Pelagic food supply and drift sea ice influence intertidal community structure along the Atlantic Canadian coast
Community structure along marine shores varies considerably even within biogeographic regions. Nearshore processes often explain broad patterns of intertidal biological variation, but most studies to date have been conducted in temperate systems largely influenced by upwelling dynamics. On the Atlantic coast of Canada, upwelling is not a major feature and sea ice constitutes an important source of variation. Thus, in 2014 we conducted a study on this coast with the objective of investigating benthic-pelagic coupling affecting intertidal communities. Essentially, we determined the latitudinal changes in rocky intertidal communities along 400 km of coastline in Nova Scotia and examined nearshore factors related to such changes. First, using field and remote-sensing data, we documented the geographic variation in coastal ice load in early spring and the nearshore abundance of phytoplankton and particulate carbon (food for intertidal filter-feeders) afterwards. Then, using cleared quadrats and mesh collectors, we measured the recruitment rate of barnacles (spring) and mussels (spring to fall) in nine wave-exposed locations spanning the studied coastal range. Lastly, we quantified the summer abundance of all macroscopic intertidal algae and invertebrates using 250 quadrats spread throughout those locations. We analyzed the results using standard statistical procedures.
In the early spring of 2014, sea ice formed in the winter in the Gulf of St. Lawrence drifted out of that gulf and scoured intertidal habitats along the northern Atlantic coast of Nova Scotia, causing high biomass losses. Central and southern shores were unaffected by ice and retained abundant algae and invertebrates. Recolonization of the disturbed shores started quickly, mainly by barnacles and secondarily by fucoid algae. Mussel recruitment along the studied coast was weak in the spring and strong in late summer and early fall. Overall, barnacle recruitment peaked at a northern location and at two nearby southern locations, while mussel recruitment peaked at one of those southern locations. The latitudinal changes in nearshore phytoplankton abundance and particulate carbon broadly coincided with those recruitment trends. The changes in the abundance of dogwhelks (the main predators of barnacles and mussels) along the coast largely followed the barnacle and mussel recruitment trends. Species richness peaked at one of the productive southern locations. We conclude that intertidal community structure along the Atlantic Canadian coast is influenced in part by drift sea ice and by nearshore food supply, which appears to cause bottom-up community regulation through barnacle and mussel recruitment.