FRIESE FRONT 1999 - Summary
At the transition on the Dutch Continental Shelf between the shallow (25 m) Texel and Vlieland Grounds and the deeper (50 m) Oyster Ground, the maximum tidal current velocity drops below a critical value, resulting in deposition of silt and detritus on the bottom. Between the 30 and 40 m contour lines the sediment has a high mud and organic carbon content, and contains an abundant fauna: the Frisian Front. During 1975-1990, there was a typical zonation of benthic species across the slope and benthic biomass at the Front was considerably higher than in adjacent areas, with half of a total of 25-40 g AFDW.m-2 accounted for by Amphiura. Mineralization of the organic matter in the rich bottom zone also seemed to lead to the occurrence of phytoplankton blooms in the overlying water. Sediment-water fluxes of ammonium and nitrate at the Front during summer were about three times higher than in the adjacent sand bottoms.
Recent observations have unsettled the supposed constancy of the Frisian Front. From 1992 to 1996 the benthic biomass at and near the Front has drastically decreased, the abundance of Amphiura even by more than a magnitude. A first survey by dredge (Triple-D) in 1998 revealed that the faunal zonation is still clearly present, at least for large-sized animals that are underrepresented in boxcore samples. The Frisian Front is still the area with highest silt and chlorophyll content of the sediment but the variation in food availability is unknown since no long term record exist of supply and quality of suspended matter. The decline in density of many species could also be due to senescence of the populations of adults in combination with recruitment failure. Annual surveys of the larval settlement and growth rate of the adults started in 1997 in an attempt to study the complex of biotic factors involved. In this framework, RIKZ and NIOZ planned a survey by Triple-D and boxcorer at the beginning of September 1999, to map the present distribution of all species and size-classes of the benthos over a large area.
In addition, a hydrographic survey was planned along a southwest-northeast axis, in the direction of the residual current from East Anglia towards the Frisian Front, to map the distribution of turbidity and nutrients. Satellite images show a marked plume of suspended matter towards the Frisian Front in the winter, when there is strong erosion of the Norfolk coast. The high turbidity, due to the strong tidal currents on the British side, prevents optimal development of phytoplankton, and nutrients in the English Coastal Water become not depleted during spring. The advection of these waters towards the Frisian Front, where the silt sinks out, then results in a vigorous diatom bloom (with primary production up to 6-8 g C .m-2.d-1) during late spring. Surprisingly, a similar phenomenon was also recorded during September 1997, when a ‘green curtain’ of diatoms was found in water with lower salinity stretched along the 30 m contour line well south of the Front. This suggested that besides the rich bottom zone the English Coastal Water could be a nutrient source also during this time of the year.
During the cruise by the Rijkswaterstaat RV Mitra a total of 67 stations was done, spread over 8 sections from 2o10’ E to 6o E. At all stations a CTD rosette sampler cast was made, and at 60 stations a vertical net haul was done in addition. The Triple-D was employed at 53 stations, and boxcorer samples were taken at 42 stations. With the ‘Meetvis’ (towed CTD) 30 sections were sailed, covering circa 2000 km in total.
An important discovery was made during the Triple-D tows at the English sections at 2o10’ and 3o E. Here, Brown Bank Clay lies at the very surface of the bottom at several locations. This tough ‘fossil’ clay is eroded during the tides and leads to a plume of fine silt in the direction of the Frisian Front. The ‘Meetvis’ transmissometer data indeed showed a clear turbidity plume. This observation modifies the present idea that the English silt plume is predominantly a phenomenon of the winter half year. If the bottom clay makes the English Coastal Water sufficiently turbid during summer, nutrients from the UK river input to the southern North Sea become possibly not depleted, and could result in offshore blooms at the Frisian Front. No ‘green curtain’ was observed near the Front during the Mitra cruise, however. The new impressions await further analysis of water samples on nutrients, suspended particulate matter, chlorophyll and clay composition, to study the distributions of related variables in more detail.
The ‘Friese Front 1999’ project will be succeeded by a series of new cruises. In January 2000, CEFAS (Lowestoft) will deploy an array of ‘smart’ moorings from the Thames plume towards the Frisian Front, in the framework of their programme ‘Transport and fate of UK nutrient input to the southern North Sea’. And summer 2000 a new series of both benthic and pelagic NIOZ-cruises will start. The pelagic programme ‘Plume & Bloom’, funded by the Foundation for Earth and Life Sciences (NWO, The Hague) will try to map the seasonal dynamics of the plankton communities in the English Coastal Water for both the turbid side near Norfolk and for the more clearer area of the Frisian Front.
