Nitrate use by plankton in the eastern subtropical North Atlantic, March-April 1989

Nitrate concentration was measured in seawater samples from the euphotic zone at the beginning and end of 12-h, daytime, in situ incubations. The changes in concentration are considered to be measurements of new production. During periods of 2-3 weeks in March-April 1989, important time scales for NO3- input to the euphotic zone (i.e. residence times) and new production were approximately 26 d at 18-degrees-N, 31-degrees-W and approximately 10 d near 33-degrees-N, 21-degrees-W. The average rate of NO3- use in the two areas was 2.63 and 0.62 mmol N m-2 (12 h)-1, or, in carbon equivalents 209 and 49 mg C m-2 d-1, respectively. These values bracket the large-scale estimate by Jenkins of new production in the nearby beta triangle of 150 mg C m-2 d-1

Benthic marine calcifiers coexist with CaCO3-undersaturated seawater worldwide

Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms, especially those building structures made of CaCO3. A large proportion of benthic marine calcifiers incorporate Mg2+ into their calcareous structures (i.e., Mg-calcite) which, in general, reduces mineral stability. The vulnerability of some marine calcifiers to ocean acidification is related to the solubility of their calcareous structures, but not all marine organisms conform to this because of sophisticated biological and physiological mechanisms to construct and maintain CaCO3 structures. Few studies have considered seawater saturation state with respect to species-specific mineralogy in evaluating the effect of ocean acidification on marine organisms. Here, a global dataset of skeletal mol % MgCO3 of benthic calcifiers and in situ environmental conditions (temperature, salinity, pressure, and [CO32-]) spanning a depth range of 0 m (subtidal/neritic) to 5500 m (abyssal) was assembled to calculate in situ seawater saturation states with respect to species-specific Mg-calcite mineral compositions (?Mg-x). Up to 20% of all studied calcifiers at depths 1200 m currently experience seawater mineral undersaturation with respect to their skeletal mineral phase (?Mg-x1200 m) of all studied calcifying species to seawater undersaturation. These observations underscore concerns over the ability of marine benthic calcifiers to continue to construct and maintain their calcareous structures under these conditions. We advocate that ocean acidification tipping points can only be understood by assessing species-specific responses, and because of different seawater ?Mg-x present in all marine ecosystems

C:N stoichiometry of the biological pump in the North Atlantic: constraints from climatological data

Recently and independently published estimates of global net community production which were based on seasonal changes of either nutrients (NO3 and PO4 (Louanchi and Najjar, 2000)) and salinity normalized dissolved inorganic carbon (NCt (Lee, 2001)) in the surface ocean indicate that the stoichiometry of new production strongly differs from the well‐established remineralization ratios in the deep ocean (the Redfield ratio). This difference appears to be most pronounced in the North Atlantic Ocean. Data quality issues as well as methodological differences in the data analysis applied in the published studies, however, make this comparison of nutrient‐ and carbon‐based estimates ambiguous. Here I present an analysis based on a combination of historical data (World Ocean Atlas and Data 1998) and empirical approaches and provide a reassessment of the C:N elemental ratio of new (export) production in the North Atlantic. It is found that the estimate of winter nutrient fields is the most crucial step in estimating basin‐scale, time‐integrated C:N ratios of new production. An approach is developed which allows an estimate of winter nitrate and total CO2 concentrations which are consistent with estimates from an isopycnal outcrop analysis where these are available. Regional trends in the spring + summer integrated C:N ratio of new production suggest an increase from high latitudes toward the subtropics. The basin‐integrated C:N ratio of new production between 40°N and 65°N is 11.4 ± 1.4, far exceeding the Redfield ratio. The bulk Corg:Cinorg rain ratio estimated for the same region is 7.7. The fate of organic carbon produced in excess of the Redfield equivalent of nitrate uptake is discussed. It is suggested that a considerable fraction of excess carbon is remineralized above the depth of the winter mixed layer

Benthic marine calcifiers coexist with CaCO3-undersaturated seawater worldwide

Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms, especially those building structures made of CaCO3. A large proportion of benthic marine calcifiers incorporate Mg2+ into their calcareous structures (i.e., Mg-calcite) which, in general, reduces mineral stability. The vulnerability of some marine calcifiers to ocean acidification is related to the solubility of their calcareous structures, but not all marine organisms conform to this because of sophisticated biological and physiological mechanisms to construct and maintain CaCO3 structures. Few studies have considered seawater saturation state with respect to species-specific mineralogy in evaluating the effect of ocean acidification on marine organisms. Here, a global dataset of skeletal mol % MgCO3 of benthic calcifiers and in situ environmental conditions (temperature, salinity, pressure, and [CO32-]) spanning a depth range of 0 m (subtidal/neritic) to 5500 m (abyssal) was assembled to calculate in situ seawater saturation states with respect to species-specific Mg-calcite mineral compositions (?Mg-x). Up to 20% of all studied calcifiers at depths <1200 m and approximately 90% of calcifiers at depths >1200 m currently experience seawater mineral undersaturation with respect to their skeletal mineral phase (?Mg-x<1). We conclude that as a result of predicted anthropogenic ocean acidification over the next 150 years, the predicted decrease in seawater mineral saturation, will expose approximately 50% (<1200 m) and 100% (>1200 m) of all studied calcifying species to seawater undersaturation. These observations underscore concerns over the ability of marine benthic calcifiers to continue to construct and maintain their calcareous structures under these conditions. We advocate that ocean acidification tipping points can only be understood by assessing species-specific responses, and because of different seawater ?Mg-x present in all marine ecosystems

Oxygen utilization rate (OUR) underestimates ocean respiration: A model study

We use a simple 1-D model representing an isolated density surface in the ocean and 3-D global ocean biogeochemical models to evaluate the concept of computing the subsurface oceanic oxygen utilization rate (OUR) from the changes of apparent oxygen utilization (AOU) and water age. The distribution of AOU in the ocean is not only the imprint of respiration in the ocean's interior but is strongly influenced by transport processes and eventually loss at the ocean surface. Since AOU and water age are subject to advection and diffusive mixing, it is only when they are affected both in the same way that OUR represents the correct rate of oxygen consumption. This is the case only when advection prevails or with uniform respiration rates, when the proportions of AOU and age are not changed by transport. In experiments with the 1-D tube model, OUR underestimates respiration when maximum respiration rates occur near the outcrops of isopycnals and overestimates when maxima occur far from the outcrops. Given the distribution of respiration in the ocean, i.e., elevated rates near high-latitude outcrops of isopycnals and low rates below the oligotrophic gyres, underestimates are the rule. Integrating these effects globally in three coupled ocean biogeochemical and circulation models, we find that AOU-over-age based calculations underestimate true model respiration by a factor of 3. Most of this difference is observed in the upper 1000 m of the ocean with the discrepancies increasing toward the surface where OUR underestimates respiration by as much as factor of 4

An unexpected nitrate distribution in the tropical North Atlantic at 18°N, 30°W—implications for new production

During a R.V. Meteor JGOFS-NABE cruise to a tropical site in the northeast Atlantic in spring 1989, three different vertical regimes with respect to nitrate distribution and availability within the euphotic zone were observed. Besides dramatic variations in the depth of the nitracline, a previously undescribed nose-like nitrate maximum within the euphotic zone was the most prominent feature during this study. Both the vertical structure of phytoplankton biomass and the degree of absolute and relative new production were related to the depth of the nitracline, which in turn was dependent on the occurrence/non-occurrence of the subsurface subtropical salinity maximum (S(max)). The mesoscale variability of the nitracline depth, as indicated from a pre-survey grid, and published data on the frequent occurrence of the S(max) in tropical waters suggest higher variability of new production and F-ratio than usually expected for oligotrophic oceans. The importance of salt fingering and double diffusion for nitrate transport into the euphotic zone is discussed

Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition

The ocean is the main source of thermal inertia in the climate system. Ocean heat uptake during recent decades has been quantified using ocean temperature measurements. However, these estimates all use the same imperfect ocean dataset and share additional uncertainty due to sparse coverage, especially before 2007. Here, we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2) – levels of which increase as the ocean warms and releases gases – as a whole ocean thermometer. We show that the ocean gained 1.29 ± 0.79 × 1022 Joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.80 ± 0.49 W watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result – which relies on high-precision O2 atmospheric measurements dating back to 1991 – leverages an integrative Earth system approach and provides much needed independent confirmation of heat uptake estimated from ocean data

Oceanic nitrogen cycling and N2 O flux perturbations in the Anthropocene

There is currently no consensus on how humans are affecting the marine nitrogen (N) cycle, which limits marine biological production and CO2 uptake. Anthropogenic changes in ocean warming, deoxygenation, and atmospheric N deposition can all individually affect the marine N cycle and the oceanic production of the greenhouse gas nitrous oxide (N2O). However, the combined effect of these perturbations on marine N cycling, ocean productivity, and marine N2O production is poorly understood. Here we use an Earth system model of intermediate complexity to investigate the combined effects of estimated 21st century CO2 atmospheric forcing and atmospheric N deposition. Our simulations suggest that anthropogenic perturbations cause only a small imbalance to the N cycle relative to preindustrial conditions (∼+5 Tg N y−1 in 2100). More N loss from water column denitrification in expanded oxygen minimum zones (OMZs) is counteracted by less benthic denitrification, due to the stratification-induced reduction in organic matter export. The larger atmospheric N load is offset by reduced N inputs by marine N2 fixation. Our model predicts a decline in oceanic N2O emissions by 2100. This is induced by the decrease in organic matter export and associated N2O production and by the anthropogenically driven changes in ocean circulation and atmospheric N2O concentrations. After comprehensively accounting for a series of complex physical-biogeochemical interactions, this study suggests that N flux imbalances are limited by biogeochemical feedbacks that help stabilize the marine N inventory against anthropogenic changes. These findings support the hypothesis that strong negative feedbacks regulate the marine N inventory on centennial time scales