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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.

June 2005


Ultraviolet Radiation and Primary Productivity: Effects May Go Beyond the Antarctic
ENSO/SAV Links in Lake Pontchartrain: More than Coincidence
Are Created Marshes Equal, Above the Ground and Below?
Elkhorn Slough's Living History

Ultraviolet Radiation and Primary Productivity: Effects May Go Beyond the Antarctic

The "hole" discovered in the ozone layer over the Antarctic revealed another kind of hole to the scientific community - the large gap in our understanding of how ultraviolet B radiation (UVB), reaching the earth's surface in larger doses, affects species and ecosystems. In the 25 years since the hole's discovery, many studies have found that the increased UVB doses have not been kind to oceanic phytoplankton communities and primary productivity, particularly in Antarctic waters closest to the hole. Detrimental effects have extended to higher trophic levels as well. But what about nearshore phytoplankton in temperate areas? Does the increased distance from the Antarctic hole mean less trouble from UVB radiation?

Investigators conducting a recent laboratory study with San Francisco Bay (SFB) phytoplankton speculated that in addition to its distance from the Antarctic, the high turbidity of the Bay might offer enough "sunscreen" to mitigate the deleterious effects of the UVB. The sunscreen did not turn out to be protective enough: When natural SFB phytoplankton samples were exposed to ambient UVB radiation in experimental containers, nutrient uptake ability (for ammonium more so than nitrate) was depressed compared to samples held in containers that filtered out UVB. This effect was perhaps due to UVB's inhibition of an enzyme used in ammonium assimilation. The authors conclude that present-day UV levels are having a damaging effect on the SFB phytoplankton community. If things are bad for phytoplankton in turbid SFB, the authors speculate that the situation might be even worse, and primary productivity might be even more affected, in other estuaries where less turbid conditions afford less sunscreen from the UV.

Source: Hogue, V. E., F. P. Wilkerson and R. C. Dugdale. 2005. Ultraviolet-B Radiation Effects on Natural Phytoplankton Assemblages of Central San Francisco Bay. Estuaries 28(2): 190-203. (View Abstract)

ENSO/SAV Links in Lake Pontchartrain: More than Coincidence

The major submersed aquatic vegetation (SAV) decline in New Orleans' Lake Pontchartrain, documented since the inception of monitoring in 1953, has been attributed to anthropogenic changes in water quality. However, it seems that more recent dramatic changes in SAV beds are actually related to a natural cycle: the El Niño Southern Oscillation (ENSO) climate pattern. Careful study of climate and SAV parameters for 1996-2003 has revealed interesting patterns and parallels. The strong 1997-1998 El Niño brought increased precipitation leading to lower salinities and reduced light penetration in the estuary. Accordingly, freshwater plants dominated, but were found mostly in depths less than 1 meter. As climate conditions shifted from wet El Niño to dry La Niña conditions, culminating in a severe drought in 2001, higher salinities were observed and the freshwater SAV species were replaced by the salt-tolerant Ruppia maritima. Better light conditions (because decreased precipitation means less runoff of particles from the land) allowed the R. maritima to utilize deeper depths. The freshwater species made a comeback with the return of El Niño in 2003.

Interestingly, the colonization of deeper waters by R. maritima occurred more quickly than the plant's life history would predict. The authors hypothesize that appropriately-timed tropical storms may have served as a rapid seed dispersal mechanism, spreading seeds into deeper water, where seedling growth was encouraged by La Niña's increased light penetration.

This study serves as a reminder that not all changes in SAV distribution are due to anthropogenic water quality issues, and that natural variations must be accounted for when establishing restoration goals. Also, these analyses indicate that for this system SAV restoration could be brought about by improving water clarity.

Source: Cho, H. J. and M. A. Poirrier. 2005. Response of Submersed Aquatic Vegetation (SAV) in Lake Pontchartrain, Louisiana to the 1997-2001 El Niño Southern Oscillation Shifts. Estuaries 28(2): 215-225. (View Abstract)

Are Created Marshes Equal, Above the Ground and Below?

At wetland restoration sites all over the country, the question of "functional equivalency" is raised. Will the new marsh do what a natural marsh does, and how long will it take for it to "act" like its natural counterparts? One way to tell is to compare primary productivity, both in the aboveground and buried parts of the plant, in created and natural areas. Such a study was undertaken at created and natural Spartina alterniflora marsh sites in the Sabine National Wildlife Refuge in southwest Louisiana. The study measured aboveground and belowground primary productivity in marshes created 3, 6, 9, and 19 years ago and in two natural marshes estimated to be about 50 years old. Aboveground productivity was highest at the youngest created site and decreased with the age of the site, approaching, but not matching, that of natural marshes at the oldest created site. Belowground the story was a little different. Productivity in the youngest created site was intermediate to that at the natural sites, decreased in the six-year-old sites, but then increased with the age of the created site. Belowground productivity levels were 20% higher than the natural marshes at the oldest site, with no indication that it would head downward toward natural levels. An analysis of the trajectories of productivity over time revealed that in terms of aboveground productivity, not only had equivalency not been reached after 19 years, but it would take 35 years for the created marshes to match the natural ones. Belowground productivity matched after only 6-8 years, but increased at the created sites as they aged, outpacing the natural marshes.

Topography of the created marshes also appeared to have an effect on productivity: where elevations of the created sites were higher than planned and topographies were heterogeneous, aboveground productivity was lower. To avoid this problem, restorations need to aim for elevations and topographies similar to natural marshes. Even then, patience is required, as functional equivalence may not be reached for decades.

Source: Edwards, K. R. and K. P. Mills. 2005. Aboveground and Belowground Productivity of Spartina alterniflora (Smooth Cordgrass) in Natural and Created Louisiana Salt Marshes. Estuaries 28(2): 252-265. (View Abstract)

Elkhorn Slough's Living History

The relatively new discipline of historical ecology integrates disparate data sets to construct and analyze environmental histories of discrete areas. These records can read much like human histories, complete with threats to an area's integrity, battles against invading enemies, and times of peace and prosperity.

Using historical maps and aerial photographs, researchers recently constructed a 150-year ecological history of central California's Elkhorn Slough, where dramatic shifts in habitat type, vegetation coverage, and human use patterns have occurred. An important goal of the project was to develop accurate, repeatable methods for conducting this type of analysis. The researchers characterized the overall progression of wetland changes in the Slough by examining parameters such as hydrology, vegetative cover and tidal channel width during a sequence of representative periods between 1853 and 2003. The Slough's story includes loss, transformation and degradation of more than two-thirds of its salt marshes, and a nearly five-fold increase in the width of its tidal channels. These changes are largely attributable to construction of an artificial navigation channel in 1947, which transformed the Slough from a depositional to an erosional system. In recent decades, intentional and accidental levee breaches have restored the area under tidal influence to near-historic levels, and most of the Slough is now protected for conservation purposes. Despite these moves toward restoration, rates of conversion of marsh to mudflat remain high and may actually be accelerating, suggesting that a new equilibrium may not be reached for several decades.

A comprehensive understanding of an estuary's history can be invaluable in suggesting hypotheses that can be tested with contemporary data, and can help in setting ecologically justifiable and achievable management and restoration goals. Given appropriate data availability, the methods outlined in this study could be used to the advantage of many other systems.

Source: Van Dyke, E. and K. Wasson. 2005. Historical Ecology of a Central California Estuary: 150 Years of Habitat Change. Estuaries 28(2): 173-189. (View Abstract)