Sustained, autonomous coastal nutrient observations aboard moorings and vertical profilers

Abstract

Recent technological advances in autonomous submersible (in situ) chemical analyzers have made the real-time determination and reporting of multiple nutrient concentrations possible. Through the use of continuous flow injection analysis, SubChem Systems, Inc's Autonomous Profiling Nutrient Analyzer (APNA) is capable of sustained, autonomous deployment aboard coastal moorings, vertical profiling moorings, and other sampling platforms — enabling the determination of coastal nutrient concentrations, spatial distributions, and temporal variability. A series of technological hurdles have been addressed to accomplish this: (1) a more compact size, (2) reduced reagent and power consumption, (3) enhanced biofouling suppression, and (4) ease of use by non-chemists. Advances in micro-fluidics enable a reduction in sample volumes leading, therefore, to a reduction in reagent and power consumption. This, ultimately, extends the duration of remote field deployments. The suppression of bio-fouling, the simplification of wet chemical techniques for easier user operation, and the automation of data analysis into engineering units have also been addressed. Currently, the performance of the APNA analyzer is being rigorously tested at a variety of highly dynamic field sites that have strong vertical, horizontal, and temporal nutrient gradients as well as substantial biofouling burden caused by episodic phytoplankton blooms. Through extended and ongoing deployments in Narragansett Bay on the URI-GSO Pier, automated data collection, analysis, and quality control routines have been designed and implemented to assess the data integrity, verify in situ calibration parameters, and create near real-time plots which can be posted and made accessible via the internet. With funding from the ONR DRI Layered Organization in the Coastal Ocean (LOCO), the APNA was deployed — in a vertical profiling capacity — in Monterey Bay during the summer of 2005 and 2006. In 2005, a time series of high-resolution vertical nutrient profiles were obtained during the deployment of the APNA on a shipboard high resolution vertical profiling package. In 2006, a time series of high-resolution vertical nutrient profiles were obtained during the deployment of the APNA on an autonomous vertical profiler. The profiler was programmed to power the APNA so it would continuously acquire nutrient concentration data while the profiler ascended through the water column. The real-time analytical results indicated that the nutrient gradients were highly correlated with gradients in chlorophyll, salinity, and temperature. The primary objective for the development of the APNA is to provide accurate, real-time nutrient distributions aboard a variety of ocean observing platforms in order to fulfill the needs of water quality managers as well as the oceanographie community. This is accomplished by using cutting edge technology combined with accepted standard analysis methods and rigorous QA/QC procedures. Long-term deployments in challenging coastal environments allow us to identify, scrutinize, and overcome the real-world obstacles encountered by in situ nutrient observing systems.