REGIME SHIFTS IN A KELP BED ECOSYSTEM AND LINKAGES WITH DEEP-LIVING SEA URCHINS

Abstract

In kelp ecosystems, abrupt shifts occur when high densities of sea urchins destructively graze attached kelp, creating unproductive barrens devoid of erect macroalgae. In this thesis, I assessed the stability of urchin barrens along temperate and polar coasts globally and investigated the role of deep-living sea urchins (Strongylocentrotus droebachiensis) in driving shallow regime shifts in Nova Scotia. Shifts between barrens and kelp beds in many parts of the world represent regime shifts, in accordance with alternative stable-state dynamics. Sea urchins in deep subtidal environments along the Atlantic coast of Nova Scotia serve as a source of adults that eventually form grazing aggregations at the deep margins of kelp beds or contribute to the pool of planktonic larvae that settle in shallow habitats. Pulses of drift kelp were generated during a strong storm event that caused large losses of kelp canopy and exported kelp to deeper regions (15 to 45 m), where sea urchins were highly associated with kelp detritus. Sea urchins rapidly located pre-weighted kelp fronds deployed at 45 and 60 m depth, and consumed them for months. Urchins collected from 60 m depth had large gonads and spawned outside of the main spring spawning season in shallow water, suggesting periodic detrital subsidy enhances their reproductive condition. Longterm dive records and coastal surveys show that kelp biomass has declined by over 84% over the past 4–6 decades, and a shift has occurred from kelp beds to rocky reefs dominated by opportunistic turf-forming and invasive algae. This shift is associated with warming sea temperatures and predicted to reduce the amount of kelp entering detrital food webs. Drift algae and sea urchins occurred across a range of benthic habitats, but most frequently within 1.5 km of the coast in depressions. We found that shallow regions where destructive grazing fronts have been documented over the past 4 decades were closer to predicted habitats of deep-living sea urchin compared to regions that remained in a kelp bed state during the same period, indicating these urchins play an important role in driving shifts from kelp beds to barrens on a coastal scale.