What controls SAV distribution in Chesapeake Bay?
SAV is distributed throughout the Chesapeake Bay and its tributaries based on several factors.
Light Availability Light is the single most important factor affecting bay grass growth. Photosynthesis, the process in which plants convert carbon dioxide (CO2) from the atmosphere into a usable form of energy, is a light-dependent process. Therefore, there must be adequate light penetrating through the water column for bay grasses to grow. The light available for underwater plants is affected by two primary factors:
- Nutrient pollution: Nutrients - nitrogen and phosphorus - that get washed into the Bay from the watershed can have an impact on water quality. While underwater grasses need nutrients to grow, too many available nutrients in the water column promote the growth of phytoplankton, epiphytes, and macroalgae. Phytoplankton blooms in the water column and blocks light from reaching the SAV growing on the bottom. Epiphytes are small plants or algae that grow directly on the leaves of SAV, preventing them from accessing sunlight and photosynthesizing efficiently. Macroalgae come in a variety of shapes, sizes, and colors, and like phytoplankton and epiphytes, are a natural and beneficial component of coastal environments, but when they grow in abundance in response to high nutrient levels, they can shade out or smother Bay grasses and cause other problems that affect ecosystem balance.
- Sediment pollution: When unconsolidated sediments wash into creeks, streams, and rivers, they eventually end up in the Bay. Depending on the type of sediment and grain size, wind energy, flow, and a number of other conditions, they can either settle to the bottom quickly or stay floating in the water column. Sediment that remains in the water column for a long period of time, or that gets resuspended into the water column from a wind event or physical disturbance to the Bay bottom, can create cloudy - or turbid - conditions that block sunlight from reaching the bay grasses growing on the bottom. Sediment loading from the watershed is caused by activities like deforestation, where too many trees are removed from an area and the soil they were rooted in becomes loose, or construction projects that move large amounts of soil around. When these unconsolidated sediment sources aren’t contained, they can wash into the Bay when it rains.
Due to light limitations in Chesapeake Bay, most of our SAV beds are found in shallow water habitats less than about six feet deep.
Salinity Salinity in Chesapeake Bay ranges from fresh to almost as salty as ocean water. In the southern part of the Chesapeake, where there’s the most oceanic influence at the mouth of the Bay, salinity generally stays above 18. This area is called the Polyhaline Zone. As you move up the Bay and the ocean water mixes with more freshwater from the rivers that feed into the Bay, the salinity is considered moderately salty and ranges from 5-18. This area is called the Mesohaline Zone. Still further up the Bay is the Oligohaline Zone, where the water is only slightly salty and ranges from 0.5 to 5. Areas with less than 0.5 salinity are considered Tidal Fresh.
Most of the SAV species found in Chesapeake Bay - over a dozen - are freshwater plants. These species live in the Tidal Fresh and Oligohaline Zones. This environment is most heavily influenced by freshwater flowing into the Bay from its rivers, so it maintains a more stable freshwater environment that allows for more diversity in its species composition. The Mesohaline Zone, where salinity fluctuates the most, is more stressful to plants, so has lower diversity. There are only four species of Chesapeake Bay SAV that thrive in this zone. Finally, the Polyhaline Zone is the saltiest area of the Bay, and is where the Bay’s only true seagrass - eelgrass - is found. Widgeon grass, the most widespread species in the Bay, is also abundant in the Polyhaline due to its very broad salinity tolerance.
The salinity of the Bay is constantly fluctuating and is dependent on things like tides, precipitation, wind, and river flow. Freshwater from the upper Bay and its tributaries pushes further south toward the mouth of the Bay when there’s a lot of rain. Likewise, when it’s dry for long periods of time, salty water pushes further up the Bay and increases salinity. Changes in salinity can lead to changes in SAV species distribution.
Temperature Rising temperatures can create stressful conditions for SAV and sometimes shifts where it can grow. Some species do well in warm water while others, like eelgrass, depend on cooler temperatures, making them vulnerable to heat events. Because of increasing Bay water temperatures, the Bay risks eventually losing much of its eelgrass. Combined with poor water clarity, conditions can be even more stressful for eelgrass, so it’s important to keep Bay waters as clean and clear as possible!
Nutrient Pollution Like all plants, bay grasses need a certain amount of nutrients, including nitrogen and phosphorus, to grow. However, excess nutrients can cause adverse effects and bay grasses can suffer as a result. Excess nutrients in the water column promote the growth of algae, which reduces or blocks the amount of light available for bay grass survival. See Light Availability above for more information.
Algae Nutrients encourage the growth of plants. However, excess nutrients can cause increases in phytoplankton, epiphytes, and macroalgae. When algae grows out of control, the water becomes cloudy and blocks the light needed by bay grasses. When algae grows directly on the leaves of bay grasses as epiphytes, it further reduces the amount of light received by the plants. See Light Availability above for more information.
Suspended Solids Sediment particles scatter and absorb light as it passes through the water column. These particles come from land and disturbances to the bottom of the bay. Turbidity measures the clarity of water and the degree to which suspended solids scatter or absorb light. In areas of high turbidity, or increased suspended solids, bay grasses do not receive adequate light for survival. See Light Availability above for more information.
Physical Disturbance Bay grasses do not typically grow in areas with heavy wave action due to bottom scouring and resuspension of sediments. Strong storms, boat propellers, anchors, and dredging can also tear up SAV beds, fragmenting or destroying the plants and their root and rhizome systems.
Sediment Type Bay grasses require sediment to anchor their roots in and provide sufficient nutrients. Although preferences for particular sediment types vary among species, most prefer more stable sediments composed of sand or mud with low organic content.
