Regional patterns of physiological condition determine giant kelp net primary production dynamics

Photoautotrophs vary the concentration of photosynthetic pigments in response to changing environmental conditions. In the ocean, the chlorophyll a to carbon ratio (Chl:C) has been used as a proxy for the physiological condition of phytoplankton, and there is laboratory evidence that the growth rate of juvenile giant kelp (Macrocystis pyrifera), a coastal foundation species, is positively related to Chl:C under nutrient-limited conditions. We examined the relative roles of nutrients and light in determining Chl:C dynamics and if fluctuations in Chl:C were related to changes in canopy biomass and net primary production (NPP) from sites spanning 750 km of the California coast. Seventy percent of the variability in canopy Chl:C was explained by a combination of photosynthetically active radiation and seawater nitrate concentration. In the periodically nutrient-limited waters of the Southern California Bight, changes in Chl:C positively resembled changes in available nitrate, whereas Chl:C negatively tracked changes in light for the more nutrient-replete central California coastline. Values of Chl:C were positively related to residuals from an autoregressive model of kelp biomass at the southern California sites indicating that Chl:C is a proxy for physiological state of the kelp canopy. NPP estimated through correlations with kelp biomass and lagged Chl:C compared well to established field-based estimates. These results open the possibility of assessing giant kelp physiological condition from estimates of Chl:C modeled from sea surface irradiance and nitrate concentration, which in turn can be used to estimate giant kelp primary production over large spatial and temporal scales using future remote sensing technologies.