Sensitivity to Natural Events
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
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Fazakerley, unpublished data, to be 20-100 years old)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