Data on the large scale distributions of phytoplankton at the level of species or functional groups are still hard to obtain. Single stations with time series records are not necessarily representative of some broader geographic region, and research cruises can give spatially extensive information, but only cover a short time scale. Two widely applied methods do provide wide coverage in time and space, satellite remote sensing of ocean colour and the Continuous Plankton Recorder (CPR), bur both have significant limitations for ecological studies.
1.5.1 Satellite remote sensing of ocean colour
Satellite instruments measure ocean colour by detecting the reflected radiance as seen through the atmosphere in number of wavebands, which correspond to high, medium and low absorption by phytoplankton pigments. The launch in 1978 of the first ocean colour sensor, the Coastal Zone Color Scanner (CZCS), was an experimental NASA mission that ended in 1986. After a gap of several years, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) was launched in August 1997. Compared to the CZCS, SeaWiFS was a much improved instrument in terms of a higher signal to noise ratio, on board calibration capabilities, and the detection of dissolved organic material which
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Moderate Resolution Imaging Spectroradiometer (MODIS) was launched in December 1999. MODIS represents an increase in capability compared to SeaWiFS with more wavebands and a higher signal to noise ratio for observing both ocean colour and (in the near infra-red) sea surface temperature. Examples of outputs from the CZCS, SeaWiFs and MODIS are given in Figure 1.4.
Figure 1.4: (A) The global distribution of chlorophyll averaged between 1978 and 1986 from CZCS data. (B) Global oceanic photoautotroph abundance, from September 1997 to August 2000, provided by the SeaWiFS Project, NASA/Goddard Space Flight Centre and ORBIMAGE. (C) A MODIS image for 12 August 2001, showing global chlorophyll distribution for a single day (http://www.pml.ac.uk).
Many studies have now been made of regional-scale distributions of chlorophyll using SeaWiFs and MODIS with respect to both seasonal and inter-annual variability
A
C B
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initiate phytoplankton blooms (e.g. Henson et al., 2006). However, they provide no information about the vertical distribution or composition of phytoplankton populations in terms of species or functional groups, although attempts to resolve the latter are now just starting (Raitsos et al., 2008).
1.5.2 Continuous Plankton Recorder
The Continuous Plankton Recorder (CPR) survey is the largest multi-decadal plankton monitoring programme in the world (Richardson et al., 2006). CPRs are towed by merchant ships and weather ships and have been sampling the North Sea and North Atlantic since the 1930s (Batten et al., 2003). This has enabled the study of large scale spatial and temporal variability in the abundance and species composition of phyto- and zooplankton populations (Batten et al., 2003; Richardson et al., 2006). The survey has an archive store in Plymouth, UK, of formalin-preserved samples dating continuously back to the 1960s (and for some areas the 1950s). Analysis is based on direct
identification and counts of organisms caught on the moving silk of the CPR, which has a mesh size of about 200 µm. In addition, an assessment is made of total phytoplankton in terms of the greenness of the silk attributable to photosynthetic pigments, which is expressed as the Phytoplankton Colour Index (PCI).
In response to recent changes in climate (global warming), large scale changes in the distributions of plankton populations in the North Atlantic have been detected by the CPR survey (Beaugrand et al., 2002; Richardson and Schoeman, 2004). In
particular, it is evident that there has been a shift by about 10o of latitude (~1000 km) of copepod communities since the 1980’s with significant bottom-up effects on the
distributions of carnivorous zooplankton. Corresponding changes in, and relationships to, phytoplankton populations are less easy to define, in part because uncertainty in the significance of smaller phytoplankton cells in determining PCI. In spite of these
uncertainties, seasonal changes in phytoplankton abundance and in diatoms (spring) and dinoflagellates (summer) have been defined by CPR data (Leterme et al., 2006;
McQuatters-Gollop et al., 2007). A general trend of increasing PCI for the NE Atlantic and North Sea has been described (Leterme et al., 2005) which can be attributed in part to increasing abundance and/or proportion (compared to diatoms) of dinoflagellates.
However, sub-areas of the NE Atlantic and North Sea appear to behave in different
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ways reflecting the complexity of the relationship between environmental variables and phytoplankton abundance.
1.5.3 Other time series
Several marine laboratories established in the late 19th and early 20th centuries started monitoring programmes in nearby coastal waters. For example, the UK Marine Biological Association (MBA) started collecting hydrographic and plankton data at station E1, about 30 km south of Plymouth in 1902, and interpretation of the
observations has made important contributions to our understanding the dynamics and variability of plankton populations (Southward et al., 2005).
Recognition of the importance of global change in influencing planktonic
ecosystems has led to a re-vitalisation of monitoring programmes. Thus, over the last 15 years, detailed observations on the plankton have been made at weekly intervals at station L4 about 15 km south of Plymouth (Rodriguez et al., 2000; http://www.pml.ac.uk).
Variables measured include vertical profiles of temperature, salinity and chlorophyll fluorescence, total and size-fractionated chlorophyll, and phytoplankton and
zooplankton species abundance. Despite the uncertainty about the degree to which a single station is representative of a wider area, it has been possible to relate variability in phytoplankton populations at L4 to the North Atlantic Oscillation (NAO) (Irigoien et al., 2000).