Case studies: UK

Case studies: Australia

The effects of storm-induced wave damage on kelp forests are frequently obvious, because entire kelp thalli are torn off and are deposited on the strand-line, forming the raw material for low-level alginate industries. Most of the recent detailed quantitative research has been carried out on kelp forests in Australia and California (e.g. by Kennelly & Underwood, 1993). Related studies have not been published for European kelp forests.

The Australian study described below was precise and methodologically sound, using control sites and artificial disturbance. Attempts to classify naturally disturbed kelp communities in Australia (Phillips et al., 1997) were less successful. Macroalgal community structure was investigated in habitats exposed to high, intermediate and low levels of physical disturbance (wave exposure). Community structure was measured by determining the biomass and diversity at both the species level and by assigning algae to functional groups (e.g. crusts, leathery thalli, etc.). Comparisons between the two approaches were made using ANOVA (analysis of variance) of biomass data and derived diversity indices. Multivariate analysis techniques of ordination, Principal Axis Correlation (PCC) and ANOSIM (analysis of similarities) showed that macroalgal assemblages were highly variable, particularly within exposure levels, when examined at both the species and functional group levels.

A less direct effect of storms is the increase in turbidity which occurs due to sediment re-suspension and higher river flows. This affects kelp biotopes because of reduced light penetration (section II.C.) and increased silt deposition. A long period of storms might have serious effects on the quantum budget of kelps living near their lower depth limit, in the same way that unusually thick snow lying over ice greatly reduces annual net growth of Laminaria solidungula in the Arctic (Dunton, 1990). Silt is also detrimental to settlement of kelp spores (see section V.D.4.), and to filter-feeding invertebrates.

The interaction between different types of disturbance is emphasised by the observations of Dayton et al. (1992) of the giant kelp forest off Point Loma, San Diego, California. A short but intense storm in January 1988 appeared to have been the most severe in perhaps 200 years. It changed age-specific kelp mortality patterns and caused the first large-scale understorey mortality in several decades. By sweeping away drift algae it resulted in intense local urchin grazing which reduced recruitment of kelp plants to the forest.

Effects of storm damage on kelp biotopes in the UK have not been researched.

Kennelly & Underwood (1993) examined the effects of experimental physical disturbances on the assemblages of understorey species living in sublittoral kelp forests in central New South Wales, Australia, across several spatial and temporal scales. Experimental clearing of kelp canopies from 3 replicate patches at four locations in each of two kelp beds mimicked damage done during storms. Subsequent macro- and microscopic sampling of cleared areas, edges of clearings and un-cleared, control areas of natural kelp forest was carried out over a 14-month period. Despite large variations in abundance and temporal fluctuations, there were some similarities in response to disturbance by several taxa in most locations.

• The area of substratum covered by holdfasts of the kelp Ecklonia radiata and encrusting algae decreased in the centres of most clearings as the plots became overgrown - first by microscopic filamentous algae and then by macroscopic foliose algae.
• The species of microscopic and turfing algae which colonised clearings were different at different sites, as were the periods after clearing before they colonised.
• The presence of turfing species in the centres of clearings seemed to preclude the successful establishment of some species of sponges and an ascidian species.
• While these changes occurred, there was an increase in the area covered by a layer of sediment and the species richness of the assemblage in the clearings declined.
• At some sites, Ecklonia juveniles recruited, but this tended to occur toward the edges of clearings where the covers of microalgae, algal turf and sediment were least.
• Assemblages on the edges of clearings often showed effects intermediate between those in the centres of clearings and those under the natural canopy.
• Species that were fairly uniform in their responses to disturbance were common, occurring in similar abundance in most places.
• Temporal variability showed that some fluctuations occurred over a matter of weeks (e.g. the microscopic organisms), others occurred over several months (foliose algae), whilst the largest time-scale involved the recovery of the disturbed kelp canopy over the full duration of the experiment.

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