Nutritional quality of giant kelp declines due to warming ocean temperatures

Giant kelp Macrocystis pyrifera forms extensive forests on temperate reefs, providing habitat and food for a diversity of marine life. Kelp biomass varies in response to changing ocean temperatures, but physiological responses as reflected in the nutritional quality of kelp tissue are poorly understood. Over a 19-year period in southern California, we found that nutritional quality of giant kelp tissue declined; nitrogen content of giant kelp tissue declined by 18%, while carbon content proportionally increased. This decline in nutritional quality was associated with increasing seawater temperatures and with regional and local scale processes including upwelling as indicated by the biologically effective upwelling transport index, the El Nino-Southern oscillation and the North Pacific Gyre oscillation. Changes in kelp stoichiometry with seawater temperature have important implications for nutrition and behavior of key consumers, such as sea urchins. Our results suggest that the consequences of projected declines in kelp abundance due to climate change may be compounded by reductions in its nutritional quality.

Automated summary

The nutritional quality of giant kelp tissue in southern California has been found to decline over time, primarily due to increasing seawater temperatures and regional/local scale processes. This decline in nutritional quality has important implications for the nutrition and behavior of marine organisms, particularly key consumers such as sea urchins. The projected declines in kelp abundance due to climate change may be compounded by reductions in its nutritional quality, indicating potential further ecological consequences.