An inter-comparison of Deep Chlorophyll Maxima characteristics from 30S
to 74S and their contribution to Net Primary Production
Abstract
Subsurface accumulations of chlorophyll, also known as deep chlorophyll
maxima (DCMs), have been studied in the tropical and temperate oceans
for decades, but they have received less attention in the Southern
Ocean. Their formation and maintenance are still under debate, as is
their contribution to phytoplankton biomass and net primary productivity
(NPP). Recently, the application of satellite-based NPP algorithms to
data from biogeochemical (BGC)-Argo floats has improved
vertically-resolved NPP estimates. Using this new approach on 247
BGC-Argo floats, 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, based on
particulate backscatter) on (1) and (2). We compare and contrast trends
in adjacent latitudinal bands in the southern hemisphere, south of 30°S,
from nitrate-limited oligotrophic waters to iron-limited high-nutrient,
low-chlorophyll (HNLC) regions. This comparison of pervasive DCMs in
oligotrophic waters with the same features in HNLC waters reveals
differences in seasonality of DCM occurrence and their contribution to
total NPP. Unlike oligotrophic DCMs, HNLC DCMs occur only during spring
and summer, and their contribution to total NPP decreases from
~40% to ~25% through the productive
season, likely linked to the availability of iron and silicate. When
DCMs are present but not accounted for, up to 45% of NPP is not
quantified. Our results highlight the importance of understanding the
vertical structure of phytoplankton stocks and productivity, with direct
impacts on global NPP estimates and, ultimately, the biological carbon
pump.