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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.

November 2010


Effects of Nutrient Enrichment Go Underground
Modeling Says Delta Smelt May Become Victims of Shrinking Habitat
Good-bye, Bycatch: Helping Crab Pots Catch Fewer Terrapins
Does Habitat Disturbance in a VA Shallow Tidal Creek Disturb the Fish?

Effects of Nutrient Enrichment Go Underground

Nutrient loading to coastal waters has long been known to bring with it myriad unwanted effects, including hypoxia, nuisance algal blooms, and food web changes. Excess nutrients have also been shown to increase above-ground biomass of marsh plants, but what happens below the marsh surface? A recent study indicates that in the case of older, deeper marsh soils, higher nutrient loadings seem to sap soil strength.

Soil strength is the capacity of the soil to withstand stresses from currents, winds, and other forces and therefore resist erosion. This parameter, not often referred to in the coastal and estuarine science literature, is measured using a shear vane, an instrument inserted into the soil that measures the resistance (torque) when a rod fitted with vanes is inserted into the soil and then rotated. In this study, above- and below-ground marsh plant biomass and soil strength were measured in experimental marsh sites in Louisiana to which N, P, and both were added over the course of 1-5 years.

The higher nutrient loadings led to larger above-ground marsh plant biomass but lower below-ground biomass (marsh plants receiving high nutrient loads do not need very deep roots to obtain nutrients, thus reducing the below-ground biomass). In the surface soil layers the nutrient additions did not seem to make a difference in soil strength. However, for older soil layers 60-100 cm below ground, the plots with experimental nutrient additions displayed significantly lower soil strength. In fact, the higher the cumulative nutrient load, the lower the soil strength over the range of loadings tested.

Why the decline in soil strength at the older time horizon? The author explains that soils at the 60-100 cm depth in these plots are at least 100 years old, predating modern-era declines in water quality. Perhaps the exposure of these soils to a new source of P speeds up soil microbes enough to cause the loss of soil strength through organic matter decomposition. Other factors are also likely at work.

Higher above-ground biomass, weaker below-ground soil strength, smaller root systems, and the likelihood of more, and more severe, storms in the future may add up to marshes’ being more easily converted to open-water habitats. Because these coastal wetlands protect shorelines and inland areas from storm surges, these results provide yet another argument for improving water quality and raise questions about the impacts of river diversions built to restore wetlands in south Louisiana.

Source: Turner, R. E. 2010. Beneath the salt marsh canopy: loss of soil strength with increasing nutrient loads. Estuaries and Coasts 33(November 2010). DOI: 10.1007/s12237-010-9341-y.

Modeling Says Delta Smelt May Become Victims of Shrinking Habitat

In many ways, delta smelt are already a species living on the edge: an annual fish with an extremely limited distribution (they are found solely in low-salinity zones in the San Francisco Estuary), they are listed as threatened by the federal government and endangered by the state of California. Because fresh water entering the San Francisco Estuary is subject to contentious water withdrawals, the fish might be at further risk. What happens to the smelt if there are future changes in freshwater outflow due to coastal development and climate change?

A modeling study examined more than 40 years of data on smelt distribution and abiotic factors in the estuary to determine habitat suitability for delta smelt and to predict habitat changes under various future scenarios. The best habitat for smelt appears to be where there is low (about 2 ppt) salinity and high turbidity, in an area between 71 and 85 km upstream of the Golden Gate Bridge. The location of optimum habitat migrates depending largely on freshwater outflow. Over the time frame studied (1967-2008), an annual habitat index developed by the investigators, incorporating both habitat quantity and quality, declined by 78%. Seven future scenarios with varying freshwater outflows were examined, ranging from present-day conditions and water demands to dryer and wetter conditions predicted by various climate change models. Under all future scenarios, the habitat index for delta smelt is predicted to decline, most of all in low-flow years.

The authors outline the uncertainty in their study, including the omission of other potentially important habitat elements and the possible additional reduction in habitat suitability with increasing water temperatures caused by climate change. However, it remains clear that in managing this endangered species it will be critical to grapple with the difficult issues of climate change and water demands in California.

Source: Feyrer, F., K. Newman, M. Nobriga, and T. Sommer. 2010. Modeling the effects of future outflow on the abiotic habitat of an imperiled estuarine fish. Estuaries and Coasts 33(November 2010). DOI: 10.1007/s12237-010-9343-9.

Good-bye, Bycatch: Helping Crab Pots Catch Fewer Terrapins

The types of places where mid-Atlantic recreational (and some commercial) crabbers find success in setting pots for blue crabs – shallow-water areas often near tidal wetlands – happen to be the same habitats favored by diamondback terrapins, the only fully estuarine turtle in the Western Hemisphere. In order to keep terrapins out of the pots, where they often drown, some states are now requiring bycatch reduction devices, or BRDs, that prevent the turtles from entering the traps. These BRDs were recently tested in Virginia shallow subtidal waters where recreational and commercial peeler pots are often located, and derelict traps sometimes drift. The investigators baited the traps infrequently to mimic the recreational fishery (of course, derelict pots are never baited). Catch of crabs and terrapins was assessed in pots with and without BRDs.

BRDs were successful in keeping terrapins out of the pots: all 51 terrapins captured during the study were found in pots without BRDs installed. Pots with BRDs collected fewer crabs, however these differences were not statistically significant because of high catch variability. Interestingly, presence of terrapins seemed to depress capture of crabs, as total crab catch per unit effort was significantly lower in traps containing trapped terrapins. An additional bonus was that BRDs also reduced incidental capture of fish.

This study likely has the most applicability for the recreational fishery, where pot sites tend to overlap more with terrapin habitat. However, the authors suggest that rather than targeting a specific crabbing group, a more effective management strategy would be to use BRDs on pots deployed in terrapin habitat, regardless of fishery sector.

Source: Morris, A. S., S. M. Wilson, E. F. Dever, and R. M. Chambers. 2010. A test of bycatch reduction devices on commercial crab pots in a tidal marsh creek in Virginia. Estuaries and Coasts 33(November 2010). DOI: 10.1007/s12237-010-9330-1.

Does Habitat Disturbance in a VA Shallow Tidal Creek Disturb the Fish?

Shallow-water habitats, such as tidal flats and creeks, are likely to be especially vulnerable to habitat disturbances such as dredging operations, and unlikely to be protected by comprehensive management plans. A recent study in a set of tidal creeks in the Chesapeake Bay system tackled the question of whether dredging has an impact on fish community characteristics, including diversity, abundance, biomass, and size structure. These parameters were measured in paired dredged and undredged tidal creeks, and related to watershed, shoreline, and physiochemical characteristics. One of the sites had been dredged six months prior to sampling, and two had been dredged several years earlier.

Generally, dredging didn’t seem to be a major factor influencing these fish populations. No significant differences were observed between the dredged and undredged sites for fish abundance, total biomass, diversity, or evenness. However, subtle differences were found in size structure and abundance of a few key species. Specifically, young-of-the-year silversides and bay anchovy were less abundant in the most recently-dredged creek, possibly indicating that this creek provides less suitable nursery habitat for some species. The results also showed that turbidity highly influenced differences in species abundances among all creeks, regardless of whether they had been dredged.

The author speculates that protective measures implemented by the state and the presence of small fringing marshes may be helping to maintain diverse fish communities at the dredged sites. In general, protective measures such as marsh preservation and restrictions on depth or timing of dredging, might mitigate development and dredging pressures at other sites as well.

Source: Bilkovic, D. M. 2010. Response of tidal creek fish communities to dredging and coastal development pressures in a shallow-water estuary. Estuaries and Coasts 33(November 2010). DOI: 10.1007/s12237-010-9334-x.