The Greater North Sea is defined (OSPAR Commission, 2000) by coordinates at 48° to 62°N, and 5°W to 12°E (Figure 4.1.1). To the south, it embraces the entire English Channel bordered by England, France, and Belgium, and to the east the waters of the Skagerrak and Kattegat, bounded by Denmark, Norway, and Sweden. These boundaries do not signify any isolation of the water mass, and exchange occurs principally through the influx of Atlantic water to the north and to a lesser extent via the Channel, and from the Baltic to the east, along with northward efflux, mainly along the Norwegian coast. An overall estimate of about one year for the flushing time of the entire North Sea disguises significant regional and temporal variations associated, inter alia, with prevailing depth, wave and tidal current regimes, and thermal stability of the water column.
The North Sea is relatively shallow, with depths generally not exceeding 100 m, except in the northern North Sea and in the Norwegian Trench (Figure 4.1.2).
Figure 4.1.1 The North Sea with depth contours and selected locations referred to in this report.
Courtesy of M. Curtis (Cefas).
Figure 4.1.2. Bathymetry of the North Sea (source: Cefas).
Generally, depths do not exceed 50 m for most of the southern North Sea, including the eastern Channel. The northern boundary of the Dogger Bank in the central North Sea is defined approximately by the 50 m contour, which also serves to distinguish latitudinally between contrasting ecological and biogeographical domains.
Patterns in bottom-water temperature and salinity in winter (Figure 4.1.3) illustrate the influence of Atlantic water in the deeper northern North Sea, and of freshwater inflows from the larger river systems in the shallower southern part. As would be expected, the patterns vary vertically, seasonally, and annually and are summarized by (for example) the OSPAR Commission (2000). Information on annual temperature trends is also given below.
Most of the North Sea is mixed in winter, but is stratified in deeper offshore waters of the central and northern parts in summer. The waters of the southern part remain mixed throughout the year, owing to the shallow bathymetry and the influence of stronger tidal currents (OSPAR Commission, 2000; Figure 4.1.4). Spatial differences in the magnitude of tidal currents can also be expected to influence both the nature and stability of bottom sediments and hence the nature of the associated benthic communities.
Climatic influences on the North Sea ecosystem over various timescales (for example, changes in the North Atlantic Oscillation (NAO) index) have been linked to changes in circulation patterns, alterations to the composition and productivity of phyto- and zooplankton
with consequences for recruitment or migration of certain commercial fish species (Anon., 2001), as well as to changes to benthic communities (see Section 5.7).
As we have a special interest in identifying the causes of any changes to the status of benthic communities between 1986 and 2000, we show trends in the winter (December–March) NAO index for the Northeast Atlantic (Figure 4.1.5) and Sea Surface Temperature (SST; Figure 4.1.6) as expressions of climatic variability. The NAO index employed in Figure 4.1.5 is a measure of the difference of normalized sea level pressures between Gibraltar and Iceland.
(The index is one of several permutations expressing contrasts in pressure data between weather stations in the vicinity of the subtropical Azores High and the polar Icelandic Low.) Positive values tend to be associated with warmer, wetter winters characterized by westerly airflows, while negative values reflect colder, more quiescent conditions characterized by northerly airflows. Following a period of persistently negative values in the 1960s, an upward trend encompassing the 1986 survey continued until the mid-1990s. After a drop in 1996, values remained near zero or positive until 2000. In the southern North Sea, annual mean values for SST were generally higher after 1986 than in preceding years, with a notable drop in 1996 (Figure 4.1.6a). This is also evident in the plot of seasonal SST anomalies (Figure 4.1.6b) and is consistent with a general warming trend in the North Atlantic over this time (ICES, 2006). The implications of changes in the NAO index and water temperatures for benthic communities are examined in Sections 5.2, 5.3, and 5.7.
(a) (b)
Figure 4.1.3. Winter (January–February) bottom-water temperature (a) and salinity (b) for the North Sea, collected during ICES International Bottom-Trawl Surveys in 2007 (from Skjoldal, 2007).
Variations in wind strength, persistence, and direction will be broadly reflected in changes to the NAO index and, mediated through the effects of wave action at the seabed, have the potential to directly influence the stability of subtidal benthic communities, especially in shallower waters. We examine spatial relationships from modelled output in Sections 5.3 and 6.1. Deducing causal relationships that might be associated with any longer term trends is complicated not only by spatial variation in the depth regime and the nature of the substratum but also, more generally, by wind-driven influences on circulation patterns in the North Sea and their demonstrable effects on system productivity (OSPAR Commission, 2000; see also
Section 5.2). Of course, storm events may occasionally result in wholesale destabilization of soft sediments and associated benthic communities in shallow waters on very short timescales (e.g. Rees et al., 1977; Rachor and Gerlach, 1978).
There is an inconsistency between observations on storminess over the past 30 years (i.e.
spanning the period between the 1986 and 2000 surveys), which appear to show no worsening trend and an increase in significant wave heights in the Northeast Atlantic (OSPAR Commission, 2000). In the North Sea, increased windspeeds over this time were also recorded off the Norwegian coast (reported in OSPAR Commission, 2000), which would be supported by trends in the NAO index (Figure 4.1.5). In the German Bight, Schroeder (2005) identified an increased frequency of windspeeds at or above 7 on the Beaufort scale in the 1990s.
Increased wave heights may be explained partly by increased fetch (reflecting changes in average wind direction) or, more prosaically, by the absence of reliable data prior to about 1960 with which to provide a longer term historical perspective on the significance of recent changes (OSPAR Commission, 2000). Further consideration of possible links between changes in North Sea benthic communities and climatic influences is given in Sections 5 and 6.
Figure 4.1.4. Peak M2 tidal streams (m/s; source: Cefas).
Finally, in a recent review of the status of European seas, Frid et al. (2003) affirm that the ocean habitat is constantly changing on scales from seconds to thousands of years and hence does not have a “normal state”. As to current uncertainties about whether the recent warming trend reflects natural or human-influenced variability, they note that a better understanding should emerge in “10–15 years”.
-3 -2 -1 0 1 2 3 4
1960 1970 1980 1990 2000 2010
Figure 4.1.5. Trends in the winter (December–March) NAO index. Arrows show the timing of the 1986 and 2000 North Sea benthos surveys (www.cru.uea.ac.uk/cru/data/nao.htm).
The superficial bottom sediments of the North Sea reflect the modern reworking of fluvial and glacial deposits and consist mainly of sands or muds. Coarser deposits, typically gravel/sand admixtures in varying proportions, are patchily located along continental coastlines and, more extensively, along the English east coast and the Channel (see Section 4.2).
(a)
(b)
Figure 4.1.6. (a) Annual mean sea surface temperature in the German Bight (Station Helgoland Roads), southern North Sea (from ICES, 2006). (b) Normalized sea surface temperature anomalies relative to the period 1971–2000, derived from seasonal data along a regular ferry route at 52°N (ICES, 2006).
Recent proposals to subdivide European seas into ecoregions (Anon., 2004) are of special interest. For the North Sea, it was recommended to include the Kattegat to the east, while the western boundary was recommended to shift to ca. 2°W, i.e. to encompass the Channel as far as its central rather than farthest westerly point. Sampling of the eastern Channel in the present (NSBP 2000) survey therefore accounts for a significant proportion of this area of the greater North Sea, defined according to ecoregion.
Inputs to the North Sea of low salinity waters are dominated by outflow from the Baltic, which may have a significant influence on parts of the eastern North Sea coastal margin.
Historically, inputs from the larger UK and continental river systems have also represented a significant source of nutrients and contaminants arising from agricultural, urban, and industrial developments along the estuaries and upriver catchment areas. Although there are continuing concerns over such inputs to parts of the North Sea, the general trend in trace element concentrations from anthropogenic sources has been downwards, reflecting concerted regulatory action to curb the quantities discharged at source, with the resolutions of the Second North Sea Conference in 1987 providing an important stimulus (Anon., 1988). Since NSBS 1986, implementation of the EU Urban Waste Water Treatment Directive (European Communities, 1991) has resulted in significant improvements to the quality of effluent discharged from sewage treatment works to estuaries and coastal waters. It also resulted in a ban on the disposal of sewage sludge to sea since 1998.
The range of human activities with the potential to influence the benthos of coastal and offshore waters includes:
• Commercial fishing/demersal fishing practices
• Oil and gas exploitation
• Shipping, including accidental oil spills, ballast-water introductions, and litter
• Coastal/offshore construction, including wind farms (larger scale developments occurring mainly since 2000)
• Dredging/disposal for port/harbour maintenance and development
• Urban/industrial discharges to estuaries and coastal waters
• Atmospheric inputs, including those via agricultural and motor vehicle emissions
• Climatic influences (to the extent that these may be affected by human activities)
• Agricultural practices/nutrient inputs via run-off
• Aquaculture
• Extraction of marine sand and gravel
• Coastal recreation/tourism/military-exercise areas
• Conservation measures
These activities are fully described in OSPAR Commission (2000) and in a more recent overview by Frid et al. (2003). A comprehensive and timely review of the status of North Sea benthic communities in relation to human activities and other influences is provided by Kröncke and Bergfeld (2003). Many of the studies identified by the authors are of a localized nature in accordance with the distribution of these activities. However, investigations of the effects of, for example, fishing, eutrophication, and climatic changes have a potentially more global relevance to the status of the North Sea benthos. Where appropriate, any relationships to changes evident from the 1986 and 2000 surveys are considered in the following sections.
References
Anon. 1988. Second international conference on the protection of the North Sea. Guidance note on the ministerial declaration. Department of the Environment, London. 13 pp.
Anon. 2001. Towards a North Sea ecosystem monitoring component as a contribution to assessment and management. Statement of conclusions from a strategic workshop in Bergen, Norway, 5–7 September 2001, co-sponsored by IOC, ICES, OSPAR, the North Sea Conferences, and EuroGOOS. Vol. 6, pp. 1–36.
Anon. 2004. ICES response to EC request for information and advice about appropriate eco-regions for the implementation of an ecosystem approach in European waters. ICES Advice 2004, 1(2): 115–131.
European Communities. 1991. Council Directive 91/271/EEC of 21 May 1991 concerning urban waste water treatment. Official Journal of the European Community, L135 (1991).
pp. 40–45.
Frid, C., Hammer, C., Law, R., Loeng, H., Pawlak, J. F., Reid, P. C., and Tasker, M. 2003.
Environmental Status of the European Seas. ICES, Copenhagen. 75 pp.
ICES. 2006. ICES report on ocean climate 2005. ICES Cooperative Research Report No. 280.
47 pp.
Kröncke, I., and Bergfeld, C. 2003. North Sea benthos: a review. Senckenbergiana Maritima, 33: 205–268.
OSPAR Commission. 2000. Quality Status Report 2000, Region II – Greater North Sea.
OSPAR Commission, London. 136 pp.
Rachor, E., and Gerlach, S. A. 1978. Changes of macrobenthos in a sublittoral sand area of the German Bight, 1967 to 1975. Rapports et Procès-Verbaux des Réunions du Conseil International pour l’Exploration de la Mer, 172: 418–431.
Rees, E. I. S., Nicholaidou, A., Laskaridou, P., 1977. The effects of storms on the dynamics of shallow water benthic associations. In Biology of Benthic Organisms, pp. 465–474. Ed.
by B. F. Keegan, P. O. O’Ceidigh, and P. Boaden. Pergamon Press, Oxford.
Schroeder, A. 2005. Community dynamics and development of soft bottom macrozoobenthos in the German Bight (North Sea) 1969–2000. Reports on Polar and Marine Research, 494:
1–181 and A.1–A.59 (Annex).
Skjoldal, H. R. (Ed). 2007. Update report on North Sea conditions – first quarter 2007.
ICES/EuroGOOS North Sea Pilot Project/Planning Group. 31 pp.
(www.ices.dk/marineworld/norsepp.asp).
4.2 Sediment particle size