All marine benthic communities are subject to natural events which may or may not prove to be catastrophic to the biotopes present. These events may present on a geological time scale (volcanic activity, tectonic movements, accumulation of sediments, all of which are likely to be chronic stresses) or on a seasonal basis (severe storms, river flooding) or may be single catastrophic changes to biotopes or their components as a result of a wide variety of local events (such as exposure of the upper subtidal zone to bright sunlight during a series of unexpectedly low tides: Hruby, 1975), which can be regarded as acute stresses.

Because of their extreme longevity and very slow rates of accumulation, maerl beds will be exposed to potential perturbations over a very long period. Maerl thalli (and hence maerl biotopes) can be classed as sensitive according to most of the criteria listed by Hiscock (1997) because they:

• are fragile (brittle)
• are long-lived (thalli of Lithophyllum dentatum have been estimated by H. Fazakerley, unpublished data, to be 20-100 years old)
• recruit poorly
• have poor larval (= spore) dispersal or no spore stage
• are unable to move away.

For maerl beds the most significant natural events affecting the biotopes on an ongoing basis are storms. Resulting water movement has been found to be very important in determining the loss rates of thalli from the beds, and the turbidity that follows storms almost certainly reduces photosynthesis and thus growth rates. A distinction should perhaps be made between the type of storms that occur on a regular basis, to which maerl beds must be adapted and perhaps depend on to prevent excessive stabilisation by algae and animals, and occasional hurricane-like storms that could cause long-term damage. Sedimentation of phytoplankton blooms is also an important seasonal event.

Effects of natural events

Pathogens

References