Led Chrono

Led Chrono

The Intertidal Environment

Rocky shores

Rocky shores are zoned according to tidal height. In the supralittoral zone highest up the shore lichens, periwinkles, isopods ( Ligia) and barnacles predominate i.e. those species which can survive some desiccation, strong winds and extremes of temperature. Vegetation here includes rock samphire, thrift and sea campion. In the Midlittotal area usually covered by high tides we have wracks, barnacles, periwinkles, limpets, mussels and whelks. This area is dominated by bladder wrack ( Fucus vesiculosis).

There is less exposure to air in this zone so more species are able to tolerate conditions here, therefore diversity increases and biological factors such as competition for space, availability of food and predation begin to determine the species present. Generally higher up the shore physical factors are more important determinants of zonation, Biological factors more important lower down.Finally , the Infralittoral region contains kelps, barnacles, limpets, mussels, starfish and crabs. This is the most amenable location which does not often dry out and has less extremes of temperature and fluctuations in salinity. Overall we see that higher up the shore we have low species diversity but high population of a species and lower down the shore we have higher species diversity but lower populations of a species. We could also include rock pools in this section which contain a large diversity of species and act as refuges from desiccation. Limpets, crabs and starfish e.g. Asterias rubens are species of interest here.

Sandy shores

Sand is produced as a result of weathering and erosion of rock. Low energy beaches with little wave action are usually composed of fine sand or silt. These are good environments for animals and show a higher diversity of species than the gravel beaches. Muddy shores show the highest diversity though. The gradient of the beach also determines the type of deposits found. Well sorted sands have high porosity which provides a good environment for burrowing species with good oxygen and drainage.

Below a certain depth on sandy shores though there is little oxygen and conditions become anaerobic marked with a black sulphur layer full of bacteria.Sandy shores have many borrowing or infaunal species e.g. amphipods, bivalve cockles, Baltic tellin and razor shell (Enis enis). There are also epifaunal species such as sandhopper. Below the tideline we have sand eel, which borrows into the sand at low tide, green shore crabs ( Carcinus maenas) and blenny. Many species also visit the sandy shores such as Flounder and Plaice, and birds such as Oystercatchers and gulls.  

The different types of estuaries and their water circulation

Estuaries are partially enclosed coastal areas where rivers enter the sea. Many of these were shaped during the last glaciation when sea levels were lower. There are several types of estuary depending on shape and water circulation patterns. Strong tidal currents flush out sediment from river mouths creating wide channels in some estuaries e.g. Chesapeake Bay.

Weak tidal currents and high sediment loads create deltas rather then estuaries. Here then is a broad classification of three types of estuary. 1) Coastal Plain Estuaries or drowned river valleys e.g. Colombia River estuary 2) Bar Built estuaries where wave action offshore forms a bar or barrier across as section of coast where rivers or streams drain into the sea. These are found in NW Europe. 3) Tectonic estuaries which are the result of movements in the Earth's crust when a block of land sinks below sea level and is flooded to create an estuary e.g. San Francisco Bay.The shape of the basin is one of the factors in determining water circulation. In wide estuaries there may be strong currents which are able to sweep more seawater upstream and a thorough mixing of seawater and freshwater results. As we move seawards estuarine water grades from freshwater to seawater through lines of equal salinity called isohalines.

Where seawater mixes upward we have areas of upwelling and more nutrients. In narrow estuaries with weak currents there is a different circulation with seawater being unable to sweep far upstream. This results in the stratifying of water with denser saline  water forming a wedge beneath which can become stagnant, anaerobic  and provide poor conditions for most species. The time it takes to flush water out of the estuary is known as the flushing time. Estuaries with long flushing times become vulnerable to pollution. It should be noted there are many variables involved in determining water circulation in an estuary in addition to shape of the basin, run-off, actions of man and storm variables. Estuaries are dynamic environments.

The adaptations found in animals living in the littoral zone.

There are many adaptations found in animals living in the littoral zone, Here is a brief discussion of the main types of adaptation. Whether the shore is rocky, a sandy beach or muddy there are several main challenges faced by organisms in the littoral zone. These may be classified as challenges due to 1) movement of the tides, 2 ) physical action of waves, 3) exposure to air 4) temperature fluctuations 5 ) changes in salinity 6) changes in levels of oxygen.

These factors also interact. For example the movement of tides leads to changes in oxygenation, desiccation and temperature for example. Now lets us consider some of the main adaptations.1 and 2) Adaptations to tides and wave action - On pebble beaches with much crushing and scouring there are relatively few organisms. On rocky shores the inhabitants adapt by attaching themselves firmly to the substrate e.g. barnacles, limpets, mussels and have thick, strong shells. Crabs use cracks to shelter in. Some animals have behavioral ways of coping i.e. they move to find shelter, whilst others have adapted physiologically. This brings us on to a consideration of 3)  exposure to air or desiccation. Some limpets have adapted to be able to lose between 30 to 70% of their water content but still survive. Barnacles are able to seal their shells very tightly, periwinkles have opercula and anemones have mucus coverings all which slow down desiccation.

4) Another set of adaptations are to cope with temperature fluctuations in the littoral zone. These are by variation in size, shell design and colour. Animals exposed higher up the shore tend to be larger than animals further down the shore in the same species e..g the mollusc Littorina littorea.  Being larger they take longer to overheat and desiccate. Shell design also helps thermoregulation with light coloured, ornamental shells shedding heat quicker than dark, smooth shells.

5) In order to cope with changes in salinity in rock pools and run-off areas some animals have adaptations although most remain osmoconformers. Estuarine animals have to be able to tolerate wider ranges in salinity (euryhaline animals) such as green shore crabs, common periwinkles and mussels. Stenohalic animals can not e.g. Thin tellin. Some fish have adapted too to a range of salinity such as Atlantic salmon, Sea trout and eels. At low tide in estuaries there are many birds with specific adaptations (shape of beak) to suit their search for food e.g. plovers, oystercatchers and curlews.

6)  Changing levels of oxygenation have led to adaptations too. Gills are easy to desiccate so some species cover them in a cavity in the mantle e.g. some molluscs. Others seal their shells, remain passive when oxygen levels are low or have increased vascularization of their mantle so they can absorb oxygen e.g. barnacles.Finally another adaptation found in animals of the littoral is the high number of species producing planktonic larvae and timing egg and spawning production to coincide with high tides. This is only a small cross section of the wide range of adaptations found in animals living in the littoral zone.

Dr Simon Harding

www.chronosconsulting.com

www.coberongreen.com

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