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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.

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.

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.

The NH4-Digiscan is an in situ analyzer designed for measuring ammonium in estuarine, coastal, and shelf waters at depths of less than 3 m. This wet chemical analyzer uses micro-solenoid pumps to propel sample and reagents, a gas diffusion cell to isolate the analyte from the matrix, and a conductivity detector for analyte detection. Instrument measurements are stable for deployments of at least 30 d. In estuarine and coastal waters, the analyzer is capable of sampling hourly and has a detection limit of 0.2 µM. In shelf waters, the NH4-Digiscan can be configured to have a detection limit of 0.014 µM. The simple chemistry, in situ capability, and high resolution sampling minimizes the use of toxic reagents, minimizes many of the problems plaguing ammonium analyses, and adequately captures the high temporal variability of coastal waters, which is often undersampled. The analyzer has been successfully deployed on coastal moorings, benthic flux chambers, and on a drifter 500 km west of Monterey Bay, California. The system can also be easily configured for laboratory bench top analysis of discrete samples.

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.

An ocean observatory that consists of an array of moored sensor platforms, telemetry, and data collection and dissemination software was designed for monitoring the biogeochemistry and physical dynamics of coastal and estuarine ecosystems. The Land-Ocean Biogeochemical Observatory (LOBO) consists of robust moorings that can withstand tidal currents and weather. The moorings are highly configurable, can be deployed in waters as shallow as 0.5 m, are relatively easy to maintain, and accommodate a complete array of standard and novel sensors. The sensors communicate with an on-board controller which relays data to shore in near-real time. Up to five LOBO moorings have been simultaneously deployed and maintained in Elkhorn Slough, California, since November 2003. Continuous hourly data of biological, chemical, and physical properties are relayed to shore, processed, and disseminated to users through a web interface in near-real time. This article describes the design, implementation, and functionality of the LOBO monitoring system.

A primary limitation of phytoplankton ecology research is the difficulty of describing patchiness and distributions of different phytoplankton groups. Chlorophyll fluorescence and optical backscatter are useful measurements that provide information about phytoplankton, but these measurements do not allow distinction of phytoplankton taxa. Traditional phytoplankton identification methods (such as microscopy, HPLC analysis, and flow cytometry) are labor intensive and therefore can provide only very limited coverage and resolution. Through lab experiments we show that the Laser In Situ Scattering and Transmissometer (LISST-100) instrument can accurately quantify phytoplankton cell dimensions for some cell shapes. Pseudo-spherical dinoflagellates are described with a single peak in the particle size distribution (PSD) at the cross-sectional dimension of the cells. Pennate diatoms are described with peaks in the PSD at the major and minor axis dimensions of the cells. Diatom cells with minor axis dimensions that vary along the major axis are described with one peak across the range of minor axis dimensions and a second peak at the major axis dimension. Through field experiments, we show that mapping the PSD in situ at high resolution permits description of patchiness and evolution of phytoplankton populations. We present two examples: (1) growing dominance of Ceratium species during a red tide bloom, and (2) concentration of Pseudo-nitzschia australis, a harmful algal bloom (HAB) species, at a water mass front. We conclude that synoptic mapping of the PSD can significantly advance phytoplankton ecology research in the coastal ocean.