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Ice age kelp forests: climate‐driven changes in kelp forest distribution since the last glacial maximum

Abstract

Kelp forest distributions are constrained by the availability of rocky substrate within the depth range tolerable for growth and reproduction, which can vary over relatively short geological timescales (millennia) due to interactions between coastal bathymetry and climate‐driven changes in eustatic sea level. Using GIS, a digital bathymetric map, sea level curves, and published kelp depth tolerances, I reconstructed changes in the size and distribution of giant kelp (Macrocystis pyrifera) forests in the Southern California Bight since the last glacial maximum. Reconstructions predicted that the total area of available kelp forest habitat for the California Channel Islands during the last glacial maximum (18.5 kyr BP; 628 square km) was greater than at present (382 square km) but less than at 16.5 kyr BP (1130 square km). Available kelp forest habitat along the southern California mainland also increased rapidly from 18.5 to 16.5 kyr BP but continued to increase with sea level rise. Differences in the effects of sea level rise on coastal geomorphology between the islands and mainland further constrained the extent of rocky substrate available to kelps. Given biomass and productivity estimates from present‐day kelp forests, these reconstructions suggest more productive and spatially extensive island kelp forests near the last glacial maximum than at present, but the opposite pattern for the mainland. These climate‐driven changes in kelp forest distribution and productivity likely had important historical impacts on the ecology and evolution of the present‐day kelp ecosystem including kelp forest exploitation by early human inhabitants of southern California.