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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 2004


Archival Data Elucidates the Fishy History of Lake Pontchartrain
Estuarine Sediments: Source or Sink for Heavy Metals?
Gaining Clarity: Model Supports Oyster Restoration to Reduce Turbidity, Encourage Seagrass Growth
Restoration of Seagrass Inhabitants Takes Some Time, Study Finds

Archival Data Elucidates the Fishy History of Lake Pontchartrain

Lake Pontchartrain, New Orleans' local estuary (despite its freshwater-sounding name), has been subjected to many of the environmental challenges of other urban estuaries: runoff of nutrients and toxics, shoreline alterations, industrial discharges, dredging and more. A recent study in the Lake set out to determine whether these actions have had an impact on fish assemblage stability over a fifty-year period for which the investigators were able to analyze data. This kind of analysis is notoriously difficult because of the high inherent variability of estuaries and the mobility of the fish assemblage compared to other biota, so an archival data set is a prerequisite for finding any impact of environmental degradation. Using a combination of trawl, beach seine and gill net data (representing demersal, nearshore and pelagic habitats respectively), the authors used canonical correspondence analysis to examine the Pontchartrain fish assemblage in four periods relative to environmental variables such as salinity, temperature and Secchi depth. The four periods varied hydrologically, and included a moderately dry period (1954), a high rainfall year (1978), a wet period (1996-98) and a drought (1998-2000).

Among their findings: salinity, temperature and Secchi depth were all related to assemblage composition, however, even more important was a unidirectional change in fish assemblage over time, indicating a response to environmental degradation. A decline in benthic Atlantic croaker and an increased occurrence of pelagic bay anchovy were primarily responsible for the changes observed. Changes in the fish assemblage were most pronounced among benthic species (perhaps attributable to alteration of bottom habitats by dredging) and less apparent in the other two habitats studied.

None of the patterns they observed would have been detected without the availability and analysis of a long-term data set, an important lesson for others interested in conducting this type of project (see also the Hurst et al. paper in the last issue of CESN).

Source: O'Connell, M. T., R. C. Cashner, and C. S. Schieble. 2004. Fish assemblage stability over fifty years in Lake Pontchartrain estuary: Comparisons among habitats using canonical correspondence analysis. Estuaries 27(5): 807-817. (View Abstract)

Estuarine Sediments: Source or Sink for Heavy Metals?

Toxic heavy metals in estuaries have long been a problem for organisms ranging from phytoplankton, which bioaccumulate these harmful contaminants, to humans, who can ingest them when consuming metal-contaminated fish and shellfish. The common perception that heavy metals are not bioavailable - that is, they cannot be incorporated into the food chain - once they become associated with sediment particles, is refuted by a large body of research recently reviewed in Estuaries.

The available data demonstrate that for bivalves living in or on the sediment and using a variety of feeding modes, sediments may actually be a source, rather than a permanent sink, of many metals. Much of the literature reviewed shows that metals can be remobilized into the food chain after ingestion by mussels and clams. The authors use a simple biokinetic model to illustrate that ingestion of particle-bound metals can be an important route of uptake as compared to uptake of dissolved metals in pore water or the water column. They also discuss how the assimilation efficiency (the fraction of metal in food taken up by the organism) can be affected by many factors, including species differences in gut passage time and gut pH, and differences among metals.

While this review provides excellent evidence that sediments do not permanently sequester metals, the authors emphasize that more research is needed on the use of radiotracers to measure bioavailability, exposure of bivalves to metals dissolved in pore waters, changes in metals concentrations in organisms during periods of changing food availability, and the relationship between toxicity and uptake route (food vs. water).

Source: Griscom, S. B. and N. S. Fisher. 2004. Bioavailability of sediment-bound metals to marine bivalve molluscs: An overview. Estuaries 27(5): 826-838. (View Abstract)

Gaining Clarity: Model Supports Oyster Restoration to Reduce Turbidity, Encourage Seagrass Growth

The loss of seagrass beds due to increased turbidity and reduced light penetration is an acknowledged problem in estuaries throughout the U.S., particularly in the mid-Atlantic. What caused the increased turbidity? Declines in filter-feeding shellfish populations have contributed, and a feedback loop involving the grasses themselves may exacerbate the losses: Seagrass beds tend to dampen water movement, causing particles to settle and water clarity to increase. A recent modeling study which set out to elucidate some of the interactions among these two mechanisms came to some valuable conclusions about shellfish restoration as well.

The relatively simple model was parameterized using both literature-derived values and the authors' own field and lab studies. Among their experimental conclusions: at high seagrass densities in spring, long reproductive seagrass shoots affected wave attenuation, in turn lowering sediment resuspension. In the lab, the investigators observed that oysters can clear more water than clams and therefore have a greater effect on light penetration.

The model simulated suspended material concentration, water clarity and seagrass shoot density as a function of shellfish biomass and filtration rate, sediment load and sediment resuspension. Model output predicted that even modest concentrations of Eastern oysters reduced suspended sediment by nearly an order of magnitude. Although the authors caution that the model includes some simplifying assumptions, such as an unrealistically uniform distribution of oysters in the modeled area, they conclude that oyster restoration projects have the potential to reduce turbidity sufficiently to enhance seagrass beds. They further suggest that in light of these results it makes sense to coordinate oyster and seagrass restoration efforts, particularly in energetic environments.

Source: Newell, R. I. E. and E. W. Koch. 2004. Modeling seagrass density and distribution in response to changes in turbidity stemming from bivalve filtration and seagrass sediment stabilization. Estuaries 27(5): 793-806. (View Abstract)

Restoration of Seagrass Inhabitants Takes Some Time, Study Finds

One of the greatest challenges of any type of habitat restoration project is to know whether it has actually worked. While the restored area may "look right," it's harder to tell if its typical residents have returned and its actual structure is equivalent to a natural area. And how long do you continue to monitor the restored site, anyway? For seagrasses, the record of restoration success has been as patchy as many of the restored areas themselves, with few projects reaching their acreage goals. However, some studies suggest that once restored beds persist, fish and crab densities are indistinguishable from those in natural beds after several years. A recent study comparing restored seagrass beds to nearby natural beds in Corpus Christi, Texas, provides some insight. The investigators looked at numbers and diversity of a range of biota in pairs of restored (3-8 years post-restoration) and natural seagrass beds.

Results indicated that full "structural equivalence" between restored and natural sites in this Gulf location probably does not occur for at least 8 years post-restoration (providing one clue as to how long monitoring must be conducted!). For fish and decapods, more taxa and individuals were collected from restored than natural seagrass beds. However, for benthos, greater densities of more taxa were observed in natural than restored beds, suggesting that benthic communities were not yet restored to their natural state. Overall, sampling period (time of year) contributed more to the observed community differences than type of seagrass bed.

An intriguing possibility that emerged from this study is that because some worm species were consistently found at higher densities in the natural seagrass beds, these species could be used as success indicators for seagrass restorations. The author also recommends that a solid monitoring program must minimally include seagrass shoot density or biomass, persistence of coverage, and biannual biota assessments for at least 5 years post-restoration, all in comparison to natural beds.

Source: Sheridan, P. 2004. Comparison of restored and natural seagrass beds near Corpus Christi, Texas. Estuaries 27(5): 781-792. (View Abstract)