Intraspecific variation and energy channel coupling within a Chilean kelp forest

The widespread importance of variable types of primary production, or
energy channels, to consumer communities has become increasingly apparent.
However, the mechanisms underlying this ‘multichannel’ feeding remain
poorly understood, especially for aquatic ecosystems that pose unique
logistical constraints given the diversity of potential energy channels.
Here, we use bulk tissue isotopic analysis along with carbon isotope
(δ13C) analysis of individual amino acids to characterize the relative
contribution of pelagic and benthic energy sources to a kelp forest
consumer community in northern Chile. We measured bulk tissue δ13C and
δ15N for >120 samples; of these we analyzed δ13Cvalues of six
essential amino acids (EAA) from nine primary producer groups (n=41) and
eleven representative nearshore consumer taxa (n=56). Using EAA δ13C data,
we employed linear discriminant analysis (LDA) to assess how distinct EAA
δ13C values were between local pelagic (phytoplankton/particulate organic
matter), and benthic (kelps, red and green algae) endmembers. With this
model, we were able to correctly classify nearly 90% of producer samples
to their original groupings, a significant improvement on traditional bulk
isotopic analysis. With this EAA isotopic library, we then generated
probability distributions for the most important sources of production for
each individual consumer and species using a bootstrap-resampling LDA
approach. We found evidence for multichannel feeding within the community
at the species level. Invertebrates tended to focus on either pelagic or
benthic energy, deriving 13–67% of their EAA from pelagic sources. In
contrast, mobile (fish) taxa at higher trophic levels used more equal
proportions of each channel, ranging from 19–47% pelagically-derived
energy. Within a taxon, multichannel feeding was a result of
specialization among individuals in energy channel usage, with 37 of 56
individual consumers estimated to derive >80% of their EAA from a
single channel. Our study reveals how a cutting-edge isotopic technique
can characterize the dynamics of energy flow in coastal food webs, a topic
which has been historically difficult to address. More broadly, our work
provides a mechanism as to how multichannel feeding may occur in nearshore
communities, and we suggest this pattern be investigated in additional
ecosystems.