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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 2010


Restoration Mixed Bag for S. Florida Manatee Winter Refuges
Florida Bay’s Methylmercury Problem Falls from the Sky, Study Says
Long-term Monitoring Supports Nutrients as Source of Chesapeake’s SAV Decline, But Small-scale Factors Matter Too
Study Shows Thin-Layer Dredged Material Disposal Has Little Long-term Impact on Chesapeake Benthic Community

Restoration Mixed Bag for S. Florida Manatee Winter Refuges

Everything’s relative: just as millions of northerly-dwelling humans flock to Florida in winter seeking refuge from the cold, some of Florida’s best-known resident wildlife head for still warmer areas. When winter water temperatures drop below 20oC, endangered Florida manatees seek warmth in power plant effluents, artesian springs, and warm, deep pools. In south Florida, where manatees do not have access to power plants or artesian springs, they seek out passive thermal refugia (PTR) in deepwater pools that occur in natural and man-made canals and basins.

A recent study mapped the manatees’ use of these pools near the Ten Thousand Islands region of southwest Florida. Aerial surveys and satellite telemetry revealed that the manatees use several PTRs within the study area, all of which are in artificial inland canals and basins. Bottom water temperatures at these refugia were significantly warmer than any other water bodies monitored during the study. Extensive monitoring at two of the PTRs, a residential development called Port of the Islands and Big Cypress National Preserve headquarters, revealed that during winter temperature inversions were maintained by a halocline formed when seasonal freshwater flow overlays warmer, saltier bottom water. Without the freshwater layer, vertical mixing rapidly eliminated warm water.

Restoration may be a mixed bag for manatee refuges in southwest Florida. Planned restorations may disrupt this winter flow in some areas, while improving conditions in others. The authors of this study suggest a number of management approaches that could achieve water quality goals while maintaining manatee habitat. For example, an uneven bottom marked by deeper holes that resist tidal mixing could be designed in some areas to create a more stable thermal environment. Flow could also be managed such that a sufficient blanket of fresh water overlays tidal water in order to establish a barrier to vertical mixing.

Source: Stith, B. M., Reid, J. P., Langtimm, C. A., Swain, E. D., Doyle, T. J., Slone, D. H., Decker, J. D. and Soderqvist, L. E. 2010. Temperature inverted haloclines provide winter warm-water refugia for manatees in Southwest Florida. Estuaries and Coasts 33(April 2010). DOI: 10.1007/s12237-010-9286-1.

Florida Bay’s Methylmercury Problem Falls from the Sky, Study Says

 For more than a decade, authorities have recommend significantly limiting consumption of many species of fish caught in Florida Bay, due to levels of methylmercury in fish tissue that exceed what is safe for human consumption. In order to address this serious ecological and economic problem, the sources of the contaminant must be known. Previous work in this system has concluded that the main source of mercury to the bay is drainage inputs from the Everglades, which has its own documented mercury problem. A recent study set out to better understand the bay’s sources of mercury, especially in relation to discharges from the mainland watershed. One fundamental reason for the study is the massive Everglades restoration that will alter the quantity, quality, timing, and distribution of water delivered to Florida Bay. More flow from the Everglades into the bay might mean even more mercury loading.

Surface water and sediment samples were collected along two transects into northeast Florida Bay and one reference site in the open bay. Total mercury (THg), methylmercury (MeHg, the more bioavailable and therefore more dangerous form), and rate of methylation (the addition of a methyl group to a compound) were measured for all samples. From this information, along with data on flow into the bay and estimates of atmospheric deposition of mercury, the relative importance of mercury inputs from various sources could be assessed. If most of the mercury loading was coming from the Everglades, the investigators would have expected to see a decreasing gradient in mercury levels from the source to the bay. However, no unidirectional gradient was observed in either THg or MeHg. Instead, concentrations of both forms were elevated at intermediate sites in mangrove transition zones. This pattern, along with results of methylation assays, indicates that direct atmospheric deposition of THg to the basins and in situ methylation are likely the most important factors determining observed mercury levels in the bay. These findings contradict previous assumptions that the salinity and sulfur content of bay sediments prevented significant methylation.

The authors recommend continuing the important work of monitoring levels of mercury in Florida Bay game fish, and suggest that research on fluxes and bioavailability of mercury in this system, as well as on sediment and water quality factors that may be controlling mercury inputs, is needed.

Source: Rumbold, D. G., D. W. Evans, S. Niemczyk, L. E. Fink, K. A. Laine, N. Howard, D. P. Krabbenhoft and M. Zucker. 2010. Source identification of Florida Bay’s methylmercury problem: mainland runoff versus atmospheric deposition and in situ production. Estuaries and Coasts(33 June 2010). DOI: 10.1007/s12237-010-9290-5.

Long-term Monitoring Supports Nutrients as Source of Chesapeake’s SAV Decline, But Small-scale Factors Matter Too

 It is very clear that submersed aquatic vegetation (SAV) matters to ecosystems and to people, and it is equally clear that there is less of it in most estuaries than there used to be. After decades of declining SAV coverage in the Chesapeake Bay, managers have set not only abundance targets for restoring beds throughout the bay, but also water quality goals in support of SAV restoration. Aerial surveys of existing beds have also been conducted for decades. A recent analysis of this long-term data set and parallel data on water quality in the bay has concluded that, as expected, long-term trends in SAV acreage at large scales in the bay are related to water quality, and to nitrogen loading and concentration. But processes at a variety of scales are also important to maintaining SAV beds.

The investigators examined trends over the entire bay, in different salinity zones separately, and in a number of case studies in smaller areas. SAV coverage for the whole bay increased by 1-28% per year from 1984 to 2006. But trends varied when each salinity zone was examined separately: SAV in the low and medium salinity areas increased by about 166 and 261%, respectively, over the 23-year period, but SAV acreage in the high salinity community is 22%lower now than it was in 1984. Declines in nitrogen loads and concentrations were consistently associated with increases in SAV. The impact of nitrogen levels could be seen in the individual case studies as well, but other factors seemed to come into play at these smaller scales, including water clarity and discrete weather events. Because the factors affecting SAV bed acreage are so complex, and so many species are involved, the authors did not uncover a “tipping point” in the relationship between nutrients and SAV coverage.

Despite the increases in SAV observed in the low and medium salinity communities, SAV acreages are still mostly below restoration targets at all spatial scales. The authors conclude that work is left to be done on nutrient loadings, and so nutrient reduction strategies should still be a priority in all salinity zones.

Source: Orth, R. J., M. R. Williams, S. R. Marion, D. J. Wilcox, T. J. B. Carruthers, K. A. Moore, W. M. Kemp, W. C. Dennison, N. Rybicki, P. Bergstrom and R. A. Batiuk. 2010. Long-term trends in submersed aquatic vegetation (SAV) in Chesapeake Bay, USA, related to water quality. Estuaries and Coasts 33(June 2010). DOI: 10.1007/s12237-010-9311-4.

Study Shows Thin-Layer Dredged Material Disposal Has Little Long-term Impact on Chesapeake Benthic Community

 Ecologists have long studied how communities react and rebound in the face of disturbances in order to learn about succession and community resilience. In the marine environment, one disturbance that can provide valuable information for both ecologists and managers is in-water disposal of dredged material removed from channels and construction projects in the coastal zone. A recent study evaluated the reaction of benthic communities to having dredged material deposited on top of them in the Chesapeake Bay. Species richness, abundance, and community composition were determined after two non-toxic dredged material disposal events in the lower Chesapeake and at two reference sites nearby. The major objective of the study was to determine how disturbance severity (defined here as the thickness of disposed sediment above the ambient sediment surface, or “overburden”) affected the patterns and rates of recovery of the macrobenthic community.

Results indicated that things seem to get back to “normal” fairly quickly, particularly when the depth of sediment deposited at the disposal site is relatively shallow. Macrobenthic communities at sites with low and medium levels of overburden (< 15 cm) were minimally affected by disposal overburdens. At the highest overburdens (>15 cm), it took 1.5 years or less for the study sites to converge with reference sites in terms of species richness, abundance, biomass, and community composition. Recolonization via immigration from nearby areas was apparently more important for reestablishing benthic communities than upward migration of animals through the new sediment layers.

These results provide some evidence that thin-layer disposal of clean material (defined by the U.S. Army Corps of Engineers as overburdens of <30 cm) might have minimal impacts on  benthic communities in the long run. It is important to note that the dredged material in this case was non-toxic; other impacts to benthos might be found if the disposed sediments contained high concentrations of contaminants.

Source: Schaffner, L. C. 2010. Patterns and rates of recovery of macrobenthic communities in a polyhaline temperate estuary following sediment disturbance: effects of disturbance severity and potential importance of non-local processes. Estuaries and Coasts 33(June 2010). DOI: 10.1007/s12237-010-9301-6.