Around two thirds of the testing area is located over a predominantly sedimentary or mixed sand and rock seabed with a low content of organic matter, a high grain size and a relatively good degree of selection. The other third is located over a rocky or predominantly rocky seabed. The shallowest zone includes two areas identified as paleochannels filled with sand and gravel sediments.
The tide of the sea zone is semidiurnal, with tidal movement generally varying between 4 m in spring tides and 1.5 m in neap tides.
The energy flow per meter of wave frontage and average wave direction in the test area stand at 21.4 kW/m and North 50º West, respectively. Data from the Bilbao buoy (www.puertos.es) on extreme wave conditions show that significant wave height can reach 11.5 m (for a return period of 50 years), with a maximum wave height of up to 22 m and an associated characteristic period of 15-16 seconds.
The typical pattern of currents in the area under study is mainly attributable to ocean circulation induced by the wind, with average values for surface currents (initial metres) of between 10 and 20 cm/s and maximum values of around 50 cm/s, These values become progressively smaller as the water column descends. Predominant directions generally run parallel to the bathymetry (NESW).
The most common range of average monthly temperature of the surface waters of the Basque coast is between 11 ºC in January-February to 22 ºC in August. Temperature values taken during the campaign in May 2008 vary between 12.85 ºC and 15.62 ºC.
Taking into consideration all available information from different studies, water salinity in the area under study shows average values of between 34.8 and 35.6 PSU, similar to the average salinity of Basque coastal seawater which stands at 35.5 PSU. In the 2008 sampling campaign, salinity values of between 35.19 PSU and 35.66 PSU were recorded.
Minimal transparency values (estimated by the depth [in metres] at which the Secchi disc is no longer visible when immersed in the water) recorded in field campaigns were 11m, whilst maximum values reached 16 m, thereby indicating very good optical properties of the water mass in the study area.
According to the results obtained by the Quality Network in the coastal area between the towns of Plentzia and Bakio (Borja et al., 2005; 2006; 2007), the quality of the water in terms of the presence (or absence) of contaminant particles is good.
Practically all the coastal area between Cabo Billano (Cape of Billano) and Cabo Matxitxako (Cape of Matxitxako) has been classified as a Natural Reserve and Marine Area of outstanding natural beauty.