Nutrient preservation, analysis precision and quality control of an oceanographic database of inorganic nutrients, dissolved oxygen and chlorophyll a from the NW Mediterranean Sea

Oceanographic data from 28 cruises performed in the NW Mediterranean Sea between 1982 and 2003 and data from historical databases, amounting to more than 100000 data points, are used to define the quality control ranges at standard level depths of dissolved inorganic nutrients, dissolved oxygen and chlorophyll a. The quality of each of the key variables and the analytical precision are assessed. The results indicate that freezing is a valid preservation method for phosphate, nitrate, silicate and nitrite in samples from the NW Mediterranean Sea, though their relative error increases (6.3%, -1.6%, -2.5% and -11.4%, respectively). In contrast, freezing nutrient samples on the cruises gathered negatively affected the analytical precision and accuracy of the ammonium analysis. The analysis precision is also estimated using different approximations, the most realistic being the use of replicate samples from the same sampling bottle. Except for phosphate and dissolved oxygen, specific quality control ranges for mixed water columns, usually found in winter, have to be defined, since the surface concentrations are particularly high due to deep water mixing. The quality-controls described in this work are an important and useful tool for validating data and for detecting erroneous or anomalous data obtained in both historical and future works in the NW Mediterranean Sea.