Effects of warming seawater temperatures on the phenology of economically valuable brown macroalgae

An understanding of brown macroalgae phenology is crucial to the growing seaweed industry in the U.S. as reproductive timing determines when sustainable wild harvest can begin or when nurseries can be established. Temperature is a critical environmental trigger for development of macroalgae reproductive tissues. The Gulf of Maine is one of the fastest warming regions in the world, making it an ideal location for our study. We focused on one intertidal and two subtidal algae species and applied different observational techniques to each. We used the point-intercept method to determine the proportion of the intertidal rockweed (Ascophyllum nodosum) population that had generated reproductive sacs at two sites in mid-coast Maine. We augmented our field data with the citizen science database, Signs of the Season Coastal Monitoring, which has reported A. nodosum fecundity along the northeast coast since 2014. For the subtidal kelps, Saccharina latissima and Laminaria digitata, we used a quadrat transect method on SCUBA to observe the proportion of the population that had developed sorus tissue at 9 sites from 44°43’26.12’’N to 43°43’26.12’’N. In either habitat type, temperature data were also recorded at the time of observation. We found the optimal temperature for A. nodosum reproduction ranges from 8.1 to 10°C. Since 2014, the reproductive peak for A. nodosum has occurred in April or May across the Gulf of Maine, but we found the peak is occurring later each year in southern Maine. In 2019, subtidal kelp in Maine had a reproductive optimum at 13.75°C; this was when the largest proportion of S. latissima was undergoing sporogenesis. Generally S. latissima was more fecund than L. digitata, despite reports that L. digitata is most fertile during the summer and S. latissima is more fertile in the fall. Predictive capacity for shifts in the timing and intensity of reproductive peaks for macroalgae associated with changes in seasonal temperatures may be critical for maintaining a sustainable seaweed industry.