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2.2.1 Physical

This section provides a very brief summary of physical information for Cardigan Bay. A more comprehensive review of the environment of Cardigan Bay can be found in Nicholls et al (1992) [16]. The following sections draw largely from this report.

2.2.1.1        Climate

Cardigan Bay is just within the boreal biogeographic region [17] and has a temperate climate. Warmer lusitanean waters extend north as far as the Celtic deep. The climate shows strong seasonality for several environmental parameters such as temperature, rainfall, water turbidity and wind strength.

2.2.1.2        Exposure

Cardigan Bay is one of the largest bays in the British Isles, measuring over 100km (60 miles) across its westernmost extent from the Llyn Peninsula to St. David’s Head. The bay has a mainly open coastline, exposed to the prevailing south-westerly and westerly winds. South-westerly gales generally occur from October through to March. As the Irish Sea is relatively sheltered, the majority of waves reaching the Cardigan Bay coast are locally generated, of fairly short period and therefore steep. A substantial swell also develops during prolonged periods of high winds. During the winter months when gales are common, the wave height exceeds 1m for about half the time, compared to about a quarter of the time during the summer months. Depending on the wind direction, small embayments within the SAC may provide some shelter during stormy conditions in the areas of New Quay, Ynys Lochtyn, Aberporth, Mwnt and in the Teifi estuary.

2.2.1.3        Turbidity

Parts of the Irish Sea have a marked seasonal variation in turbidity and this is particularly true in Cardigan Bay. This is, in part, due to the shallow nature of the Bay, but largely due to the seasonality of wind strength and rainfall.

During the summer months, suspended sediments settle out in the relatively calm bay, reducing turbidity. During the winter when high winds increase bottom sediments are mixed throughout the water column and produce turbid surface waters. Turbidity of inshore waters is strongly affected by outflow from the rivers. Within the SAC, the turbidity round the Aeron, Ina, and Teifi rivers, as well as smaller outflows, increases markedly when the rivers are in spate. Sediments from the Gwaun and Nevern Rivers adjacent to the southern boundary of the site are also carried into the SAC by tidal currents, and together these turbid waters often form darker coloured bands that spread out from the estuaries and follow the line of the coast.

The mean annual sea temperature in St. George’s Channel is just over 11°C with little variation [18], although shallow coastal areas are subject to larger fluctuations. The mean surface temperature at the mouth of the Bay in August is 15°C. Within Cardigan Bay, there are fairly large fluctuations in water temperature, influenced by proximity to land, as well as in response to seasonal changes, and the shallowness of the bay itself.

Figure 10.         Mean surface sea temperature in winter.

Reproduced from the UK digital marine atlas courtesy of CEFAS

Local water temperatures are affected by the substantial input of fresh water into Cardigan Bay, and this also affects the Bay’s salinity and water quality, particularly locally. Seasonal stratification occurs in the shallower waters of Cardigan Bay, due to weak tidal currents, with warmer, fresher water lying above colder, more saline water. Surface temperatures of coastal waters are lowest in February (5°C in the inner part of Cardigan Bay to 7.5^°C in the outer part), and warmest in July/August when they may reach 20°C inshore and about 14-16^°C offshore [19].

Average sea bed temperatures are only slightly lower than those of the surface waters in winter, but the difference in summer is greater with temperatures up to 2°C lower at depth [20].

Figure 11.  Mean surface sea temperature in summer.

Reproduced from the UK digital marine atlas courtesy of CEFAS

 

 

 

2.2.1.5        Salinity

Within the Irish Sea the annual mean salinity is characterised by a decrease from south to north and from the centre of the channel to the sides. Salinity within Cardigan Bay is influenced not only by incoming Atlantic water, but also by freshwater input from rainfall, run-off from rivers and estuaries within the bay as well as the Severn, and the effects of evaporation, currents and mixing. Surface salinities within the Bay in summer are generally less than 34 parts per thousand, decreasing towards the shore. Mean salinity in the outer parts of the Bay ranges from <34.5ppt in August to <34.6ppt in February, while in the inner waters, average salinity ranges from <34.2ppt in August dropping to about <33.3 in February [21]. During the summer months when the inshore waters of Cardigan Bay are stratified, salinity also varies with depth with fresher water overlying more saline water, particularly near the mouths of rivers and estuaries.

Rainfall into the Irish Sea contributes a volume of water equal to about one third of the riverine input. Cardigan Bay receives an average freshwater flow from rivers of 113m ³s^-1. Of this, the Teifi contributes 31m ³s^-1, the Dyfi 39m³s^-1 and others a total of 43m ³s^-1 [22]. The Teifi has a comparatively small estuary with a proportionally greater outflow of freshwater than larger estuaries in the Bay [23]. Rivers adjacent to the SAC include the Aeron, Ina and Teifi, which contributes the greatest input of freshwater into this area. Smaller streams also flow into the SAC, and freshwater from the rivers Nevern at Newport and the Gwaun at Fishguard also affect salinity, particularly in the southern inshore waters of the SAC. River discharges are highly variable; the peak flow of a major river in flood can be up to 400 times the flow during a drought. The largest inputs to Cardigan Bay occur during the months December to February and the smallest in July [24].

2.2.1.6        Stratification & Fronts

Satellite infra red images show the surface waters of Cardigan Bay to be warmer than the water outside the bay in St. George’s Channel.

Figure 12.         Stratification of the water column in the summer months.

Reproduced from the UK digital marine atlas courtesy of CEFAS

Studies have shown that for at least part of the summer months stratification of the water occurs within the bay with warm freshwater overlying denser, cooler and more saline, waters. It is thought that the weak currents within the Bay, particularly the northern corner, contribute to this.

The stratification breaks down along a line running south of Trwyn Cilan and along this ‘front’ there are strong horizontal gradients of surface temperature, salinity, density and turbidity [25]. Figure 11 shows stratification of the water column in summer. Conditions for frontal formation are most likely to occur along the log10(H/U**3) = 2.0 units contour.

2.2.1.7        Tides

The Irish Sea is a relatively enclosed body of water with moderate tidal ranges. For southern Cardigan Bay, mean spring tidal ranges are approximately 4-5m. Tides in this area are predominantly semi-diurnal, with a period of 12h 25min between two successive high or low waters. High or low water times are progressively later as one goes north along the Cardigan Bay coast. The tide enters the Bay from St. George’s Channel with a weak average flow northwards of both surface and bottom currents, running north during flood tides and south during the ebb [26] .

Tidal current strength is generally low within the bay (max 1.8 kts) and little is known about water transport patterns. The weakest tidal currents are within Tremadoc Bay, increasing to the south and west. Currents are slightly stronger near headlands and estuaries; the strongest currents along the SAC coast run between Cardigan Island and the mainland.

2.2.1.8        Sediment transport

There is limited knowledge on the sediment movement within and into Cardigan Bay. Estimations of coastal sediment transport regimes are being made as part of the Ceredigion Shoreline Management Plan.

2.2.1.9        Geology & Geomorphology

The geology of the sea bed influences the type and abundance of marine life that lives there. The type of rock, its hardness, solubility, inclination and orientation are all important to some degree.

The geology of Cardigan Bay has been recorded by Dobson and Whittington [27] as consisting of an almost complete arc of Pre-Cambrian and Lower Palaeozoic rocks (the basement rocks) cradling a post-Palaeozoic sedimentary basin (with Mesozoic and Cainozoic sediments). It shows a south-west to north-east trend characteristic of the Irish Sea as a whole, extending from St. George’s Channel to the coastline of Tremadoc Bay, where the Mesozoic to Tertiary basin is strongly faulted against the basement. These features are shown on the British Geological Survey Solid Geology sheet [28] for the area which specifically excludes the Quaternary sediments, that completely cover Cardigan Bay except for small areas of exposed basement rocks.

During the middle to late Pleistocene, the area was subject to periods of intense erosion during glaciations, and sequences of glacial and glacio-marine sediments were deposited in the area, particularly in the Celtic Trough. Quaternary deposits are shown on the BGS Quaternary Geology sheet [29] for the area. These are widespread, and the only exceptions are limited to areas of rock which outcrop close inshore off Bardsey and the north Pembrokeshire coast.

The distribution of seabed sediments is largely dependent on tidal current speed; gravels occur where the currents are strongest and muds where water energy is lowest. The coastal areas of Cardigan Bay are generally dominated by sands, with narrow intrusions of gravel around the sarnau, and in the ‘gutter’ areas adjacent to New Quay. Gravelly sediments and areas of exposed boulders and bedrock mainly occur in regions dominated by strong tidal currents or wave action, such as headlands [30]. A map of sediment distribution in Cardigan Bay is given in Figure 7.

2.2.1.10      Topography

Cardigan Bay is a relatively shallow and gently sloping embayment of the Irish Sea, reaching 50m generally in the outer parts of the bay towards St. George’s Channel. Most of the cSAC is less than 30m deep, with deeper areas of 30-40m depth off Aberporth and in the south western corner of the site. Because of the general shallowness of the bay, wind and wave action dominate the physical dispersion processes.

Detailed information on the bathymetry within Cardigan Bay is quite poor. A general indication can be gleaned from the Admiralty charts for the area (see Appendix 2 ) or, far better, from recent acoustic surveys (see Figure 6).

[16] Nichols, C., Havard, M. and Warren, L.M. 1992. An evaluation of the use and efficacy of statutory and non-statutory designations for the conservation of Cardigan Bay. Report for The Countryside Council for Wales, University of Wales, Cardiff.