La interpretación en los servicios públicos

2.1 Intertidal areas

In this study, we will focus on intertidal habitats and more specifically on sandy beaches. Here we briefly introduce some general features of intertidal areas and then elaborate more on sandy beaches.

The intertidal zone, also called littoral, is the area between low and high water marks and forms part of many landscapes that appeal to humans, allowing the formation of important

24 recreational and tourist economies. This is an area of a foreshore and seabed which is exposed to air at low tide and submerged at high tide. Throughout the intertidal area, prominent zonation occurs. Zonation is evident from the occurrence of specific organisms in distinct bands. These distinct bands result from many complex physical and biological factors that affect marine organisms. In other words, the zonation of organisms is the reflection of their response to both physical and biological factors (McLachlan and Jaramillo 1996).

Based on the duration of submergence, the intertidal zone can be subdivided into three subzones: 1) the high tide zone (upper mid-littoral) is covered by water during spring high tide only, so it experiences dry periods daily; it is a highly saline environment and spends much of its time as a semi-terrestrial habitat; 2) the middle tide zone (lower mid-littoral) is regularly covered and uncovered (twice a day when tides are semidiurnal) by tides; 3) the low tide zone (lower littoral) which borders on the shallow subtidal zone and is mostly submerged and is dry only at the lowest tides (Doty 1946). This zone often harbours the highest biodiversity of macrobenthos (Armonies and Reise 2000). Intertidal animals experience physiological stress during low tide and species inhabiting the upper intertidal zone are often more tolerant to thermal and desiccation stress than those found in other zones (McMahon 1990; Short 1999). Climatic factors (temperature, humidity and wind) and also other factors (such as desiccation and light penetration) all affect community structure in the intertidal zone. Consequently, the only animals to be found in areas regularly emerged by the tide, are those that have adapted to these more variable and extreme conditions. The adaptations may be behavioural (i.e. movements or actions), morphological (i.e. characteristics of external body structure), and/or physiological (i.e. internal functions of cells and organs) (Giere 2009).

2.2 Sandy beaches: Definition, characteristics and classification

Beaches form the border areas between the land and water bodies such as oceans and seas. This strip of nature formed by a deposit of sediment between land and sea represents one of the world’s most dynamic natural environments (Schwartz 2005; Pilkey et al. 2011). Beaches in many places of the world are of substantial touristic value, drawing millions of visitors worldwide. There is no single, agreed-upon definition of a beach. The term "sandy beach" can be used to describe a wide range of environments, from high-energy open-ocean beaches to

sheltered estuarine sand flats (McLachlan 1983). One fairly broad definition refers to a beach

as an "accumulation of wave-washed, loose sediment that extends between the outermost breakers and the landward limit of wave and swash action" (Leatherman 1988). Our use of the term (sandy) beaches roughly corresponds to this definition.

25 In comparison to other coastal ecosystems such as rocky shores and seagrass beds, sandy beaches (also called sandy shores) are among the most simple systems in terms of habitat complexity (Reise 2001). They are distributed worldwide in many temperate and tropical areas, where they constitute an important habitat for a variety of fauna. Due to their ecosystem services and economic benefits to mankind (e.g. harvestable natural resources, storm buffers, recreation and tourism), sandy beaches are considered an important element of the coastal ecosystems with economic, social and cultural importance to humans (Costanza et al. 1997; Costanza 1999; Pilkey et al. 2011). Besides, beaches function as natural filters responsible for the re-mineralization of substances, which then return to the sea as nutrients (Coull and Chandler 2001; McLachlan and Brown 2006).

Sandy beaches are commonly found in association with coastal dunes and occur along approximately 20 % of the world’s coast. Marine sandy beaches are composed of a mixture of quartz and carbonate sands from terrestrial and marine origin, respectively (Masselink and Pattiaratchi 2001).

The beach is often divided into the following zones (Fig. 1-10): 1) upper beach or back shore: area between high tide line and primary dune; 2) swash zone: area where waves rush up the face of the beach and retreat seaward (usually remains saturated), and 3) surf zone: area between the low water line and the point where breakers form.

26 Fig. 1-10. Cross-section of an idealized beach showing the common beach zones and some other features at both high and low tide, adapted from: (http://www.marine.tmd.go.th/marinemet_html/lect20.html)

Sandy beaches are shaped principally by the interacting physical forces of waves, tides and

sediment movements. In fact, the physical structure of sandy beaches can be defined in terms

of sediment, waves and tides. The interaction of breaking waves, tides, slope and sediment texture determines the morphology of the beach as well as the circulation patterns of the surf zone (Wright and Short 1984; Masselink and Short 1993), collectively resulting in the so- called "morphodynamic state of a beach" (Carter 2013). Wright and Short (1984) proposed a morphodynamic continuum of beaches with two endpoint-type beaches and several intermediate types between them. These endpoints are dissipative and reflective beaches. The factors that determine the morphodynamic beach state or type include mainly the wave height, wave period and particle size of the beach sediment. Accordingly, beach state can be estimated by an index called Dean’s dimensionless fall velocity (also called parameter Ω). Ω reflects the interaction between wave height, wave period and sediment fall velocity of sand particles (Wright and Short 1984).

Ω = Hb/Ws.T

where Hb is the average height of a wave (in cm) at the point where the wave starts to break,

Ws is sediment fall velocity in cm/s, and T is wave period in seconds. When Ω < 1, beaches

27 are intermediate beaches. Thus, coarser sediments (larger particles sink faster) and long wave periods contribute to low values of Ω and hence to more reflective beaches, whereas high waves with shorter periods, running over finer-grained sediments typically result in a dissipative morphodynamic beach state (Kaiser and Attrill 2011).

Dissipative beaches are a product of large waves moving over fine sands, resulting in a flat or low gradient beach face and wide surf zone. Waves start to break far from the shore in a series of spilling breakers that dissipate their energy along the broad surf zones. Dissipative beaches usually have rather stable morphologies, and exhibit minimal shoreline change with only a gentle slope (Short and Hesp 1982) (Fig. 1-11). Wave pressure on interstitial water is limited, hence organic matter can partly accumulate, leading to high microbial activity and steep geochemical profiles.

Intermediate beaches are located between the high energy dissipative and the lower energy reflective beaches. The two most distinguishing characteristics of intermediate beaches are 1) a surf zone and 2) a cellular rip circulation (rip currents) commonly associated with rhythmic bar and beach topography (Schwartz 2005). They are produced by moderate to high waves (0.5 - 2.5 m), fine to medium sands and longer wave periods (Short and Wright 1983) (Fig. 1- 11).

Reflective sandy beaches lie at the lower energy end of the beach spectrum. These beaches are a product of lower waves and coarser sand. Their morphology consists of a steeper and narrower surf zone which is sometimes absent. Waves surge or break straight on the shore generating fast swashes with short periods. During the breaking process, part of the wave energy is "reflected" back to the sea by the very steep beach face (Fig. 1-11). Wave pressure on interstitial water is large, resulting in strong pore water flushing and very limited or no retention of organic matter. Geochemical profiles are therefore more homogeneous.

Sandy beaches have also been divided into three categories based on tidal range: microtidal, mesotidal and macrotidal. Areas with high tidal ranges (> 4 m) are referred to macrotidal, where waves reach further up the shore, while areas with lower tidal ranges are considered mesotidal (range = 2 – 4 m) or microtidal (range < 2 m) (Davies 1964). However, depending on wave energy and sediment particle size, even microtidal beaches can belong to any morphodynamic beach state (Wright and Short 1984; Masselink and Short 1993), illustrating nicely that tidal range is a poor criterion to properly classify beaches and reflect their true morphodynamic diversity.

28 Figure 1-11 A schematic drawing showing the main beach morphodynamic types, Adapted from Short and Wright (1983).

In document Actas del Congreso CNLSE de la Lengua de Signos Española 2015 (página 165-168)