Seasonal dynamics in a nearshore isotopic niche and spatial subsidies from multi-trophic aquaculture

A poorly understood food web dynamic concerns possible seasonal variation in spatial subsidies associated with multi-trophic aquaculture and their effects on extractive and naturally occurring organisms. We used the stable isotopes δ¹³C and δ¹⁵N and circular statistics to investigate niche overlap across a year-long period at an experimental multi-trophic aquaculture facility in British Columbia, Canada. A two-source mixing model revealed that particulate organic matter was the most important food source for all sample invertebrates (mean range 40%–98%) compared with farm effluent (mean range 3%–35%). There were significant month-to-month changes in δ¹³C and δ¹⁵N for all species except for the brooding transparent tunicate (Corella inflata). We did not detect any directionality for the entire community, but did identify variable directional shifts for each species, suggesting resource partitioning driven by competition and (or) morphology-based differences in feeding strategies. This was further supported by seasonal variation in inter- and intraspecific isotopic niche widths. Isotopic niche overlap among co-occurring invertebrates appeared to be stronger during winter and summer than autumn months. Our study provides valuable insights on the role of multi-trophic derived effluent on a nearshore marine community composed of both natural and cultured species within the same feeding guild.