FRIESE FRONT 1999 - Introduction
The discovery of the Frisian Front
In the late seventies, Creutzberg and co-workers of the Netherlands Institute for Sea Research (NIOZ, Texel) found a rich bottom zone in the transition area between the shallow (25 m) Texel and Vlielands Grounds, and the deeper (50 m) Oyster Ground (Creutzberg et al. 1984). This bottom zone, called the 'Frisian Front', is located approximately between 53o30'N, 4oE and 54oN, 5oE. Experimental work on sedimentation and resuspension by Creutzberg & Postma (1979) indicates that silt and detritus, kept in suspension by the large turbulence in the sandy Southern Bight, is transported northeastwards with the residual currents and ultimately deposited at this Frisian Front, the zone with a depth between 30 and 40 m. Here, the maximum tidal current velocity drops below a critical value, resulting in sedimentation of the suspended particulate matter, inorganic and organic.
The rich benthic fauna of the Frisian Front is explained by this import of organic material from adjacent, more turbulent areas (Creutzberg et al., 1984). The bottom front is very muddy, with high concentrations of organic carbon, and contains a markedly rich benthic fauna arranged in a typical zonation of species. South of the Frisian Front a community with the bivalve Tellina fabula occurs in the fine-sand area till a depth of 30 m (Creutzberg 1986). Nearer to the bottom front, just below the 30 m contour line, a muddy, very-fine sand zone is characterized by high densities of the bivalve Nucula nitidosa and the sea-urchin Echinocardium cordatum. The fauna assemblage at the front, at ca. 35 m depth, is characterized by high densities of the bivalve Abra alba, and particularly high densities of the brittle star Amphiura filiformis. North of the Frisian Front, at depth > 40 m, the fauna composition changes more gradually. Amphiura filiformis remains abundant but the characteristic components in this deeper area are the gastropod Turritella communis and the polychaete Chaetopterus spec. (Creutzberg et al. 1984). Macrofauna surveys by Van Veen grab and boxcorer in the period 1975-1987 showed that macrofauna biomass peaks in the bottom front, with total ashfree dryweight ranging 25-40 g AFDW.m-2. Nearly half of the biomass is by Amphiura filiformis. In southern areas, and in the Tellina zone, biomass is much lower, ranging 4-7 g AFDW.m-2. To the north of the Front, and in the Oyster Ground, biomass ranges 11-16 g AFDW.m-2, with predominance by Chaetopterus spec. (Baars et al. 1991, De Gee et al. 1991).
The decrease of the macrofauna abundance during 1992-1997
The spatial pattern of the benthic fauna of the Frisian Front was supposed to be very robust, though it was clear that patchiness and interannual variation were not negligible. Data from the period 1986-1991 again show the typical zonation of species, with a clear peak in biomass in the transition zone (Lavaleye 2000). However, in the middle of the nineties it became obvious that many of the macrofauna species of the Frisian Front were steadily decreasing in density. The numbers of Amphiura filiformis at a central station of the Frisian Front, where previously a high density (> 1000 .m-2) was observed, decreased drastically from 1992 to 1995 (Baars et al. 2000). The numbers of Amphiura at this station are about one order of magnitude lower in the years 1995-1996 than in the past. A similarly large decrease in abundance was noticed for the worm Chaetopterus spec. at a more northern station in the Oyster Ground. These marked changes renewed the interest in the Frisian Front. A series of cruises was done by workers of the NIOZ department Marine Ecology during 1997 and 1998. The preliminary results from boxcorer samples show that changes in abundance have occurred not only at the transition zone but over a large area. For example, the dense populations of Tellina and Nucula at the upper part of the slope, as described by Creutzberg (1986), have virtually disappeared in 1997-1998 (Lavaleye 2000).
In a preliminary analysis of the present situation, Lavaleye (2000) concludes that the sediment characteristics of the area seem not to have changed, the Frisian Front is still the area with highest silt content. Also, it is not likely that the decline in density of many species is caused by mortality due to severe winters or to the effect of beam trawl fisheries. Lavaleye (2000) presumes that a complex of biotic factors is involved, including variation in food availability and in predation pressure. Moreover, the density shift in the Amphiura population resembles long term changes observed in other areas in the past, and could be due to senescence of the old Amphiura population and failing recruitment.
In December 1998, also a first survey was done with the Deep Digging Dredge (Triple-D), a bottom dredge developed at NIOZ (Bergman & Van Santbrink, 1994). Compared to the limited sample size by boxcorer (0.07 m2), the Triple-D samples a much larger surface of the seabed: during a tow of 125 m the upper sediment layer from a total surface of 25 m2 is washed into a net with a mesh size of 0.7 cm. The 1998 Triple-D survey showed that the Frisian Front pattern still exists (Bergman 2000). Typical species as Tellina fabula, Nucula nitidosa, Abra alba and Turritella communis were found in the zonation from shallow to deep as described previously (Creutzberg et al. 1984). Maximum densities for these species ranged from a few to some tens per m2, which implies that the zonation of these larger-sized specimens (as caught by the Triple-D) is hard to detect any more by boxcorer.
Phenomena in the waters over the Frisian Front
During cruises along a central south-north section over the Frisian Front during summer and autumn 1982, a conspicuous phenomenon was found in the water. There was a persistent chlorophyll a maximum near the enriched bottom zone, irrespective of the presence or absence of the tidally-induced front between mixed and stratified waters in summer (Creutzberg 1985). This chlorophyll maximum is attributed to the mineralization of accumulated organic matter in the enriched zone and the subsequent mixing of 'new' nutrients from the bottom into the overlying water column. In Creutzberg's group, the working hypothesis evolved that in spring the pelagic system over a large area in the southern North Sea would feed the Frisian Front, and that in summer the Frisian Front would feed nutrients to the local pelagic system (cf. Baars et al. 1991, De Gee et al. 1991).
During spring 1986 a series of pelagic measurements revealed that the Frisian Front area is also characterized by a vigorous, local diatom bloom during late spring, with primary production up to 6-8 g C .m-2.d-1 (Baars et al. 1991). This would be due to the advection of nutrient-rich English Coastal Water. The East Anglian waters are characterized by strong tidal currents, and are much more turbid than those on the Dutch side of the southern North Sea. This difference is most pronounced in the winter period, when a marked silt plume stretches north-eastwards to the Frisian Front (Van Raaphorst et al. 1998). The high turbidity on the British side prevents optimal development of phytoplankton during spring, and consequently, nutrients in these waters are not depleted during May/June (Brockmann et al. 1990). The advection of these nutrient-rich waters by the residual currents towards the Frisian Front, then results in a bloom once the silt has sedimented in the calmer waters of the Frisan Front. The late spring bloom of 1986 stimulated the zooplankton development, resulting in biomasses of 10-20 g C .m-2 at the beginning of summer, much larger than in surrounding waters (Baars et al. 1991).
The observation of a 'green curtain' in September 1997
Actual measurements of the sediment-water fluxes of nutrients showed that the Frisian Front is an important source for ammonium and nitrate during summer (Raaphorst et al. 1992, Lohse et al. 1995). DIN-flux observed in August 1989 and in August 1991 was 1.5-2 and 1.2 mmol.m-2.d-1 respectively. The DIN-flux in the sandy sediments in the south was only about onefifth of this amount, that in the Oyster Ground about half. In calm periods, with little wind, the residence time of the water above the Front is in the order of one month. The sediment-water flux of nutrients could then result in an increase of the chlorophyll a concentration of circa 2 µg.l-1, a.o. under the assumption that the summer phytoplankton predominantly consists of non-sedimenting species. This increase would largely explain the difference in chlorophyll concentration as observed by Creutzberg (1985) during Sept. 1982: about 4 µg.l-1 near the Front versus < 2 µg.l-1 in more southern or northern waters. Similar differences were also found during Sept. 1983, July 1984 and Aug. 1985, but chl maxima near the Front were not observed after the windy summers of 1986 and 1987 (Baars et al. 1991).
Thereafter, observations on chlorophyll during late summer cruises to the Frisian Front lack, except for a cruise in Sept. 1990 when a remarkably high chl max of > 15 µg.l-1 was found in the waters well south of the Front. In Sept. 1997, the renewed interest in the Frisian Front led to a spatially more extensive pelagic survey in addition to the benthic programme. Again, a clear chl maximum was observed and, for the first time, it was found that this had the suggested shape of a 'green curtain', extending southwest-northeast parallel to the countour lines (Baars 2000). Chl in the 'curtain' was about 8 µg.l-1 (versus ca. 2 µg.l-1 in the adjacent waters) and the phytoplankton consisted of diatoms > 20 µm. Surprisingly, the 'green curtain' lie again well south of the enriched bottom zone, above the shallowest zone (< 30 m) right before the slope. The curtain was in the southernmost part of a watermass with relatively high concentrations of nutrients and with salinity < 35 in between waters with salinity > 35. This suggested that the influx of English Coastal Water during late summer, could be an important cause for blooms near the Front, like in the spring situation.
It is noteworthy that a CZCS image of 3 September 1980 (Reid et al. 1990) shows large suspended matter loads along the British coast with a clear plume stretched towards the Frisian Front. So it could well be that also outside the winter period the plume is present occasionally and that the nutrients by the English river input are not fully depleted during summer before the English Coastal Water becomes less turbid in the area of the Frisian Front.
So the key to understanding the pelagic productivity of the Frisian Front could probably mainly lay in the advection of English Coastal Water, and a new programme should map the hydrographic conditions over a large area, along a southwest-northeast axis, in the direction of the residual currents.
The Mitra cruise and further studies planned
The decrease in the abundance of typical species at the Frisian Front evoked the interest of the North Sea Directorate (Rijkswaterstaat, Rijswijk), responsible for monitoring life in the Dutch sector of the North Sea. For the year 1999 a series of NIOZ-cruises by RV Pelagia was already planned to map the distributions of the benthic fauna, including studies on the larval occurrence and settlement. In these cruises, no deck time was available for dredging by Triple-D, or for extensive hydrographic surveys including English waters. The North Sea Directorate offered use of the RV Mitra to the National Institute for Coastal and Marine Management (RIKZ , The Hague) for a Frisian Front cruise, in cooperation with NIOZ, dedicated to these topics. In addition to dredging, also boxcoring was planned, to get a complete picture of the fauna, and also to fill the late summer gap in the Pelagia programme.
Meanwhile the hypothesis on the role of the English Coastal Water at the Frisian Front had led to the British programme 'Transport and fate of UK nutrient input to the southern North Sea' by the Centre for Enviroment, Fisheries and Aquaculture Science (CEFAS, Lowestoft). In January 2000, CEFAS will deploy an array of 'smart' moorings from the Thames plume towards the Frisian Front, in order to record continuously the hydrography. 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.
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Cruises to the Frisian Front, 1999-2001 |
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1999 |
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Pelagia |
4-11 June |
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benthic prog. |
(NIOZ-MEE) |
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Pelagia |
12-16 July |
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benthic prog. |
(NIOZ-MEE) |
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Mitra |
30 Aug-10 Sep |
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joint benthic + pelagic prog. |
(RIKZ + NIOZ) |
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Pelagia |
18-22 Oct |
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benthic prog. |
(NIOZ-MEE) |
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Pelagia |
8 - 12 Nov |
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benthic prog. |
(NIOZ-MEE) |
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2000 |
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Corystes |
19 - 26 Jan |
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Transport and fate of UK nutrient input |
(CEFAS) |
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Corystes |
25 Feb-1 Mar |
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Transport and fate of UK nutrient input |
(CEFAS) |
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Corystes |
24 - 30 Mar |
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Transport and fate of UK nutrient input |
(CEFAS) |
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Corystes |
further cruises |
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Transport and fate of UK nutrient input |
(CEFAS) |
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(to be decided) |
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Pelagia |
3-7 July |
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benthic prog. |
(NIOZ-MEE) |
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Pelagia |
10-21 July |
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Plume & Bloom 1 |
(NIOZ-BIO) |
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Pelagia |
4-8 Sept |
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benthic prog. |
(NIOZ-MEE) |
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Pelagia |
11-22 Sept |
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Plume & Bloom 2 |
(NIOZ-BIO) |
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Pelagia |
26 Oct- 4 Nov |
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Plume & Bloom 3 + benthic prog. |
(NIOZ-BIO/MEE) |
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2001 |
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??? |
Jan or Feb? |
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Plume & Bloom 4 ? |
(CEFAS-NIOZ?) |
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??? |
12 d March/April |
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Plume & Bloom 5 |
(NIOZ-BIO) |
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??? |
12 d May/June |
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Plume & Bloom 6 |
(NIOZ-BIO) |
References
Baars, M.A., G.C.A. Duineveld, F.C. van Duyl, A. de Gee, G.W. Kraay, M.F. Leopold, S.S. Oosterhuis, W. van Raaphorst and C. Westra, 1991. The ecology of the Frisian Front. Observations on a biologically enriched zone in the North Sea between the Southern Bight and the Oyster Ground. ICES C.M. 1991/L:25, Session Q, 21 pp.
Baars, M.A., 2000. The 'green curtain' near the Frisian Front in September 1997: an offshore diatom bloom in advected English Coastal Water? In: The Frisian Front revisited. New observations on the benthic and pelagic communities in the transition zone between the Southern Bight and the Oyster Ground (M.A. Baars, M.J.N. Bergman and M.S.S. Lavaleye). NIOZ-Rapport 2000-x, xx-xx.
Baars, M.A., M.J.N. Bergman and M.S.S. Lavaleye, 2000. The Frisian Front revisited. New observations on the benthic and pelagic communities in the transition zone between the Southern Bight and the Oyster Ground. NIOZ-Rapport 2000-x, xxx p.
Bergman, M.J.N., 2000. Spatial variation in invertebrate megafaunal populations across the Frisian Front in 1997-1998. In: The Frisian Front revisited. New observations on the benthic and pelagic communities in the transition zone between the Southern Bight and the Oyster Ground (M.A. Baars, M.J.N. Bergman and M.S.S. Lavaleye). NIOZ-Rapport 2000-x, xx-xx.
Bergman, M.J.N. and J.W. van Santbrink, 1994. A new benthos dredge ('Triple-D') for quantitative sampling of infauna species of low abundance. Neth. J. Sea Res. 33: 129-133.
Brockmann, U.H., R.W.P.M. Laane and H. Postma, 1990. Cycling of nutrients in the North Sea. Neth. J. Sea Res. 26: 239-264.
Creutzberg, F., 1985. A persistent chlorophyll a maximum coinciding with an enriched benthic zone. In: P.E. Gibbs (Editor), Proc. 19th Europ. Mar. Biol. Symp., Cambridge University Press, pp. 97-108.
Creutzberg, F., 1986. Distribution patterns of two bivalve species (Nucula turgida, Tellina fabula) along a frontal system in the southern North Sea. Neth. J. Sea Res. 20: 305-311.
Creutzberg, F. and H. Postma, 1979. An experimental approach to the distribution of mud in the southern North Sea. Neth. J. Sea Res. 13: 99-116.
Creutzberg, F., P. Wapenaar, G. Duineveld and N. Lopez-Lopez, 1984. Distribution and density of the benthic fauna in the southern North Sea in relation to bottom characteristics and hydrographic conditions. Rapp. P.-v. Réun. Cons. Int. Explor. Mer 183: 101-110.
Gee, A. de, M.A. Baars & H.W. van der V
