CESN Main Page

Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

February 2006


Can Mid-Atlantic Marshes and Tidal Ponds Keep Their Heads above Water?
Shoreline Alterations Turn up the Heat on Puget Sound Beaches
A Green Light for Restoration: Transplanted Georgia Marsh Plants Thrive in Dieback Sites
What Happens to the Oxygen? Bacteria and Hypoxia in the Potomac River

Can Mid-Atlantic Marshes and Tidal Ponds Keep Their Heads above Water?

The constant tug of war between sediment accretion and land subsidence causes the surface elevation of marshes, tidal ponds, and other coastal features to rise and fall over time. If these processes cannot keep pace with expected sea level rise, critical areas may be lost to inundation by the sea. One of the many reasons for concern about potential loss of marshes and coastal ponds is that they serve as essential habitat for so many species, including many species of waterbirds. A recent study of elevation changes in four mid-Atlantic sites focused on the implications of those changes for the waterbirds found at each site.

The study determined accretion rates and elevation changes in Spartina-dominated marshes and associated unvegetated ponds in MA, NJ, and VA. Elevation trajectories were compared to long-term tidal records at each site, which provide estimates of sea level rise. The authors also examined waterbird use at the sites. At all sites except the one in MA, net elevation trends indicate that the marsh/pond complexes are not keeping pace with current sea level rise, suggesting that these marshes will eventually be lost to inundation by future increases in sea level if conditions remain unchanged. These losses will likely spell trouble for many of the species of waterbirds observed nesting and feeding at the sites. For example, loss of ponds could reduce wintering habitat for American black duck and other species of concern. Because these ponds also serve as alternative habitat for many shorebirds during flooding of intertidal mudflats, the increased flooding that will accompany sea level rise will force those birds elsewhere. Some more adaptable species may seek alternative habitat in nearby agricultural areas but it is unclear how other species, including yellowlegs, willets, and whimbrels, will adapt.

Source: Erwin, R. M., D. R. Cahoon, D. J. Prosser, G. M. Sanders, and P. Hensel. 2006. Surface elevation dynamics in vegetated Spartina marshes versus unvegetated tidal ponds along the mid-Atlantic coast, USA, with implications to waterbirds. Estuaries and Coasts 29(1): 96-106. (View Abstract)

Shoreline Alterations Turn up the Heat on Puget Sound Beaches

While the average beachgoer views the nearshore zone as simply the part of the beach on which to take a walk or build a sand castle, this area performs a number of underappreciated ecological functions, including serving as foraging, spawning, rearing, and migrating habitat for a wide variety of species. Even fish use this zone, like the beach-spawning surf smelt, a recreationally-important species in Puget Sound, WA. These fish make their way into the upper intertidal zone at high tide and lay their eggs, which incubate in the sand and gravel until they hatch 10 to 21 days later, at which time the larvae are swept out to sea by the high tide.  

Though largely undocumented, anthropogenic changes to nearshore areas, including bulkheading, pier building, and vegetation removal, must have ecological impacts. In Puget Sound, where at least one-third of the shoreline has been modified in some way, alteration of one beach has clearly changed the microclimate (the sediment temperature and humidity of the air close to the ground) of the upper intertidal zone. Daytime light intensity and daytime and nighttime air and substrate temperatures were found to be significantly higher, and relative humidity lower, on a bulkheaded beach where shade-providing vegetation has been removed as compared to an unaltered beach. Most striking was the average daily peak substrate temperature difference of 11oC (higher on the altered beach). These differences appear to have had major consequences for the surf smelt: The proportion of smelt eggs containing live embryos at the altered beach was approximately half of that at the natural site.

When proposed shoreline modifications are evaluated on a case-by-case basis, the potential for localized and hidden impacts like the ones documented in this study is often overlooked. More comprehensive and detailed studies of the ecology of nearshore species – including intertidally spawning fishes – would be helpful in determining whether population- and ecosystem-level impacts result from the cumulative effects of these shoreline modifications.

Source: A Rice, C.. 2006. Effects of shoreline modification on a northern Puget Sound beach: Microclimate and embryo mortality in surf smelt (Hypomesus pretiosus). Estuaries and Coasts 29(1): 63-71. (View Abstract)

A Green Light for Restoration: Transplanted Georgia Marsh Plants Thrive in Dieback Sites

In 2001 salt marshes all over Georgia were laid bare by an unprecedented dieback that affected both dominant salt marsh plants there, Spartina alterniflora and Juncus roemerianus. By the end of the event more than 800 hectares had been affected. Can the impacted areas be restored using transplants from nearby healthy stands, or are those areas still under the influence of whatever caused the diebacks in the first place, destined to remain bare until nature takes its course?  

Results from a transplant study conducted in two of the affected marshes indicate that active restoration might work well at GA sites. Nearly 100% of transplants of both plant species survived after being moved to dieback areas and significant plant growth was observed in both healthy and dieback areas. In some cases growth was actually greater in dieback areas than control sites unaffected by the dieback, perhaps due to elevated porewater ammonium concentrations observed in dieback soil samples. Other porewater parameters were similar between the experimental and control sites. These results suggest that restoration via transplanting has a good chance of success.  

Similar major dieback events have occurred recently in such widespread areas as New England, the Carolinas, and the Gulf of Mexico. This study provides hope that restoration can be successful after these events, but better monitoring of marsh systems may help detect the early stages of these diebacks in order to determine their causes.  

Source: Ogburn, M. B., and M. Alber. 2006. An investigation of salt marsh dieback in Georgia using field transplants. Estuaries and Coasts 29(1): 54-62. (View Abstract)

What Happens to the Oxygen? Bacteria and Hypoxia in the Potomac River

Sometimes the biggest water quality problems have the tiniest sources. For example, hypoxia is usually caused by elevated bacterial respiration during the breakdown of overabundant organic matter. While phytoplankton and particulate organic matter (POC) are usually considered to be the sources of organic matter that contribute to the problem, chlorophyll concentrations are actually not always correlated with dissolved oxygen (DO) levels. So what is feeding the bacteria? In the Potomac River, the second largest tributary to Chesapeake Bay, bacterial respiration and hypoxia appear to be correlated more with dissolved organic matter in a form available for microbes to use (dissolved microbially labile organic carbon or DiMLOC) than POC.

A study of the relationships among bacterial abundance, chlorophyll, organic matter, and DO at three stations along the river’s salinity gradient found very high bacterial abundance (2-5 times higher that nearby estuaries). In contrast to other estuaries where most bacteria tend to be associated with suspended particles, nearly all the bacteria were small, free-living cells.

Hypoxia and anoxia at the mesohaline study site were associated with the highest bacterial counts and the highest concentrations of DiMLOC. On the other hand, chlorophyll concentration was not correlated with bacterial abundance or DO. These results suggest that in this system oxygen depletion is driven by bacterial response to dissolved organic matter rather than particulate matter. Eutrophication modelers might want to consider revising some models to reflect the importance of this pathway.

Source: Hamdan, L. J., and R. B. Jonas. 2006. Seasonal and interannual dynamics of free-living bacterioplankton and microbially labile organic carbon along the salinity gradient of the Potomac River. Estuaries and Coasts 29(1): 40-53. (View Abstract)