iiUSGS science for a changing world

Global Change and Submerged Aquatic Vegetation Research

Communities of submerged aquatic vegetation (SAY) are important components of many freshwater, brackish, and marine aquatic ecosystems. They prevent erosion by baffling the impacts of waves, especially from storms. These aquatic plant communities remove nutrients and other pollutants from river and runoff inputs to coastal areas, preventing their entry into surrounding waters. They provide nursery habitat for fish, shrimp, and other species, as well as forage for wintering water­ fowl and endangered species such as sea turtles and manatees. Unfortunately, not only have the distribution and abundance of seagrasses in the northern Gulf of Mexico declined precipitously during the past 50 years, most notably from widespread deterioration of water quality, but submerged aquatic plant communities are also susceptible to long-term environmental changes that are predicted to accompany global climate change.

The direct effect of temperature on rapidly deteriorating. plant physiology may ultimately Research has begun to yield influence community composition and information on the responses of latitudinal species occurrence. submerged aquatic vegetation commu­ Increases in the concentration of nities to some of the environmental dissolved inorganic carbon, including consequences expected to accompany both carbon dioxide and bicarbonate, global change. may affect submerged plants directly and may have indirect effects by Seagrass Disturbance increasing the growth of attached and Overwashes from storms frequently suspended algae. deposit sandy sediments in seagrass Global sea level has risen between beds growing leeward or behind 10 and 25 em over the past 100 years. protective barrier islands, such as the The Intergovernmental Panel on Chandeleur Islands of Louisiana (Fig. Climate Change (IPCC, 1995) expects 1). In Chandeleur Sound, turtle grass that average sea level will continued to ( Thalassia testudinum) inhabits increase as a result of thermal expan­ nutrient-rich protected areas, while sion and melting of glaciers and ice manatee grass (Syringodium filiforme) sheets. Models using the IPCC's "best dominates sandy areas subject to estimate" values of climate sensitivity frequent overwash. Increased storm and ice melt suggest an increase in sea disturbances and overwash will alter Fig. 1. Storm overwash fan deposited level of about 50 em from the present the community composition of gulf on a seagrass bed in the Chandeleur to 2100. Continued rise in sea level seagrass beds by promoting increases Islands, Louisiana. will increase the depth of coastal in the abundance of manatee grass. waters and will cause inland and Since different species of seagrass Effects on Plant upstream salinity intrusion, both of support different food webs, changes Photosynthesis in the community composition of which will affect submerged vegeta­ As atmospheric carbon dioxide seagrasses will be propagated into tion. While there is uncertainty increases with global change, concen­ changes at higher levels in the food regarding the influence of global trations of dissolved carbon dioxide chain. warming on the frequency and and bicarbonate are expected to Frequent severe overwash and intensity of storm events, sea-level increase in aquatic ecosystems. The barrier island erosion may even lead rise alone has the potential for photosynthetic responses of some to total loss of all seagrasses from an increasing the severity of storm surge, submerged aquatic vegetation species area. particularly in areas where coastal to additions of carbon dioxide were habitats and barrier shorelines are measured in the laboratory. Photosynthetic capacity increased by consumer organisms such as wintering as much as two to eight times ambient waterfowl. levels during certain times of the year Effects of Increased in three freshwater species- wild­ Salinity celery ( Vallisneria americana), coontail ( Ceratophyllum demersum ), As sea level rises, many communi­ and hydrilla (Hydrilla vertic illata)­ ties of fresh and brackish submerged and in a seagrass species, shoalgrass aquatic vegetation will experience (Halodule wrightii). Only widgeon salinity increases. In some cases, an grass (Ruppia maritima) from an area inland migration of communities rich in dissolved carbon did not might be possible, but in other cases respond to additions of carbon salinity stress will change the commu­ dioxide. Alteration of photosynthesis nity composition. Where salinities can lead to changes in plant growth, increase from freshwater to which may disrupt species dependent oligohaline (0.5-5.0 parts per thou­ on these aquatic plants by altering Fig. 2. Wild-celery and shoalgrass sand), community composition will be plant biomass and seasonal timing of mesocosms receiving enhanced dominated by species which are strong biomass availability. atmospheric concentrations of carbon competitors for light, nutrients, and dioxide. other factors. Two nuisance exotic Elevated Carbon Dioxide species, milfoil (Myriophyllum and Growth of Submerged spicatum) and hydrilla, often Vegetation much that the host plant cannot outcompete other species at low Freshwater wild-celery and survive. Both species produced more salinities (Table). Increased hydrilla shoalgrass were grown for 6 weeks in root and rhizome biomass relative to can lower water quality and oxygen mesocosms (Fig. 2) receiving four leaf biomass. This change in biomass concentrations, possibly causing fish times the atmospheric concentration production suggests that climate kills. Under mesohaline (5-18 parts of carbon dioxide. While biomass did change would favor grazing organisms per thousand salinity) conditions, not increase in either species, other that preferentially consume below­ community structure will be strongly effects were found. Biomass of ground tissue. Leaf tissues of wild­ governed by the salinity tolerances of epiphytes (plants depending on celery had higher ratios of carbon to the species. Salt-tolerant species such another plant for mechanical support) nitrogen than plants grown under as wild-celery, widgeon grass, and on shoalgrass increased significantly. normal carbon dioxide concentrations. sago pondweed (Potamogeton Excess epiphytic growth has been High carbon-to-nitrogen ratios tend to pectinatus) are predicted to dominate shown to reduce light to plants so provide poorer quality forage for submerged aquatic vegetation commu­ nities at higher salinities. Salinity­ Table. Potential changes in community composition of various species of submerged induced changes in community aquatic vegetation as a result of increased salinity. Given this assemblage of species, those species marked with an asterisk are expected to dominate the community. composition will induce changes in animal communities adapted to A. Potential community composition determined from maximum salinity tolerances of individual species. An X indicates species presence at the given salinity. specific plant associations. Salinity %a 0 4 8 16 Hydrilla X X Grassleaf mud-plantain X X X Milfoil X X X X Wild-celery X X X X Sago pondweed X X X X For more information, contact Widgeon grass X X X X Hilary A. Neckles B. Predicted community structure based on salinity tolerance and competitive Glenn R. Guntenspergen performance. William M Rizzo Salinity %a or 0 4 8 16 Thomas C. Michot U.S. Geological Survey Hydrilla * X National Wetlands Research Center Grassleaf mud-plantain * * X 700 Cajundome Blvd. Milfoil * * X X Lafayette, LA 70506 Wild-celery X * X X 318-266-8633 Sago pondweed X X * * [email protected] Widgeon grass X X * * http:/ /www.nwrc.gov

U.S. Department of the Interior USGS FS-090-97 U.S. Geological Survey ~ June 1997