Widespread kelp-derived carbon in pelagic and benthic nearshore fishes suggested by stable isotope analysis

• We collected two species of nearshore fish at eight northeast Pacific Ocean sites.
• Fish carbon and nitrogen isotope values were measured to estimate energy sources.
• Kelp-derived carbon contributions were prominent for nearshore fish at all sites.
• Isotopes of nearshore fishes suggest strong benthic-pelagic coupling.

Kelp forests provide habitat for diverse and abundant fish assemblages, but the extent to which kelp provides a source of energy to fish and other predators is unclear. To examine the use of kelp-derived energy by fishes we estimated the contribution of kelp- and phytoplankton-derived carbon using carbon (δ13C) and nitrogen (δ15N) isotopes measured in muscle tissue. Benthic-foraging kelp greenling (Hexagrammos decagrammus) and pelagic-foraging black rockfish (Sebastes melanops) were collected at eight sites spanning ∼35 to 60°N from the California Current (upwelling) to Alaska Coastal Current (downwelling) in the northeast Pacific Ocean. Muscle δ13C values were expected to be higher for fish tissue primarily derived from kelp, a benthic macroalgae, and lower for tissue primarily derived from phytoplankton, pelagic microalgae. Muscle δ13C values were higher in benthic-feeding kelp greenling than in pelagic-feeding black rockfish at seven of eight sites, indicating more kelp-derived carbon in greenling as expected. Estimates of kelp carbon contributions ranged from 36 to 89% in kelp greenling and 32 to 65% in black rockfish using carbon isotope mixing models. Isotopic evidence suggests that these two nearshore fishes routinely derive energy from kelp and phytoplankton, across coastal upwelling and downwelling systems. Thus, the foraging mode of nearshore predators has a small influence on their ultimate energy source as energy produced by benthic macroalgae and pelagic microalgae were incorporated in fish tissue regardless of feeding mode and suggest strong and widespread benthic-pelagic coupling. Widespread kelp contributions to benthic- and pelagic-feeding fishes suggests that kelp energy provides a benefit to nearshore fishes and highlights the potential for kelp and fish production to be linked.

Map of black rockfish (left) and kelp greenling (right) sampling locations in the northeast Pacific Ocean with pie charts representing the relative carbon contributions from kelp (brown) and phytoplankton (green) primary producers estimated using carbon (δ13C) and nitrogen (δ15N) stable isotope values of muscle tissue. Black rockfish feed pelagically in the nearshore water column and kelp greenling feed benthically in nearshore waters. Mixing models were used to estimate carbon contributions based on site-specific phytoplankton and kelp (Table 2) and trophic discrimination factors (mean ± SD) of 1.0 ± 1.0‰ for δ13C per trophic level. Image credits: kelp and phytoplankton Jane Thomas and Tracey Saxby, IAN Image Library (http://ian.umces.edu/imagelibrary) and fish pictures, Washington Department of Fish and Wildlife.Figure optionsDownload high-quality image (169 K)Download as PowerPoint slide