Coupled “Ferrybox” Ship of Opportunity and Satellite Data Observations of Plankton Succession Across the European Shelf Sea and Atlantic Ocean
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CM 2004/P:28 Not to be cited without prior reference to the author Coupled “FerryBox” Ship of Opportunity and satellite data observations of plankton succession across the European Shelf Sea and Atlantic Ocean From April 2002, a robust minimum maintenance system (FerryBox) to measure temperature, conductivity, and chlorophyll fluorescence has run continuously (except in January) between Portsmouth, UK and Bilbao, Spain. It provides an un-aliased view of all changes in the phytoplankton community driven by physical process and changes in nutrient supply. The ferry-box samples eutrophic harbours, the open-shelf (seasonally stratified and un-stratified), an up-welling shelf break and seasonally oligotrophic ocean water. Monthly calibrations have collected samples for chlorophyll-a, HPLC pigments, phytoplankton samples and nutrients (nitrate, phosphate and silicate). The 2 dimensional scale is provided by integration with satellite images. Additional quasi-synoptic data from a separate research cruise was collected during the growth of an intense bloom (>50 mg/m3 chlorophyll-a) of Karenia mikimotoi in the western English Channel in July 2003. The data from these different sources has been combined to provide insight into the complex development of this bloom. B. A. Kelly-Gerreyn1*, M. A. Qurban1, D. J. Hydes1, P. Miller2, L. Fernand3 1Southampton Oceanography Centre, Waterfront Campus, Empress Dock, Southampton, SO14 3ZH, UK; 2Remote Sensing Data Analysis Service, Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK.; 3Centre for Environment, Fisheries and Aquaculture Sciences, Pakefield Road, Lowestoft, NR33 0HT, UK Keywords : low salinity intrusion, Karenia mikimotoi, ship of opportunity, FerryBox, wind, river outflow, Loire, Gironde, western English Channel Introduction Hydrographic data have been collected in the English Channel since the beginning of the 20th century (Matthews, 1907 quoted in Pingree, 1980). Much of this data has come from dedicated research cruises operating over limited time periods or from fixed time series stations such as E1 off the coast of Plymouth (Southward et al, 1975). Given this long oceanographic history, a coherent understanding of commonly observed features in the English Channel and its western approaches has yet to be fully established. Such features include the regular appearance of large summer blooms (>100 µg Chl l-1) of the red-tide forming dinoflagellate Karenia mikimotoi (Pingree et al, 1977, Holligan et al, 1984; note that K. mikimotoi was previously and mistakenly identified as Gyrodinium aureolum). These high biomass concentrations are reached in some summers despite apparently insufficient fuel to do so: nutrient - concentrations at the end of winter are approximately 8µM NO3 in the western English Channel. The required nutrient source is likely to be the nutrient rich waters below the pycnocline (Holligan et al, 1984) which K. mikimotoi may access on account of both its ability to vertically migrate (Koizumi et al 1996) - although this ability is contentious (Dahl and Brockmann, 1989, Kristiansen et al, 1995) - and its association with frontal areas (Holligan, 1979, Le Corre and L' Helguen, 1993) - K. mikimotoi is often observed as a subsurface chlorophyll maximum associated with the 1 CM 2004/P:28 pycnocline. Environmental concern over this species has been raised over reports associating it with fish kills and water column anoxia (Tangen, 1977, Blasco et al, 1996). Ecological significance can be estimated in terms of the contribution of K. mikimotoi to total annual primary production in the western English Channel. Holligan et al (1983) provide data, which suggest that this contribution may be as high as between 28 and 47%. The many physiological and environmental characteristics of this organism can be found in a number of reviews (Dahl and Tangen, 1993, Gentien, 1998). Lacking from previous studies is an understanding of why interannual differences exist in the intensity of the blooms of K. mikimotoi. We present data (temperature, salinity, chlorophyll fluorescence) collected at high frequency (1Hz) between April 2002 and July 2003 from a ship of opportunity (PandO Ferry, The Pride of Bilbao) operating year-round between Portsmouth (UK) and Bilbao (Spain). The unprecedented detail in the seasonal dynamics of important oceanographic parameters along a track which crosses many water types - eutrophic harbours, coastal well mixed and stratified waters, a shelf break and deep (4000m) oligotrophic waters - is in stark contrast to the limited, albeit important, data sets gathered in the past (e.g. Holligan et al, 1984). These new data are combined with two additional and concurrent sources (remotely sensed satellite data (Seawifs and AVHRR) and data from a dedicated research cruise in July 2003) together with recently published data to examine possible causes for the differences in bloom intensity of K. mikimotoi between 2002 and 2003 in the western English Channel. This paper is closely aligned to another presentation in this conference (Qurban et al, CM 2004/N:09). Study area and data sources (FerryBox and satellite) The main details of the study area and the sources of data can be found in the associated paper by Qurban et al (2004 CM 2004/N:09). Briefly, the ferry route (Figure 1) is that of the P & O European Ferries Ltd ship “Pride of Bilbao” between Portsmouth (UK, 50.8°N, -1.1°E) and Bilbao (Spain, 43.4°N, -3.0°E). Operations began in April 2002 and remain ongoing. This route has been incorporated into the EU framework V “FerryBox” project (http://www.Ferrybox.org/). The distance is approximately 1000 km and the journey time is about 35 hours. Repeat rates are between 4 hours and 4 days depending on location. Measurements of temperature, conductivity and chlorophyll fluorescence (Chelsea Technologies Group MiniPack) are made at 1Hz from samples taken from the ship’s cooling water supply at 5 metres depth. These data are downloaded weekly when the ship is in Port (Portsmouth). In 2003 (and 2004), manned monthly calibration crossings have collected samples for salinity, nutrients (nitrate, phosphate, silicate) and extracted chlorophyll. Satellite data (Seawifs and AVHRR weekly composites) are provided by the Remote Sensing Data Analysis Service (RSDAS) at the Plymouth Marine Laboratory. These data come from pixels along the route of the Ferry. The percentage of the weekly composite satellite data that is usable is high (close to 100%, Qurban et al, 2004). Cruise data A multidisciplinary research cruise on board the RV Corystes (Cruise 8, Principal Investigator: L. Fernand) was undertaken in the western English Channel during 26th – 9th July (Cor 8/03) and 14th – 27th August 2003 (Cor 11/03). Here , we concentrate on data from the first cruise ( Cor 8/03). Towed undulating CTD sections were 2 CM 2004/P:28 performed using Scanfish, a computer-controlled vehicle towed at 3.0 - 4.5 m s-1. A comprehensive description of its use and the subsequent data analysis can be found in Brown et al (1996) and Fernand (1999). Nominally, the instrument was set to profile between 4 m below the surface, as determined by the pressure sensor of the CTD, and to within 5 m of the sea bed, a height controlled by an onboard altimeter. Separation of profiles was equivalent to conventional CTD profiles at approximately 150 - 500 metres horizontal separation, the latter largely dependent on bottom depth. The Scanfish vehicle was fitted with a Falmouth Scientific Inc. Integrated (FSI) CTD. Calibration of the temperature sensor was performed pre- and post cruise in the laboratory, and values were found to be accurate to ±0.001°C. Salinity calibration of the Scanfish CTD was undertaken at sea. Water samples were drawn hourly from a pumped ship board system, with the inlet at 4 m, and salinities determined using a Guildline Portasal. Comparisons were then made with values from the FSI CTD as it reached the top of its profile at approximately 4 m depth. By allowing for the distance of the instrument behind the ship and the residence time of the ship board sample in the ship's pipe work, derived salinities were determined with a standard deviation of ±0.007. Salinity and density were calculated according to international standard procedures for seawater (UNESCO, 1981). All salinities quoted hereafter were determined using the practical salinity scale (UNESCO, 1978). For chlorophyll a, known volumes were filtered through Whatman glass fibre (GF/F) filters in triplicate. Pigments were extracted in acetone and analysis was carried out on board ship. A Turner Designs Model 10 filter fluorometer was used to measure extracted pigment fluorescence, and phaeopigments after acidification with HCl, following the method described by (Tett, 1987). The fluorometer was calibrated using a solution of pure chlorophyll a (Sigma Chemical Co.) with concentration being determined spectrophotometrically. The % error of chlorophyll a analyses was < 2% relative to Turner certified reference material. Samples were also taken in triplicate and analysed on board for nitrate and nitrite (hereafter nitrate), ammonium, phosphate and silicate using a Skalar Auto analyser according to (Kirkwood, 1996). The detection limit for nitrate, ammonium, phosphate and silicate analyses were 0.1 mmol m-3, 0.2 mmol m-3, 0.02 mmol m-3 and 0.1 mmol m-3 respectively. The % error for all nutrient analyses was < 5% relative to Ocean Scientific International standards. Almost 300 stations were sampled in each cruise (Figure 2) Results 1D horizontal structure: FerryBox data Sea surface salinity, temperature and chlorophyll-fluorescence (5 minute averages) between Portsmouth and Bilbao are shown for 2002 and 2003 in Figure 3a-f. Note that gaps in Figure 3 are periods when the MiniPack sensors were not logging because of technical failures. Salinities varied between 34.2 (near the coast) and 35.6 (in the Bay of Biscay) in both years (Figure 3a,b). Salinities are similar below 48°N in 2002 and 2003. Off the coast of Brest (~48°N), there is a freshwater signal in March 2003 (34.2) and in April 2002 (34.3).