Enhanced silica ballasting from iron stress sustains carbon export in a frontal zone within the California Current

Abstract

Nutrient dynamics, phytoplankton rate processes, and export were examined in a frontal region between an anticyclone and a pair of cyclones 120 km off the coast in the southern California Current System (sCCS). Low silicic acid: nitrate ratios (Si:N) and high nitrate to iron ratios (N:Fe) characteristic of Fe‐limiting conditions in the sCCS were associated with the northern cyclone and with the transition zone between the cyclones and the anticyclone. Phytoplankton growth in low‐Si:N, high‐N:Fe waters responded strongly to added Fe, confirming growth limitation by Fe of the diatom‐dominated phytoplankton community. Low Si:N waters had low biogenic silica content, intermediate productivity, but high export compared to intermediate Si:N waters indicating increased export efficiency under Fe stress. Biogenic silica and particulate organic carbon (POC) export were both high beneath low Si:N waters with biogenic silica export being especially enhanced. This suggests that relatively high POC export from low Si:N waters was supported by silica ballasting from Fe‐limited diatoms. Higher POC export efficiency in low Si:N waters may have been further enhanced by lower rates of organic carbon remineralization due to reduced grazing of more heavily armored diatoms growing under Fe stress. The results imply that Fe stress can enhance carbon export, despite lowering productivity, by driving higher export efficiency.

Diatom Transcriptional and Physiological Responses to Changes in Iron Bioavailability across Ocean Provinces

“…This greater number of differentially expressed genes in Pseudo-nitzschia when compared to C1 suggests the C2 diatom community in the DFB treatment experienced a higher degree of Fe stress during the incubation period. The initially low dissolved Si(OH) 4 :NO 3 ratio at this site furthermore implies a possible increase in the Si:N ratios of Fe-stressed diatoms Marchetti and Cassar, 2009;Brzezinski et al, 2015). Interestingly, concentrations of domoic acid (DA), a neurotoxin produced by Pseudo-nitzschia, was 90 pg mL −1 in initial seawater (T 0 ) and exceeded 3,000 pg mL −1 in the control treatment by T 1 (Supplementary Figure 3).…”

Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction

“…Fe limitation has been shown to drive enhanced export in the CCE by increasing silicification by diatoms that continue to take up Si despite reduced organic matter production (31). The increased cellular Si:N ratios amplify the ballasting effect of diatoms, leading to higher sinking rates of aggregates and fecal pellets that contain diatoms.…”

“…5 C and D), consistent with Fe limitation. The ratio of nitrate:dissolved Fe (micromolars of NO -3 :nanomolars of dFe) is another diagnostic feature of Fe limitation in the CCE, with ratios in excess of 5 indicating significant potential for Fe limitation of diatoms (31)(32)(33). Consistent with our interpretation of Fe limitation in the main axis of the front, we found NO - 3 :dFe values of 10 to 26 in the upper 50 m during cycle 1 and consistently >5 on the coastal edge of the E-Front transects.…”

Iron-Binding Ligands in the Southern California Current System: Mechanistic Studies

“…However, only one station from each cycle was sampled in this particular study, so sampling locations will simply be referred to as stations. Each station has been given the same number as the cycle to which it belongs (for example, station 1 was part of cycle 1) in order to compare to other studies from the same cruise (e.g., Brzezinski et al, 2015;Krause et al, 2015).…”

“…on non-metallic hydroline (Cutter and Bruland, 2012) according to the methods described in Brzezinski et al (2015). Filtered samples for dFe analysis were placed in 250 mL low-density polyethylene (LDPE) bottles, acidified to pH 1.8 (Optima HCl) and stored for at least 3 months until analysis in the lab.…”