04 - Aquatic Plants of the Chesapeake Bay



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Common Aquatic Plants in Chesapeake Bay






In the 1950's and 1960's there used to be extensive beds of aquatic grasses along every shore in the Chesapeake Bay. They covered the bottom from 1 foot below mean low water out to 8 or nine feet of water. An estimated 600,000 acres of grass lined the shore, so much that it was then considered a nuisance. Homeowners drug bedsprings through the water to rip up the grass so they could swim and run outboards through the wide and thick mats. Now we typically have less than 40,000 acres of grasses.

Grasses provided the cover and food for fish and crabs. They were a safe place for crabs to molt. They filtered the bay water resulting in much better visibility than we have today now that the grasses have largely disappeared. Back then it was not unusual to be able to see 4 or 5 feet to the bottom at the height of summer. Now in most places visibility is under one foot.

At the head of the bay on the Susquehanna flats, there was a massive aquatic grass "meadow" miles long and a mile wide. Today there are few clumps of grasses struggling to reclaim their territory. Not only is the clarity of the water diminished, but the nursery for small fish, young fish and molting crabs has been greatly diminished.

All is not yet lost as there are places were grass beds remain. I am writing this page after a kayak paddle from Gunpowder State Park on Dundee Creek in which there is a thick mat of grass along all of its shores. The water clarity in Dundee creek contrasts strongly with that in the Gunpowder itself which is mostly devoid of grasses. On this quiet day, we were able to glide over the grass mats , observe the different grass types, watch the little fish dart about, see clam shells on the bottom and see the bottom at 6 feet.


Having enjoyed the afternoon among the grasses I turned to the internet to identify the grass types we had encountered, and to gather material for a web page. I quickly found a very complete on informative site on Chesapeake Bay Aquatic Grasses. It is so extensive that there is no reason to extract anything for this page so I simply refer you to it.

In addition, here is a web site from the University of Florida for identification of many different types of underwater grasses in regions other than just the Chesapeake bay.

Th aquatic grasses we saw in Dundee Creek were: Eel grass, Eurasian Milfoil and Hydrilla

Lorie W. Staver and J.Court Stevenson, Center for Environmental and Estuarine Studies, University of Marvland System, Horn Point Environmental Laboratory, Cambridge, MD

In the early 1980's Hydrilla verticillata was introduced to Chesapeake Bay in both the Potomac River and at Susquehanna Flats near Havre de Grace. Hydrilla is an effective competitor with native submersed aquatic vegetation (SAV) and initially invaded the landward margins of existing beds as soon as water temperatures warmed in early summer. As the growing season progressed Hydrilla invaded into deeper areas by sending out lateral shoots along the bottom. Eventually, vertical shoots emerged from the bottom shoots forming dense canopies which limited the light to competitors. Not only does Hydrilla have higher productivity (5.4 mg 02gzdw-1h-1) in shallow waters than native SAV, but it can more effectively use bicarbonate ions as an inorganic carbon source than competitors. This physiological adaptation is especially important since pH in the surface waters of dense Hydrilla canopies often reach a pH of 9.0 - 10.5 in late afternoon making CO2 unavailable as a carbon source. In deeper waters where pH is lower. native SAV, such as Vallisneria, have slightly higher productivity (3.8 mg 02 gzdw-1h-1) than Hydrilla (2.8 mg 02 gzdw-lh-1) and may be able to uptake phosphorus more efficiently which is often the limiting nutrient. Hydrilla was found to have little tolerance to salinity in mesocosms and only maintains itself in tidal freshwater sections of the Bay. However, in these areas Hydrilla has a very marked impact in the shallows. Not only is the dense vegetation important habitat for a variety of consumers (e.g. fish), Hydrilla can sequester large amounts of nutrients in its biomass (500-1000 g m-2) as the season progresses. Transects indicate that nutrients in the overlying waters may be lowered in the afternoon when beds are most productive and diel sampling of nutrients every two hours suggest Hydrilla beds can attenuate nutrient pulses from the riverine and land sources.

From Anne Arundel Community College SAV lab. Native grasses of the middle (mesohaline) Chesapeake Bay.





Guide to Identifying Underwater Grasses

Guide_to_Underwater_Grasses.pdf


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