ESTOC

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Project title and acronym

UV photometer operation and improvement in the top pelagic layer of an oligotrophic station

Host facility

European Station for Time series in the Ocean Canary Islands (ESTOC)

Modality of Access

MoA2 – Partially remote (the presence of the user is required at some stage, e.g. for installing and uninstalling an instrument)

Description

In order to investigate biogeochemical cycling and dynamics of nutrients, measurement capabilities with sufficient spatial and temporal resolution are required. Nitrate as one of the most relevant nutrient in aquatic ecosystems is by now a standard parameter for marine observations and regulations. Optical measurements utilizing ultraviolet (UV) in-situ spectrophotometers have promised to be an excellent method for nitrate sensing, since their operation is fast, reagent-free thus providing a powerful tool as they are applicable to long- term observations. Particularly in coastal waters the determination of nitrate by optical methods  is  very  challenging  due  to  various  influencing  of  present  water  constituents, especially  coloured  dissolved  organic  matter  (CDOM)  and  suspended  particulate  material (SPM). Furthermore physical properties of seawater, e.g. salinity and temperature, have a strong  influence  on  the  computation  of  nitrate  concentrations  from  absorption  spectra. Having this in mind, new approaches in the computation of nitrate in coastal waters have been achieved by reducing the degrees of freedom via a stepwise simplification of the spectra considering all influencing parameters by different correction approaches before calculating nitrate concentrations. Within various field campaigns a rapid and high-resolution mapping of nitrate was possible with good correlation to reference sampling. Thus it demonstrates the potential for in-situ observations and regulatory actions as well as for long-term monitoring observations. Detailed information of the current methodology of nitrate calculations regarding temperature and salinity dependencies of instrumentation and surroundings can be found  in  the  listed  and  available  relevant  publications  (see  below  section).  Processing methods will be constantly validated and improved in different study sites, e.g. in estuaries with  steep  salinity  gradient  and  in  open ocean observations  with  low  nutrient  seawater proofing the detection limit. The ESTOC station will offer a good opportunity to test and validate the implemented methods to determine temporal nitrate fluxes within an oligotrophic/subtropical region enabling a comparison to so far investigated shelf sea and coastal areas. Besides the determination of nitrate the usage of an UV process spectrophotometer with a spectral wavelength range in the deep UV (200- 360 nm) allow the calculation of further parameters from the absorption spectra afterwards, e.g. the spectral absorption coefficients at 254 nm (SAC254). This parameter can be related to water quality assessment and organic pollution and is strongly correlated to present DOC concentrations.