Highly productive nearshore ecosystems in temperate regions such as the northern Gulf of Alaska (NGOA) are often supported by macroalgae and phytoplankton at the base of their food webs. Availability of these basal producers likely varies seasonally and with environmental factors, suggesting nearshore food webs within the NGOA may be susceptible to effects of climate change like warming waters, marine heatwaves, and rapidly melting glaciers. Our goal was to understand how changes in the proportional contributions of carbon from these two primary producer sources may affect performance of nearshore consumer species. We used carbon and nitrogen stable isotope analysis to investigate trophic use of filter-feeding mussels (Mytilus trossulus), pelagic-feeding Black Rockfish (Sebastes melanops), and benthic-feeding Kelp Greenling (Hexagrammos decagrammus) in four different NGOA regions: Western Prince William Sound, Kenai Fjords National Park, Kachemak Bay, and Katmai National Park and Preserve. All three focal species used a mix of phytoplankton- and macroalgal-based pathways in each region but macroalgae were the major source of carbon in the diet of all three species. There was a significant increase of macroalgal-based contribution to mussels in 2017 compared to previous years, possibly reflecting the large pulse of detritus from the 2016 macroalgae die-off during the Pacific Marine Heatwave. We used annual growth increments in each of these consumers as a measure of performance across the four regions to better understand the effect of variation in primary production sources to secondary production. Results for mussels suggest that the relationship between growth and carbon sources varies by region, possibly related to regional differences in environmental variables, such as temperature. There was no relationship between carbon source and growth for Kelp Greenling and Black Rockfish but the range of macroalgal contribution values in the samples of both species was small and relationships may occur if tested over a larger geographic range with larger differences in macroalgal and phytoplankton contributions. The only exception was for Black Rockfish in Kachemak Bay, where growth increased with higher macroalgal contribution to the diet. Mussels during the Pacific Marine Heatwave showed significantly less growth compared to before and after the heatwave. With the high reliance on the macroalgal trophic pathway in all three nearshore consumers, coupled with the significant effect the heatwave had on both macroalgal contribution and growth in mussels, nearshore ecosystems may see big changes with the predicted increase in heatwave frequency and duration due to climate change.
