Seagrasses of North America
The North American continent spans many countries and experiences a broad range of ecosystems extending from tropical to arctic. Within this continent’s range, there are four regions for seagrass distribution throughout North America: Pacific coast, Atlantic coast, Gulf of Mexico and the Caribbean. Throughout these regions, there are 26 species of seagrass from 8 genera, with 3 of these species (2 genera: Cymodocea and Enhalus) found outside this range on United States Minor Outlying Islands. Two species of seagrass, Halophila hawaiiana (Hawaii) and Halophila johnsonii (Florida) are endemic to the United States and one species, Halophila bermudensis, is endemic to Bermuda.
The Pacific coast extends from the Baja Peninsula in Mexico north through Alaska. There are 8 species of seagrass found within this region (Halodule wrightii, Phyllospadix scouleri, Phyllospadix serrulatus, Phyllospadix torreyi, Ruppia maritima, Zostera asiatica, Zostera japonica and Zostera marina) covering approximately 1,000 km2; however, this area may be much larger.2 The surf grasses (Phyllospadix serrulatus, Phyllospadix scouleri and Phyllospadix torreyi) dominate the rocky subtidal and intertidal zones and are endemic to the northeast Pacific Ocean. Zostera marina is the dominant seagrass species in this region and is typically found growing in the subtidal, intertidal and estuaries in areas composed of soft substrates. This species of seagrass is found as far north as Alaska and is the only species of seagrass to extend into the Arctic Circle. Ruppia maritima is found in many of the brackish water environments throughout the entire Pacific coast region. Two species, Zostera asiatica and Zostera japonica were introduced to this region. It is unsure how Zostera asiatica was introduced to this area; however, it is thought that oyster shipments for oyster enhancement programs introduced Zostera japonica to the eastern Pacific.
Based on the distribution of seagrass along the Atlantic coast, this region can be further divided into three distinct subregions: the western north Atlantic, the mid-Atlantic and east coast of Florida. The western north Atlantic region spans from Quebec south to New Jersey. Zostera marina is the dominant seagrass in this area inhabiting a variety of benthic environments in the subtidal and intertidal zones in depths down to 12 meters. Ruppia maritima has a patchy distribution throughout this region and is common in brackish and fresh water environments. The mid-Atlantic region includes Delaware, Maryland, Virginia and North Carolina. Throughout this region there are numerous estuaries and barrier island coastal lagoons comprised of quartz sand and fine grain sediments with seagrass present in most of the shallow environments. Halodule wrightii, Zostera marina and Ruppia maritima are the predominant seagrass species in this region covering an area of 292 km2. 2 From North Carolina south to mid-Florida, seagrass is absent from the Atlantic coast and reappear from Mosquito Lagoon south through Biscayne Bay. There are 7 species of seagrass found along Florida’s east coast covering an area of approximately 2,800 km2 with most of this area located in the Indian River Lagoon. These species include Thalassia testudinum, Syringodium filiforme, Halodule wrightii, Halophila decipiens, Halophila engelmannii, Halophila johnsonii and Ruppia maritima.2 Halophila johnsonii is the only seagrass to be listed on the Endangered Species list and is endemic to Florida. It is only found along the east coast from Sebastian Inlet south to Virginia Key. Thalassia testudinum is a climax species in this region and is one of the more common seagrasses on the east coast of Florida, along with the pioneer species, Syringodium filiforme and Halodule wrightii. Ruppia maritima is very common in the northern limits of this region found predominantly in the Indian River Lagoon.
Gulf of Mexico
Seagrass distribution throughout the Gulf of Mexico spans Florida, Alabama, Mississippi, Louisiana and Texas in the United States, Cuba and Mexico. Approximately 9,888 km2 of seagrass was measured along the Gulf coast of Florida in 1995.2 However, this area of coverage did not include the large, deep meadows of Halophila decipiens in Big Bend which would account for an additional 20,000 km2. Florida Bay and the Big Bend region account for the majority of seagrass coverage on Florida’s Gulf coast with the mid-region (Lee County to Pinellas County including Tampa Bay) and the Florida Panhandle making up the remaining coverage. Florida Bay has experienced the greatest declines in this region mainly due to mechanical damage from recreational boating activities. Alabama’s Gulf coast does not support a vast area of seagrass, however, Halodule wrightii has been observed in Mobile Bay, as well as occasional observations of Ruppia maritima. Seagrass species diversity used to be relatively high in Mississippi with the presence of Halodule wrightii, Halophila engelmannii, Ruppia maritima, Syringodium filiforme and Thalassia testudinum. After the late 1960’s, most of this seagrass was lost and now only small densities of Halodule wrightii and Ruppia maritima remain. While the majority of Louisiana’s Gulf coast is dominated by fresh and brackish marshes, Halodule wrightii, Halophila engelmannii, Ruppia maritima, Syringodium filiforme and Thalassia testudinum are present and well established in the Chandeleur Islands in the southeast bayous of Louisiana. Unlike the patchy distributions along the Gulf coasts of Alabama, Mississippi and Louisiana, Texas has approximately 951 km2 of seagrass along its Gulf coast. Halodule wrightii is the dominant species in this region with Halophila engelmannii and Ruppia maritima occurring in lesser quantities. Thalassia testudinum is dominant along the western edge of the Galveston Bay system with Syringodium filiforme the dominant species at Laguna Madre. Continuing west along the Gulf coast, seagrass is present from the northeastern most boundary of Mexico south to the Yucatán Peninsula. Total seagrass coverage for Mexico is approximately 500 km2 and includes Thalassia testudinum, which is the dominant seagrass in this region, Halodule wrightii, Syringodium filiforme, Halophila decipiens, Halophila engelmannii and Ruppia maritima.
The Caribbean makes up the final region within the North American seagrass distribution range. The Caribbean region can further be subdivided into the western Caribbean, southern Caribbean and the Lesser and Greater Antilles. There are seven seagrass species recorded through the Caribbean: Thalassia testudinum, Syringodium filiforme, Halodule wrightii, Ruppia maritima, Halophila baillonii, Halophila decipiens and Halophila engelmannii. These seagrasses are typically found in the reef lagoons, as well as forming thick meadows in sheltered bay and estuaries. Thalassia testudinum is the dominant seagrass species; however, it only occurs north of Venezuela. Syringodium filiforme and Halodule wrightii have a wide distribution and are fairly common throughout the Caribbean. Ruppia maritima is common throughout the shallow, brackish bays of this region. The three Halophila species are wide spread throughout the Caribbean. Halophila decipiens is distributed throughout the deeper waters, up to 30 meters in this region, with Halophila engelmannii found in the shallow waters of the Bahamas, Florida and the Greater Antilles and Halophila baillonii restricted to the Lesser Antilles.
1den Hartog, C. (1970) The sea-grasses of the world. Verhandelingen der Koninklijke Nederlandse Akademie van Wetenschappen, afd, Natuurkunde, Tweede Reeks 59: 1-275.
2Green E.P and Short F.T. (2003) World Atlas of Seagrasses. Prepared by the UIMEP World Conservation Monitoring Centre, University of California Press, Berkeley, USA
3Phillips, R.C. and Menez, E.G. (1988) Seagrasses. Smithsonian Contributions to the Marine Sciences: 34-89.