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The most accurate calculation of nitrate concentration from the ultraviolet (UV) absorption spectrum of seawater requires that the absorption signal due to bromide in seawater be removed before nitrate concentrations are computed. Recent work suggests that the UV absorption spectrum of bromide in seawater has a pressure dependence. Neglect of this signal could add a bias when nitrate concentrations at high pressure are computed from UV measurements. Laboratory tests were conducted to determine the pressure dependence of the bromide absorption in seawater. Our results confirm the existence of a pressure coefficient in the bromide spectrum. The percentage change in bromide molar absorptivity is wavelength and temperature independent. The effect of pressure on the absorptivity of sea salt (ESW), which is dominated by bromide ion, can therefore be calculated as ESW pressure=ESW 1 dbar∗ (1 – 0.0 26 ∗ Pressure (dbar)/1000) The correction amounts to an error of around 0.95 μM nitrate at 1000 dbar. The pressure correction should be used in the calculations of nitrate concentrations from UV absorption spectra at high pressures.

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality-controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long-term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1 +/- 1%, nitrate to within 0.5 +/- 0.5 mu mol kg(-1), and pH to 0.005 +/- 0.007, where the error limit is 1 standard deviation of the fleet data. The bio-optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m(-3) or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.

Six profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and 10 float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 m of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly 3 months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one toward the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m−2 yr−1 using nitrate and 1.5 ± 0.7 mol C m−2 yr−1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles, and these models yield NCP values that are two to three time higher than the nitrate-based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle.

Reagent-free optical nitrate sensors [in situ ultraviolet spectrophotometer (ISUS)] can be used to detect nitrate throughout most of the ocean. Although the sensor is a relatively high-power device when operated continuously (7.5 W typical), the instrument can be operated in a low-power mode, where individual nitrate measurements require only a few seconds of instrument time and the system consumes only 45 J of energy per nitrate measurement. Operation in this mode has enabled the integration of ISUS sensors with Teledyne Webb Research's Autonomous Profiling Explorer (APEX) profiling floats with a capability to operate to 2000 m. The energy consumed with each nitrate measurement is low enough to allow 60 nitrate observations on each vertical profile to 1000 m. Vertical resolution varies from 5 m near the surface to 50 m near 1000 m, and every 100 m below that. Primary lithium batteries allow more than 300 vertical profiles from a depth of 1000 m to be made, which corresponds to an endurance near four years at a 5-day cycle time. This study details the experience in integrating ISUS sensors into Teledyne Webb Research's APEX profiling floats and the results that have been obtained throughout the ocean for periods up to three years.

Widespread contamination of surface water with nitrate-nitrogen (NO3-N) has led to increasing regulatory pressure to minimize NO3-N release from agricultural operations. We evaluated the use of wood chip denitrification bioreactors to remove NO3-N from tile drain effluent on two vegetable farms in Monterey County. Across several years of operation, denitrification in the bioreactors reduced NO3-N concentration by an average of 8 to 10 milligrams per liter (mg L−1) per day during the summer and approximately 5 mg L−1 per day in winter. However, due to the high NO3-N concentration in the tile drainage (60 to 190 mg L−1), water discharged from the bioreactors still contained NO3-N far above the regulatory target of < 10 mg L−1. Carbon enrichment (applying soluble carbon to stimulate denitrifying bacteria) using methanol as the carbon source substantially increased denitrification, both in laboratory experiments and in the on-farm bioreactors. Using a carbon enrichment system in which methanol was proportionally injected based on tile drainage NO3-N concentration allowed nearly complete NO3-N removal with minimal adverse environmental effects.

Chemical fluxes into and out of permeable seafloor sediments, such as those found on much of the continental shelf, are difficult to measure using conventional techniques. To overcome this difficulty, the eddy correlation method was adapted by oceanographers to determine oxygen flux across the sediment-water interface. In this article, we demonstrate that the eddy correlation method can also be used to measure nitrate flux across the sediment-water interface. A modified ISUS (In Situ Ultraviolet Spectrophotometer) optical nitrate sensor was used to measure dissolved nitrate at a frequency of 1.8 Hz. These observations were combined with vertical velocity measurements to calculate nitrate fluxes. Oxygen fluxes were also determined using a fast oxygen electrode system. The results for nitrate and oxygen flux compare well with simultaneous benthic flux chamber measurements made at a muddy sediment site at 95 m depth in Monterey Bay on the central California coast. The nitrate fluxes determined by the eddy correlation technique were considerably higher than benthic flux chamber values at sandy sediment sites at similar depths to the north and south of Monterey Bay. This difference is expected in permeable sediments where lateral flow through sediments will ventilate chamber water.

Improvements in the data processing algorithm and calibration procedures have greatly increased the accuracy of nitrate measurements using an in situ ultraviolet spectrophotometer (ISUS). Two major changes in the algorithm involve application of a temperature-dependent correction to the bromide spectrum and then using the observed temperature and salinity to subtract the bromide component before fitting nitrate. By reducing the degrees of freedom in calculating nitrate concentrations, the accuracy of the ISUS instrument is substantially improved. The new algorithm was tested in environments ranging from the Southern Ocean to oligotrophic eastern Pacific seawater and found to be applicable at all temperatures and depths. The standard error of the estimate for regression between ISUS nitrate values and discrete samples measured by standard wet chemistry methods for the combined data set is reduced by greater than 2-fold (1.4 down to 0.65 µM) using the new algorithm. This corresponds to a 5-fold reduction in variance (2.0 down to 0.4 µM2). Although biofouling and calibration drift remain issues for any instrument deployed in situ for long periods of time, using the measured salinity and temperature to correct the ultraviolet spectra before the nitrate calculations will reduce the impacts of these confounding processes.

We report on a direct and autonomous measure of new production based on time series observations with ISUS nitrate sensors deployed for several years on oceanographic moorings offshore of Monterey Bay, California. The amplitude of diel cycles of surface nitrate is correlated with rates of primary production measured by 14C uptake. The drawdown of nitrate averaged over a year is about 70% of the fixed nitrogen needed for biomass production. Phytoplankton biomass predicted from the diel nitrate uptake and a fixed rate constant for nitrate loss (grazing and export) matched observations over the annual cycle. New production rates determined with the moored sensors are highly correlated with nitrate concentration and the intensity of upwelling. The implication is that arrays of moorings with chemical sensors can now be used to estimate new production of biomass and ecosystem processes over multiple temporal and spatial scales.

We report measurements of dissolvable and particulate iron, particulate Al, nutrients and phytoplankton biomass in surface waters during the termination of one upwelling event and the initiation of a second event in August 2000. These events occurred in the area of the Ano Nuevo upwelling center off the coast of central California. The first event was observed after ∼8 days of continuous upwelling favorable winds, while the second event was observed through the onset of upwelling favorable winds to wind reversals ∼3 days later. Coincident with the upwelling signatures of low temperature and high salinity were significantly elevated concentrations of nitrate and silicate with average concentrations greater than 15 and 20 μM, respectively, during both upwelling events. Dissolvable Fe concentrations (TD-Fe) were significantly higher in the second event, 6.5 versus 1.2 nM Fe found in the first event. Nitrate was reduced by ∼5 μM day −1 within this second upwelled plume as compared to a drawdown of ∼2 μM day −1 within the first plume. Silicate was reduced in a ratio of 1.2 mol Si:mol NO 3 in the high Fe waters of the second plume as compared to a ratio of 2.2 in the lower Fe waters of the first plume. The observed differences in nutrient utilization are consistent with some degree of iron limitation. The area of increased dissolvable Fe in the second upwelling event was coincident with elevated particulate Fe concentrations, indicating the particulate pool as a possible source of the observed increase in TD-Fe. The elevated particulate Fe in surface waters was a result of resuspended sediments in the bottom boundary layer (BBL) of the shallow shelf being transported to the surface during upwelling. Particulate (and dissolvable) iron concentrations were significantly reduced as upwelling continued. This was most probably due to a decoupling of the BBL from upwelled source waters as the upwelling front moved offshore and/or reduced turbulence in the BBL as upwelling continued. The observed reduction in both particulate and dissolvable Fe, as upwelling continued to deliver macronutrients to surface waters, may result in varying levels of Fe limitation.

Sustaining measurements of water quality parameters in sensitive watershed habitats is challenging, particularly during critical periods such as spring blooms, and major run off events. Direct, in situ measurements of nitrate have been difficult to obtain until the development of the ISUS (In Situ Ultraviolet Spectrometer, Johnson & Coletti 2002) and more recently the next generation SUNA (Submersible Ultraviolet Nitrate Analyzer). These chemical-free optical sensors have now been operating in a wide range of challenging environments over the past five years. Performance evaluations of these instruments, lessons learned, and environmental insights obtained will be discussed, as well as results from these sensors under the recent ACT (Alliance for Coastal Technologies) Nutrient Sensor Technology Demonstration Program.