Discussion of sensitivity

Examples of the sensitivities of kelps

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 occur on a geological time scale (volcanic activity, tectonic movements, accumulation of sediments) 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. A local coastal erosion event such as a landslip; unusually warm calm weather resulting in reduced oxygen concentrations in the water; prolonged heavy rain; the outbreak of an algal disease; toxic red tides - all can result in perturbations to a kelp biotope on a local scale. The level of perturbation can be such that surveying a single site in different years can result in the site being classed as a different community type. Hruby (1975) reported that one of his hard-substratum sites in British Columbia changed dramatically due to an "unpredictable environmental perturbation" i.e. the exposure of the fixed quadrat areas he was monitoring to bright sunlight during a series of unexpectedly low tides.

Kelp sporophyte plants are not classed as sensitive according to most of the criteria listed by Hiscock (1997) because they:

• are not fragile, but are capable of surviving enormous mechanical stress from wave action
• are relatively tolerant of pollution (both organic and inorganic - in comparison to many animals tested)
• reach maturity within a few years (e.g. 2 years for Saccorhiza polyschides; up to 6 years for Laminaria hyperborea; see linked table)
• show strong recruitment under normal circumstances
• have good "larval" (for kelps, spore) dispersal.

However, kelp sporophyte plants are sensitive by two of Hiscock’s (1997) criteria, and an additional one peculiar to Laminaria:

• They are long-lived (Laminaria hyperborea plants can live up to 25 years).
• They are unable to move away, being fixed to the substratum, and unlike some other seaweeds, cannot reattach and continue growing after removal.
• A particular anatomical feature of Laminaria, its formation of a single meristematic zone, make thalli susceptible to damage as they will die if this area is removed (e.g. by storm action, which frequently leaves dying stipes denuded of their meristems) or killed, e.g. by desiccation caused by unusually low tides.

The sensitivities of the zoospores, the gametophyte plants, the gametes and the early stages of the developing sporophyte have been studied under laboratory conditions (Lüning, 1990) but these have not been extended to field conditions. Little research has been undertaken on the chemical sensitivities of the zoospores, gametophytes and gametes of kelp species. In Norway, experiments using sampling devices at varying distances from the kelp forest showed the dispersal range of a population of Laminaria hyperborea plants to be at least 200 m (Fredriksen et al., 1995). Large numbers of zoospores were found in samples taken from the water column at a depth of 5 m both within and 50 and 200 m from the kelp forest.

Laboratory experiments showed that spores probably swim as long as they can and thereafter sink passively out of the water column and settle.

The specific sensitivities of other species in the kelp biotopes cannot be addressed in this review. Of the thousands of species which may be present, the sensitivities of only a handful have been investigated. The key species in kelp beds remain to be confirmed. Once established, however, the sensitivities of the key species will define the tolerance of the biotopes in which they are present.

Kelp biotopes have been well studied in regard to two types of natural events. The first is the long-term effect of El Niños in California, and the second, a possibly indirectly related phenomenon, is that of urchin barrens where dense aggregations of various sea urchin species destroy most or all of the macroalgal vegetation, leaving bare crustose corallines. Both these topics are discussed in more detail later in this section. Interactions between different forms of disturbance, such as the complex set of biological relationships and abiotic aspects of the environment that are involved in the formation of urchin barrens, are particularly important for kelp biotopes. For this reason, urchins and their predators are components of kelp biotopes that have been examined in particular detail in several parts of the world.

Caution in interpreting studies showing long-term changes in kelp biotopes was highlighted by Kennelly & Underwood (1992), working in Australia, on a study providing background information relevant to the effects of human and natural perturbations on these systems. Assemblages of macro- and microscopic species in two sublittoral kelp forests at each of four locations on the coast of central New South Wales were variable from site to site, both within and among locations. Complex patchiness was detected at all spatial scales examined. In addition, temporal changes in fauna and flora were variable from site to site. Of 38 taxa which showed detectable fluctuations in abundance, 26 showed trends unique to one forest and 11 showed consistent patterns in both forests within a location. Only juvenile kelp (Ecklonia radiata) showed similar fluctuations in abundance in both forests in most locations.

Thus, the structure and dynamics of kelp bed systems vary and do not conform to predictions of simple models about processes in habitats dominated by one or a few large organisms. Kennelly & Underwood (1992) concluded that when kelp forests are described using quantitative data at several spatially replicated scales, few general conclusions can be made about the structure of these assemblages.

• It cannot be emphasised too strongly that our present understanding of the natural fluctuations in the species assemblages, populations, distribution and diversity of species in kelp beds is very limited.

Storm damage

Climate change effects

Ecological relationships

References