- The nutrients
Malcolm Woodward from the Plymouth Marine Laboratory (PML) and Sarah Breimann, PhD student at the National Oceanographic Center in Southampton University are both analyzing nutrients during the FRidge cruise. They are measuring ammonium, silicate, phosphate, nitrate and nitrite in sea water collected from both rosettes (Stainless steel and titanium). These analyses are very important for two reasons. The first one is that nutrients are a key to understanding phytoplankton productivity in the world’s ocean and so to identify nutrient depleted regions such as Atlantic gyres. The second one is that, as nutrients have generally a well-known profile in the water column, this allows us to check that the Niskin/Go-Flo bottles were closed properly and at the good depths without any mechanical issues.
To do the measurements on the cruise, Malcolm Woodward and Sarah Breimann are using two types of analyzers depending on the concentration of the nutrients:
- A Five channel nutrient autoanalyzer (AA3) for nutrient measurements at low μM concentrations. It is composed of samplers, mixing coils, heater, cooling mixing coils and digital colorimeters. The classic colorimetric analytical technique is used with pH reaction conditions controlled to measure the nutrients detected at different wavelengths.
- A Nanomolar nutrient analyzer for measurements until nM levels. It is composed of samplers, reaction coils, a teflon-membrane diffusion cell and a fluorescence detector. Here, Ammonia is measured by fluorescence; the sample reacts with chemicals and buffers that increase the pH and then the Ammonia moves across a 5 micron Goretex Teflon membrane forming a fluorescent species in a quantity that is proportional to the ammonia concentration.
Generally, two replicates of 60ml non- filtered sea water in HDPE Nalgene bottles are collected. One is used to measure ammonia in the Nanomolar nutrient analyzer, and the other is analyzed in the autoanalyzer (AA3). This sea water was then analysed as soon as possible because of the short life time of some nutrients (especially ammonia).
The difficulties of these measurements are that nutrients have to be analyzed as soon as possible after the sampling as they are time sensitive (especially ammonia with a short half-life). This can be very challenging especially when the rosettes are deployed frequently.
- The dissolved oxygen
On the Fridge cruise Azyyati Binti Abdul Aziz, PhD student from Universiti Malaysia Terengganu, received a NF-POGO visiting fellowship to measure dissolved oxygen. Dissolved oxygen chemistry measurements are very important in most oceanographic cruises. Indeed, to correct performance of the oxygen sensors, they need to be calibrated frequently (correction of the drift, temperature and pressure influences). To do so, samples have to be taken at the very beginning of the niskin sampling and in glass bottles without any bubbles because they are sensitive to atmospheric oxygen contamination. The sampling methodology used by Azyyati followed the GO-SHIP protocol (Langdon, 2010). The dissolved oxygen is immediately fixed with reagent (Manganous Chloride MnCl2) and Alkaline Iodide. Then oxygen bottles are shaken thoroughly and stored in a dark container until the precipitate had settled. Samples have to be analysed by titration with a standardized sodium thiosulfate (Na2S2O3) solution within 24 hours of sample collection. This method is based on the modified Winkler method (Carpenter, 1965) by using a PC controlled potentiometric titration system (Metrohm Titrando 888). Prior to the analysis, reagent blank (to measure the oxygen in the reagents) and standardisation (using certified OSIL iodate standard) are run. This process is repeated until reproducible results are achieved. These reagent blank and standardisation data are used for the final calculation of the dissolved oxygen concentration. Then, prior to analyses, sulphuric acid is added to each sample in order to dissolve the precipitate and release the iodate ions. Immediately samples are titrated with sodium thiosulfate. Calculation of dissolved oxygen concentration followed the HOT protocol and Grasshoff (1983). All the data is forwarded to the physical team to calibrate the oxygen sensor on the CTD. The obtained oxygen sensor data will be used for physical oceanographers to characterize water masses, for chemical oceanographers to study the production and destruction of organic matter and for biological oceanographers to determine rates of photosynthesis and respiration. (C. Langdon, 2010).
- The chlorophyll analysis
Valerie Le Guennec, PhD student from University of Liverpool and National Oceanography Center in Liverpool is in charge of chlorophyll analysis on this cruise. To do so, she collects samples from both rosettes in dark bottles. Then samples are GF-F fitered on 0,7μm. The filtered samples are put in vials with added acetone to extract chlorophyll-a. The vials are wrapped in aluminium foil and stored in the fridge for 24 hours. After this time, acetone that has been in contact with the filter is analysed with a fluorometer Turner Design. The Rfu (Raw fluorescence unit) value obtained from the machine is then used to calculate the concentration of chlorophyll-a. Before each measurement, the stability and the background of the machine is checked with the measurement of a standard and an acetone blank. All the data is forwarded to the physical team to calibrate the chlorophyll sensor on the CTD.
Ammonium Analysis: search for the holy grail? from Malcolm Woodward. Carpenter, J.H. (1965). The Chesapeake Bay Institute Technique for the Winkler oxygen method. Linnol. Oceanogr., 10, 141-143. Grasshoff, K. Ehrhardt, M, and K. Kremling (1983). Methods of Seawater Analysis. Grasshoff, Ehrhardt and kremling, eds. Verlag Chemie GmbH. 419 pp. HOT protocol: http://hahana.soest.hawaii.edu/hot/protocols/chap5.html. Langdon.C. (2010). Determination of Dissolved Oxygen in Seawater by Winkler titration using the Amperometric Tcehnique. The GO-SHIP Repeat Hydrography Manual: A Collection of Expert Report and Guidelines, IOCCP Report No. 14, ICPO Publication Series No. 134, Version 1.