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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.

December 2007


ASSETS Recommended for Use in Chinese Estuaries
Oceanographic Portrait of Glacier Bay Provides Insights About Fjords
Modern Technology Sheds Light on an Ancient Species

ASSETS Recommended for Use in Chinese Estuaries

China has become a global economic engine, but like many engines, it is producing significant amounts of pollution as a side effect of its function. A recent international effort to determine the best method to assess the eutrophication status of Chinese estuaries compared the utility of two simple nutrient indices (including one recommended by the Chinese National Guidelines for Marine Ecological Survey) and the ASSETS method used in the U.S. by NOAA. The ASSETS method produces an integrated index of eutrophication status based on primary (e.g., chlorophyll a concentration) and secondary (e.g. dissolved oxygen concentration) symptoms. It also incorporates information on influencing factors, such as nutrient load, and future outlook for improvement.

These methods were used to evaluate two Chinese estuaries: the Changjiang (Yangtze) Estuary and Jiaozhou Bay. The Changjiang Estuary, China’s largest, experiences frequent eutrophication problems including harmful algal blooms and bottom water hypoxia. Jiaozhou Bay is of modest size and is heavily used for shellfish aquaculture. Changjiang was rated as “eutrophic” by both nutrient indices, and by ASSETS as “bad.” Furthermore, the estuary’s outlook is negative, as its status may deteriorate due to expected agricultural expansion. Jiaozhou Bay was rated as “eutrophic” by one index, but the other could not be applied. In contrast, ASSETS rated it as “high” (corresponding to low eutrophication), and its future outlook is positive as nutrient loads are expected to decrease as attention to water quality in this area increases. The extensive aquaculture of filter-feeding bivalves here also helps to keep chlorophyll levels low.  

The authors determined that ASSETS provided a more complete picture of eutrophication in these estuaries than either of the index approaches. Further, they recommend that in a more extensive national assessment approximately 50 Chinese estuaries should be screened, and, the authors assert, ASSETS is the tool for the job.

Source: Xiao, Y. J., J. G. Ferreira, S. B. Bricker, J. P. Nunes, M. Y. Zhu, and X. L. Zhang. 2007. Trophic assessment in Chinese coastal systems – review of methods and application to the Changjiang (Yangtze) Estuary and Jiaozhou Bay. Estuaries and Coasts 30(6): 901-918. (View Abstract)

Oceanographic Portrait of Glacier Bay Provides Insights About Fjords

Fjords are just not like other estuaries, and because they are rare in the U.S. (only Washington’s Puget Sound qualifies in the lower 48), oceanographic studies of these unique systems are rare too. One place that has many glacial fjords is Alaska, with Glacier Bay being a prime example. Results of the first comprehensive broad-scale analysis of physical and biological oceanographic conditions in Glacier Bay were recently published.

CTD casts were taken at 24 stations throughout the bay for ten years (1993 to 2002) and meteorological parameters were recorded. The observed patterns suggest that freshwater input was highest in summer, thanks to glacier and snowmelt. This glacial meltwater plays a critical role in water column stabilization which promotes phytoplankton blooms by allowing phytoplankton to remain in the photic zone. The competing forces of freshwater discharge in the upper fjord and strong tidal currents at the bay’s entrance impacted the degree of vertical mixing. The meeting of these processes in the central deep basins of the bay led to optimal conditions for production of high and sustained chlorophyll a levels from spring through fall. This productivity, in turn, supports rich populations of fish, birds, and marine mammals (although these trophic levels were not assessed in this study). External physical factors that appeared to have the most impact on the physical properties of Glacier Bay surface waters included air temperature, precipitation, and day length.

Changes are in store for Glacier Bay. Fishing is being phased out in the national park there, which may have an ecological effect. Global climate change will almost certainly lead to faster and earlier glacial melting, which might have consequences for the entire food web. This characterization of Glacier Bay will provide managers with baseline information critical to managing this system, and will also aid in understanding of glacial fjords generally..

Source: Etherington, L. L., P. N. Hooge, E. R. Hooge, and D. F. Hill. 2007. Oceanography of Glacier Bay, Alaska: implications for biological patterns in a glacial fjord estuary. Estuaries and Coasts 30(6): 927-944. (View Abstract)

Modern Technology Sheds Light on an Ancient Species

Scientists who used acoustic telemetry to study horseshoe crab movements in a Maine bay have concluded that the crabs in this neck of the woods tend to stay close to home.

In recent years, concern has been growing about the status of these “living fossils.” Fishery restrictions and closures have been implemented to combat overfishing, especially in the mid-Atlantic as more information points to the critical role horseshoe crabs play in estuarine ecosystems. At the northern limit of their range, where densities are lower and distribution more limited, horseshoe crabs may be particularly vulnerable to overexploitation and large-scale habitat disruption. The Maine study endeavored to fill some of the gaps in our understanding of horseshoe crab ecology.

Crabs tagged with acoustic transmitters in two small subembayments of Taunton Bay exhibited generally small home ranges (< 70 ha), which shrunk to only about 10% of that size in winter. Tagged crabs from both subembayments ranged farther during the June-July breeding season, but not the 4 km distance between embayments required for populations to mix. Movement to and from subtidal wintering areas seemed to be triggered by bottom water temperatures rising above (or falling below) 10oC.

The apparent isolation of these resident populations implies a potentially enhanced susceptibility to overfishing, more so than in wider-ranging southern populations. In these waters, managers should consider measures to protect the crabs’ limited wintering range, perhaps even closing the subtidal winter habitat to disruptive human uses during that time.

Source: Moore, S., and S. Perrin. 2007. Seasonal movement and resource-use patterns of resident horseshoe crab (Limulus polyphemus) populations in a Maine, USA estuary. Estuaries and Coasts 30(6): 1016-1026. (View Abstract)