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Topographical wave exposure indices allow objective assessment of the degree of wave action at coastal sites. We present a grid-based method for rapidly calculating indices at fine spatial resolutions along whole coastlines, and evaluate the power of candidate indices in predicting composition of rocky shore communities. The method has 3 stages: (1) a grid is created from a vector-based digital coastline using geographical information systems (GIS) software; (2) for every coastal cell, wave fetch is determined as the distance to the nearest land cell in 16 angular sectors, using coarse-, medium- and fine-resolution searches of the surrounding cells up to a distance of 200 km; (3) wind energy (average wind speed and proportional occurrence) in each sector is calculated for nearby coastal sites. We calculated the average fetch in each sector (F) and the sum of products of fetch and wind energy (W). A total of 57 species were surveyed at 185 sites in west Scotland for determination of trends with wave indices. Average wave fetch with a 200 m scale grid explained >50% of the variation in the first principal component of the species-sites abundance matrix, with shore extent explaining another 10%. Incorporating wind data in the indices had a negligible effect on predictive power. Species diversity explained 61% of the variance in the second principal component and declined from low to high pelagic primary productivity. Separating direct physical effects from biological effects, such as food supply or grazing could potentially help us better understand the processes structuring biological communities on rocky shores.

For more information see https://www.sams.ac.uk/people/researchers/burrows-professor-michael/

Tidal range is generally between 4 and 5 m; highest tidal ranges are found in the inner Solway Firth where the mean spring tidal range can be between 7 and 8 m. Tidal range is at a minimum in areas known as amphidromic points. One of these points occurs in Scottish waters between Islay and the Mull of Kintrye; another amphidromic point can be found in the north east of the North Sea. Tidal range decreases with distance offshore from the North East coast.

Data source for NMPi Mean Spring Tidal Range layer: Atlas of UK Marine Renewable Energy Resources (see https://www.renewables-atlas.info/)

Sea surface waves are mainly caused by the effects of wind on the surface of the sea. Their height is predominantly determined by the fetch (i.e. distance wind has blown over) and length of time of the wind forcing. Some local modification of wave height can be caused by tides. The seabed also influences waves in shallow water as waves will become steeper and higher as they approach the shore. Wave power is the quantification of the power transmitted by a wave moving across the sea surface. In general, larger waves are more powerful but wave power is also determined by wave speed, wavelength, and water density. Within Scottish waters, the wave climate is mainly influenced by conditions in the North Atlantic ocean, where the fetch is long enough to establish large, regular waves known as swell. The north and west of Scotland (Hebrides, Orkney and Shetland) are most exposed to these conditions. On the east coast of Scotland, conditions in autumn and winter may also be rough in the North Sea because the wind direction can lead to a large fetch. Moreover, the Moray Firth is also relatively exposed because of its shoaling bathymetry and exposure to the North Sea.

Data source: Atlas of UK Marine Renewable Energy Resources (see https://www.renewables-atlas.info/)

Annual Mean Significant Wave Height - The average height of the highest 1/3 of waves. The wave height is greatest in the Atlantic, moderate in the North Sea and least in the immediate coastal zone where the values are generally <1.2m.

Data source: Atlas of UK Marine Renewable Energy Resources (see https://www.renewables-atlas.info/)