Trophic pathways and their relationship to growth in nearshore consumers across the northern Gulf of Alaska

Abstract

Highly productive nearshore ecosystems in cold-temperate regions, such as the Northern Gulf of Alaska (NGOA), are supported by macroalgae and phytoplankton at the base of their food webs. Biomass of these primary producers varies seasonally, suggesting that longer-term environmental change could also shift biomass and/or range of basal producers. Our goal was to determine how proportional contributions of organic matter from macroalgae and phytoplankton to the diet of nearshore consumer species in the NGOA vary in space and time, and if these differences affect consumer growth. We used carbon and nitrogen stable isotope analysis to investigate diet sourcing of filter-feeding mussels (Mytilus trossulus), pelagic-feeding Black Rockfish (Sebastes melanops), and benthic-feeding Kelp Greenling (Hexagrammos decagrammus) in four different NGOA regions: Katmai National Park and Preserve, Kachemak Bay, Kenai Fjords National Park, and Western Prince William Sound. All three focal species used a mix of phytoplankton- and macroalgal-based pathways in each region; however, macroalgae were the major source of organic matter in the diet of all three species. We used recent (last annual) growth recorded by shells or otoliths 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 on secondary production. The relationships between organic matter source and growth were neutral to positive for all three focal species, and only significantly positive for mussels, and for Black Rockfish in a single region. As the values for macroalgal contribution were almost always greater than 50 %, the total range of macroalgal contribution in comparison to growth was small and showed that, across this range, one source did not consistently support higher growth. The Pacific Marine Heatwave (PMH) with heat spikes in 2014-2016 and 2019 provided a natural experiment to test the effects of strong climate variation on source contributions to mussels and their growth. Mussels had high variation in macroalgal contribution to their diet, with the lowest contributions during the PMH heat spikes and highest after. This could reflect lower macroalgal primary production during PMH, but a subsequent large pulse of macroalgal detritus from a documented macroalgal die-off late in the PMH may have supplied ample food for mussels after the PMH. Mussels also grew slowest during the PMH compared to the years before and after. While macroalgal contributions were also low at this time, mussel growth rate was more strongly related to macroalgal contributions during the PMH than before or after. Therefore, macroalgal production could be an especially important pathway to support coastal consumers during specific climate regimes. With the high reliance on the macroalgal trophic pathway in all three nearshore consumers, coupled with the significant effect of the PMH on both macroalgal contribution and growth in mussels, we should expect changes in nearshore food-web pathway use and consumer growth in relation to future climate variations. Results here suggest that some of these effects could be muted in the nearshore system based on flexibility in consumers to rely on different primary producer pathways but clarifying relationships among climate, trophic pathways, and growth is important, as such changes in nearshore species could have cascading impacts on many higher trophic level predators.