Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River plume

Abstract

Abstract. The nutrient-rich waters of the Amazon River plume (ARP) support dense blooms of diatom-diazotroph assemblages (DDAs) that introduce large quantities of new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers, are likely to directly fuel vertical carbon flux. To investigate the factors controlling DDA blooms, we develop a five phytoplankton (cyanobacteria, diatoms, unicellular microbial diazotrophs, DDAs, and Trichodesmium), two zooplankton model and embed it within a 1/6° resolution physical model of the tropical and subtropical Atlantic. The model generates realistic DDA blooms in the ARP and also exhibits basin-wide primary production, nitrogen fixation, and grazing rates consistent with observed values. By following ARP water parcels with synthetic Lagrangian drifters released at the river mouth we are able to assess the relative impacts of grazing, nutrient supply, and physical forcing on DDA bloom formation. DDA bloom formation is stimulated in the nitrogen-poor and silica-rich water of the ARP by decreases in grazing pressure when mesozooplankton (which co-occur in high densities with coastal diatom blooms) concentrations decrease. Bloom termination is driven primarily by silica limitation of the DDAs. In agreement with in situ data, this net growth niche for DDAs exists in a salinity range from ∼20–34 PSU, although this co-occurrence is coincidental rather than causative. Because net growth rates are relatively modest, bloom formation in ARP water parcels depends critically on the time spent in this ideal habitat, with high DDA biomass only occurring when water parcels spent >23 days in the optimal habitat niche.

Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

“…A warming ocean will increase stratification, decreasing nutrient flux from depth (Doney et al 2012); under these conditions N 2 -fixation may increase in importance, making diazotrophy important in fueling secondary production. Furthermore, our estimates of grazing can be incorporated into existing biogeochemical models for this region (Cooley et al 2007;Stukel et al 2014) to understand how changes in precipitation, temperature, or other factors may impact biogeochemical cycling, and to predict energy transfer in future ocean food webs.…”

“…Observed changes to the hydrological cycle in the Amazon basin (Gloor et al 2013) and increasing temperature predicted with climate change (Doney et al 2012) could directly impact the Amazon River discharge which is linked with both the El Niño Southern Oscillation (ENSO) and sea surface temperature in the tropical north Atlantic (Richey et al 1989;Espinoza et al 2011). Furthermore, these measurements will improve existing empirical and biogeochemical models of this dynamic region (Cooley et al 2007;Stukel et al 2014), as well as provide important comparisons with other major rivers discharging into the oceans (e.g., the Mississippi, Mekong, and Congo).…”

Factors controlling the seasonal distribution of pelagic Sargassum

“…HYCOM model fields have been widely utilized in studies of particle dispersal (Coles et al 2013 Rypina et al 2013, Stukel et al 2014 and model validation studies have been published for the global domain (Chassignet et al 2003(Chassignet et al , 2009). Here we concentrate on how well the model particles compare with the limited set of surface observational drifter data to ensure the surface currents relevant to Sargassum dispersal are captured reasonably, and to identify potential regions of divergence.…”

Amazon River influence on nitrogen fixation and export production in the western tropical North Atlantic

“…Carpenter et al (1999) argued that nitrogen fixed by DDAs can readily enter the food web through direct grazing, in contrast to nitrogen fixed by Trichodesmium spp., which are grazed by only a small number of zooplankton species. Recent studies have additionally demonstrated the importance of zooplankton grazing in controlling plumeassociated DDA populations (Stukel et al 2014) and in moving DDA-derived N into the planktonic food web (Loick-Wilde et al in press). The low d 15 N of particles associated with H. hauckii also suggests that diazotrophic N is also likely retained in the upper water column as detritus and as rapidly recycled excreta (especially NH 1 4 ).…”