|
western English Channel Associated Investigators: Related Web Sites: Plymouth Station L4 (4 13 W 50 15 N) is located about 16 km southwest of Plymouth in the western English Channel. The station is about 50 m deep and is influenced by seasonally stratified and transitional mixed-stratified waters (Pingree and Griffiths, 1978). Since 1988, zooplankton have been collected on a weekly basis using a WP-2 net (200 µm mesh) towed vertically from the seabed (~54 m depth) to the surface. Samples are split, and organisms are counted and identified to major taxonomic groups and families. For some groups, particularly copepods, organisms are identified at species level; sex and life stages are determined for some targeted species (e.g. Calanus helgolandicus). The SST has been measured since the beginning of the time-series, using a mercury-in-glass thermometer immersed in an aluminium bucket of water collected at the surface. Since 1992, water samples collected from a depth of 10 m with a Niskin bottle have been analysed to determine abundance and estimate carbon biomass of phytoplankton and microzooplankton. Organisms are counted and identified at genus or species level using inverted microscopy. At the same time, Chlorophyll a triplicate measurements are made using a Turner fluorometer after filtering of sea surface water samples and extraction. Additionally, since 2002, water column profiles of CTDf measurements have also been available for temperature, salinity, and fluorescence. The L4 data are maintained at the Plymouth Marine Laboratory and are available online at the Western Channel Observatory website. The seasonal cycle of SST is characterized by strong seasonality, with a winter minimum of 9+/-1 C in March and a summer maximum of 17+/-1.5 C in August. The water column is subject to a weak seasonal stratification, with a thermocline temperature difference of 2-3 C in summer. The seasonal cycle of Chlorophyll a and phytoplankton are both characterized by two peaks. The first occurs during late April/early May, with an intense, short period corresponding to the spring Diatom bloom, followed by a second peak in late summer/early autumn, with a lower magnitude, but a longer period (~2 months), corresponding to the late summer Dinoflagellate bloom. The seasonal cycle of zooplankton is characterized by a maximum peak in abundance in April, followed by a slight decrease until August, when the summer phytoplankton bloom leads to a second increase in zooplankton abundance. Zooplankton abundance remains variable until October, followed by a decrease in November/December, with the lowest abundance in January/February, which also corresponds to the lowest values in Chlorophyll a and phytoplankton abundance. The mesozooplankton community at L4 is dominated by copepods, which represent 69-74% of the total zooplankton abundance and are the most abundant species in the top ten species ranking. Each year, the ten species listed in this table (out of more than 70 different species and groups identified at the site) have represented more than 80% of the total zooplankton population, and their relative composition has remained fairly constant over the 20 years of the time-series. In contrast, the major phytoplankton groups show clearly visible trends in their composition, with Diatom abundance decreasing and coccolithophores abundance increasing. It is possible that copepod abundance may be following (with a 1-2 year lag) phytoplankton abundance and this should be investigated further. Although the community composition seems to be stable, the interannual variation in zooplankton abundance is important, but does not demonstrate any long-term trend. Nevertheless, periods with high abundance are observable (e.g. 2000/2004), but do not seem to be related to temperature or phytoplankton variations. Furthermore, zooplankton abundance observed at L4 is not synchronized with the CPR abundances observed for the corresponding area, suggesting that station L4 may be influenced by the nearshore and currents. The values for average SST in the Plymouth L4 area over the last 100 years show that water temperatures have been rising steadily for the last 100 years. The last three years (2005-2007) have been especially warm, with temperatures higher than any recorded in the last 100 years. These high-temperature years correspond to below-average copepod abundance since 2005, but the trend is too short to establish any clear relationship. Local CPR copepod abundance has also been decreasing with increasing water temperatures over the last 50 years, so this may be a trend to watch for as sampling continues. REFERENCES: Pingree, R. D., and Griffiths, D. K. 1978. Tidal fronts on the shelf seas around the British Isles. Journal of Geophysical Research, 83: 4615-4622. |