Shorelines worldwide comprise of extensive sandy beaches, dominating both temperate and tropical coastlines. The biodiversity of these sandy beaches is made up of macrofauna, meiofauna and insects. Diversity patterns help illustrate the effect that human pressures can have on coastal environments and the need for an integrated management approach in order to maintain these levels of coastal diversity.
The coastal area (coastal zone) is an extremely dynamic environment where interface between sand, water and air are always observed. This interface is composed of specific gradients or boundaries whose dimensions range from a few nanometres to a kilometre or more.
Coastal areas contain:
- maritime zone
- "sea" zone
- littoral or intertidal zone, where specialized fauna and flora at the sea-land interface is found
Beach types based on morphodynamic scale of Short & Wright (1983):
- intermediate (a few types)
Beach types based on the degree of exposure Brown & McLachlan (1990):
- very sheltered
- very exposed
Sandy shorelines are some of the most extensive intertidal systems worldwide, dominating most of the temperate and tropical coastlines, where they represent both excellent recreational assets and buffer zones against the sea . Despite their initial barren and sterile appearance, many sandy beaches sandy littoral localities might even be considered as highly productive . Shallow marine sands that appear to consist of clean mineral grains only harbour a microscopic community of microorganisms (bacteria, fungi, protozoa), meio- and macrofauna organisms that in its diversity rivals that of terrestrial ecosystems. Every single individual of this sedimentary community affects the processing of organic matter within the sediment and thereby enhances the biocatalytical filtration capacity of the permeable sand beds.
Biodiversity of sandy beaches
Most invertebrate phyla are represented on sandy beaches, either as interstitial forms or as members of the macrofauna . The macrofaunal forms are by far the better known. Some of them are typical of intertidal sands and their surf zone, while others are more characteristic of sheltered sandbanks, sandy muds or estuaries and are less common on open beaches of pure sand .
Macrofauna of the 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 those 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 . 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 abundance population. Water movement is important parameter controlling macrofaunal distribution on beaches.
In contrast to the wave-swept surface sand 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. 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 the macrofauna in biomass in some cases. The dominant taxa of sandy beach meiofauna are nematodes and harpacticoid copepod with other important groups including turbellarians, oligochaetes, gastrotrichs, ostracods and tardigdades.
Terrestrial insects and vertebrates are frequently ignored in accounts of sandy beaches. These animals are usually a conspicuous component of the ecosystem, often rivalling the aquatic macrofauna in terms of biomass and having a significant impact on the system with regard to predation and scavenging.
Latitudinal biodiversity patterns of meiofauna from sandy littoral beaches
Recently, large-scale patterns of marine biodiversity were the subject of many discussions (e.g.). These attempts to develop a general picture of diversity in the sea are hampered by the small number of key studies, the varied sampling protocols applied, the different diversity indices and the varying levels of taxonomic resolution. A general trend of species impoverishment towards the poles was reported for some taxa (e.g., corals, gastropods), but this does not hold for others (e.g., amphipods and decapods crustaceans). In addition, it seems probable that there is a cline in increasing diversity from the arctic to the tropics, but the cline from the Antarctic to the tropics is far less well-established since the Antarctic has high diversity for many taxa. Broad latitudinal gradients in species richness are illustrated for open-ocean pelagic and deep-sea taxa, but some debate continues to surround evidence for shallow-water systems, particularly for non-calcareous taxa. Gray (1997) stated that marine biodiversity is higher in benthic (bottom-related) rather than in pelagic (in the water column) systems, and on coasts rather than in the open ocean, since there is a greater range of habitats near the coasts. A good comparison of multispecies macrofaunal assemblages inhabiting the same type of habitat (sublittoral, fine sediment bottom) showed little if any difference among tropical, temperate and arctic sites in terms of diversity. There was some dispute on how far the observed latitudinal patterns are size-dependant and small bodied taxa (Protoza and meiofauna) tend to be more ubiquitous and their richness is less latitude dependant compared to large organisms.
In coastal environments the interactions between coastal morphology, land-ocean exchanges, meteorological and tidal conditions, create a highly complex and finely scaled network of environmental boundaries. These boundary conditions explain why coastal waters have both higher species richness and a richer ecosystem than their oceanic counterparts . Sandy beaches are among the most ‘simple’ systems in terms of habitat complexity in comparison to other coastal ecosystems as, for example, rocky shores, algae and seagrass beds. Biodiversity and biomass of interstitial organisms are rather low. However, recent findings have shown that marine sands transfer energy very effectively, and that chemical and biological reactions take place faster there than in fine-grained sediments.
The maximum meiofauna densities reported in the study of Kotwicki et. al (2005) ranged between 15 and 4312 individuals 10 cm sq. The reported densities rank among the meiofauna densities in sandy beaches reported in available literature. In general, high meiofauna density can be found in intertidal muddy estuarine habitats, while much lower values are recorded in the deep sea. In fine sediments such as organic rich muds, meiofauna densities of 104 individuals 10 cm sq and more are common. The available meiofauna data showed a large within-site (within-region) variation in the temperate zone while there was very little variation within the meiofauna densities of the antarctic and arctic zones. These patterns demonstrate that attempts to project global biodiversity from the results of regionally based studies must include the significant variation in diversity among sampling sites. The low salinity effect on meiofauna occurrence was not clear – two brackish water locations have the same range of meiofauna density as full marine sites (Kotwicki et. al 2005). As was reported in cited literature, the lower salinity was not associated with decrease of meiofauna.
In terms of taxonomical composition, the meiofauna taxa that were encountered during study of Kotwicki et. al (2005) are similar to those of muddy sediments. There were no meiofauna taxa found that were restricted to shallow permeable sediments only. The percentage composition, on the other hand, differed significantly between the different study sites along the latitudinal gradient. In general, nematodes dominate benthic meiofauna communities comprising more than half of the total meiofauna abundance. This was indeed the case for most sampling sites except for both polar regions (arctic and antarctic), where turbellarians were the dominant meiofauna group.
Some small macrofaunal crustacean species (Cumacea, Amphipoda, Mysidacea) that can occasionally be found in meiofauna samples, were absent in the littoral zone of polar waters. Small size in macrofauna is often associated with a fast development, r-strategy and warm environmental conditions that permit fast egg incubation and growth. That strategy is unlikely to be fruitful in cold regions. These results support the hypothesis that warm regions support fast growing, smaller and more abundant organisms, and cold regions are dominated by larger and less abundant meiofauna. Macrofauna taxa may contribute to the meiofauna size class only in the tropics. A lower number of taxa was collected in both polar sites and, this in combination with slightly higher diversity in the temperate and tropic zones, supported the general pattern of diversity increase towards lower latitude. When only ‘meiofauna sensu stricto’ (i.e., without the small macrofaunal organisms) were taken into account, no clear latitudinal change could be found.
However, data at species level can give a more detailed and perhaps different outcome. Average number of nematode species from sandy littoral sites ranges between 50 and 60 species in warm temperate localities (Italy), cold temperate (Baltic) and slightly less are reported from arctic Svalbard (Gheskiere, Ghent University, personal communication). The results of the classification illustrated the clear difference between the polar sampling sites on the one hand and the more temperate beaches on the other hand. Archambault and Bourget (1996) showed that large-scale heterogeneity explains a larger proportion of the variance in macrofauna species richness than substratum heterogeneity on a more local scale. In this context, it is fairly reasonable to refer to the important human pressure on temperate beaches. Recent studies (e.g.) focused on the effects of recreational pressure (trampling, beach cleaning and nourishment). Weslawski et al. (2000) suggested that a highly diverse meiofauna and diatom assemblage in undisturbed beaches may act as an effective biological filter for some types of pollutant, while less diverse, but more abundant biota in disturbed areas are more effective in processing organic matter (self-cleaning of the beach). Largely as a result of conflicting uses of coastal habitats, losses of marine diversity are highest in coastal areas. The best way to conserve marine diversity is to conserve habitat and landscape diversity in the coastal area. Marine protected areas are only a part of the conservation strategy needed. A framework for coastal conservation should include integrated coastal area management, where one of the primary goals is sustainable use of coastal biodiversity.
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