Effects of natural events

Storm damage

Weather

Climate changes

Storm damage

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 al., 1992).

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:

February

March

April

June

July

August

70%

47%

25%

13%

11%

10%

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H. Fazakerley (unpublished data) likewise observed a loss of 100% of marked thalli from monitored areas during strong winter storms in Mannin Bay, Connemara.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.

Weather

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, in press).

Climate changes

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 1-3C 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.

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