Pelagic-benthic coupling in kelp forests of central California

Abstract

Pelagic-benthic coupling in the marine environment influences productivity, trophic interactions, and community structure in nearshore ecosystems. It is also intrinsically variable as oceanographic conditions, species abundances, and the availability of basal resources shift. Kelp forests are home to a nearshore fish assemblage that accesses not only energy derived from kelp but also from pelagic, phytoplankton-based primary production, providing a clear example of pelagic-benthic coupling. Here, I used a combination of stomach content analysis and carbon and nitrogen stable isotope analyses to explore rockfish use of kelp- and phytoplankton-based carbon in kelp forests in central California from 2013-2016. I examined connections between rockfish diet and 2 large-scale events that influence the influx of pelagic-based energy to the kelp forest: seasonal upwelling and the recruitment of juvenile rockfish. I show that phytoplankton-based carbon use was higher in species that forage in the water column (~65%) as opposed to species on or near benthic substrates (~50%), and that both pelagic and benthic foragers consumed a diversity of prey taxa advected to the kelp forest from pelagic environments. Furthermore, multiple lines of evidence indicate the importance of variable juvenile rockfish recruitment in understanding rockfish use of phytoplankton-based energy. This research advances our understanding of how ecosystem connectivity influences species’ diets and the many ways that cross-ecosystem subsidies shape the marine environment. Results suggest that to truly understand how kelp forest communities function, we must give more attention to the dynamics of pelagic energy sources and the conditions that lead to variability in pelagic-benthic coupling.