Determining the extent to which weather-related abiotic factors influence daily variation in early benthic phase mortality of intertidal marine invertebrates

Populations of marine intertidal invertebrates vary substantially in abundance from year to year. These differences might be partly due to cohorts experiencing 60 – 99% mortality during the first few days after the transition from pelagic to benthic environments. However, the causes of this mortality, including the role of weather conditions experienced during low tide, are not well understood. This study aimed to improve our understanding of the influence of low tide environmental stress on survival through the early benthic phase by (1) determining the influence of temperature and desiccation stress on mortality of newly settled mussels and the ontogeny of sensitivity to these stresses through the early benthic phase; (2) examining the mortality of barnacle cyprids from settlement to metamorphosis and mortality of early juveniles up to the age of 10 days after metamorphosis; and (3) documenting the frequency of occurrence of lethal conditions occurring in the field at low tide during the recruitment season. Laboratory experiments revealed that newly settled Mytilus trossulus of 1-2 mm shell length experienced a temperature tolerance threshold at 34°C and a desiccation tolerance threshold at a vapour pressure deficit level of 1.01 kPa. Mussels became highly tolerant to desiccation stress when they reached a size of 3 mm shell length, suggesting a size threshold of desiccation tolerance between 2-3 mm shell length. This size closely corresponds to the size at which some studies have reported juvenile M. trossulus relocate from protective filamentous algal habitat to adult habitat, suggesting ontogenetic shifts in habitat use by juvenile M. trossulus may be a response to changing vulnerability to desiccation stress. A field survey of Balanus glandula recruitment revealed that cyprid and juvenile mortality varied greatly among daily cohorts and was significantly higher under Fucus spp. cover than on bare surfaces. Contrary to expectations, cyprid mortality was not significantly influenced by weather-related abiotic conditions. This was likely due to the study being conducted during a La Niña cycle that may have had lower temperature and desiccation stress than is typical for Barkley Sound. Intertidal temperatures above the threshold tolerance levels for newly settled mussels were uncommon during the recruitment season, suggesting that temperature stress is not likely an important factor influencing early benthic phase mortality of mussels. The desiccation stress threshold level for newly settled mussels was frequently exceeded for several hours during the recruitment season, however, suggesting that desiccation stress may be an important factor influencing early benthic phase mortality in mussels that settle on open surfaces. In contrast, cyprid mortality on bare surfaces was not significantly influenced by desiccation stress or any other weather-related abiotic factors. I concluded that the survival of newly settled mussels likely depends upon the presence of protective microhabitats created by filamentous and fucoid algae, whereas barnacles experience reduced survival through the early benthic phase in the presence of fucoid algae. Changes in survival of these algae due to climate change could therefore have extensive influence on mortality through the early benthic phase and may subsequently affect population and community structure.