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Downeast ME, which is located within the northeastern-most part of the state, extends from the mouth of the Bay of Fundy (NS/NB/ME) to Mount Desert Island, ME. Its flora consists of 208 major macroscopic, multicellular, marine and estuarine algae, including 59 green, 70 brown, and 79 red algae. The number of species at 42 Downcast towns was highly variable, ranging from 118-128 taxa/town at open coastal and tidal rapid sites to 3-12 taxa at sheltered coastal and inner estuarine areas. Detailed studies at 27 individual sites between Eastport and Bar Harbor, ME identified 154 species. Seven open coastal and a Cobscook Bay tidal rapids within these 27 individual sites had the highest number of taxa (141 taxa, 91.6%). Fewer species (93 taxa, 60.4%) were found between Moose Neck and the Narraguagus River that had more sheltered coastal, estuarine, or riverine habitats with reduced salinities and higher surface water temperatures. Eight freshwater taxa were identified at the tidal headwaters of several rivers, including five green (Bolboehaete repanda, Mougeotia sp. Spirogyra sp., Stigeoclonium sp., and Zygnema sp.) and three red algae (Audouinella hermannii, Batrachospermum gelatinosum, and Lemanea fucina). The total flora from Downcast ME and eight other areas from the Bay of Fundy to the Great Bay Estuary consisted of 336 taxa. The number of total seaweed taxa and their occurrence values were highly variable, with the Bay of Fundy having the highest values (i.e. 253 taxa or 75.3%), Downeast the second highest (208 taxa or 61.9%), and Penobscot Bay the lowest (98 taxa or 29.1%). The percent of shared Downeast seaweeds varied from 62.7% (Penobscot Bay) to 85.9% (Bold Coast), while the number of shared taxa varied from 96 (Penobscot Bay) to 170 (Bay of Fundy). The seaweed flora from Downcast represents similar to 38% of the 535 seaweeds currently recorded from the NW Atlantic. The floristic ratios for the 42 Downcast towns and eight contiguous NW Atlantic areas had a cold-temperate ratio of 2.0. However, a few towns and individual sites had warm temperate ratios of 3.0-5.0 due to few brown algae resulting from low salinities and variable water temperatures. Nine warm-water or disjunct estuarine seaweeds were recorded from a few Downeast estuarine sites. In addition, eight introduced seaweeds were identified from Downeast ME or 25% of the 32 known introductions for the NW Atlantic, while 17 introduced taxa (53.1%) are known for Downeast ME plus the other eight areas.