Where the Land Meets the Sea, Extreme Conditions on The Rocky Shore.
The rocky intertidal shore represents a unique and extreme marine habitat. Unlike the persistent conditions of the icy Polar Regions and continually crushing depths of the deep sea vents, the rocky intertidal shore experiences a broad spectrum of physical variables. In this sense, the Intertidal rock shore is perhaps more extreme, in that the changes in conditions over varying time scales can be great in comparison to other habitats. And yet we see in this environment high productivity and diversity of intertidal flora and fauna as can be seen in the Image below. This brief article will address the changes in physical conditions on the rocky intertidal shore and attempt to highlight their significance in comparison to other marine habitats.
It is no surprise that the physical conditions at the rocky intertidal shore change over time. As with almost all marine habitats (other than those at depths impenetrable by light) the rocky shore experiences the perpetual setting and rising of the sun and the annual cycle of the seasons. However, what sets apart the intertidal zone from other habitats is the cycle of the tides and the interface between the aquatic and atmospheric environment.
The spring and neap tidal cycle acts to create variation in the amount of light attenuated through the varying depth of water that submerges the shore. Over diurnal scales, we see dramatic changes in light levels through the daily rise and fall of the tides acting to submerge and expose the shore . This means that the organisms on the rocky intertidal shore will be exposed to direct sunlight at low tides and restricted levels of light at high tides. Over periods of seconds to, we see large variations in light through passing clouds and shading from algal fronds. Comparing theses changes in light to subtidal environments we may see diurnal variation in light intensity in subtidal environments but no exposure to direct sunlight.
The tidal cycle also impacts the temperatures that intertidal organisms are exposed to. In winter and summer months at temperate climates the difference between air and water temperatures may be large. Thus, organisms will be exposed to sudden changes in temperature with each tide that are varied in their duration dependent on their height relative to mean tide. Luning and Freshwater (1988) find that some intertidal macroalgae are able to tolerate temperatures from -1.5°c to 28°C, compared to reef building corals in tropical latitudes that experience seasonal changes in temperature of only 4-5°C .
Salinity stress comes in two forms at the rocky intertidal shore, with the influence of freshwater from rainwater and river input acting to reduce salinity and the effects of evaporation acting to increase salinity. Using intertidal rock pools at high shore levels as an example we can see that with increasing exposure time in warm dry conditions (such as those in the second image) we see increased evaporation which can lead to a significant reduction in salinity (from 35psu to 100psu). In contrast with a natural rapid influx of freshwater from rainfall, rock pools in arctic Norway showed dramatic changes from 115psu to 20psu in several hours. In comparison, minimum and maximum salinity in coastal waters of Scotland we see experience annual minimum and maximum variation of 7psu.
In addition to the aforementioned physical stresses, the intertidal rocky shore is subject to additional kinetic energy in the form of breaking waves as in the image below. This means that organisms on the rocky shore must be able to remain attached even when pummelled with waves and flow speeds of 8m/s in some instances, comparatively low compared to maximum surface water velocities of 3m/s(14) in the gulf stream.
In summary, the physical stresses experienced on the rocky intertidal shore are varied over many temporal scales, uniquely the intertidal rocky shore is at the interface between the aquatic and atmospheric environment. This means that intertidal organisms experienced exaggerated fluctuations in Salinity, Temperature and Light that otherwise remain consistent in other habitats. These physical factors accumulate to produce a unique stressful environment that you might agree, is in every sense extreme.