Description: The availability of dissolved iron (dFe) exerts an important control on primary production. Recent ocean observation programs have provided information on dFe in many parts of the ocean, but knowledge is still limited concerning the rates of processes that control the concentrations and cycling of dFe in the ocean and hence the role of dFe as a determinant of global primary production. We constructed a three-dimensional gridded dataset of oceanic dFe concentrations by using both observations and a simple model of the iron cycle, and estimated the difference of processes among the ocean basins in controlling the dFe distributions. A Green's function approach was used to integrate the observations and the model. The reproduced three-dimensional dFe distribution indicated that iron influx from aeolian dust and from shelf sediment were 7.6 Gmol yr and 4.4 Gmol yr in the Atlantic Ocean and 0.4 Gmol yr and 4.1 Gmol yr in the Pacific Ocean. The residence times were estimated to be 12.2 years in the Atlantic and 80.4 years in the Pacific. These estimates imply large differences in the cycling of dFe between the two ocean basins that would need to be taken into consideration when projecting future iron biogeochemical cycling under different climate change scenarios. Although there is some uncertainty in our estimates, global estimates of iron cycle characteristics based on this approach can be expected to enhance our understanding of the material cycle and hence of the current and future rates of marine primary production.

Description: Bacterial metabolism largely drives the sequestration of refractory organic matter in the ocean. However, a lack of understanding exists regarding the abundance and reactivity of bacterial particulate organic matter (POM). Here we report the bacterial contributions to suspended POM collected in the oligotrophic Western Pacific Warm Pool (WPWP). Around 27% of particulate organic carbon (POC) and ∼39% of particulate nitrogen (PN) in the surface ocean were derived from bacteria. Most of the bacterial POM (∼87%) was labile or semi-labile, and ∼85% of bacterial POM was removed between depths of ∼100–300 m. Bacterial POM constituted only ∼8% and ∼13% of refractory POC and PN, respectively. The rapid cycling of bacterial POM in upper waters was likely related to oligotrophic conditions and facilitated by higher temperatures in the WPWP. Taken together, these observations indicate that bacterial POM plays a crucial role in supplying energy for bacterial respiration. Key Points We assess bacterial contributions to suspended particulate organic matter (POM) in the Western Pacific Warm Pool on the basis of D-amino acid biomarkers Bacterial organics constitute 27% of surface ocean particulate organic carbon (POC) and 39% of particulate nitrogen (PN), but majority (∼87%) is labile or semi-labile Rapid cycling of bacterial POM in the upper ocean results in a contribution of only ∼8% to refractory POC and ∼13% to PN

Description: Key Points: - Glacier-derived particles release 2–46% of labile particulate lead (Pb) upon mixing with seawater - Pb dynamics in glacier fjords are characterized by a rapid release of dissolved Pb followed by readsorption on a timescale of hours-to-days - Dissolved Pb release from the Greenland Ice Sheet is likely within the range 0.2–1 Mmol yr−1 Higher than expected concentrations of dissolved lead (dPb) have been consistently observed along glaciated coastlines and it is widely hypothesized that there is a net release of dPb from glacier-derived sediments. Here we further corroborate that dPb concentrations in diverse locations around west Greenland (3.2–252 pM) and the Western Antarctic Peninsula (7.7–107 pM) appear to be generally higher than can be explained by addition of dPb from glacier-derived freshwater. The distribution of dPb across the salinity gradient is unlike any other commonly studied trace element (e.g., Fe, Co, Ni, Cu, Mn, and Al) implying a dynamic, reversible exchange between dissolved and labile particulate Pb. Incubating a selection of glacier-derived particles from SW Greenland (Ameralik and Nuup Kangerlua) and Svalbard (Kongsfjorden), with a range of labile particulate Pb (LpPb) content (11–113 nmol g−1), the equivalent of 2–46% LpPb was released as dPb within 24 hr of addition to Atlantic seawater. Over longer time periods, the majority of this dPb was typically readsorbed. Sediment loading was the dominant factor influencing the net release of dPb into seawater, with a pronounced decline in net dPb release efficiency when sediment load increased from 20 to 500 mg L−1. Yet temperature also had some effect with 68 ± 22% higher dPb release at 11°C compared to 4°C. Future regional changes in dPb dynamics may therefore be more sensitive to short-term suspended sediment dynamics, and potentially temperature changes, than to changing interannual runoff volume.

Description: An insufficient supply of the micronutrient iron (Fe) limits phytoplankton growth across large parts of the ocean. Ambient Fe speciation and solubility are largely dependent on seawater physico-chemical properties. We calculated the apparent Fe solubility (SFe(III)app) at equilibrium for ambient conditions, where SFe(III)app is defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to organic matter formed at a free Fe3+ concentration equal to the solubility of Fe hydroxide. We compared the SFe(III)app to measured dissolved Fe (dFe) in the Atlantic and Pacific Oceans. The SFe(III)app was overall ∼2 to 4-fold higher than observed dFe at depths less than 1000 m, ∼2-fold higher than the dFe between 1000-4000 m and ∼3-fold higher than dFe below 4000 m. Within the range of used parameters, our results showed that there was a similar trend in the vertical distributions of horizontally averaged SFe(III)app and dFe. Our results suggest that vertical dFe distributions are underpinned by changes in SFe(III)app which are driven by relative changes in ambient pH and temperature. Since both pH and temperature are essential parameters controlling ambient Fe speciation, these should be accounted for in investigations of changing Fe dynamics, particularly in the context of ocean acidification and warming. Key Points Apparent iron solubility is driven by ambient pH, temperature (T) and dissolved organic carbon (DOC), and showed a 6-fold variation between surface (pH= 8.05 on the total scale, DOC= 71.8 µmol L-1, T= 20.4 °C) and deep oceanic waters (pH= 7.82, DOC= 38.6 µmol L-1, T= 1.1°C). Higher values of apparent iron solubility were determined for deep Atlantic and Pacific waters, with lower values in subtropical gyres. Calculated apparent iron solubility showed a similar trend in vertical distribution to dissolved iron, highlighting the importance of considering the impact of changes in ambient physico-chemical conditions on seawater iron chemistry.

Description: Atmospheric aerosol deposition into the low latitude oligotrophic ocean is an important source of new nutrients for primary production. However, the resultant phytoplankton responses to aerosol deposition events, both in magnitude and changes in community composition, are poorly constrained. Here, we investigated this with 19 d of field and satellite observations for a site in the subtropical North Atlantic. During the observation period, surface dissolved aluminum concentrations alongside satellite-derived aerosol and precipitation data demonstrated the occurrence of both a dry deposition event associated with a dust storm and a wet deposition event associated with strong rainfall. The dry deposition event did not lead to any observable phytoplankton response, whereas the wet deposition event led to an approximate doubling of chlorophyll a, with Prochlorococcus becoming more dominant at the expense of Synechococcus. Bioassay experiments showed that phytoplankton were nitrogen limited, suggesting that the wet deposition event likely provided substantial aerosol-derived nitrogen, thereby alleviating the prevalent nutrient limitation and leading to the rapid observed phytoplankton response. These findings highlight the important role of wet deposition in driving rapid responses in both ocean productivity and phytoplankton community composition.

Description: Subtropical gyres cover 26%-29% of the world's surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time-series and the Bermuda Atlantic Time-series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N-2 fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone (EZ) in subtropical gyres as a two-layered vertically structured system: a nutrient-depleted layer above the top of the nutricline in the well-lit upper ocean and a nutrient-replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long-term variations in key biogeochemical parameters in both of these EZ layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies. Key Points Subtropical gyres display larger spatiotemporal dynamics in biogeochemical properties than previously considered An improved two-layer framework is proposed for the study of nutrient-driven and biologically mediated carbon export in the euphotic zone Future research will benefit from high-resolution samplings, improved sensitivity of nutrient analyses, and advanced modeling capabilities