North Sea phytoplankton pigments 2010 to 2011

quarterly

10L water samples were taken from niskin bottles on a rosette fired at various depths, or from the continuous flow ferrybox water supply (approx. 4m water depth) in adverse weather conditions. Water samples (typically 1000ml) were prefiltered with 200 micron gauze to remove larger zooplankton and debris, then filtered through a Whatman GFF filter, and immediately frozen in a -80 degrees C freezer on board. On return to shore, samples were transferred to a -80 degrees C freezer for a storage period of 1-2 months before shipping of samples on dry ice to an accredited HPLC laboratory (DHIWater Quality Institute; Horsholm,Denmark) for chlorophyll a (Chl a) quantification and full accessory pigment analysis. Pigments were extracted adding 6 ml of 95 percent acetone (and an internal standard, vitamin E) to each filter, at 4 degrees C for 20 hours. Samples were then filtered through 0.2 micron Teflon syringe filter into HPLC vials, together with DHI mixed pigments. Buffer and samples were injected on HPLC (Shimadzu LC-10A HPLC system with LC Solution software) in the ratio 5:2 using a pre-treatment program and mixing in the loop before injection. The HPLC method used was the HPL method by Van Heukelem and Thomas (2005). Pigment data from the surface stations were quality data controlled in several steps: first, with an initial comparison of HPLC Chl a against independent measures of chlorophyll fluorescence from the fluorometers on the ship’s FerryBox and CTD system. This step corrected a small number of mislabelled samples. In a second step, anomalies within a sample were detected using methods described by Aiken et al. (2009), e.g. regression of total accessory pigments against Chl a concentration and search for outliers. Diagnostic pigment analysis was then used on the quality controlled data set to relate the composition of specific accessory pigments to the relative contribution of different size classes to the total phytoplankton biomass. The designation of specific accessory pigments to algal taxonomic groups of different size, e.g. fucoxanthin and peridinin for large-cell diatoms and dinoflagellates, has been widely established in the biological oceanographic literature (Uitz et al., 2006, 2008). The equations used to estimate the contribution of pico-phytoplankton (0–2 microns), nano-phytoplankton (2–20 microns) and micro- or net phytoplankton (greater than 20 microns) were later modified by Hirata et al. (2008, 2011) and Brewin et al. (2010). The various methods differ in the degree to which the marker pigments chlorophyll b (Chl b) and 19-hexfucoxanthin (19-hex) are attributed to the three size classes. Here, Chl b and 19-hex were assigned equally to the picophytoplankton and nano-phytoplankton size classes. Pico-phytoplankton are therefore represented by zeaxanthin, Chl b, and 19-hex; nano-phytoplankton are represented by 19-hex, 19-but, alloxanthin, and Chl b; and micro-phytoplankton are represented by fucoxanthin and peridinin. Results are expressed as a proportion of the total Chl a concentration for each station.