SBC LTER: REEF: Net primary production, growth and standing crop of Macrocystis pyrifera in Southern California

The giant kelp Macrocystis pyrifera forms the foundation for highly dynamic and productive marine forests on shallow subtidal reefs in coastal temperate regions of the world. It is believed to be one of the fastest growing autotrophs on Earth and its high productivity supports a diverse array of marine food webs. In the early 2000’s we developed a method for estimating biomass and net primary production (NPP) of giant kelp over the long-term so as to better understand their variation and the mechanisms that control them. Our method involves monthly field measurements of standing crop and loss rates that we combine with a model of kelp biomass dynamics to estimate instantaneous growth rates and NPP for each season of each year. We published this method, along with five years of NPP estimates, in 2008. Since then, we have improved our approach to account for several previously unresolved sources of biomass loss. These improvements have led to a near doubling of our prior estimates and show that NPP of a persistent kelp stand (Mohawk) averages ~5.5 kg dry mass m-2 y-1, and results from the rapid growth (~3.5% per day) of a relatively small standing biomass (~ 0.4 kg dry mass m-2 on average) that turns over ~ 12 times annually. Here we present a new data set with revised estimates of seasonal biomass, growth and NPP for the five years (2002-2006) covered by our previous publication along with an additional decade (2007-2016) of data. We also present updated relationships for predicting giant kelp biomass and NPP from much more easily obtained measurements of frond density that are intended to replace and supersede those presented in our earlier publications.