Alternative stable states of a kelp-bed ecosystem

The long-term goals of my research program are: 1) to investigate biological and physical processes that govern large-scale shifts between two alternative states of the rocky subtidal ecosystem in eastern Canada, kelp beds and sea urchin barrens; 2) to determine the consequences of these state-shifts on ecosystem function and services; and 3) to develop predictive models of community structure and dynamics that aid in assessment of ecosystem health and management of natural resources. Connectivity between the shallow system, which undergoes cyclical alternations in state, and adjacent assemblages in deeper water has emerged as an important theme in my current research. Deeper habitats provide a thermal refuge for sea urchins, the dominant grazers, from a disease that decimates their populations at depths above 25 m. As kelps and other seaweeds recolonize the shallow zone after an urchin die-off, these deep-living sea urchins form feeding aggregations (fronts) along the lower margin of emergent kelp beds. These fronts migrate shoreward to begin a process of destructive grazing that ultimately reinstates the barrens state. The flow of organic matter and energy from shallow seaweed beds to deeper habitats is another form of connectivity. Kelp fronds continuously erode and fragment, or are dislodged during storms. Waves and currents convey these drifting fragments and particulate matter to deeper water, where it serves as food source for sea urchins and other consumers. The proposed research will address several critical and interconnected elements of alternative-state dynamics, as they apply to the coast of Nova Scotia: 1) the mechanism of introduction of the pathogen that drives the transition to the kelp-bed state; 2) the potential for accumulated disease resistance in sea urchins, which would enhance the resilience of the barrens state; 3) the distribution and abundance of deep-living sea urchins that repopulate the shallow zone after mass mortality events; 4) the importance of kelp debris in supporting growth and reproduction of these deep-living sea urchins; and 5) the potential of sea urchin harvesting as a means of regulating grazing intensity to maintain a dynamic balance between states in transition.