The Contribution of Deep Chlorophyll Maxima to Net Primary Production in the Southern Ocean

Abstract

Deep chlorophyll maxima (DCMs) have long been studied in the northern hemisphere but have received less attention in the Southern Ocean. Their contribution to phytoplankton biomass and net primary productivity (NPP) is poorly resolved. Recently, the application of satellite NPP algorithms to biogeochemical (BGC)-Argo float data has improved vertically resolved NPP estimates. Using this approach on 12,700 BGC-Argo profiles south of 30°S, we report (1) subsurface (below the mixed layer) estimates of NPP, (2) the contribution of subsurface NPP to total NPP, and (3) the influence of DCMs and deep biomass maxima (DBMs) on (1) and (2). When DCMs are present (n = 2,119 profiles), subsurface NPP is 217 ± 106 mg C m⁻² day⁻¹ compared to 82 ± 92 mg C m⁻² day⁻¹ for all profiles. We further compare observations across seasons in four water masses from nitrate-limited oligotrophic waters north of the subtropical front to iron-limited regions further south, including the sea ice zone. Low-latitude DCMs (i.e., 30–44°S), show the highest contribution to column-integrated NPP. However, DCMs occur across all frontal zones and contribute significantly to total NPP when present. Rather than missing subsurface NPP associated with DCMs, the satellite Carbon-based Productivity Model (CbPM) tends to mistakenly assume DCMs below the mixed layer, overestimating NPP. This situation is somewhat ameliorated in the ferricline version of the CbPM due to better nutricline-euphotic depth alignment. Our results highlight the importance of understanding the vertical structure of phytoplankton stocks and productivity, with direct impacts on global NPP estimates and, ultimately, climate model projections.