Abstract
AbstractGravitational sinking of photosynthetically fixed particulate organic carbon (POC) constitutes a key component of the biological carbon pump. The fraction of POC leaving the surface ocean depends on POC sinking velocity (SV) and remineralization rate (Cremin), both of which depend on plankton community structure. However, the key drivers in plankton communities controlling SV and Cremin are poorly constrained. In fall 2014, we conducted a 6‐week mesocosm experiment in the subtropical NE Atlantic Ocean to study the influence of plankton community structure on SV and Cremin. Oligotrophic conditions prevailed for the first 3 weeks, until nutrient‐rich deep water injected into all mesocosms stimulated diatom blooms. SV declined steadily over the course of the experiment due to decreasing CaCO3 ballast and—according to an optical proxy proposed herein—due to increasing aggregate porosity mostly during an aggregation event after the diatom bloom. Furthermore, SV was positively correlated with the contribution of picophytoplankton to the total phytoplankton biomass. Cremin was highest during a Synechococcus bloom under oligotrophic conditions and in some mesocosms during the diatom bloom after the deep water addition, while it was particularly low during harmful algal blooms. The temporal changes were considerably larger in Cremin (max. fifteenfold) than in SV (max. threefold). Accordingly, estimated POC transfer efficiency to 1,000 m was mainly dependent on how the plankton community structure affected Cremin. Our approach revealed key players and interactions in the plankton food web influencing POC export efficiency thereby improving our mechanistic understanding of the biological carbon pump.
Highlights
Phytoplankton fix approximately 50‐Gt carbon per year, which is comparable to the annual primary production of the terrestrial biosphere (Field et al, 1998; Longhurst et al, 1995)
In this study we used in situ mesocosms to investigate the influence of a subtropical plankton community structure on particulate organic carbon (POC) sinking velocity (SV) and C‐specific remineralization rates (Cremin)
In accordance with earlier studies, we find that SV is accelerated by CaCO3 ballast, while the influence of biogenic silica (BSi) ballast was less obvious
Summary
Phytoplankton fix approximately 50‐Gt carbon per year, which is comparable to the annual primary production of the terrestrial biosphere (Field et al, 1998; Longhurst et al, 1995). The majority of the organic biomass generated by phytoplankton is consumed and remineralized in the surface ocean, while 11–27% is exported below the euphotic zone (Field et al, 1998; Henson et al, 2011) This export flux maintains a permanent surface‐to‐depth CO2 gradient, which allows the ocean to store significantly more atmospheric CO2 than it would without this biological carbon pump (BCP; Volk & Hoffert, 1985). The efficiency of the BCP through gravitational sinking can be determined empirically by fitting a power law function to, for example, depth‐resolved sediment trap fluxes (Martin et al, 1987) This provides the b value, which quantifies the transfer efficiency from the surface to the deep ocean. Packed and heavily ballasted particles will sink rapidly and provide relatively little surface area for bacterial remineralization
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