Climate and fishing drive regime shifts in consumer‐mediated nutrient cycling in kelp forests

Joey Peters, University of California Santa Barbara, Department of Ecology, Evolution and Marine Biology, Santa Barbara, United States, Daniel Reed, University of California Santa Barbara, Santa Barbara, CA, United States and Deron E Burkepile, University of California Santa Barbara, Department of Ecology, Evolution, and Marine Biology, Santa Barbara, CA, United States
Globally, anthropogenic pressures are reducing the abundances of marine species

and altering ecosystems through modification of trophic interactions. Yet, consumer

declines also disrupt important bottom‐up processes, like nutrient recycling,

which are critical for ecosystem functioning. Consumer‐mediated nutrient dynamics

(CND) is now considered a major biogeochemical component of most ecosystems,

but lacking long‐term studies, it is difficult to predict how CND will respond

to accelerating disturbances in the wake of global change. To aid such predictions,

we coupled empirical ammonium excretion rates with an 18‐year time series of the

standing biomass of common benthic macroinvertebrates in southern California

kelp forests. This time series of excretion rates encompassed an extended period

of extreme ocean warming, disease outbreaks, and the abolishment of fishing at

two of our study sites, allowing us to assess kelp forest CND across a wide range of

environmental conditions. At their peak, reef invertebrates supplied an average of

18.3 ± 3.0 μmol NH4

+ m−2 hr−1 to kelp forests when sea stars were regionally abundant,

but dropped to 3.5 ± 1.0 μmol NH4

+ m−2 hr−1 following their mass mortality due

to disease during a prolonged period of extreme warming. However, a coincident increase

in the abundance of the California spiny lobster, Palinurus interupptus (Randall,

1840), likely in response to both reduced fishing and a warmer ocean, compensated

for much of the recycled ammonium lost to sea star mortality. Both lobsters and sea

stars are widely recognized as key predators that can profoundly influence community

structure in benthic marine systems. Our study is the first to demonstrate their

importance in nutrient cycling, thus expanding their roles in the ecosystem. Climate

change is increasing the frequency and severity of warming events, and rising human

populations are intensifying fishing pressure in coastal ecosystems worldwide. Our

study documents how these projected global changes can drive regime shifts in CND

and fundamentally alter a critical ecosystem function.