Sandy shore habitat
This article describes the habitat of sandy shores. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems.
Contents
Introduction
Sandy shores or beaches are loose deposits of sand, including some gravel or shells, that cover the shoreline in many places. They make up a large portion of the world’s ice-free coastlines. Beaches serve as buffer zones or shock absorbers that protect the coastline, sea cliffs or dunes from direct wave attack. It is an extremely dynamic environment where sand, water and air are always in motion. Beaches also provide important coastal recreational areas for a many people. Fine-grained sand beaches tend to be gently sloping and quite flat.
Formation
Sandy beaches are soft shores formed by deposition of particles that have been carried by currents and waves. The transported material is in part derived from shore erosion, but the major part is generally derived from land and transported by rivers to the sea in the Holocene era or earlier. A more comprehensive introduction to beach formation is given in Coastal Hydrodynamics And Transport Processes. The two main types of beach material are quartz (=silica) sands of terrestrial origin and carbonate sands of marine origin. The carbonate sand is weathered from mollusk shells and skeletons of other animals. Other material includes heavy minerals, basalt (=volcanic origin) and feldspar.
Characteristics
The grain size of sand varies from very fine to very coarse. The particle diameter is shown in the table below. Quartz sands have a slightly lower density ([math]\sim 2.6 \; g.cm^{-3}[/math]) than carbonate sands ([math]2.7 - 2.95 \; g.cm^{-3}[/math]). The quartz particles are generally more rounded. Calcium carbonate particles sink more slowly in water due to their more irregular shapes, even if their density is higher.
| Generic Name | Particle Diameter (mm) |
|---|---|
| Very coarse | 1.0 to 2.0 |
| Coarse | 0.50 to 2.0 |
| Medium | 0.25 to 0.50 |
| Fine | 0.125 to 0.25 |
| Very Fine | 0.0625 to 0.125 |
Porosity is the volume of void space in the sand. It is the volume of water needed to saturate a given weight of dry sand. Most sands have a porosity of about 30 to 40 % of the total volume. The finer a sand the greater its porosity.
Permeability is the rate of flow or drainage of water through the sand. Fine sands have lower permeabilities due to their smaller pore sizes.
Penetrability is related to particle size and porosity. It can be important to the macrofauna. All species must be able to burrow into the substratum. To determine the penetrability, the proportion of clay and silt and the water content are very important.
The two basic beach types are dissipative and reflective. Together with the intermediate types, there are six major microtidal beach types. Beach types can also be based on the degree of exposure. This ranges from very sheltered over sheltered and exposed to very exposed.
For further details see: Shoreface profile, Coastal and marine sediments.
Functioning and adaptations
The intertidal beach zone is covered part of the day by water and is part of the day exposed to air. Swash and tides supply nutrients and food. When the tide retreats, waste products, eggs and larvae are taken away. Organisms living on sandy shores have adapted to this dynamic environment.
Burrowing on high-energy sandy beaches must be rapid and effective. This is because the animals must not be swept away by uprushing and downrushing water. In contrast with rocky shores, desiccation is not an overriding concern, because the animals can retreat into the substratum or below the water table. Intertidal filter-feeders cannot feed while the tide has retreated. Many species of the meiofauna use vertical tidal migrations through the sand column. Other species move up and down the beach with the tides. There is a difference between directional stimuli (such as light, slope of the beach, water currents) and nondirectional stimuli (such as disturbance of the sand, changes in temperature, hydrostatic pressure). Directional stimuli act as orientational signs, while nondirectional stimuli act as releasing factors. The predominant feeding types are filter-feeding and scavenging. Animals on low-energy sandy beaches adapt their respiration differently from those on surf-swept beaches. Some adaptations are an increased ventilation rate, an increased ventilation efficiency, reduced metabolic rate or other ways of energy economy. Many sheltered-shore animals are facultative anaerobes as an adaptation to ebb tides. Other animals in oxygenated surf-swept beaches are essentially aerobic. The majority of the intertidal animals have a high tolerance to variability in their environment, even exceeding what is necessary for survival in their particular habitats. Some species bury themselves to escape high temperatures; others cool by evaporation, by entering the sea or by absorbing water from the substratum. Another problem for intertidal animals is the time of reproduction. There is variation in the number of eggs, the anatomy of the reproductive organs, the morphology of egg shells, times of breeding, mating behavior and developmental stages. Some species adapt by reproducing frequently (iteroparous) or by reproducing just once in a year (semelparous). There are also species that follow the lunar cycle to reproduce at the right time. To avoid predation, several behaviors have developed. The first one is deep burrowing. Another one is migration with the tide to escape predation. Crabs impress predators by holding their chelae open and aloft. According to circumstances, animals can modify their behavior. This is called phenotypic plasticity.
Several groups of vertebrates make use of sandy beaches for foraging, nesting and breeding. Turtles nest on the backshore of sandy beaches. Birds use the beach for foraging, nesting and roosting. Seals use several areas of the beach for nesting, molting, breeding and raising pups. Other terrestrial animals such as otters, baboons, raccoons, lions,… may descend onto the beach to forage. [2]
Biota
The distribution and abundance of the sediment infauna is mostly controlled by complex interactions between the physicochemical and biological properties of the sediment. [3]
The physicochemical properties are:
- Grain size
- Water content
- Flushing rate of water through the sediment
- Oxidation-reduction state
- Dissolved oxygen
- Temperature
- Light
- Organic content
The biological properties are:
- Food availability and feeding activity
- Reproductive effects on dispersal and settlement
- Behavior that induces movement and aggregation
- Intraspecific competition
- Interspecific competition and competitive exclusion
- Predation effects
Most invertebrate phyla are represented on sandy beaches, either as interstitial forms or as members of the macrofauna [2]. The macrofaunal forms are by far the better known. Some of them are typical of intertidal beaches and the surf zone, while others are more characteristic of sheltered sandbanks, sandy muds or estuaries and are less common on open beaches of pure sand [2].
Macrofauna
Macrofauna of sandy beaches are often abundant and, in some cases, attain exceptionally high densities. Their main feature is the high degree of mobility displayed by all species. These animals may vary from a few mm to 20 cm in length. The macrofauna community consists of organisms too large to move between the sand grains. The macrofauna of sandy beaches includes most major invertebrate taxa although it has been recognised that molluscs, crustaceans and polychaetes are the most important. There is a tendency for crustaceans to be more abundant on tropical sandy beaches or more exposed beaches and molluscs to be more abundant on less exposed and on temperate beaches although there are many exceptions of this and polychaetes are sometimes more abundant than either of these taxa. Generally crustaceans dominate the sands towards the upper tidal level and molluscs the lower down level [2]. Physical factors, primary wave action and particle size of the sand largely determine distribution and diversity of the invertebrate macrofauna of sandy beaches. Food input and surf-zone productivity may determinate the population abundance.
Meiofauna
In contrast to the wave-swept beach surface inhabited by most of the macrofauna, the interstitial system is truly three-dimensional, often having great vertical extent in the sand. The porous system averages about 40% of the total sediment volume. Its inhabitants include small metazoans forming the meiofauna, protozoans, bacteria and diatoms[2]. The meiofauna is defined as those metazoan animals passing undamaged though 0.5 to 1.0 mm sieves and trapped on 30 mm screens. On most beaches the interstitial fauna is rich and diverse, even exceeding in some cases the macrofauna in biomass [2]. The dominant taxa of sandy beach meiofauna are nematodes and harpacticoid copepod with other important groups including turbellarians, oligochaetes, gastrotrichs, ostracods and tardigdades. See Latitudinal biodiversity patterns of meiofauna from sandy littoral beaches for a more detailed description of the latitudinal biodiversity patterns of meiofauna on sandy beaches.
Insects
Terrestrial insects and vertebrates are frequently ignored in accounts of sandy beaches. These animals are usually a conspicuous component of the ecosystems, often rivalling the aquatic macrofauna in terms of biomass and having a significant impact on the system with regard to predation and scavenging.
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References
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