The important role of seaweed in nearshore food webs

Seaweeds in the intertidal in the Gulf of Alaska.

U.S. Geological Survey/Sarah Traiger

In the highly productive nearshore waters of the North Pacific, macroalgae (seaweeds) play an important role in the transfer of organic material through the food web. We collected mussels (an intertidal bivalve), kelp greening (a benthic-feeding fish), and black rockfish (a pelagic-feeding fish) from several sites across the Gulf of Alaska. Using carbon and nitrogen stable isotopes, we examined the primary source of organic material, the impact on growth rates and whether there was a notable difference of the source of carbon during the Pacific Marine Heatwave (PMH; 2014-2016), and any potential impacts to growth.

Macroalgae was found to be the predominant organic matter source, contributing more than 60% to the diet of mussels, kelp greenling, and black rockfish. Macroalgae significantly influenced mussel growth, where a positive relationship between macroalgal contribution and growth was observed. However, the same relationship was not evident in the fish species. During the PMH, there was a noticeable decrease in macroalgal contributions to mussel diets. This corresponded with an observed decline in some macroalgal species during the PMH. This decrease in macroalgae contribution to the diet of mussels also corresponded with a reduction in mussel growth compared to other time periods.

In general, the findings suggest that macroalgae plays a crucial role in supporting cold-temperate nearshore food webs, potentially more so than phytoplankton. Therefore, conservation and management efforts focused on protecting and enhancing macroalgae habitats are likely to benefit these ecosystems, especially under changing climate scenarios and during marine heatwaves.

Relative Importance of Macroalgae and Phytoplankton to Nearshore Consumers and Growth Across Climatic Conditions in the Northern Gulf of Alaska

Abstract
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.

Corliss, K., V. von Biela, H. Coletti, J. Bodkin, D. Esler, and K. Iken. 2024. Relative importance of macroalgae and phytoplankton to nearshore consumers and growth across climatic conditions in the northern Gulf of Alaska. Estuaries and Coasts doi: 10.1007/s12237-024-01371-6