A Synopsis of Introduced and Native Seaweeds from 14 Open Coastal and Estuarine Sites within Southern Maine and New Hampshire, U.S.A.

ABSTRACT Floristic studies of introduced and native seaweed populations from 14 open coastal and estuarine sites within southern Maine and New Hampshire were documented between 1965 and 2017. A total of 186 seaweed taxa were recorded, including 15 introduced (8.1%) and 171 native (91.9%) species. The highest species diversity (123 taxa) occurred at the open coastal Seapoint Beach site in Kittery, Maine, and an estuarine tidal rapid site at Dover Point, New Hampshire, with 106 taxa. The numbers of introduced species per site ranged from 2–11. The percent occurrence patterns for the 15 introduced seaweeds were highly variable, with Codium fragile subsp. fragile and Ulonema rhizophorum being restricted to single sites (7%), Melanothamnus harveyi occurring at 12 sites (86%), and Agarophyton vermiculophyllum and Dasysiphonia japonica at 13 sites (93%). The geographical origins and initial collection dates for the 15 introduced taxa were highly variable, with their initial collections ranging from 1848 (M. harveyi) to 2007 (D. japonica). Codium fragile subsp. fragile had the most protracted period between its initial occurrence at Orient Point, New York, in 1957 and in northern New England 40 years later (i.e., 1997). By contrast, several other introduced taxa had rapid geographic expansions within five years. The numbers and abundance of introduced species in the Gulf of Maine have increased dramatically between 1986 and 2017, with nine recorded in 1986 and 15 in 2017. Two native warm-water red algae (Agardhiella subulata and Gelidium crinale) showed sudden northward expansions during 2016 and 2017, respectively. Their presence may be associated with global warming, which is presently occurring at an alarming rate within the Gulf of Maine and exceeds many areas within the world.