Relative importance of macroalgae and phytoplankton to nearshore consumers and growth across climatic conditions in the northern Gulf of Alaska

Macroalgae and phytoplankton support the base of highly productive nearshore ecosystems in cold-temperate regions. To better understand their relative importance to nearshore food webs, this study considered four regions in the northern Gulf of Alaska where three indicator consumers were collected, filter-feeding mussels (Mytilus trossulus), pelagic-feeding Black Rockfish (Sebastes melanops), and benthic-feeding Kelp Greenling (Hexagrammos decagrammus). The study objectives were to (1) estimate the proportional contributions of macroalgal and phytoplankton organic matter using carbon and nitrogen stable isotopes, (2) determine if macroalgal use affected consumer growth using annual growth rings in shells or otoliths, and (3) describe changes in organic matter use and growth during the Pacific Marine Heatwave (PMH; 2014–2016) in one consumer, mussels. Macroalgae were the major organic matter source (> 60%) to the diet for all three consumers. The relationships between macroalgal contribution and growth were neutral for both fish species and significantly positive for mussels. During the PMH, mussels had a drop (> 10%) in macroalgal contributions and grew 45% less than in other time periods. Simultaneously, the relationship between macroalgal contribution and mussel growth was strongest during the PMH, explaining 48% variation compared to 3–12% before or after the PMH. Collectively, the results suggest that macroalgae is likely more important to cold-temperate nearshore food webs than phytoplankton. Management actions aimed at conserving and expanding macroalgae are likely to benefit nearshore food webs under all climate scenarios and especially during marine heatwaves.