Title:

A peek under the canopy: Does rockweed facilitate intertidal oyster spread in Great Bay?

The rockweed Ascophyllum nodosum is a prominent foundation species in the northeastern United States intertidal zone, where it relieves temperature and desiccation stress for organisms beneath its canopy. In recent years, a novel association between rockweed and eastern oysters (Crassostrea virginica) has been observed in the intertidal zone – before then, eastern oysters were thought to exist only subtidally. In Great Bay, NH, we examined the abiotic and biotic factors underlying how rockweed facilitates eastern oyster expansion into the intertidal zone. We conducted a full factorial field experiment from May 2025 through January 2026 with two levels of rockweed cover (covered by rockweed and bare) and three levels of predator exclusion (full exclusion cage, partial cage, and no cage). We found that rockweed increased relative humidity (13.5 percentage points higher) and reduced temperature ranges (3.9°C) compared to bare plots. Adult oyster mortality was low across all treatments in summer, but rose to 29.5% by January 2026, with mortality occurring due to extremely cold temperatures as well as crushing by ice scour. Rockweed cover increased probability of oyster survival by 10% compared to bare plots and decreased likelihood of being crushed by 5x. Mortality of spat was not affected by rockweed and was driven by predation, likely by non-native green crabs. Our work demonstrates the importance of conducting facilitation studies across seasons, as most impacts were observed in winter rather than summer. This study may also inform management of the rockweed “fishery” in Maine, where this oyster-rockweed association has also been observed.

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