Effects of natural events
In areas that are exposed to the prevailing wind and the open ocean, both local as well
as distant storms may affect the swell conditions. Large swells can produce oscillatory
currents at proportional depths and where maerl beds are found in exposed shallow areas
the stability of the surface layers may be completely disrupted as a result. Maerl beds
can form underwater dune systems (Keegan, 1974), and are widely reported to exhibit
ripples and various-sized megaripples, which have been specifically related to storm
conditions of various intensities (Hall-Spencer, 1995a). The onset of calmer periods of
weather may re-stabilise the surface, but a preponderance of perennial, opportunistic
algal species would be expected. In such an area the species composition would be
unpredictable over both temporal and spatial scales, especially in the short term (Hily et
Storm-related damage as a result of increased river discharges and increased turbidity
of the coastal waters may affect maerl biotopes, but these effects have not been studied.
Salinity reduction could affect species with narrow salinity tolerances.
Case studies UK and elsewhere
Hall-Spencer (1995a) has studied the effects of storm damage on maerl in Scotland and
work is continuing in Alicante, Galicia and Brittany under the BIOMAERL programme. Despite
the occurrence of several winter storms that extensively affected the maerl at 10 m depth,
the survival of permanently marked megafaunal burrows showed that only the coarse upper
layer of maerl was moved while the underlying layers, including the burrows, were stable
(BIOMAERL, in press). Following the storms, infaunal organisms renewed their burrow
linings within a week. At 38 m off Alicante, maerl was not obviously affected by a major
storm, with the exception of additional silt deposition.
As part of an experiment to measure growth rates of maerl species in the Ria de Vigo,
Spain (Adey & McKibbin, 1970) some indication was obtained of the movement of maerl
thalli within the study area. At a depth of 5-6 m in a part of the ria exposed to heavy
swell during periods of south-westerly winds (winter months) the following loss rates for
individually tagged rhodoliths on the surface of the maerl bed were found:
Data abstracted from Adey & McKibbin, 1970.
H. Fazakerley (unpublished data) likewise observed a loss of 100% of marked thalli from
monitored areas during strong winter storms in Mannin Bay, Connemara. The thalli, although
not necessarily destroyed, were moved outside the study area.
Severe disturbance of the maerl epifloral community was reported for maerl beds in
Galway Bay (Maggs, 1983a), with the deeper beds showing a less marked drop in total algal
abundance during the winter months than the shallower beds. Doty (1971) found that in
Hawaii storms were the principal factor governing total algal biomass, and the structure
of the community studied by Lieberman et al. (1979) was also controlled by seasonal
abundance resulting from storm mobilisation of the substratum.
Annual weather cycles cause the seasonal pattern of species abundances and species
richness in maerl communities referred to in the Biodiversity chapter. Although macroalgae
and maerl fauna are not directly affected by nutrient availability, winter
remineralisation of sea water causes increases in dissolved nutrients that result in
spring phytoplankton blooms. In the Clyde Sea, the spring diatom bloom eventually settles
out on the maerl leading to high BOD and anoxic conditions so that large infauna such as
the urchin Spatangus purpureus come out of the sediment to obtain oxygen (BIOMAERL,
It is only in recent years that the potential effects of climate change (whether
natural or accelerated by anthropogenic influences) on the natural environment have been
considered in depth, because of the enormous amounts of computing power required for
modelling studies. Most research effort has been directed towards the effects of
anthropogenic climate change, as natural changes in climate are thought to proceed on a
geological time scale so are unlikely to influence biotopes from one generation of
scientists to the next.
Even in the relatively short term, global warming of the anticipated 1-3°C within the next century could have an effect on the composition
of maerl beds in the UK, in that the cold-intolerant species Lithothamnion corallioides
might be able to extend its distribution northwards, and L. glaciale might retreat
northwards. Associated effects of global warming have been predicted to include changed
rainfall patterns and storm systems, both of which would affect maerl by increased water
turbidity and sediment deposition, as discussed above. Changes in sea level could affect
these slow-growing algae, some beds of which are estimated to be about 8000 years old.
Maerl biotopes in some parts of the EU and possibly in parts of the UK are thought to
be very long-lived and as such the maerl beds may be stratified. It should be possible to
determine marine palaeoclimatic information from such maerl beds in the same way that
terrestrial palaeoclimatic information is obtained from peat accumulations and stratified
lakebed sediments. The occurrence of relict dead maerl beds off the Fal estuary and nearby
Cornish coast and in the rade de Brest suggests that natural changes, perhaps in currents
and sedimentation load, have killed the maerl (J. Hall-Spencer, pers. comm.). The dead bed
near the Fal represents many centuries of maerl growth, being 17 km long, 2 km wide and c.
30 cm deep (Anon., 1993).
The geological literature available on palaeoclimate assessment based on the study of
coralline algae is considerable. Hall-Spencer (pers. comm.) has found, at depths of 1 m in
the maerl bed, shells of molluscs that are now extinct in Scotland but still occur further
north. This suggests that the maerl bed dates back to the last ice age. Attempts at
assessing the palaeoclimatic conditions present during the formation of fossil and
semi-fossil maerl deposits have been made in several parts of the world. Foster et al.
(1997) investigated the rhodolith beds in the Gulf of California, looking at the
morphology of the rhodoliths in both modern and fossil deposits, attempting to correlate
the branching density of the live rhodoliths to wave motion. Freiwald et al. (1991)
used maerl deposits to reconstruct holocene climatic changes.