Diversity, patterns of adaptation, and stability of Nova Scotian kelp beds

Abstract

There are two alternate community states in the rocky subtidal of the Atlantic coast of Nova Scotia, an unproductive sea urchin/coralline alga community, and highly productive kelp beds dominated by Laminaria longicruris. Disease-induced mortality of the sea urchins (Strongylocentrotus droebachiensis) triggered a switch from the first state to the second and provided a unique opportunity to study (1) the ability of L. longicruris to recover its former dominant status, and (2) its stability when competing with other seaweeds and when perturbed by storms and grazers other than urchins. Rates of recolo- nization of L. longicruris depended on the proximity of a refugial source of spores. When reproductive plants were nearby, a closed canopy developed within 18 mo of urchin mortality. When a reproductive population was several kilometres away, there was sparse recolonization for 3 yr, then a massive recruitment occurred with closure of the canopy in the 4th yr. Laminaria is clearly the competitive dominant in the seaweed community. Manipulative experiments showed that the kelp limits the abundance of several understory species, but there was no evidence that the abundant annual seaweeds limited kelp recruitment. When sea urchins were rare, the density and growth rates of Laminaria were influenced mostly by intraspecific competition. When the canopy of adult plants was removed there was a dramatic increase in kelp recruitment, but the recruits that grew in dense patches in the clearings were significantly smaller than those of a similar age that grew more sparsely beneath the canopy. Once the kelp recovered from destructive grazing and formed a mature forest, it was able to maintain its dominance, even in habitats subject to severe nutrient stress for 8 mo of the year. For most ofthe year mortality and erosion of laminae outweighed the effects of recruitment and growth, and the canopy declined, especially during winter when storms were frequent. Erosion was exacerbated by grazing of the gastropod Lacuna vincta. However, in late winter and early spring, recruitment and rapid growth restored the canopy. When severe storm damage was simulated by completely removing Laminaria in patches, the kelp rapidly recolonized and soon outgrew other seaweeds. Unlike the competitive dominants in kelp bed systems in the northeast Pacific, L . longicruris in Nova Scotia manifests multiple patterns of adaptation that enable it to dominate early and late stages of succession in a range of habitats of different levels of nutrient stress and of disturbance from storms and grazers. The principal threat to the stability of the kelp beds is destructive grazing by sea urchins. We suggest that the consid- erable differences between the dynamios of kelp beds in Nova Scotia and those of the northeast Pacific, and the high degree of stability of L . longicruris stands in Nova Scotia, is attributable to the low diversity of kelps and therefore low levels of competition in Nova Scotia, and to the multiple adaptations of L . longicruris that enable it to tolerate several stresses and disturbances. We argue that the dynamics of community organization, and therefore the stability properties of this system are determined primarily by biological interactions and not by physical variables. This differs from the kelp communities in the northeast Pacific, in which both biological and physical factors influence dynamics significantly at a primary level. We offer a qualitative model of the dynamics of community structure in Nova Scotia that may be viewed as a set of deterministic "subroutines," in which each subroutine describes the outcome of a particular biological interaction. The subroutine(s) that predominate at one point in time and space are probably determined mostly by physical hydrographic variables that have a large stochastic component.