Carbon Isotope Composition of Mysids at a Terrestrial-Marine Ecotone, Clayoquot Sound, British Columbia, Canada

The relative contribution of summertime terrestrial versus marine carbon to an estuary on coastal British Columbia, Canada was explored using stable carbon isotopic ( 13C values) analysis of mysid crustaceans (Malacostraca: Peracarida: Mysidacea). We hypothesized that landscape linkages between the forested upland and adjacent inshore marine waters, via river, groundwater and overland flows, may influence carbon content and metabolism in the coastal zone. We sampled 14 stations spatially distributed in a grid and found 13C compositions of mysids ranged from 15·2 to 18·4‰. There was, however, no obvious spatial distribution of 13C values relative to the estuarine gradient in Cow Bay. Heavy tidal mixing is suggested to disperse marine and terrestrial carbon throughout the entire bay. From a temporal perspective however, mysid 13C signatures became enriched over the sampling period (mid-July to mid-August), which is representative of a stronger marine influence. This may arise because mysids are exposed to greater marine-derived carbon sources later in the summer, a decrease in freshwater input (and hence terrestrial carbon), changes in phytoplankton or macrophyte community structure, or that mysids preferentially feed on marine food sources. Overall, the recorded isotopic values are characteristic of marine organic carbon signatures suggesting that in summer, despite the proximity to shore, little or no terrestrial carbon penetrates the food web at the trophic level of mysids. This notwithstanding we believe there is a strong need for additional study of carbon flows at the marine-terrestrial interface, especially for disturbed watersheds.