Subsurface Chlorophyll-a Maxima in the Southern Ocean

Abstract

Our review of the literature has revealed Southern Ocean subsurface chlorophyll-a maxima (SCMs) to be an annually recurrent feature throughout the basin. Most of these SCMs are different to the "typical" SCMs observed in the tropics, which are maintained by the nutrient-light co-limitation of phytoplankton growth. Rather, we have found that SCMs are formed by other processes including diatom aggregation, sea-ice retreat, eddies, subduction events and photo-acclimation. At a local scale, these SCMs can facilitate increased downward carbon export, primary production and food availability for higher trophic levels. A large proportion of Southern Ocean SCMs appear to be sustained by aggregates of large diatoms that form under severe iron limitation in the seasonal mixed layer. The ability of large diatoms to regulate their buoyancy must play a role in the development of these SCMs as they appear to increase buoyancy at the SCM and thus avoid further sinking with the decline of the spring bloom or naturally iron fertilized blooms. These SCMs remain largely unobserved by satellites and it seems that ship-based sampling may not be able to fully capture their biomass. In the context of the Marine Ecosystem Assessment of the Southern Ocean it is important to consider that this phenomenon is missing in our current understanding of Southern Ocean ecology and future climate scenarios. The broader implications of SCMs for Southern Ocean ecology will only be revealed through basin-wide observations. This can only be achieved through an integrated observation system that is able to harness the detailed information encapsulated in ship-based sampling, with the increased observational capacity of fluorometers on autonomous platforms such as those in the biogeochemical Argo (BGC-Argo) and the Marine Mammals Exploring the Ocean Pole to pole (MEOP) programs. The main challenge toward achieving this is the uncertainties associated with translating fluorescence to chlorophyll-a concentrations. Until this translation is resolved, the reporting of subsurface fluorescence maxima (SFMs) in place of SCMs could still yield valuable insights with careful interpretation.

Deep Chlorophyll Maxima in the Global Ocean: Occurrences, Drivers and Characteristics

“…At low latitudes, DCMs emerge as an almost inherent characteristic of [Chla] profiles, with the highest occurrences recorded at 20°S; at higher latitudes (above 35°), DCMs become a rather rare feature among the measured profiles. This latitudinal delineation confirms previous observations at a regional scale (Ardyna et al, 2013;Baldry et al, 2020;Cullen, 2015;DiTullio et al, 2003;Estrada et al, 2016;Holm Hansen et al, 2004;Kawamiya et al, 2000;Mignot et al, 2014;Parslow et al, 2001;Ravichandran et al, 2012;Thushara et al, 2019). This delination is also globally coherent with the satellite-based prediction put forward by Silsbe and Malkin (2016): a gradient of increasing occurrences from the poles to the Equator.…”

Impact of Mesoscale Eddies on Deep Chlorophyll Maxima

“…The low occurrence of DCMs at high latitudes is related to their highly seasonal dynamics both in terms of hydrographical conditions and surface primary production. At those latitudes, DCMs occur during a short period (i.e., in summer) when the water column is stratified and organic material in the upper layer undergoes post‐bloom depletion (Ardyna et al., 2013 ; Baldry et al., 2020 ). The high occurrence of DCMs in subequatorial and subtropical water systems confirms the quasi‐permanence of upper‐water stratification and organic depletion at these latitudes (Herbland & Voituriez, 1979 ; Letelier et al., 2004 ; Mignot et al., 2014 ).…”

“…Deep Chlorophyll Maxima (DCM) are ubiquitous phytoplanktonic features observed in the global ocean (Baldry et al., 2020 ; Cornec et al., 2021 ; Cullen, 2015 ; Mignot et al., 2014 ; Silsbe & Malkin, 2016 ), and more specifically in stratified systems (Cullen, 2015 ; Estrada et al., 1993 ). These active phytoplankton layers at depth are often indiscernible via satellite observation (Joint & Groom, 2000 ; Platt et al., 1988 ).…”

St Helena Marine Water Quality: Background Conditions and Development of Assessment Levels for Coastal Pollutants

“…Field measurements clearly showed the presence of a sub-surface chlorophyll maximum in the thermocline region. These chlorophyll maxima are a frequent occurrence in many marine ecosystems (e.g., Herbland et al, 1985;Bauerfeind, 1987;Wasmund et al, 2005;Lett et al, 2007;Lund-Hansen, 2011;Cullen, 2015;Baldry et al, 2020) and are thought to occur due to the interplay of many factors including light, nutrient availability and grazing. Discrete samples showing that chlorophyll values in surface waters may be higher than data from remote sensing suggest that surface sensed chlorophyll data should be taken as indicative.…”