Lori Adornato

Summer blooms of diatom-diazotroph assemblages and surface chlorophyll in the North Pacific gyre: A disconnect

[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. DDA blooms were dominated by either Hemiaulus hauckii (in the central Pacific in 2003 and the eastern Pacific in 2002) or by Rhizosolenia (eastern Pacific in 2002), both with nitrogen‐fixing Richelia symbionts. The 2002 DDA bloom was measured a week prior to the development of a satellite‐ observed chlorophyll bloom at the same location. In contrast, the 2003 Hemiaulus bloom was not within a clearly defined satellite feature. Although DDA abundance increased 10 4 –10 5 ‐fold relative to the background and they dominated the net plankton (≥5 m mo r >10 mm chl size) fraction, the in situ chl (maximum ≤0.11 mg m −3 ) 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.

In situ spectrophotometric measurement of dissolved inorganic carbon in seawater

Autonomous in situ sensors are needed to document the effects of todays 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 indicators 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.

Evaluation of reagentless pH modification for in situ ocean analysis: determination of dissolved inorganic carbon using mass spectrometry

RATIONALE In situ analytical techniques that require the storage and delivery of reagents (e.g., acidic or basic solutions) have inherent durability limitations. The reagentless electrolytic technique for pH modification presented here was developed primarily to ease and to extend the longevity of dissolved inorganic carbon (DIC) determinations in seawater, but can also be used for other analytical methods. DIC, a primary carbon dioxide (CO(2)) system variable along with alkalinity, controls seawater pH, carbonate saturation state, and CO(2) fugacity. Determinations of these parameters are central to an understanding of ocean acidification and global climate change. METHODS Electrodes fabricated with electroactive materials, including manganese(III) oxide (Mn(2)O(3)) and palladium (Pd), were examined for potential use in electrolytic acidification. In-line acidification techniques were evaluated using a bench-top membrane introduction mass spectrometry (MIMS) setup to determine the DIC content of artificial seawater. Linear least-squares (LLSQ) calibrations for DIC concentration determinations over a range between 1650 and 2400 µmol kg(-1) were obtained, using both the novel electrolytic and conventional acid addition techniques. RESULTS At sample rates of 4.5 mL min(-1), electrodes clad with Mn(2)O(3) and Pd were able to change seawater pH from 7.6 to 2.8 with a power consumption of less than 3 W. Although calibration curves were influenced by sampling rates at a flow of 4.5 mL min(-1), the 1σ measurement precision for DIC was of the order of ±20 µmol kg(-1). CONCLUSIONS Calibrations obtained with the novel reagentless technique and the in-line addition of strong acid showed similar capabilities for DIC quantification. However, calculations of power savings for the reagentless technique relative to the mechanical delivery of stored acid demonstrated substantial advantages of the electrolytic technique for long-term deployments (>1 year).

High-resolution chemical sensor for unattended underwater networks

Autonomous underwater sensors are the best solution for continuous detection of chemical species in aquatic systems. The Spectrophotometric Elemental Analysis System (SEAS), an in situ instrument that incorporates both fluorescence and colorimetric techniques, provides high-resolution time-series measurements of a wide variety of analytes. The use of Teflon AF2400 long-pathlength optical cells allows for sub-parts-per-billion detection limits. User-defined sampling frequencies up to 1 Hz facilitate measurements of chemical concentrations on highly resolved temporal and spatial scales. Due to its modular construction, SEAS can be adapted for operation in littoral or open ocean regions. We present a high-level overview of the instruments design along with data from moored deployments and deep water casts.

Development of a portable carbon system sensor for ocean acidification research

Lowering of seawater pH due to increased anthropogenic carbon dioxide, known as ocean acidification (OA), has garnered attention from scientists and lawmakers around the globe. A great deal of research involves projecting how OA will affect aquatic organisms and systems in the future. It is generally recognized that many of the laboratories engaged in OA research do not possess the facilities or expertise necessary to measure carbon system parameters at the precision required to produce reliable, actionable results. To address this deficiency, researchers and engineers from SRI and USF are undertaking a major advancement in carbon-system sensors. Drawing on our experience in developing and deploying our multi-parameter inorganic carbon analyzer (MICA) instruments, we are building a new system (MICA III) that is easy to use and requires virtually no supporting infrastructure. Advances in commercial technology and refinement of our methods enable us to dramatically reduce the size and cost of MICA instrumentation. MICA III measures three parameters: pH, dissolved inorganic carbon (DIC), and total alkalinity (TA). Our instrument includes innovative designs for temperature compensation and total alkalinity measurements, and uses an embedded display/user interface to produce a truly portable (about the size of a large briefcase) sensor for studies of OA and the CO2 system. Our goal is to develop an instrument that can be easily deployed by both chemists and non-chemists to produce carbon system data fully consistent with best-practice protocols. This article discusses overall objectives, highlights key design elements, and presents results from initial hardware evaluations.

Potential impacts of climate change on acoustic propagation in the Arctic

Some forecasts show surface pH in the Arctic dropping from 8.1 to 7.6 over the next 100 years. This substantial decrease may cause changes in acoustic transmission at frequencies where the pH-dependent borate absorption plays a role, below about 5 kHz. In many cases, upward refraction of sound in the near-isothermal Arctic waters causes ice or surface scattering effects to dominate transmission. However, recent observations in the Canada Basin show that 700 and 900-Hz sound can be fully ducted beneath the Pacific Summer Water, with no ice interaction, and be detectable over distances of a few hundred kilometers. In this situation, the received signal level is controlled largely by cylindrical spreading and absorption. Here, for a wide band of frequencies, the effects of probable pH reductions and reduced absorption are investigated using a few models of pH depth profiles. There is potential for increased signal levels of 5 dB or more for 200-km propagation if the duct waters have significantly reduced pH.

Semi-fuel cell studies for powering underwater devices: integrated design for maximized net power output

Use of sensor systems in water bodies has applications that range from environmental and oceanographic research to port and homeland security. Power sources are often the limiting component for further reduction of sensor system size and weight. We present recent investigations of metal-anode water-activated galvanic cells, specifically water-activated Alcells using inorganic alkali peroxides and solid organic oxidizers (heterocyclic halamines), in a semi-fuel cell configuration (i.e., with cathode species generated in situ and flow-through cells). The oxidizers utilized are inexpensive solid materials that are generally (1) safer to handle than liquid solutions or gases, (2) have inherently higher current and energy capacity (as they are not dissolved), and, (3) if appropriately packaged, will not degrade over time. The specific energy (S.E.) of Al-alkali peroxide was found to be 230 Wh/kg (460 Wh/kg, considering only active materials) in a seven-gram cell. Interestingly, when the cell size was increased (making more area of the catalytic cathode electrode available), the results from a single addition of water in an Al-organic oxidizer cell (weighing ~18 grams) showed an S.E. of about 200 Wh/kg. This scalability characteristic suggests that values in excess of 400 Wh/kg could be obtained in a semi-fuel-cell-like system. In this paper, we also present design considerations that take into account the energy requirements of the pumping devices and show that the proposed oxidizers, and the possible control of the chemical equilibrium of these cathodes in solution, may help reduce this power requirement and hence enhance the overall energetic balance.