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2015
Brzezinski, MA, Krause JW, Bundy RM, Barbeau KA, Franks P, Goericke R, Landry MR, Stukel MR.  2015.  Enhanced silica ballasting from iron stress sustains carbon export in a frontal zone within the California Current. Journal of Geophysical Research-Oceans. 120:4654-4669.   10.1002/2015jc010829   AbstractWebsite

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.

Bundy, RM, Abdulla HAN, Hatcher PG, Biller DV, Buck KN, Barbeau KA.  2015.  Iron-binding ligands and humic substances in the San Francisco Bay estuary and estuarine-influenced shelf regions of coastal California. Marine Chemistry. 173:183-194.   10.1016/j.marchem.2014.11.005   AbstractWebsite

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity <2) to the mouth of the estuary (salinity >26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand,salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5 nmol L-1 and strong ligand (log K-Fel, Fe'(cond) >= 12.0) concentrations equal to 139.5 nmol L-1. The weakest ligands (log K-Fel, Fe'(cond) < 10.0) were always in excess of dFe in low salinity waters, but were rapidly flocculated within the estuary and were not detected at salinities greater than 7. The strongest ligands (log K-Fel, Fe'(cond) > 11.0) were tightly coupled to dFe throughout the estuary, with average excess ligand concentrations ([L]-[dFe]) equal to 0.5 nmol L-1. Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5 mu g L-1 to 67.5 mu g L-1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters. (C) 2014 Elsevier B.V. All rights reserved.

2012
King, AL, Buck KN, Barbeau KA.  2012.  Quasi-Lagrangian drifter studies of iron speciation and cycling off Point Conception, California. Marine Chemistry. 128:1-12.   10.1016/j.marchem.2011.11.001   AbstractWebsite

The distribution and speciation of dissolved Fe (dFe) were measured during four quasi-Lagrangian drogued drifter studies (similar to 4 d duration each) that were conducted in the southern California Current System in May 2006 and April 2007. Three of the four drifter studies were within the coastal upwelling regime and one drifter study was in a warm-core anticyclonic eddy. Incubation bottle experiments were also conducted to determine the degree of phytoplankton Fe limitation and to assess changes in the concentration of Fe-binding ligands. In the coastal upwelling drifter studies, in situ dFe (1.4-1.8 nM) and macronutrients were initially high and declined over time. Fe addition incubation experiments indicated that the phytoplankton community was not Fe limited at the beginning of the coastal upwelling drifter experiments (when mu M nitrate:nM dFe ratios were similar to 7-8). By the end of two of the three drifter studies (when mu M nitrate:nM dFe ratios were similar to 12-19), Fe addition resulted in larger nitrate and silicic acid drawdown, and larger accumulations in chlorophyll a, particulate organic carbon and nitrogen, and diatom and dinoflagellate-specific carotenoid pigments. Fe speciation was measured in situ in three of the four drifter studies with stronger L-1-type ligands found to be present in excess of dFe in all samples. In Fe speciation incubation experiments. L-1-type ligand production was observed in conjunction with phytoplankton growth under Fe-limiting conditions. The results presented here support and add a quasi-Lagrangian perspective to previous observations of dFe and macronutrient cycling over space and time within the California coastal upwelling regime, including Fe limitation within the phytoplankton community in this region and the biological production of Fe-binding ligands concomitant with Fe limitation. (C) 2011 Elsevier B.V. All rights reserved.

2011
King, AL, Barbeau KA.  2011.  Dissolved iron and macronutrient distributions in the southern California Current System. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006324   AbstractWebsite

The distribution of dissolved iron in the southern California Current System (sCCS) is presented from seven research cruises between 2002 and 2006. Dissolved iron concentrations were generally low in most of the study area (<0.5 nM), although high mixed layer and water column dissolved iron concentrations (up to 8 nM) were found to be associated with coastal upwelling, both along the continental margin and some island platforms. A significant supply of iron was probably not from a deep remineralized source but rather from the continental shelf and bottom boundary layer as identified in previous studies along the central and northern California coast. With distance offshore, dissolved iron decreased more rapidly relative to nitrate in a transition zone 10-250 km offshore during spring and summer, resulting in relatively high ratios of nitrate: dissolved iron. Higher nitrate: dissolved iron ratios could be the result of utilization and scavenging in addition to an overall lower supply of iron relative to nitrate in the offshore transition zones. The low supply of iron leads to phytoplankton iron limitation and a depletion in silicic acid relative to nitrate in the coastal upwelling and transition zones of the sCCS.

2007
King, AL, Barbeau K.  2007.  Evidence for phytoplankton iron limitation in the southern California Current System. Marine Ecology-Progress Series. 342:91-103.   10.3354/meps342091   AbstractWebsite

Observations of phytoplankton iron limitation in the world's oceans have primarily been confined to high-nutrient, low-chlorophyll (HNLC) regimes, found in the western equatorial and subarctic Pacific, Southern Ocean, and coastal upwelling zones off California and Peru. We investigated the potential for phytoplankton iron limitation in coastal transition zones (50 to 200 km offshore) of the southern California Current System, a weak upwelling regime that is relatively low in nutrients (< 4 mu mol nitrate 1(-1)) and low in chlorophyll (< 1 mu g chl a 1(-1)). In grow-out incubation experiments conducted during summer, July 2003 and 2004, phytoplankton responded to nanomolar iron additions, despite the non-HNLC initial conditions, Observed changes in phytoplankton and nutrient parameters upon iron addition were significant, although markedly lower in amplitude relative to typical grow-out experiments in HNLC regimes. While we cannot disprove alternate explanations for the observed limitation of phytoplankton growth, such as a proximate grazing control, our results indicate that phytoplankton growth in the southern California Current System is, at times, limited by the supply of iron. Based on our findings and the results of previous studies in this region, we suggest that phytoplankton biomass is generally limited by the supply of nitrate, while iron, directly or indirectly, influences macronutrient utilization, community species composition, and phytoplankton spatial and temporal distribution.