Multi-scale variation in salinity: a driver of population size and structure in the muricid gastropod Nucella lamellosa

Abstract

The abiotic environment varies continuously at a variety of temporal scales. While this variation is known to be ecologically important, multiple scales of variability are rarely explicitly considered in ecological studies. Here, we combine field observations and laboratory experiments to determine the individual and population level effects of short-term (tidal) and longer-term (seasonal and interannual) salinity variation on the dogwhelk Nucella lamellosa in the Strait of Georgia, British Columbia, Canada. The Fraser River heavily influences surface salinity in the Strait of Georgia, which varies with season, depth, and distance to the river mouth. At low salinity sites, N. lamellosa population size decreased following high outflow years, with fewer juveniles present, as opposed to high salinity sites, which had higher population densities in all years. Sustained salinity exposure in the laboratory caused developmental delay of encapsulated embryos and complete mortality at 9 and 12 psu. Juvenile dogwhelks (<30 mm shell length) and those from a high salinity site experienced higher mortality in low salinity conditions than larger individuals and those from a low salinity site. The inclusion of a 3 h daily exposure to 20 psu, simulating high tides in a stratified water column, enabled N. lamellosa to survive otherwise low salinity conditions for considerably longer. Overall, our results suggest that seasonal and interannual variation in salinity have a profound influence on N. lamellosa populations and that shorter-scale fluctuations can moderate these seasonal and interannual effects. It is likely that similar multi-scale environmental effects will determine survival and population dynamics in many species.