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.

December 2003


Is The Mangrove Restored? Ask the Crabs
Water Quality Impacts on Fish Hard to Find in a Tidal Creek
Dennis, Floyd and Irene Provide Lessons About Hurricane Impacts
Effects of Nitrogen Loading on NE Marshes Characterized

Is The Mangrove Restored? Ask the Crabs

Evaluating progress of environmental restorations and management initiatives is always critical but never easy. The definition of "success" depends on management goals for a particular site, and the choice of indicators can be difficult. A recent study of crab communities in managed mangrove forests provides one excellent example of the use of an ecological indicator as a habitat restoration assessment tool.

After decades of large-scale mangrove destruction, mangrove forest restoration projects are now being undertaken throughout Southeast Asia. The functional importance of crabs in these mangrove ecosystems led a team of investigators to examine brachyuran crab community structure as a potential indicator of habitat status. Crab communities were characterized at a variety of sites in four mangrove areas with different management histories. The sites included former tin mining areas and shrimp farming ponds that had been actively restored, plantations left to recover naturally, and mature, undisturbed forests.

The differences observed in crab community structure among sites were related most strongly to the previous use and management history of the site and the age of the forest stand. More individuals and species of ocypodid (fiddler) crabs were found at disturbed and younger sites, while mature forests were dominated by grapsids (shore crabs). The crab community at the tin mining site was the most different from all of the others, attributed by the investigators to higher soil temperatures and more frequent tidal inundation associated with the previous mining activities at the site. Although the investigators caution that it is difficult to separate natural and management-induced environmental influences in this study, brachyuran crab community structure is likely to be a useful indicator of habitat status, and therefore restoration success, in mangrove forests.

Source: Ashton, Elizabeth C., Peter J. Hogarth and Donald J. MacIntosh. 2003. A comparison of crab community structure at four mangrove locations under different management systems along the Melaka Straits - Andaman Sea coast of Malaysia and Thailand. Estuaries 26(6): 1461-1471. (View Abstract)

Water Quality Impacts on Fish Hard to Find in a Tidal Creek

Decades of research have shown that human disturbances such as development and timber harvest can have detrimental effects on estuarine water quality, potentially impacting the many species of nekton (fish and crustaceans) that use estuarine nursery grounds. However, results of a study of nekton use of a small tidal creek in North Carolina whose watershed had undergone extensive logging indicate that water quality is not the only determinant of habitat use.

Nekton using nursery areas in tidal creeks or "branch estuaries" prefer the uppermost low-salinity reaches, but that preference might get them into trouble: disturbances such as timber harvest can depress salinity and dissolved oxygen and increase water temperatures in these upper reaches, creating potentially stressful water quality conditions. Investigators surveyed water quality parameters and nekton populations in Isaac Creek, a sub-estuary of the Neuse River, for 8 years, during which extensive timber harvest activities took place (60% of the forest in the creek's watershed was harvested). They found little or no relationship between periods of potentially stressful water quality conditions and nekton abundance in the mid and upper portions of the creek except under extreme conditions. While tree harvest did result in increased frequency of stressful water quality conditions, including low dissolved oxygen and depressed salinity, these conditions did not translate into decreased nekton use of the upper reaches of the creek during the three post-harvest years.

The study serves as a reminder that habitat quality is complex: except under extreme conditions, other contributors to habitat quality such as food availability and habitat structure may be more important than water quality in these small branch estuaries. The authors also recommend a cautious approach to application of laboratory-derived stress thresholds to field conditions.

Source: Kirby-Smith, William W., Martin E. Lebo, and Robert B. Hermann. 2003. Importance of water quality to nekton habitat use in a North Carolina branch estuary. Estuaries 26(6): 1480-1485. (View Abstract)

Dennis, Floyd and Irene Provide Lessons About Hurricane Impacts

When a hurricane blows through the mid-Atlantic landscape, it often leaves devastating calling cards: coastal flooding, power outages, damaged buildings, and more. Estuaries are often altered by these storms too, as barrier islands are breached and increased freshwater input drives down salinity. It seems that hurricanes may leave another estuarine change in their wake: shifts in phytoplankton community composition, perhaps having implications for the rest of the food web.

North Carolina's unlucky Pamlico Sound and its watershed were battered by three sequential hurricanes in 1999 (Dennis, Floyd and Irene), prompting a team of scientists to evaluate the storms' lasting impacts on the Sound. They monitored water quality parameters and phytoplankton community structure for more than two years after the 1999 storms, and found many hurricane-induced impacts. The water residence time of both the Sound and one of its major sub-estuaries, the Neuse, was substantially reduced, leading to depressions of salinity and dissolved oxygen and increases in stratification, nutrients, chlorophyll, dissolved organic carbon and turbidity. Most of these parameters returned to normal background levels 2-8 months after the storms, with one notable exception: the phytoplankton community composition continued to change throughout the study period. These changes likely occurred in response to the lower salinities and higher nutrient levels induced by the storms. The investigators speculate that post-hurricane declines in fish and shellfish catches in the Sound may be related to these changes at the base of the food web. Especially because Atlantic hurricane activity is predicted to increase, managers should be aware that these storms can cause long-term ecosystem changes.

Source: Peierls, Benjamin L., Robert R. Christian and Hans W. Paerl. 2003. Water quality and phytoplankton as indicators of hurricane impacts on a large estuarine ecosystem. Estuaries 26(5): 1329-1343. (View Abstract)

Effects of Nitrogen Loading on NE Marshes Characterized

The patchwork of plant species in salt marshes has been shown to shift in response to changes in environmental conditions and human-generated stressors. A new study provides evidence that one factor affecting characteristics of the marsh plant community is elevated nitrogen (N) loads associated with residential development.

From previous experiments, it was known that "fertilizing" marshes with N causes Spartina alterniflora (salt marsh cord grass) to outcompete Spartina patens (salt hay). Could this effect or others be observed in marshes experiencing a range of N inputs from wastewater? Over the course of two years, scientists measured a variety of physical and plant characteristics (marsh elevation, slope, area, flood tide height, plant height, species richness, etc.) at ten Narragansett Bay, RI fringing salt marshes with varying degrees of development in their watersheds. N loading was also estimated for all marshes. At sites with more development and higher nitrogen loading they found higher densities, extents and heights of the tall form of S. alterniflora. These sites had lower densities and extents of S. patens and the short form of S. alterniflora, as well as lower numbers of plant species overall.

The investigators caution that the study is not definitive because confounding physical factors made it unclear whether the observed trends resulted from differences in N load or physical attributes of the study sites. Although more research is needed, the study suggests that salt marsh vegetation may be significantly affected by development-associated N loading. Managers beware: changes in marsh vegetation may mean changes in ecological niches for other plants and animals, and may affect marsh biodiversity and overall function.

Source: Wigand, Cathleen, Rick McKinney, Mike Charpentier, Marnita Chintala, and Glen Thursby. 2003. Relationships of nitrogen loadings, residential development, and physical characteristics with plant structure in New England salt marshes. Estuaries 26(6): 1494-1504. (View Abstract)