Sea Technology

OCT 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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12 st / October 2017 www.sea-technology.com resolved. Operation in optically dense or turbid water can reduce optical transmission, reducing the strength of the signal. To compensate, the SUNA has adaptive sampling capabilities, varying the length of time over which each measurement is made. Integra- tion times vary from 200 to 500 ms, increasing in turbid and optically poor conditions. On the other hand, absor- bance greater than 1.3 in a wavelength channel can significantly bias the re- gression and is, therefore, not used in processing. If less than 10 wavelength channels are available, the multivari- ate regression cannot be performed and nitrate concentration is not calcu- lated. Biofilms and window scratching can also alter light transmission to the sensor. The source of this error can be prevented by keeping the optics clean and using the optional Hydro-Wiper external anti-fouling system. Light-source aging causes sensor drift as the intensity of the light emitted from the light source decreases with time. This can be corrected by the user by just regularly updating the spectrum of reference deionized water. Instruc- tions for updating the reference spec- trum and the calibration file are provid- ed in the SUNA manual. Interference with overlapping absorption spectra from other materials in the light path affects sensor accuracy. Bromide, ni- trite, bisulfide, CDOM and suspended particulates have absorbance spectra similar to nitrate, and if not removed, nitrate concentrations are biased. This was found in early field testing of the SUNA's technological predecessors, where the temperature dependence of bromide absorbance in seawater caused a bias. This bias is corrected by changing the baseline seawater absor- bance spectrum using in-situ seawater temperature and salinity. The baseline seawater absorbance spectra are cor- rected, then subtracted from the mea- sured absorbance before using multi- variate linear regression to determine nitrate concentration. Gathering robust CTD measurements concurrently with SUNA nitrate measurements allows the application of the seawater correc- tion in post-processing using Sea-Bird UCI software, or in situ when properly interfaced, as on Argo floats. CDOM and particulate suspended matter (turbidity) affect accuracy be- cause they absorb at all wavelengths and therefore have absorbance spectra that overlap nitrate. CDOM and tur- bidity spectra cannot be parameter- ized with independent measurement, such as the seawater correction, as there is no universal model for their concentration and composition. John- son and Coletti have used site-specific absorbance spectra in post-process- ing, which could be used to correct for CDOM and NTU, but this type of correction is still under research. Typi- cally, this correction only needs to be applied in regions with high sediment loads or phytoplankton blooms. Field Validation Case Study The King County Department of Natural Resources and Parks in Se- attle, Washington, conducts long-term marine monitoring to assess baseline conditions and trends in Central Puget Sound. Routine nutrient data, includ- ing nitrate, have been collected since 1994. Twice monthly observations are collected at several marine stations in Puget Sound using a suite of biogeo- chemical sensors while concurrently collecting discrete water samples at multiple depths. This example en- ables the direct comparison of nitrate concentrations made with the SUNA nitrate to those made with traditional wet chemistry methods. A shipboard profiling carousel is outfitted with sensors to measure tem- perature, salinity, depth, dissolved oxygen, transmissivity, fluorescence and nitrate. Five water-sampling bot- tles mounted on the carousel are used to collect discrete measurements of bacteria levels (fecal coliform indica- tors and enterococcus); nutrient levels (dissolved inorganic nitrogen, total nitrogen, orthophosphate and silica); pigment levels (chlorophyll-a and pheophytin-a); and physical param- eters (dissolved oxygen, salinity and total dissolved solids). Here, we focus on the SUNA nitrate sensor and discrete samples of nitrate and nitrite nitrogen collected at Jef- ferson Head Station on April 4, 2017. The SUNA V2 is configured with a 10- mm path length and is controlled by a Sea-Bird 25plus CTD. The CTD col- lects measurements in real time during the downcast and upcast and triggers the acquisition of discrete water sam- ples by closing the sampling bottles at specific depths during the upcast. These bottle samples are then used to measure the nitrate concentration in the water using a colorimetric analy-

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