SAV and its substrate: spatial, temporal, and historical trends (NOAA)
Principal Investigators: Cindy Palinkas, Evamaria Koch
The primary cause of submersed aquatic vegetation (SAV) decline is generally thought to be water quality impairment by excess suspended sediments and nutrients, which affects light penetration (e.g., Dennison et al., 1993). However, other, sometimes co-varying, factors also can be responsible for the decline, including temperature, salinity, waves, sediment type and accumulation (Koch, 2001). It is likely that a combination of water-column and substrate requirements exist for healthy SAV, satisfying the needs of both the leaf and root systems.
Comparing sites with relatively stable SAV beds and those that have historically supported beds but may not presently be vegetated (“declining beds”) lends insight into why some areas of Chesapeake Bay are presently unvegetated despite meeting the water-column requirements. In the Chesapeake Bay region, healthy SAV beds typically are composed of 6-10% fine (mud) material and have 1-5.3% organic content (Batiuk et al., 1992). It is thought that organic content >5% and fine (mud) material >35% is limiting to SAV (Barko and Smart, 1983; Koch et al., in prep), but the specific substrate requirements for SAV are not well understood.
Study Sites
Site maps of stable and declining beds in mesohaline Chesapeake Bay are shown in the figure below. Sampling stations are indicated by circles and are color-coded by median diameter. Green shading represents the weighted density of SAV beds 1978-2007 (data from VIMS aerial photgraphy); denser and more persistent beds are darker green. Shoreline coloring represents the erosion rate observed by the VIMS Center for Coastal Resources Management. The two relatively stable beds are in Trippe Bay (T) and Crab Alley Bay (CB); the two declining beds are in Herring Bay (H) and Casson Point (CS). Note the scale is ~10x finer for Casson Point than the other sites; grain-size transitions are fairly abrupt at this site.
Data from the aerial photographs are ground-truthed via manual surveys, providing information on the specific SAV species present in the region. Likely SAV species present at our study sites are:
Crab Alley Bay: Ruppia maritima, Potamogeton perfoliatus, Zannichellia palustris, Stuckenia pectinata
Trippe Bay: R. maritima, S. pectinata
Casson Point: R. maritima
Herring Bay: R. maritima, Z. palustris
Summary of results:
The results of this study can be summarized by the following conceptual models of sedimentary processes in SAV beds (symbols courtesy of the UMCES IAN Symbol Library).
Stable Beds: Stable beds are composed of sandy sediment with a low organic content and support healthy SAV beds in most years. At Trippe Bay, Ruppia maritima and Stuckenia pectinata have been observed in previous years. Accumulation rates are high at inshore and offshore locations within the beds. The shoreline nourishes the beds with sandy sediments. When sandy shorelines are hardened, such as occurs at Crab Alley Bay (CAB), the transport of sand from the shoreline is essentially eliminated, allowing finer and more organic material from offshore to dominate sediment deposition. This also results in a slightly higher accumulation rate at the offshore core location relative to the inshore. While sediment conditions at CAB are still suitable for SAV (R. maritima, Potamogeton perfoliatus, Zannichellia palustris, and S. pectinata have been observed), they may become unsuitable in the future, if the trend of increasing fine and organic content continues.
Declining beds: Both declining beds have low accumulation rates that may prevent SAV from establishing due to seed loss from erosion/predation prior to germination. The low rates result from reduced supply of sediments and/or trapping by established SAV beds. At Casson Point (CP), transitions are abrupt, both at the shoreline and in the nearshore, from sandy beach to muddy marsh material. In years with greater amounts of sand (from the shoreline and/or longshore transport), SAV has been observed at CP (Zostera marina (not shown), R. maritima); previous studies indicate a layer >2-cm thick is suitable for SAV. The underlying substrate at CP is muddy marsh material; while organic content is elevated, it remains <5%. Sediment grain size and organic content in Herring Bay are similar to Trippe Bay. The absence of SAV at Herring Bay may be due to the lower accumulation rates and/or degraded water clarity. While long-term data are not available, secchi depths <1 m were recorded in summer 2008
For more information, please contact the lead PI, Cindy Palinkas