Computation of nitrate concentrations in turbid coastal waters using an in situ ultraviolet spectrophotometer

In order to investigate biogeochemical cycling and dynamics of nutrients, measurement capabilities with sufficient spatial and temporal resolution are required. New ultraviolet-based methods and instrumentation for in situ determination of nitrate provide a powerful tool for these purposes. Here, we present a full dataset obtained by an UV-based process spectrophotometer (ProPS) during a cruise in 2008 in the south-eastern North Sea. Due to highly turbid conditions and mixing water masses, an improved calculation algorithm was performed on the UV absorption data. Nitrate concentrations ranged from 0 to 9 μmol/L on very short timescales, mainly due to tidal effects. Comparison of continuous optical nitrate measurements was performed against conventional wet-chemical analyses of discrete water samples which lead to a standard deviation of 1.7 μmol/L NO3−. High resolution measurements performed at a tidal inlet demonstrated the capability to map nitrate dynamics in turbid coastal environments.

Research highlights
► We present a new method for in situ determination of nitrate in turbid waters.
► UV spectrophotometer data is coupled with salinity and temperature sampling.
► A full dataset from a coastal inlet in the south-eastern North Sea is presented.
► Comparison against wet-chemical analyses lead to an accuracy of 1.7 μmol/L NO3−.
► High resolution sampling enabled time-space mapping of nitrate dynamics.