Mapping the spatial variability of plankton metabolism using nitrate and oxygen sensors on an autonomous underwater vehicle

Abstract

It is now possible to make highly resolved vertical (,1 m) and horizontal (3 m at constant depth and ,400 m when undulating from surface to 100 m) measurements of nitrate concentration in the upper ocean using sensors deployed on propeller-driven autonomous underwater vehicles (AUVs). The ability to make such highly resolved measurements opens the possibility of detecting small-scale anomalies in nutrient fields that are created by locally high rates of primary production. We employed an in situ ultraviolet spectrophotometer optical nitrate sensor and Sea-Bird oxygen sensor mounted on the Monterey Bay Aquarium Research Institute Dorado AUV to investigate the spatial variability of nitrate and oxygen in Monterey Bay, California. The Dorado conducts missions up to 100 km in length. A very simplified diffusion and reaction model for nitrate, ignoring horizontal processes and advection and assuming steady state, would require that consumption rate 5 K Z 3 h 2 NO { 3 /hZ 2 (K Z 5 vertical eddy diffusion coefficient). Water column regions exhibiting elevated nitrate uptake rates are identified from anomalies in the second derivatives of the vertical concentration profiles relative to the second derivative of conservative properties. The relationship between chlorophyll a fluorescence and the inferred productivity rates exhibits coherent spatial variability across the bay.Patchiness in plankton distributions is a characteristic feature of the upper ocean (Martin 2003(Martin , 2005. The role of nutrients in regulating patchy distributions remains poorly understood. Moored nutrient sensors have only recently reached a state such that they can generate time series data that provide useful information on ecosystem processes (McGillicuddy et al. 1998;Sakamoto et al. 2004;Johnson et al. 2006). The ability to make high-spatial-resolution measurements of nutrient distributions in both horizontal and vertical dimensions within the ocean could provide important insights into the interaction of phytoplankton and their environment. However, mapping nutrient distributions at the same scales at which chlorophyll fluorescence intensity can be measured has been a challenge (Martin 2003). Underway mapping of surface nutrient distributions is feasible using shipboard chemical analyzers (Sakamoto et al. 1998;Woodward and Rees 2001). Relatively few such studies have been made that incorporate vertical resolution because of the difficulty in pumping water from depth while ships are underway. Specialized pumping systems are required in this case to deliver seawater to shipboard laboratories. Hales and Takahashi (2004) reported on an extensive set of nutrient and inorganic carbon measurements using a SeaSoar system with an integral pump to deliver water to the ship. Hales et al. (2005) demonstrated measurements with vertical resolution in nutrient measurements of several meters using submersible pumps towed behind a slowly moving ship to supply high-speed, automated chemical analyzers (Hales et al. 2004) operated in shipboard laborat...

Observations of turbulent mixing in a phytoplankton thin layer: Implications for formation, maintenance, and breakdown

“…5). Johnson and Needoba (2008) observed a similarly strong relationship between nitrate and temperature over a 6-month period in Monterey Bay, with only modest nonstationarity due to biological activity and surface cooling and heating. The condition of stationarity allows nitrate to be inferred from direct temperature measurements acquired during the layer observations on 19 August, using a cubic smoothing spline fit to the data (Fig.…”

“…1). The AUV was equipped with a suite of sensors including Sea-Bird temperature sensors and an ultraviolet spectrophotometer optical nitrate sensor (Johnson and Needoba 2008), which was calibrated with nitrate measurements from water samples taken during the LOCO study. The details of the data processing and the development of associated numerical models are provided in parallel with the analysis that follows.…”

Guidelines Towards an Integrated Ocean Observation System for Ecosystems and Biogeochemical Cycles

“…Optical sensors for dissolved nitrate are now available (Johnson and Coletti, 2002). In combination with autonomous platforms, this sensor can be used to track nutrient injection events that may stimulate productivity in oligotrophic regions (e.g., Sakamoto et al, 2004) or to map plankton metabolism (Johnson and Needoba, 2008).…”

Progress in multidisciplinary sensing of the 4-dimensional ocean

“…AUVs, such as Autosub, can play major roles in exploring long distances under ice in remote regions (e.g., Nicholls et al, 2008). Examples of other important work being conducted with AUVs include: studies of high resolution ocean optics (in optical wavelengths and in space; Glenn et al, 2004), ice-ocean turbulent exchanges (e.g., Hayes and Morison, 2008), mapping of plankton and distributions of nutrient and oxygen concentrations (e.g., Johnson and Needoba, 2008), and mapping of coral reefs (e.g., Shcherbina et al, 2008) and bays (e.g., . Deployment of large AUVs from ships at sea is difficult and may require special deck gear opposed to small AUVs, drifters, floats and gliders, some of which can be deployed from aircraft or near shore.…”

“…The MOSEAN CHARM mooring, which was located in the Santa Barbara Channel, was used to test a variety of optical and chemical sensors and associated anti-biofouling approaches. It should also be noted that new chemical sensors including an osmoanalyzer (e.g., Jannasch et al, 1994) and an in situ ultraviolet spectrophotometer (Johnson and Needoba. 2008) have been successfully deployed in oligotrophic waters, with both capturing nutrient injection events associated with mesoscale eddies (e.g., see McNeil et al, 1999 andSakomoto et al, 2004).…”