How does light intensity affect the distribution of organisms?

Answer 1

You may have seen this shore profile in the tides section. Note the environmental gradient, that is, the gradual change in abiotic (non-living) factors across the habitat. The first factor considered below, desiccation, is the result of the tide. As the water slowly drops so the shore is exposed to drying (called emersion, the opposite of immersion). As you can appreciate, this emersion becomes less the further down the shore you are. On neap tides it may be that days occur when the lower shore never dries out. Likewise, on neaps the upper shore may never get water on some days. This is an environmental gradient of desiccation. The tide is a primary factor as it not only affects the amount of water present but temperature, salinity any many others.

Below is a summary of the main factors which can influence the distribution of organisms on the seashore.

DESICCATION occurs as a result of emersion at low tide; influences the upper and middle shore.

WAVE ACTION more wave action means the water splashes higher and so the zones occur higher up on the shore. The strong force produced by powerful wave action will determine not just whether that organism can remain attached to the rock but also may have an effect on its growth. E.g. Bladderwrack displays substantial variation in its shape, size and number of bladders. See Ballantine's Exposure Scale.

LIGHT is needed for photosynthesis. Seaweeds need to be in seawater for this to occur. However, the water will filter off some of the wavelengths of light and reduce the intensity. Small algae, e.g. some of the red algae, will photosynthesise with very little light and occur under other larger algae. Seaweeds in mid-lower shore require accessory pigments to absorb lower amounts of light penetrating the water.

TEMPERATURE: immersion in water buffers against temperature change. Upper shore species will have to tolerate the greatest variation in temperature whilst it has least affect in the lower. High temperatures will increase the affect of drying out. Increases salinity in pools

ASPECT is the direction the shore faces. South facing will have more illumination and warmth, but dries faster; north is cooler, darker and less likely to dry out. Thus, on a north facing slope community bands will be wider and higher up the shore. Catenella (red alga) colonises north aspect whilst on south facing ones the lichen, Lichina replaces it.

SLOPE. A flatter shore may provide a greater area of substrate for colonising and will not drain as fast as a steeper one.

TURBIDITY is the cloudiness of the water. Large amounts of plankton can increase the turbidity, as will detritus and sewage pollution. This restricts the light reaching the algae on the rocks.

SUBSTRATE. The hardness and size of rocks and boulders will influence an organisms ability to attach itself. Soft rocks will be suitable for burrowers, e.g. piddocks. Large boulders and rocks give good shelter for animals and the angle of dip of the rock strata may produce more crevices and pools. If stones are too small they will be mobile, moving around in the surf and so prevent any organism from attaching itself to the rock.

FRESHWATER. Seepage of water from the cliff can dilute the seawater. Few of the organisms on the shore can tolerate salinity changes. Enteromorpha is so tolerant it is a good indicator of freshwater on rocky shores. Upper shore rockpools are vulnerable to salinity variation as water runs off the cliff.

BIOTIC. These are the biological factors influencing the community. Algal turf, like Osmundea and Chondrus, will slow down the drainage on the shore and reduce desiccation. Grazing is very important. A high concentration of limpets will reduce the establishment of the normally dominant brown seaweed. Removal of limpets from a shore, e.g. due to oil pollution, results in a sudden "bloom" of algal growth, usually green. Inter-specific competition occurs when niches of different species overlap. Knotted Wrack occupies a similar position in the middle shore to Bladderwrack. The latter survives wave action better than the former, which is found on sheltered shores. Where they both occur competition allows the former to dominate as it lives for many years longer. The fucoid algae have a "whiplash" affect, where water movement causes a sweeping action of the alga across the rock and prevents the attachment of algae spores and the settling of planktonic larvae. In this way it competes with barnacles. If the later does manage to become established it may push out the wrack. Populations cannot become established unless juvenile forms are available to colonise the rocks. This is called Supply-side Ecology. Most organisms living on the shore use the sea to disperse their young. The seaweeds have microscopic spores and animals like topshells have larvae which settle on to the rock to begin growth. If these young are not available the populations cannot get established.