Unique pelagic time series at the WCO
The WCO contains some of the longest marine time series in existence, with temperature profile measurements at the E1 site dating from 1903. The main sites, entitled L4, L5 and E1, have been sampled for plankton, fish and associated environmental variables for over a century and an excellent description of the sampling history is provided by Southward et al. 2005 . In 1988 intensive (weekly resolution) pelagic sampling resumed at the most inshore L4 station, and this intensive weekly coverage has continued, weather permitting, ever since. Fig. 1 below shows just some of the key pelagic measurements catalogued in full here.
Initial net sampling of zooplankton was soon supplemented with phytoplankton measurements, then came full profiling and a suite of biogeochemical parameters. Shipboard measurements were supplemented at higher resolution in 2008 by the L4 buoy. These various measurements are described in the data pages, but in combination they provide both a long time series and excellent high-resolution insights into the multiple factors driving shelf sea seasonality. There have been many shorter-term intensive process studies made at the WCO sites that benefit from the longer-term context provided by the time series.
The L4 site is 13 km SSW of Plymouth in water ~54 m deep and, while transitionally stratified, it receives periodic coastal influence, for example from flood water discharge from the rivers Tamar and Plym emanating from Plymouth. The furthest offshore site is E1, 40 km SSW of Plymouth, and this provides a better representation of open shelf, seasonally stratified water. This site, in over 70 m of water, is more weather-dependent to sample and has lower sampling resolution. The intermediate site, L5 near the Eddystone reef, is sampled by the MBA.