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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.

August 2007


Critical Tool for American Samoa Mangrove Restoration is Community Support
Working “Backward,” Seagrass Depth Limits Can Lead to Nutrient and TSS Loading Limits
Ripped to Shreds: Mechanical Shredding Works to Wipe Out Water Hyacinth
Listening for Fish in the Shallow Waters of Barataria Bay

Critical Tool for American Samoa Mangrove Restoration is Community Support

Mangrove forests are one of the most threatened ecosystems on earth: roughly 50% of the global mangrove area has been lost since 1900 and losses continue at a global average annual rate of about 2.1%. Restoration is challenging, nowhere more so than in the Pacific Islands. A recent restoration project in Tutuila Island, American Samoa endeavored to demonstrate potential restoration techniques to use in the region, and revealed the importance of community participation in such projects.

Saplings of two mangrove species (Bruguiera gymnorrhiza and Rhizophora mangle) were planted inside sediment-filled tires (to raise surface elevation) and adjacent to rebar or other physical support. B. gymnorrhiza saplings were also planted with no physical support (no tire or rebar). After six months no differences in survival were observed among treatments, although survival differed significantly between the two species. Survival of B. gymnorrhiza and R. mangle averaged 45% and 21% respectively. The high mortality of R. mangle may have been due to root damage during transplantation, the seaward location of the zone where they were planted, or human disturbance. Monitoring will be needed for many years at this site to truly evaluate restoration success.  

Despite the modest survival rate of the transplants, one of the successes and lessons of this project was the involvement of the local community. The project site was stripped of mangrove trees in 1979 by the adjacent landowners. Members of this same family, still living on property adjacent to the restoration site, endorsed and participated in the project which the authors say was critical to project execution. The community outreach methods employed here make the project a potential model for mangrove restoration in other Pacific Island countries.

Source: Gilman, E., and J. Ellison. 2007. Efficacy of alternative low-cost approaches to mangrove restoration, American Samoa. Estuaries and Coasts 30(4): 641-651. (View Abstract)

Working “Backward,” Seagrass Depth Limits Can Lead to Nutrient and TSS Loading Limits

Setting load limits for nutrients and suspended solids in order to achieve restoration goals or calculate EPA-required total maximum daily loads (TMDLs) can be very complex. These parameters are particularly important in systems where seagrass restoration is a management goal. Since nutrients and TSS impact water clarity, and water clarity impacts seagrass growth and depth limits, can this relationship be turned around such that desired seagrass depth limits are used to back-predict appropriate load limits? This approach was used for watersheds of the Indian River and Banana River Lagoons, FL. The system was divided into segments, and segment-specific regression relationships between nutrient/TSS loadings and seagrass depth limits were established using seagrass cover and water quality data from 1943, 1996, 1999, and 2001. These highly significant relationships were then turned around so that desired depth limits specified by the Florida Department of Environmental Protection could be used to determine the load limits necessary to achieve those goals. These load limits can be used to calculate load allocations, waste load allocations, and TMDLs. The authors point out that this simple approach is likely to be widely applicable in similar systems with long residence times. In order to carry out this type of calculation, though, long-term or multi-year data (or both) are needed to construct the initial regressions.

In the same journal issue, an article by Duarte et al. explored the relationship between seagrass depth limits and light attenuation in 424 published reports of this relationship. This extensive review revealed that the regression analysis presented in a previous publication by some of the same authors overestimates the depth limit of seagrasses growing in shallow, turbid conditions. Seagrasses colonizing turbid waters have higher light requirements than those growing in clearer waters, and there appears to be a shift in the relationship between seagrass depth limit and light attenuation at a Kz value of 0.27m-1. The authors therefore recommend that separate equations to predict depth limits be used in waters with attenuation coefficients above and below the Kz = 0.27m-1 threshold.

Source: Steward, J. S., and W. C. Green. 2007. Setting load limits for nutrients and suspended solids based upon seagrass depth-limit targets. Estuaries and Coasts 30(4): 657-670. (View Abstract)

Source: Duarte, C. M., N. Marbà, D. Krause-Jensen, and M. Sánchez-Camacho. 2007. Testing the predictive power of seagrass depth limit models. Estuaries and Coasts 30(4): 652-656. (View Abstract)

Ripped to Shreds: Mechanical Shredding Works to Wipe Out Water Hyacinth

The invasive water hyacinth (Eichhornia crassipes) is one of the most noxious aquatic weeds in North America, growing rapidly and often forming impenetrable masses. Concerns have been raised that removal of the plant might actually make things worse, ecologically speaking. For example, one inexpensive removal method with potentially damaging consequences is shredding the weeds and leaving them to die and decompose in place. Would that approach release nutrients and chemicals stored in the plant tissues into the water, causing water quality problems? A recent study found that under some conditions the shred-it-and-leave-it method would have a negligible effect on water quality.  

To reach that conclusion, investigators conducted four shredding events at two sites, a well flushed marsh and a poorly flushed irrigation ditch in the Sacramento-San Joaquin River Delta system. Water quality parameters were measured before and after the shredding. At some sites and times increases in BOD, turbidity, and nutrient concentrations were observed after shredding. At the tidally flushed site the few observed effects were brief, and the site returned to baseline conditions after four days. Changes in the irrigation ditch were more pronounced and persistent. In one case, the site went anoxic for several weeks, and a fish kill occurred 16 days after the shredding. These impacts were less pronounced when the shredding was undertaken in the spring when weed biomass was low.  

The localized water quality changes observed in the study would have limited impacts on the overall nutrient budgets of the Delta (an estimated 0.1% to 9.6% increase in abundance of carbon, nitrogen and phosphorous in the Delta water column) because water hyacinth covers less than 10% of the Delta in any year. One concern that warrants further research is the potential release and subsequent methylation of bioaccumulated mercury from the plant tissue. Another concern is the potential for rapid regrowth and dispersal of the shredded plant material; the authors suggest that future studies should identify conditions that minimize regrowth rates.

Source: Greenfield, B. K., G. S. Siemering, J. C. Andrews, M. Ryan, S. P. Andrews, Jr. , and D. F. Spencer. 2007. Mechanical shredding of water hyacinth (Eichhornia crassispes): effects on water quality in the Sacramento-San Joaquin River Delta, California. Estuaries and Coasts 30(4): 627-640. (View Abstract)

Listening for Fish in the Shallow Waters of Barataria Bay

Acoustics isn’t just for concert halls: hydroacoustics has been widely used as a tool for surveying fish populations in freshwater and marine systems. A recent study in Barataria Bay, LA, used hydroacoustic techniques to examine fish biomass and size distribution in estuarine habitats, and in the process demonstrated the utility of their new approach in very shallow estuarine waters. Although there are many advantages to the use of noninvasive acoustic techniques, these methods are not always accurate, particularly in very shallow water, as the backscatter from non-fish items (e.g., entrained air bubbles) in the water column tends to interfere with the fish signals. While traditional hydroacoustic approaches use a downward looking (vertical) transducer, the LA survey demonstrated that horizontal beaming results in less interference in very shallow water (<2 m) allowing for better estimation of fish biomass and size distribution. The method used was groundtruthed by collecting fish in the surveyed areas with gill nets and trawls, and also by pointing the acoustic equipment at an empty exclosure to ensure that most of the area’s scattering sources were actually fish.

The paper highlighted a study conducted in March 2003 which found higher fish biomass in low salinities than high salinities, and larger fish associated with oyster shell habitats than mud flats. Just as important, though, was the confirmation of the utility of this technique in very shallow waters. Those conducting similar surveys or monitoring may want to consider the method.

Source: Boswell, K. M., M. P. Wilson, and C. A. Wilson. 2007. Hydroacoustics as a tool for assessing fish biomass and size distribution associated with discrete shallow water estuarine habitats in Louisiana. Estuaries and Coasts 30(4): 607-617. (View Abstract)