Southern Ocean Phytoplankton Blooms Observed by Biogeochemical Floats

Abstract

The spring bloom in the Southern Ocean is the rapid-growth phase of the seasonal cycle in phytoplankton. Many previous studies have characterized the spring bloom using chlorophyll estimates from satellite ocean color observations. Assumptions regarding the chlorophyll-to-carbon ratio within phytoplankton and vertical structure of biogeochemical variables lead to uncertainty in satellite-based estimates of phytoplankton carbon biomass. Here, we revisit the characterizations of the bloom using optical backscatter from biogeochemical floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling and Southern Ocean and Climate Field Studies with Innovative Tools projects. In particular, by providing a three-dimensional view of the seasonal cycle, we are able to identify basin-wide bloom characteristics corresponding to physical features; biomass is low in Ekman downwelling regions north of the Antarctic Circumpolar Current region and high within and south of the Antarctic Circumpolar Current. Plain Language SummaryThe advent of satellites has allowed us to observe the ocean surface on unprecedented scale. One of the major biogeochemical findings from these observations was that phytoplankton in the Southern Ocean repeatedly went through a dramatic phase of growth every spring basin wide. Phytoplankton, however, exist not only at the ocean surface but also in the interior, generally in the top 100 m. In our study, we revisit the characterization of this spring growth using autonomous floats that vertically profile biogeochemical properties including phytoplankton concentration, providing three-dimensional estimates of phytoplankton. Our results support the conventional knowledge of there being a robust seasonal cycle in phytoplankton within and south of the Antarctic Circumpolar Current region, an energetic eastward current that wraps around Antarctica.

Southern Ocean BGC-Argo Detect Under Ice Phytoplankton Growth Before Sea Ice Retreat

“…These findings are broadly in agreement with those presented by Uchida et al (2019), who analysed the same dataset and found that early growth initiated in August/September in the region south of ∼60°S. However, the authors do not explicitly investigate growth in relation to the release of melt waters in the SSIZ, and appear to conclude that melting generally initiates growth through the release of iron trapped in sea ice, as well as the relief of light limitation.…”

Weddell Sea Phytoplankton Blooms Modulated by Sea Ice Variability and Polynya Formation

“…Since the floats are being advected and may sample high‐frequency events like synoptic storms, each float time series is smoothed by a 30‐day running mean to highlight variability at seasonal time scales (Uchida et al, 2019). The median float displacement over the course of a full bloom cycle from October to May was 150 km, or 19 km over a 30‐day period.…”

Evidence for the Impact of Climate Change on Primary Producers in the Southern Ocean

“…Essentially, either phytoplankton are distributed relatively evenly through the mixed layer (no DCM, higher mixed-layer attenuation), or are present in a narrow band of elevated concentration at the base of the mixed layer (typically co-located with the nutricline: Cullen, 2015) forming a DCM below a relatively oligotrophic mixed-layer. It is difficult to forecast which of these will occur without good knowledge of the factors involved (inter alia water column structure, nutrient supply/demand, incident irradiance, photoadaptive-capability of phytoplankton species present, loss terms including grazing and sinking; Parslow et al, 2001;Kemp et al, 2006;Cullen, 2015;Carranza et al, 2018;Uchida et al, 2019), but we hypothesize that satellite data may be able to tell us which of these situations has occurred after the event.…”

“…In addition, mixed layers can contain vertical structure in optical properties (Carranza et al, 2018). To evaluate the utility of E DCM as an indicator of DCM, a comparison was made between E DCM and the amount of phytoplankton biomass in the DCM from three Southern Ocean Carbon and Climate Observations and Modeling (SOCOMM project; Riser et al, 2018;Uchida et al, 2019) profiling drifters between 2013 and 2018. Phytoplankton biomass in the DCM was proxied from these float measurements as the depth-integrated phytoplankton carbon biomass (C p ) minus the surface phytoplankton carbon biomass multiplied by the mixed layer depth, C p (surf), following Uchida et al (2019).…”

“…Within the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO), we present new analyses of the spatial and seasonal patterns of near-surface chlorophylla concentration (chl-a) and satellite-based proxies of primary production in the Southern Ocean from Earth-observing satellites, and relate this to information from ships, shorestations, and autonomous instruments. We consider both primary production in the surface mixed-layer (from the surface to the depth of the seasonal pycnocline, at ∼50-150 m) and in the 'deep chlorophyll maximum' (DCM; Cullen, 2015;Carranza et al, 2018;Uchida et al, 2019). New approaches to track changes in primary production in the DCM based on satellite observations are proposed.…”