Nitrate
It is an essential resource, which, together with available light, determines the amount of organic matter that can be synthesized by phytoplankton through photosynthesis. As such, its concentration is also a key variable of biogeochemical and ecosystem models. At present, research still lacks sufficient amount of nitrate concentration datasets that would allow in particular to better constraint such models and improve our understanding.
2) How is it measured ?
Nitrate concentration is generally measured through chemical determination performed by an autoanalyser on discrete water samples. New in situ sensors, based on the optical determination of nitrate absorption in the UV are now sufficiently mature to be implemented on autonomous platforms (cf. PROVNUTS http://www.oao.obs-vlfr.fr/robots-a-sensorssm/profiling-floatssm/nitratepf and VASQUE http://www.oao.obs-vlfr.fr/projectssm/other-ongoing-projectssm/100-vasque ). The sensitivity of these methods is admittedly weak (~1 mM) compared to chemical analysis. However, this platform-sensor combination will allow to acquire tremendous amounts of data; it will also allow to systematically locate the nitracline (zone in the water column where nitrate concentration increases) which is a key feature to be addressed in biogeochemical studies.
3) Where is more information available?
Johnson et al. (2010). Nitrate supply from deep to near-surface waters of the North Pacific subtropical gyre. Nature 465, 1062-1065.
Sakamoto et al. (2009). An improved algorithm for the computation of nitrate concentrations in seawater using an in situ ultraviolet spectrophotometer. Limnology and Oceanography, Methods, 7, 132-143.
Johnson et al. (2006). Diel nitrate cycles observed with in situ sensors predict monthly and annual new production. Deep Sea Research, I 53(3), 561-573.
Johnson, K.S. & Coletti, L.J. (2002). In situ ultraviolet spectrophotometry for high résolution and long-term monitoring of nitrate, bromide and bisulfide in the ocean. Deep Sea Research, I 49, 1291-1305.