[1] The discovery of large summer chlorophyll blooms in oligotrophic regions of the ocean has led to questions about the relationship between these blooms and the frequently cooccurring outburst of nitrogen-fixing phytoplankton. We compared diatom-diazotroph assemblage (DDA) abundance to size-fractionated chlorophyll (chl) and satellite ocean color chlorophyll estimates to evaluate how DDAs affected ocean color estimates in the eastern and central North Pacific gyre at 28-30°N. ) never reached the 0.15 mg m −3 threshold used to define satellite-observed chlorophyll blooms in oligotrophic waters. The DDA blooms were not evident in the in situ fluorometer data; however, the blooms occurred within high beam attenuation features observed in the transmissometer data. Trichodesmium was not a component of either diatom bloom although elevated levels of Trichodesmium were observed at two stations where DDAs were not abundant. While DDA blooms and satellite ocean chlorophyll blooms are sometimes coincident, our data do not support that DDAs are the sole source of the satellite-observed chlorophyll in summertime blooms. DDA blooms are likely underreported in the North Pacific, particularly in the waters west of Hawaii, due to their frequent lack of distinctive ocean color, fluorescence, and chlorophyll signatures. The source of the ocean color signature in the blooms remains elusive, but scattered literature observations suggest that cooccurring members of the near-surface flora such as the small pennate diatom Mastogloia may play an important role.

Autonomous in situ sensors are needed to document the effects of today's rapid ocean uptake of atmospheric carbon dioxide (e.g., ocean acidification). General environmental conditions (e.g., biofouling, turbidity) and carbon-specific conditions (e.g., wide diel variations) present significant challenges to acquiring long-term measurements of dissolved inorganic carbon (DIC) with satisfactory accuracy and resolution. SEAS-DIC is a new in situ instrument designed to provide calibrated, high-frequency, long-term measurements of DIC in marine and fresh waters. Sample water is first acidified to convert all DIC to carbon dioxide (CO2). The sample and a known reagent solution are then equilibrated across a gas-permeable membrane. Spectrophotometric measurement of reagent pH can thereby determine the sample DIC over a wide dynamic range, with inherent calibration provided by the pH indicator's molecular characteristics. Field trials indicate that SEAS-DIC performs well in biofouling and turbid waters, with a DIC accuracy and precision of ∼2 μmol kg(-1) and a measurement rate of approximately once per minute. The acidic reagent protects the sensor cell from biofouling, and the gas-permeable membrane excludes particulates from the optical path. This instrument, the first spectrophotometric system capable of automated in situ DIC measurements, positions DIC to become a key parameter for in situ CO2-system characterizations.

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Accurate, high-resolution profiles of nitrate and phosphate distributions in the open ocean are difficult to obtain using conventional techniques. Concentrations typically range from low nanomolar levels in the stratified euphotic zone to micromolar levels below the nutricline. With multiple pumps, a heating cartridge, a long-path-length cell, and a multiwavelength spectrometer, the reconfigured Spectrophotometric Elemental Analysis System (SEAS) provides the capability to fully ascertain the distributions of nitrate and phosphate in the upper 200 m of the oligotrophic ocean. By utilizing a 15 cm path length and multiple wavelength spectrophotometry, SEAS can detect nitrate concentrations from 2 nM to 20 microM and, with a 50 cm path length, can accurately measure phosphate concentrations from 1 nM to 1 microM. SEAS is capable of collecting auxiliary data from up to four separate instruments, including a CTD, a fluorometer, a PAR sensor, and a second SEAS instrument. Sampling frequency depends on peripheral instrument selection and ranges from 0.4 to 0.75 Hz.