[1] The thermohaline structure across the tidal fronts of the continental shelf off Patagonia is analyzed using historical and recent summer hydrographic sections. The near-summer tidal front location is determined on the basis of the magnitude of vertical stratification of the water column as measured by the Simpson parameter. Sea surface and air CO 2 partial pressures based on data from eleven transects collected in summer and fall from 2000 to 2004 are used to estimate CO 2 fluxes over the shelf. The near-shore waters are a source of CO 2 to the atmosphere while the midshelf region is a CO 2 sink. The transition between source and sink regions closely follows the location of tidal fronts, suggesting a link between vertical stratification of the water column and the regional CO 2 balance. The highest surface values of Chl a are associated with the strongest CO 2 sinks. The colocation of lowest CO 2 partial pressure (pCO 2 ) and highest Chl a suggests that phytoplankton blooms on the stratified side of the fronts draw the ocean's CO 2 to very low levels. The mean shelf sea-air difference in pCO 2 (DpCO 2 ) is À24 matm and rises to À29 matm if the shelf break front is included. Peaks in DpCO 2 of À110 matm, among the highest observed in the global ocean, are observed. The estimated summer mean CO 2 flux over the shelf is À4.4 mmol m À2 d À1 and rises to À5.7 mmol m À2 d À1 when the shelf break area is taken into account. Thus, during the warm season the shelf off Patagonia is a significant atmospheric CO 2 sink.

The Pacific Northwest National Laboratory (PNNL) is evaluating the performance of adsorption materials to extract uranium from natural seawater. Testing consists of measurements of the adsorption of uranium and other elements from seawater as a function of time using flow-through columns and a recirculating flume to determine adsorbent capacity and adsorption kinetics. The amidoxime-based polymer adsorbent AF1, produced by Oak Ridge National Laboratory (ORNL), had a 56-day adsorption capacity of 3.9 ± 0.2 g U/kg adsorbent material, a saturation capacity of 5.4 ± 0.2 g U/kg adsorbent material, and a half-saturation time of 23 ± 2 days. The ORNL AF1 adsorbent has a very high affinity for uranium, as evidenced by a 56-day distribution coefficient between adsorbent and solution of log KD,56day = 6.08. Calcium and magnesium account for a majority of the cations adsorbed by the ORNL amidoxime-based adsorbents (61% by mass and 74% by molar percent), uranium is the fourth most abundant element adsorbed by mass and seventh most abundant by molar percentage. Marine testing at Woods Hole Oceanographic Institution with the ORNL AF1 adsorbent produced adsorption capacities 15% and 55% higher than those observed at PNNL for column and flume testing, respectively. Variations in competing ions may be the explanation for the regional differences. Hydrodynamic modeling predicts that a farm of adsorbent materials will likely have minimal effect on ocean currents and removal of uranium and other elements from seawater when farm densities are <1800 braids/km2. A decrease in uranium adsorption capacity of up to 30% was observed after 42 days of exposure because of biofouling when the ORNL braided adsorbent AI8 was exposed to raw seawater in a flume in the presence of light. No toxicity was observed with flow-through column effluents of any absorbent materials tested to date. Toxicity could be induced with some non-amidoxime based absorbents only when the ratio of solid absorbent to test media was increased to part per thousand levels. Thermodynamic modeling of the seawater−amidoxime adsorbent was performed using the geochemical modeling program PHREEQC. Modeling of the binding of Ca, Mg, Fe, Ni, Cu, U, and V reveal that when binding sites are limited (1 × 10–8 binding sites/kg seawater), vanadium heavily outcompetes other ions for the amidoxime sites. In contrast, when binding sites are abundant, Mg and Ca dominate the total percentage of metals bound to the sorbent.

Under physically isolated conditions, net community production (NCP) can be accurately estimated from the rate of oxygen evasion to the atmosphere derived from local mixed layer oxygen/argon measurements. We use a simple box model to demonstrate that, when physical inputs are negligible, the sea‐to‐air flux of biological oxygen (bioflux) represents the average NCP exponentially weighted over the past several residence times of oxygen in the mixed layer. This new weighting scheme shows that there is no apparent lag between bioflux and exponentially weighted time‐averaged NCP. Furthermore, a strict steady state assumption is unnecessary to this relationship. However, this widely used O2/Ar method is not effective in dynamic coastal zones where low oxygen water upwells to the surface. Yet these zones are highly productive and their episodic productivity needs to be quantified. We use a quasi‐2‐D version of the Regional Ocean Modeling System, including oxygen and argon as prognostic variables, to explore the application of this method and the relationship between NCP and bioflux in a coastal upwelling system. We show that bioflux is an accurate measure of NCP over large regions of time and space. Bioflux is most biased near the shore following upwelling favorable winds, where bioflux is sometimes negative (flux from the atmosphere to the ocean) and even positive bioflux values can severely underestimate NCP. Assessing a range of model variables that are easily observed in the field, we show that sea surface temperature is the most effective at identifying bioflux measurements that are likely to be biased.

Increasing levels of nutrients, persistent hypoxia, harmful algal blooms, and increased frequency of fish kills are degrading the ecological health of the Salish Sea. An improved version of a diagnostic hydrodynamic and biogeochemical model (nutrients, phytoplankton, carbon, dissolved oxygen, and pH) of the Salish Sea has been developed with the ability to simulate characteristic circulation and water quality features. Sensitivity tests were conducted to assess the responsiveness of the system to land-based (rivers and wastewater sources) nutrient loading. The influence of Fraser River on the magnitude of estuarine exchange with the Pacific Ocean and nearshore habitat was examined given that it contributes nearly half of the total freshwater discharged to the Salish Sea. A large region of hypoxia in Hood Canal that extends over 30-40 km during its peak was reproduced and attributed primarily to the existence of a two-layer classic fjord-type circulation and a nearly stagnant deep bottom layer that occupies nearly 60% of the water column. Nitrate mass in the euphotic zone from land-based and oceanic sources is depleted to near-zero limiting levels during summer. Under such conditions, the Salish Sea is responsive to changes in nutrient loads entering the euphotic zone directly. A hypothetical scenario involving the elimination of land-based nutrient sources results in notable water-quality improvement, featuring a reduction in algal biomass (≈5.4%), reduction in sediment oxygen demand (≈17.1%), and significant reduction in hypoxic area (≈39%) and exposure in area-days to bottom layer hypoxia (≈62%) within the Salish Sea. KHANGAONKAR ET AL.4735

[1] Recently, independent concerns about declining oxygen and pH conditions in the coastal ocean have emerged. In coastal upwelling regions, hypoxia can be driven by onshore advection of oxygen-depleted offshore waters as well as by local biological consumption triggered by high productivity. As both mechanisms can also decrease pH and carbonate saturation states, coupled studies of oxygen and carbon are imperative. A quasi two-dimensional model coupling carbon, oxygen, and nitrogen was developed for the summer wind-driven upwelling region off southern Vancouver Island, using the Regional Ocean Modeling System. The physical model is coupled to an ecosystem module that tracks 11 state variables and allows nonfixed C:N ratios for detritus and dissolved organic matter. Given uncertainties in sediment parameterizations in biophysical models, three sediment models are compared and discussed. Results demonstrate that sediment-associated processes play a dominant role in consuming oxygen from, and releasing inorganic carbon to, the bottom waters over the shelf. This study also examines the unique characteristics of the southern Vancouver Island shelf. Two key features distinguish this region from other shelves in the California Current System and protect inner shelf waters from severe hypoxia and corrosive (i.e., undersaturated in aragonite) conditions. First, the near-shore Vancouver Island Coastal Current provides a source of oxygen and nutrients and forms a barrier that prevents upwelled waters (depleted in oxygen and rich in carbon) from penetrating the inner shelf. Second, the greater width of the shelf dilutes these upwelled offshore waters and reduces their penetration onto the shallower shelf region.

No abstract

Changes in sea ice thickness and extent have corresponded with substantial changes in net primary production (NPP) in the Arctic Ocean. In recent years, observations of massive phytoplankton blooms under sea ice have upended the previous paradigm that Arctic NPP was driven largely by growth in the marginal ice zone and open water periods. Here, a new 1‐D biogeochemical model capable of simulating ice algal and phytoplankton dynamics both under the ice and in open waters is applied in the northern Chukchi Sea for the years 1988–2018. Over this period, substantial under‐ice (UI) blooms were produced in all but four years and were the primary drivers of interannual variation in total NPP. While NPP in the UI period was highly variable interannually due to fluctuations in ice thickness and the length of the UI period, UI NPP accounted for nearly half of total NPP between 1988 and 2018. Further, years with high UI NPP had reduced annual zooplankton grazing, indicating an intensification in the mismatch between phytoplankton and zooplankton populations and possibly altering the partitioning of food between benthic and pelagic ecosystems. These results demonstrate that the often‐overlooked ice covered period can be highly productive in the Arctic Ocean, and that the northern Chukchi Sea has been amenable to UIB formation since at least 1988.

While ocean acidification was first investigated as a global phenomenon, coastal acidification has received significant attention in recent years, as its impacts have been felt by different socio-economic sectors (e.g., high mortality of shellfish larvae in aquaculture farms). As a region that connects land and ocean, the Salish Sea (consisting of Puget Sound and the Straits of Juan de Fuca and Georgia) receives inputs from many different sources (rivers, wastewater treatment plants, industrial waste treatment facilities, etc.), making these coastal waters vulnerable to acidification. Moreover, the lowering of pH in the Northeast Pacific Ocean also affects the Salish Sea, as more acidic waters get transported into the bottom waters of the straits and estuaries. Here, we use a numerical ocean model of the Salish Sea to improve our understanding of the carbonate system in Puget Sound; in particular, we studied the sensitivity of carbonate variables (e.g., dissolved inorganic carbon, total alkalinity, pH, saturation state of aragonite) to ocean and freshwater inputs. The model is an updated version of our FVCOM-ICM framework, with new carbonate-system and sediment modules. Sensitivity experiments altering concentrations at the open boundaries and freshwater sources indicate that not only ocean conditions entering the Strait of Juan de Fuca, but also the dilution of carbonate variables by freshwater sources, are key drivers of the carbonate system in Puget Sound.