Date of Award

Fall 2026

Abstract

Eastern oysters (Crassostrea virginica) are ecologically and economically important reef-building bivalves that support estuarine habitats as well as United States fisheries and aquaculture farms. While eastern oysters occur subtidally throughout their range, intertidal oysters are uncommon north of the Chesapeake Bay. Recent research has shown that rockweed Ascophyllum nodosum may facilitate eastern oyster expansion into the intertidal zone at northern latitudes, but mechanisms underlying this interaction are poorly understood. Characterizing intertidal oyster survival and behavior is key to better understanding their distributions in the northeastern United States. In Chapter 1, we conducted a full-factorial field experiment across seasons in Great Bay, New Hampshire, to understand abiotic and biotic factors that contribute to rockweed facilitation of oysters. From May 2025 to January 2026, we examined effects of 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 adult oyster mortality was low across all treatments in summer, but increased greatly during winter, with mortality occurring due to extremely cold temperatures as well as crushing by ice scour. In winter, oysters were more likely to survive under rockweed cover compared to bare plots and were five times less likely to be crushed. Juvenile oyster (spat) mortality was not affected by rockweed cover and was driven by predation, possibly by non-native green crabs. Results in this chapter show that winter, not summer, is the most stressful time for adult oysters in this region, underscoring the importance of examining facilitative interactions across seasons. In Chapter 2, we conducted the first long-term (10-month), in situ monitoring experiment of eastern oyster valve gaping behavior using high-frequency sensors. In Great Bay, we measured the behavior of 48 oysters across subtidal, rockweed-covered intertidal, and bare intertidal habitats from June 2025 to April 2026. Bare intertidal oysters did not survive the winter, while nearly all rockweed-covered intertidal and subtidal oysters survived through the end of the experiment. We found that when oysters were submerged and temperatures were warm, valve gaping behavior was similar across habitats; however, intertidal oysters were more likely to be open compared to subtidal oysters when temperatures were cold during winter. These novel results from Chapter 2 reveal differences in intertidal and subtidal eastern oyster behavior across seasons. Findings suggest that in the winter, intertidal oysters may not enter the quiescent state that subtidal oysters do, possibly to ensure continued survival under harsh conditions. Together, these chapters shine light on environmental factors, habitat characteristics, and behavioral adaptations that influence intertidal eastern oyster survival and distributions in Great Bay, New Hampshire.

Document Type

Master's Thesis

First Advisor

Easton R White

Second Advisor

Brittany M Jellison

Third Advisor

David M Burdick

Department or Program

Biological Sciences

Degree Name

Master of Science

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