Recent patterns in potential phytoplankton limitation along the Northwest European continental coast

The European coastal zone is characterized by high anthropogenic riverine nutrient loads, and turbid waters along the continental North Sea coastal stretch. Riverine nutrient inputs (N and P) have increased since the 1950ʹs and have decreased again since the mid 1980ʹs. Using the approach of Cloern (Cloern, J.E., 1999. Aquatic Ecology 33, 3–16), we analyzed potential limitation patterns of light and nutrients (DIN, PO4, Si) for seven long-term monitoring stations along the North Sea coast. As available, data from 1990 until 2005 were analyzed. Underwater irradiance was the dominant factor potentially limiting phytoplankton growth. Seasonal limitation patterns strongly differed between the investigated sites. Si limitation was the most common among the stations followed by PO4 limitation. Only one station with optimum light conditions (northern Wadden Sea) resulted a potential DIN limitation. An increase in nutrient limitation, or co-limitation of light and nutrients, was observed off the Dutch coast (DIN and PO4), and in the German Bight near Helgoland (DIN, PO4, Si) within the period from 1990 until 2005. However, we observed no clear effects of nutrient reduction measures in shallow areas of the Wadden Sea during the period studied. We conclude that long-term nutrient limitation patterns are consistent and site dependent, and strongly linked to the local hydrography and light conditions of the water column.