Case studies: UK

Case studies: California

It is only in recent years that the potential effects of climate change (whether natural or anthropogenic) on the natural environment have been considered in depth, because of the enormous amounts of computing power required for modelling global 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 and so are unlikely to significantly influence biotopes from one generation of scientists to the next.

Evidence for natural changes in coastal ecosystems has to be separated from the relatively intense effects of anthropogenic impacts, especially those which may occur in heavily populated areas. Data on changes in kelp standing stocks is available from several sites over long periods (decades) because it has been and continues to be harvested for extraction of alginates. In southern California there are highly productive ecosystems based on the primary production of the giant kelp, Macrocystis pyrifera, which also support a variety of documented fisheries. The most detailed study on the possible effects of decadal climatic change is that of Dayton and co-workers, e.g. Dayton et al. (1992), described below.

Standing stocks of Laminaria hyperborea were compared over a period of two decades, from 1946 onwards, as part of a survey of seaweed resources around the Scottish coasts carried out by the Institute of Seaweed Research. Walker (1956) and Walker & Richardson (1957) described how the standing stock varied from year to year, and followed an 11-year cycle that corresponded to a cycle of sunspot activity. Although their methodology was primitive, (spring grabs were used to sample the kelp) their results were surprising and interesting, as more recent ecological studies have shown similar wide-ranging effects which follow sunspot cycles. The cycles of sun-spots may be a visible indicator of solar activity that has a profound effect on patterns of physical changes in the global environment. Increased sunspot activity is thought to have an effect on the climate of the Earth in that the associated increase in the strength of the solar wind is thought to provide additional protection from cosmic rays. This in turn results in a reduction in global cloud cover and a consequent temperature rise (H. Powell, pers. comm. - from p.9 of "The Observer" 12/4/98 ).

Because of the importance of Macrocystis to the diverse assemblage within the

Californian kelp forest community, Tegner et al., (1996) focused their research on changes in giant kelp populations. Canopy maps of the Point Loma kelp forest near San Diego show major changes over the last century, which have been ascribed to a variety of different causes, including fluctuations in seawater temperature. To understand the role of physical forcing on interannual variability in the Macrocystis canopy at Point Loma, they compared two 31-year kelp data sets with available physical records.

Annual average surface water temperature was significantly correlated with the kelp harvest, but explained only 24% of the variance. Because the canopy of giant kelp is susceptible to disturbances that may not affect plant survival, they also evaluated two subsurface measures: stipe number (as an index of individual plant growth) and stipe density (as a measure of the carrying capacity of the area). Both stipe measures were sensitive to interannual variability in surface water temperature for the period 1983-95 and were more sensitive indicators than the survival of plants.

Plant size and the carrying capacity of the area were very low following the 1992-93 El Niño conditions and the anomalously warm year of 1994. Comparison with historical stipe data from 1957, 1973, and 1974 indicated a reduction in standing biomass of up to two-thirds since 1957. There was a strong inverse trend between median plant size and the sums of anomalies in Scripps Institution of Oceanography Pier surface water temperature, calculated quarterly for three years. Tegner et al. (1996) concluded, however, that these large, inter-decadal changes in biomass could be explained by the location of the data sets within multi-year warm and cold periods (i.e. precisely when the samples were taken during the temperature fluctuations). They argued that the sensitivity of stipe counts to surface temperature supported their incorporation into ongoing and future Californian kelp forest research.

The same team also described more community-orientated aspects of this study. Dayton et al. (1992) studied the effects of various types of disturbance for three decades on the populations of several species of kelp in the Point Loma kelp forest, with a view to addressing the question "do large-scale episodic events override biological mechanisms as major community structuring processes?" The most sensitive population factors they studied were recruitment, density, and survivorship. There were marked differences between the decades with regard to the intensity of the disturbances. Compared with the 1980s, the two preceding decades were relatively benign. The 1980s had two extreme disturbance events:

• the 1982-1984 El Niño-Southern Oscillation (ENSO) was the most severe El Niño event of the century, and resulted in very warm, nutrient and oxygen depleted water in Californian coastal areas;
• in 1988-1989 a severe storm was followed by a strong La Niña event marked by unusually cool, nutrient-rich water.

The massive disturbances of the 1980s obliterated much of the biological structure in the kelp forest. Certainly the anomalies caused many longer term effects including:

• which, in turn, resulted in between-area variation in recovery rates from the disturbances. outbreaks of understorey algae such as the foliose brown alga Desmarestia ligulata
• intraspecific competition
• changes in grazing patterns, etc.,

However, in all cases the variation between different sites within the kelp bed was overshadowed by the overwhelming competitive dominance of Macrocystis pyrifera. Most of the patches of understorey algae on the transect lines (some of which had persisted for 7 years) died out by the end of 1990. The population biology of Macrocystis was remarkably similar in most areas, as the cohort longevity and survivorship curves were very similar, and the plant and stipe densities tended to level off in only a few years. Thus large-scale episodic events such as El Niños, La Niñas and rare storms may have dramatic impacts, but small-scale responses such as "density-vague" recruitment (i.e. neither density dependent nor density independent) and survival of robust species allow prompt recovery, often to pre-existing patterns. The one exception was a site which was marked by increased sea urchin grazing and resultant poor kelp recruitment throughout the latter half of the 1980s. A subsequent urchin disease event later reduced the grazing pressure in this area and led to kelp recruitment in the autumn of 1991.

Next Section                     References