentrations >60 and 85 μmol L–1, respectively; Smith and Nelson, 1985; Smith et al, 1996). Growth of diatoms in austral summer is apparently supported by recycled iron, new inputs from the atmosphere, unique acclimations to low iron concentrations, or an exceptional ability to use low bioavailable iron (Smith and Kaufman, 2018)
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