Biomass loss reduces growth and resource translocation in giant kelp Macrocystis pyrifera

The biomass dynamics of primary producers have important implications for the structure and function of ecosystems. Along the wave-swept coastline of central California, USA, biomass removal by wave action is a key driver in the primary productivity of giant kelp forests, yet the mechanisms of regrowth within giant kelp Macrocystis pyrifera are not well understood. To examine the physiological consequences of biomass loss on Macrocystis, a manipulative experiment was used to simulate biomass removal by wave action. Growth rates were measured as the number of new fronds produced through time, and the δ13C and δ15N values of juvenile fronds were used as a proxy for carbon and nitrogen translocation in support of growth. The experimental removal of biomass significantly constrained the growth of new fronds and, under extreme levels, led to mortality. The growth rate and isotopic composition of juvenile fronds on sporophytes with a portion of canopy biomass intact recovered to pre-disturbance values within 4 mo. In contrast, a reduction in growth rates as well as a permanent depletion in δ13C and δ15N values was observed when the canopy was completely removed and the magnitude scaled with biomass loss. These results suggest that the translocation of carbon and nitrogen to juvenile fronds from nearsurface biomass is a critical process affecting growth in giant kelp. The spatial variability and physiological consequences of biomass loss among individuals may therefore play an important role in the biomass dynamics of giant kelp forests across multiple temporal and spatial scales.