Distribution of Rockweed Macroalgae (Ascophyllum and Fucus Spp.) and Associated Eastern Oysters (Crassostrea virginica) in the Great Bay–Piscataqua River Estuary, New Hampshire.

McKown, J.G.; Grizzle, R.; Burdick, D.M.; Moore, G.E.; Ward, K., and White, L.E., 2026. Distribution of rockweed macroalgae (Ascophyllum and Fucus spp.) and associated eastern oysters (Crassostrea virginica) in the Great Bay–Piscataqua River Estuary, New Hampshire. Intertidal macroalgae known as rockweed (Ascophyllum nodosum and Fucus spp.) occur on cobble and rocky outcrop shorelines in northern New England. Eastern oysters (Crassostrea virginica) have long been thought to occur only in subtidal waters in northern New England but recently have been observed in the intertidal zone of the Great Bay Estuary, New Hampshire (GBE) underneath rockweed canopies that likely provide buffering from thermal extremes and desiccation during low tides. To better characterize this rockweed-and-oyster habitat in GBE, a remote sensing and field study was conducted to (1) map the distribution of rockweed, (2) survey intertidal oyster and other bivalve populations associated with rockweed, and (3) assess potential environmental drivers of rockweed and oyster distributions. Pixel-based classification with manually applied thresholds of common spectral indices and elevation were used on publicly available aerial imagery for the entire GBE (70.4 km2) to estimate rockweed coverage. Intertidal bivalves and rockweed characteristics were surveyed at 24 random sites throughout the estuary. Rockweed covers approximately 144 acres in GBE (κ = 90.5%), distributed primarily in Portsmouth Harbor, Piscataqua River, Little Bay, and Great Bay. Rockweed coverage dropped substantially moving up tidal rivers, where water salinity would remain at or below 10 psu for longer than 10 days consecutively. Intertidal oysters were primarily found in Great Bay, Little Bay, and Oyster River, with densities ranging between 0.6 and 10.0 m−2. Multiple size classes suggested successful spat colonization over the past 5 years. At the estuary scale, oysters were found only within 1 km of oyster farms and natural reefs. The intertidal oyster population was estimated at 647,000 ± 65,000 individuals, or roughly 3% of the previously mapped subtidal population. Rockweed should be considered a priority estuarine habitat for management based on its wide distribution and ability to facilitate oyster expansion into the intertidal zone. [ABSTRACT FROM AUTHOR]