Community Dynamics on a Subtidal Cobble Bed Following Mass Mortalities of Sea Urchins

Abstract

The macrobenthic community of a shallow subtidal cobble bed at Eagle Head on the Atlantic coast of Nova Scotia [Canada] was monitored from ca 4 mo before to 41 mo after mass mortalities of sea urchins Strongylocentrotus droebachiensis in September 1983. Biomass of erect algae on cobbles increased from 0 to between 50 and 75 g (dry weight) m-2 after the sea urchin die-off, and consisted mainly of small and sparsely distributed plants of Fucus evanescens and Chondrus crispus. In contrast, algal biomass increased from 0 to ca 2.8 kg m-2 on an adjacent boulder ridge where large kelps (Laminaria longicrurus, L. digitata, Saccorhiza dermatodea) formed a dense canopy. The macroflora of cobble beds at 2 other sites was similar in biomass and composition to the cobble bed at Eagle Head. At a third site, the sea urchin population had recovered and precluded the establishment on non-clacareous algae on the cobbles. In the absence of sea urchins, herbivourous molluscs, mainly perwinkles Littorina littoriea, limpets Notoacmaea testudinalis and chitons Tonicella rubra, dominated the cobble macrofauna, reaching densities of up to ca 150, 400, and 1300 ind. m-2 respectively at Eagle head. However, these grazers (and other organisms) suffered mass mortalities in summer 1986, when thick mats of drift algae inundated the bottom creating anoxic conditions. Suspension consisted mainly of small bivalves of which juvenile mussels (Modiolis modiolis, Mytilis edulis) were most abundant. Predators/scavengers included crabs, lobsters, whelks, sea stars, and polychaetes, many of which were small and seasonally active. With the exception of juvenile mussels, which increased in abundance (but rarely reached adult size), there was no evidence of a numerical response among macrofaunal populations to the elimination of sea urchins.