Barren coarse sand shores
Ecological and functional relationships
Community and population patterns of distribution and abundance in exposed sandy beaches have been assumed to be primarily controlled by specific species responses to the hydrodynamic climate and sediment characteristics which are intimately linked, a scenario where biological interactions do not appear to play a critical role (McLachlan, 1983). Furthermore, there is a conspicuous lack of information concerning the effects of biotic factors e.g. competition, on the structure and distribution of sandy beach populations, as it is likely that detection of intra- and interspecific competition in such a dynamic environment is very complex (Branch, 1984). However, competition for space and food is unlikely to be a limiting feature in this high energy environment, as the faunal population of mobile amphipods and isopods is extremely small and they swim in the water column at high tide in search of food, only sheltering temporarily in the sediment at low tide (Peterson, 1991). Consequently, no single species can be considered a keystone species whose activity is essential to the structure of the community.
Seasonal and longer term change
The LGS.BarSnd and LGS.BarSh biotopes will be sensitive to seasonal changes in the hydrodynamic regime, and as a result of increased wave action and water movement, sedimentary disturbance is likely.
Habitat structure and complexity
- The hydrodynamic regime (tides, waves and residual currents) together with the underlying physiography and geology create the conditions for a given substrata to develop.
- Grain size, shape and degree of sorting are most important in determining porosity and permeability which influence drainage. Drainage is critical in determining the moisture content, oxygen saturation, organic content and the depth of the reducing layer (if present). Permeability increases with coarse substrate and better sorting, and drainage also increases on steeper beaches. Consequently, the sediment diameter of the coarse sand of this biotope (0.25 - 2 mm diameter) ensures that it is freely-draining.
- In the LGS.BarSnd biotope a macrophyte community is absent owing to the lack of stable substrata. However, in the LGS.BarSh biotope a temporary covering of the green algae Ulva may develop (via attachment to larger pebbles and cobbles) during the period of relative stability in the summer.
Macroalgal productivity within the LGS.BarSnd biotope is likely to be very low. Macroalgae (if present) occur in extremely low abundance and is typically absent owing to the lack of a stable substratum. Therefore the benthic microalgae (microphytobenthos e.g. diatoms, flagellates and euglenoides) are probably the most significant primary producers of the depositing shore and are confined to the interstices of the illuminated sediment surface. The phytoplankton of the sea also becomes a temporary part of the sandy beach ecosystem when the tide is in and primary producers from other environments may appear on the shore. These are invariably macroalgae that have become detached from rocky substrata and have been washed up. Eventually they decompose on the beach and contribute to the energy budget of the shore system. Consequently, most productivity on the mobile sandy shore may be categorized as secondary, derived from detritus and allochthonous organic matter. In the LSG.BarSh biotope also represented by this review, a temporary covering of the green algae Ulva sp. may develop during periods of relative stability during the summer and consequently contribute to the productivity of the biotope.
The burrowing amphipods Bathyporeia pelagica
and Pontocrates arenarius
and the isopod Eurydice pulchra
may occur in the LGS.BarSnd and LGS.BarSh biotopes at extremely low abundance. If these species are found in any greater abundance the biotope should be classed as LGS.AEur. However, the recruitment processes of these species may be summarized as follows:
- Eurydice pulchra breeds between April and August once sea temperatures rise above 10°C, and the highest number of juveniles occurs around the periods of maximum summer temperatures. Males and females pair during their nightly swimming on falling spring tides and mating occurs in the sand once the female has completed her moult. Incubation of the embryos in the brood pouch takes some 7-8 weeks and after release of the young the female returns to the non-breeding condition (J. Fish, pers. comm.). Juvenile Eurydice pulchra first appear in July, the minimum length being 1.7 mm (J. Fish, pers. comm.). Although the first juveniles may reach sexual maturity before the onset of winter, they begin breeding in the following spring and die during their second autumn after a total life span of approximately 15 months. Mid-summer juveniles also mature to breed the following summer and only reached 12 months of age before dying. In contrast, the last broods appearing as late as October, do not mature until late the following summer. They breed in their second October and then over-winter for a second time, producing a second brood in the spring before dying of at 18-20 months old (Fish, 1970; Jones, 1970; Hayward, 1994).
- Bathyporeia pelagica may breed throughout the year, but the greatest reproductive activity occurs during spring and late summer/autumn. Males and females pair whilst swimming and mate on the night-time ebb tides following each new and full moon. Development of an egg to the stage when it is released as a juvenile takes just 15 days to complete. The over-wintering population of Bathyporeia pelagica consists largely of juvenile animals. These mature in spring to form the majority of the next breeding population and eventually die in June and July, after a life span of about one year (Fish & Preece, 1970). Bathyporeia pilosa has a similar recruitment cycle.
- In Pontocrates arenarius from Irish Sea coasts, breeding has been recorded throughout the year (Fish & Fish, 1996).)
Time for community to reach maturity
Beaches are dynamic environments, even when they are neither gaining nor losing sediment they are subject to short-term changes in response to wave regimes and weather conditions. Beach profiles show alteration as beach-face sands are re-cycled and decline as the component sand grains are reduced in calibre by attrition and weathering. In some locations these trends are marked by accretion as new sandy sediment arrives and the coastline advances. Whilst in other locations there is a loss of sandy sediment, marked by diminishing beach volumes and coastline retreat (Bird, 1983). As a consequence of the dynamic nature of the habitat the faunal component of the biotope is very sparse and low in species richness. Therefore, the community might be considered 'mature' only a few days or weeks after the last spring tide or drying event, as the mobile species migrate into the biotope from adjacent areas carried in as surf plankton.
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This review can be cited as follows:
Barren coarse sand shores.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=16&code=1997>