Temperature, Salinity, and Fluorescence in Southern North Sea: High-Resolution Data Sampled from a Ferry

/ In 1991 and 1992, temperature, salinity, and fluorescence were measured by automatic continuous registration using instrumentation on a ferry crossing the southern North Sea daily along a transect between Zeebrugge, Belgium, and Hull, United Kingdom. The temperature ranged between 4 and 21 degrees C off the Belgian coast and between 6 and 17 degrees C in the middle of the transect. Salinity varied between 34 and 35.5 PSU in the offshore part of the transect, but showed much larger variation along the Belgian and UK estuarine coasts. Fluorescence, which was used as a measure of phytoplankton biomass, was highest at the continental coast and lowest near the English coast. Spring blooms of phytoplankton were found along the continental coast and in the channel-influenced water; in 1991 the blooms were denser than in 1992. Some summer blooms were also recorded. Water masses could be distinguished on the basis of salinity and fluorescence patterns. The general patterns in the data are compared with the literature and discussed in relation to river discharge, light penetration, and wind speed and direction. Finally, the value of automatic and frequent measurements of fluorescence for monitoring phytoplankton is compared with less frequent observations at selected stations. It is concluded that accurate information about phytoplankton biomass can only be established from the high-frequency data. KEY WORDS: Monitoring; Sampling; North Sea; Ferry; Fluorescence; Salinity; Temperature