Marine net community production (NCP), a metric of ecosystem functionality, is often estimated as the residual term in a mass balance equation that aims to describe upper ocean variations in the time series of a chemical tracer. The advent of biogeochemical (BGC) Argo profiling floats equipped with nitrate, pH, and oxygen sensors has enabled such NCP estimation across vast ocean regions. Floats typically drift at 1000 m depth between profiling from ~2000 m to the surface every 10 days, resulting in quasi-Lagrangian time series that can reflect different upper ocean water masses over time. Yet, limited information about real-time horizontal tracer gradients often leads to lateral processes being omitted during tracer budget closure, which can bias the residual-term NCP estimates. To determine the potential magnitude of such biases, we developed a method to quantify and adjust for the impact of lateral float movement across horizontal tracer gradients using DIC as our case study. We evaluated the method by extracting artificial float profiles from a depth-resolved, observation-based DIC product to generate an artificial DIC time series. We then estimated NCP before and after accounting for horizontal gradient effects and compared the results to NCP estimates from an artificial DIC time series extracted at a fixed location along the float trajectory. Testing 10 biogeographical domains with moderate to substantial horizontal DIC gradients, our method significantly improved the precision (by ~50 to ~80%) and accuracy (by ~10 to ~100%) of regional NCP estimates. This method can be applied to other tracers with multi-month-long residence times.

The seasonal variability of phytoplankton vertical distribution is investigated in the South Pacific where observations are scarce and scattered. We used 13 BioGeoChemical-Argo floats deployed across diverse oceanic environments. The seasonal latitudinal displacement of the Tasman front induces transitions from mesotrophic to oligotrophic conditions. This shift results in Chlorophyll-a concentration vertical distribution changing from bloom types to Subsurface Chlorophyll Maxima (SCM) types, with intermediate hybrid types between these extremes. Such hybrid profiles frequently occur in the equatorial Pacific, highlighting a large-scale pattern rather than local island mass effect. In oligotrophic regions, seasonal variations of light availability and stratification dynamics below the mixed layer likely relate SCMs to an increase in carbon biomass or photoacclimation. A biomass increase is frequently observed, contrary to previously reported, suggesting that subsurface phytoplankton production may have been largely underestimated. This calls for further observations of the water column in these remote undersampled open ocean areas.

Deep Chlorophyll Maxima (DCMs) are ubiquitous in low-latitude oceans, and of recognized biogeochemical and ecological importance. DCMs have been observed in the Southern Ocean, initially from ships and recently from profiling robotic floats, but with less understanding of their onset, duration, underlying drivers, or whether they are associated with enhanced biomass features. We report the characteristics of a DCM and DBM (Deep Biomass Maximum) in the Inter-Polar-Frontal-Zone (IPFZ) south of Australia from CTD profiles, shipboard-incubated samples, a towbody, and a BGC-ARGO float. The DCM and DBM were ~20 m thick and co-located with the nutricline, in the vicinity of a subsurface ammonium maximum characteristic of the IPFZ, but ~100 m shallower than the ferricline. Towbody transects demonstrated that the co-located DCM/DBM was broadly present across the IPFZ. Large healthy diatoms, with low iron requirements, resided within the DCM/DBM, and fixed up to 20 mmol C m-2 d-1. The BGC-ARGO float revealed the DCM/DBM persisted for >3 months. We propose a dual environmental mechanism to drive DCM/DBM formation and persistence within the IPFZ: sustained supply of both recycled iron within the subsurface ammonium maxima and upward silicate transport from depth. DCM/DBM cell-specific growth rates were considerably slower than those in the overlying mixed layer, implying that phytoplankton losses are also reduced, possibly as a result of heavily silicified diatom frustules. The light-limited seasonal termination of the observed DCM/DBM did not result in a ‘diatom dump’, rather ongoing diatom downward export occurred throughout its multi-month persistence.

A document by Jennifer K McWhorter. Click on the document to view its contents.