Aquatic Vegetation
Chlorophyll-containing vegetation, from trees to phytoplankton, supply critical needs to nearly all life on our planet. In addition to producing oxygen, these “primary producers” use water and energy from sunlight to fix carbon, converting gaseous carbon dioxide into a solid form that contains stored energy. The stored energy, as biomass, forms the base of the food chain and is then used by nearly all other organisms as an energy source. Vegetation communities can be either terrestrial or aquatic but all are controlled by the relative availability of water, nutrients, and light, with other limiting factors being temperature and available substrate.
Terrestrial vegetation is dominated by vascular plants and accounts for slightly more than 50% of the earth’s total primary production. The remaining production is from aquatic vegetation, dominated by oceanic microalgae or phytoplankton. Because of the availability of water and nutrients, terrestrial wetlands provide the highest density of fixed carbon in both soils and living biomass and serve to connect the terrestrial environment with the oceanic through the flows of water, essential materials, and organisms.
In addition to maintaining the atmosphere, wetlands provide for groundwater recharge to ‘meter’ supplies of freshwater and allow wetlands to persist though the dry season. Wetland vegetation slows water flow through friction, allowing suspended sediments to settle out, reducing supplies of solids, and associated nutrients and toxics to downstream regions, and accomplishing water purification. Wetlands can both take nutrients from solution to use for plant growth, and can slowly remobilize nutrients as microbial action breaks down plant debris and soils. Wetlands provide critical habitat for numerous critical species by supplying both physical refuges and food or food webs for both juveniles and adults. In addition to the benefits of diverse and productive wetlands, streams and estuaries, humans receive additional value from wetlands in the form of flood control and protection from erosion and storms. Wetlands are also directly used for recreation, and the production of foodstuffs and fuel.
The ecosystem functions provided by wetlands, however, can be exceeded. Too much water, sediments, nutrients, or toxics can overwhelm the metering or removal capabilities. Changes or loss of plant species can reduce habitat value; loss of wetland area can reduce the number of organisms supported. Excessive nutrients downstream can result in an overgrowth of nuisance species or phytoplankton blooms.
Seagrasses are one of the most downstream of the aquatic plant communities. As they respond to changes in water quality, they serve to integrate the effects of changes in the watershed and terrestrial wetlands. Increased nutrients supplied to an estuary generally result in increased drift macroalgae, increased epiphytic organisms growing on seagrass blades, and increased phytoplankton in the water column, all of which reduce the amount of sunlight available at depth, and result in the loss of the deepest grasses. Increased freshwater flow and the associated brown color of the water also reduce light at depth during high rainfall periods, making grasses sensitive to climatic cycles as well. As seagrasses are stationary, with comparatively modest seasonal variations, they are relatively easy to monitor and an effective ‘yardstick’ to gauge watershed and wetland improvements.
Submitted by: Kellie Dixon, Mote Marine Laboratory
