Synthesis of U.S. Geological Survey Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management

Chapter 11: Submerged Aquatic Vegetation and Water Clarity
By Nancy B. Rybicki and Jurate M. Landwehr

Underwater grasses, known as submerged aquatic vegetation (SAV), provide food for waterfowl populations as well as vital habitat for juvenile fish and shellfish. Historically, the Chesapeake Bay supported a diverse and abundant community of SAV; however, the acreage has declined substantially since the 1960s. The decline has been linked to poor water clarity due to a combination of increased suspended sediment and persistent algal blooms. The CBP has a goal to double the number of SAV acres by 2012. The USGS summarized its findings related to (1) water clarity, and (2) the influence of exotic species on SAV acreage.

USGS research on SAV minimum light requirements has identified the water-clarity conditions needed to support SAV in different salinity zones of the Bay. The minimum light requirements, defined as the amount of surface light reaching the bottom, are 13 percent for the freshwater SAV community and 22 percent for the more brackish waters (Carter and others 2000; Kemp and others, 2004). Many fluctuating factors, such as quantity of river flow and suspended matter in the water column, contribute to the variability in water clarity (fig. 11.1). To determine which water column constituents best explain variation in water clarity during the SAV growing season (April to October), the USGS analyzed factors influencing water clarity at 63 mid-channel water-quality-monitoring stations (fig. 11.2) throughout the Chesapeake Bay (Landwehr, 2005). The analysis indicated that the most important factor affecting water clarity is total suspended solids (TSS), which includes organic matter (phytoplankton, other planktonic organisms, bacteria, and organic detritus) and inorganic solids (clay, silt, and sand). For the Potomac River and the eight major tributaries, TSS was the primary explanatory variable for water clarity at 54 of the 63 stations. At eight stations in the more saline portions of the York, Rappahannock, Patuxent, and Choptank Rivers (fig. 11.2), chlorophyll-a concentration (an indicator of phytoplankton biomass) was the primary explanatory variable. Assuming that the inorganic component of TSS is greater than the organic component in most regions of the Bay and that the attenuation from inorganic solids exceeds attenuation from organic solids (Cerco and Moore, 2001), these results indicate that strategies to reduce sediment loads could improve water clarity more than strategies to reduce nutrient loads in most locations. At the other eight locations, these data would indicate that water clarity could improve with nutrient reduction and subsequent reduction in phytoplankton.

Figure 11.1. Conceptual diagram of factors affecting water clarity. Impacts of sediment, nutrients, algal blooms, and epiphytic growth on submerged aquatic vegetation (SAV) can affect the amount of sunlight reaching the plants. USGS research on the light requirements for SAV in different salinity zones was used to help set the water-quality standards in the estuary.

Figure 11.2. Water-clarity monitoring sites in the Chesapeake Bay estuary. Investigations have shown that factors affecting water clarity vary in different areas of the estuary. The results indicate that managers need to further utilize information about the primary cause of degraded water clarity to better focus sediment- and nutrient-reduction strategies.

The USGS also analyzed information from both shallow water sites (nearer the shoreline) and mid-channel sites (further from the shoreline) to assess factors affecting water clarity in these different areas. In 2002, the Maryland Department of Natural Resources (MD DNR), in partnership with the USGS, measured water quality at 10 shallow water sites within the Chesapeake and Maryland Coastal Bays (fig. 11.2). Regression analysis showed that in 2002, a dry and low-flow year, nutrients and organic suspended solids best explained light attenuation at the shallow water monitoring sites (Baldizar and Rybicki, 2006). These results indicate that nutrient reduction and subsequent reduction of organic solids would have a greater impact on water clarity than reduction of sediments (inorganic solids) during low-flow conditions. The regression analysis of the mid-channel data from the nine Bay tributaries showed a different result. The results of mid-channel analysis indicate that TSS is the dominant factor impacting water clarity at most sites in the estuary. Given these results, managers should remain focused on both sediment- and nutrient-reduction strategies to improve water clarity in the estuary. The results also indicate that additional data analysis is needed to evaluate factors affecting water clarity during other flow conditions.

The USGS also addressed the occurrence of invasive aquatic plants in the estuary (Rybicki and Landwehr, 2007). Exotics are expanding their range annually, yet few studies have summarized the conditions and impacts of this expansion within the context of water-quality restoration efforts. Hydrilla, the dominant exotic species in the upper tidal Potomac River, occurs in rivers, lakes, and estuaries throughout the world. The USGS conducted a long-term, quantitative study of SAV diversity following the colonization of hydrilla to the fresh and upper oligohaline part of the Potomac Estuary between Washington, D.C. and Maryland Point. Using information from annual field surveys and aerial photographs, USGS scientists created a database to document which species occurred in SAV beds in different sections of the Potomac River system. They recorded the percentage of total coverage and biomass each species attained annually. In comparing species coverage with water-quality composition, they discovered that, with the reduction of nitrogen concentration, hydrilla coverage expanded but so did the diversity of plant species. Hydrilla did not crowd out native species; indeed, native species increased. In addition, hydrilla is a good winter food source for waterfowl communities, which have increased significantly over this period.

Seagrass in Round Bay in the Severn River, Annapolis, Maryland. Photograph by Jane Thomas, IAN Image Library (www.ian.umces.edu/imagelibrary/).

References

Baldizar, J.B., and Rybicki, N.B., 2006, Primary factors influencing water clarity at shallow water sites throughout the Chesapeake and Maryland Coastal Bays, in Proceedings of the Joint 8th Federal Interagency Sedimentation Conference and 3rd Federal Interagency Hydrologic Modeling Conference, April 2–6, 2006, Reno, Nevada, in CD-ROM file ISBN 0-9779007-1-1.

Carter, V., Rybicki, N.B., Landwehr, J.M., and Naylor, M.D., 2000, Light requirements for SAV survival and growth, in Kemp, M., Batiuk, R., and others, eds., Chesapeake Bay submerged aquatic vegetation water quality and habitat-based requirements and restoration targets: A second technical synthesis: U.S. Environmental Protection Agency 903-R-00-014, 217 p.

Cerco, C.F., and Moore, K., 2001, System-wide submerged aquatic vegetation model for Chesapeake Bay: Estuaries, v. 24, no. 4, p. 522–534.

Kemp, W.M., Batiuk, R., Bartleson, R., Bergstrom, P., Carter, V., Gallegos, C.L., Hunley, W., Karrh, L., Koch, E.W., Landwehr, J.M., Moore, K.A., Murray, L., Naylor, M., Rybicki, N.B., Stevenson, J.C., and Wilcox, D.J., 2004, Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water quality, light regime, and physical-chemical factors: Estuaries, v. 27, no. 3, p. 363–377.

Landwehr, J.M., 2005, Determining the “Best” model for explaining water clarity variation during SAV seasons within the tidal tributary rivers of the Chesapeake Bay watershed, [abs] in the Proceedings of Estuarine Research Federation Meeting, October 16–20, 2005, Norfolk, Virginia.

Rybicki, N.B., and Landwehr, J.M., 2007, Long-term changes in abundance and diversity of macrophyte and waterfowl populations in an estuary with exotic macrophytes and improving water quality: Limnology and Oceanography, v. 52, no. 3, p. 1,195–1,207.

Floating mats of green algae and horned pondweed on the North Fork of the Tred Avon River in Easton, Maryland. Photograph by Jane Hawkey, IAN Image Library (www.ian.umces.edu/imagelibrary/).