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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.

February 2007


In Chesapeake, Benthic Community Shows that Where Development is Up, Estuarine Health is Down
Telling the Phragmites Twins Apart: Native Prefers No Salt and Little Nitrogen
Fish Index Proves Useful in Taking the Pulse of South African Estuaries
Bioenergetics Study Reveals Tough Choice for Invasive Crabs: Get Eaten or Use Excess Energy

In Chesapeake, Benthic Community Shows that Where Development is Up, Estuarine Health is Down

Decades of research and monitoring have established that coastal development is often accompanied by impacts such as hypoxia, algal blooms, and decreased biodiversity. One set of tools that has proven useful in quantifying the links between development and environmental health are easily-interpretable benthic indices based on macroinvertebrate community composition. Two such indices, B-IBIN and the W-value, were used in an extensive two-year study of 23 Chesapeake Bay sub-watersheds to assess the condition of nearshore tidal habitats. After calculating the benthic indices for all sites and accounting for variable environmental conditions, the indices were then correlated with land use categories to determine the impact of development on estuarine health. The highest benthic index scores (indicating the healthiest sites) were associated with forested watersheds, while the lowest were found in developed watersheds. Further, the authors identified shoreline and watershed land use thresholds above which degradation consistently occurred: areas in which more than 10-12% of the land is developed were likely to exhibit degraded ecological conditions.  

These indices are highly recommended by the study’s authors because they can integrate effects of multiple environmental stressors. Also, benthic macroinvertebrates make good indicator organisms because they are easy to collect and respond rapidly to environmental stress. Use of these indices, in combination with measures of landscape and habitat condition, can be used to guide further research on ecological processes and environmental quality.

Source: Bilkovic, D. M., M. Roggero, C. H. Hershner, and K. J. Havens. 2006. Influence of land use on macrobenthic communities in nearshore estuarine habitats. Estuaries and Coasts 29(6B): 1185-1195. (View Abstract)

Telling the Phragmites Twins Apart: Native Prefers No Salt and Little Nitrogen

Most studies of the common reed, Phragmites australis, focus on its invasive nature and the ecological and management implications of its takeover of disturbed aquatic ecosystems. But there is a non-invasive native Phragmites haplotype (genetically distinct lineage) as well, which has largely been displaced by the invasive haplotype in many areas. A recent study of the two haplotypes along Virginia’s Rappahannock River revealed some interesting differences between the relatives, which may have important management implications. The native Phragmites was found only at sites where the salinity was 0, while the invasive plant exhibited a much wider range of salinity tolerance, from 0-11. The invasive haplotype was generally found in a monoculture in ecologically disturbed areas (near highway bridges, development, etc.) while the native Phragmites was found in small patches interspersed with a mix of other wetland plants. Finally, nitrogen concentrations in non-native haplotype leaves were significantly higher than the native plants, while carbon concentrations were lower, perhaps indicating that the invasives have a higher N requirement.

Based on these results, managers who want to keep invasive Phragmites at bay while protecting the native version should focus on minimizing upland disturbance and nutrient enrichment in tidal freshwater marshes. Biological control might also be an option, as the authors observed that a fungal pathogen preferentially attacked the invasive haplotype in an attempted greenhouse study.

Source: Packett, C. R., and R. M. Chambers. 2006. Distribution and nutrient status of haplotypes of the marsh grass Phragmites australis along the Rappahannock River in Virginia. Estuaries and Coasts 29(6B): 1222-1225. (View Abstract)

Fish Index Proves Useful in Taking the Pulse of South African Estuaries

Indices of environmental quality that integrate multiple measures of ecological condition serve many purposes: they elucidate the effects of multiple environmental stressors, and they can serve as effective communications tools to convert complex information about a system’s health into an easily understood metric for policy makers and the general public. One such condition index was developed and tested in a recent survey of South African estuaries. The Estuarine Fish Community Index (EFCI) integrates 14 metrics representing four aspects of fish communities and ecology: species diversity and composition, species abundance, nursery function, and trophic integrity. The South African survey examined 190 estuaries encompassing a broad spectrum of biogeographic regions and morphological estuary types. After calculating the EFCI value for each estuary using data from extensive field surveys, the index was used to classify the health of each system, ranging from “very poor” to “very good.” Results indicated that the more degraded estuaries tended to be smaller systems with restricted mouths.

Based on comparisons with independent assessments of estuarine health in some of the sampled systems, the EFCI effectively differentiated between good- and poor-quality systems. It was also reproducible: in estuaries where calculation of EFCI was carried out in multiple years, 88% of the duplicated indices resulted in similar system classifications.

While the EFCI, like other indices of this type, does not provide information about the causes of system impairment, it is still valuable as an assessment and communication tool. It is also useful in making inter-system comparisons over large spatial and temporal scales. Readers are reminded that an index such as the EFCI should be used to complement, rather than replace, other measures of estuarine health.

Source: Harrison, T. D., and A. K. Whitfield. 2006. Application of a multi-metric fish index to assess the environmental condition of South African estuaries. Estuaries and Coasts 29(6B): 1108-1120. (View Abstract)

Bioenergetics Study Reveals Tough Choice for Invasive Crabs: Get Eaten or Use Excess Energy

The European green crab, Carcinus maenas, gets around. Native to the northeast Atlantic, it has made itself at home around the world, establishing itself as one of the most successful marine invasive species. In Washington’s Willapa Bay, where the crab has been found since 1998, monitoring data indicate that the crabs favor intertidal areas during the summer, especially extensive Spartina meadows. Researchers set out to determine whether there is a particular energetic incentive for the crabs to seek out these habitats, using a bioenergetics framework. This approach is based on the simple premise that, energetically speaking, what goes in must go out. In other words, consumption of food (“energy in”) is equal to the sum of energetic expenditures: respiration, growth, egestion, and excretion. In order to determine if the Willapa Bay green crabs were making the “right” habitat choice from an energetics standpoint, a commonly-used bioenergetic model was parameterized using literature values derived from previous studies of C. maenas and related species. Ground-truthing of actual crab habitat use was carried out in conjunction with ongoing monitoring surveys.  

Results revealed that while the crabs were most often found in intertidal Spartina stands, the most energetically favorable habitats (as measured by growth efficiency) would be in deeper waters. Deeper, colder-water areas have two energetic advantages over the shallower areas: the crabs’ metabolism is slower there, decreasing their energetic demands, and the crabs can spend more time foraging since they’re not periodically exposed by low tides.

If their habitat choice is less than energetically ideal, what is driving their distribution? The authors suspect that the invasive crabs are avoiding native Dungeness and rock crabs, who prefer the deeper habitats and prey on and compete with the green crabs. The native crabs do not use the Spartina meadows, so the green crabs have that habitat to themselves. Ironically, in this system (and throughout the Pacific Northwest), Spartina itself is an invasive species, here creating a new favorable habitat for another invader.

Source: McDonald, P. S., K. K. Holsman, D. A. Beauchamp, B. R. Dumbauld, and D. A. Armstrong. 2006. Bioenergetics modeling to investigate habitat use by the nonindigenous crab, Carcinus maenas, in Willapa Bay, Washington. Estuaries and Coasts 29(6B): 1132-1149. (View Abstract)