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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 2009


Long-term Records Show that for Nitrogen, Losses are as Important as Loads
Fluorescence and Reflectance Indices are Promising as Early Warning Signals of Effects of Herbicide on Marsh Plants
For Coastal Ecosystems, Maybe You Can’t Go Home Again
Invasive Plants Maybe Not Such Bad News for Northern Gulf of Mexico Invertebrates

Long-term Records Show that for Nitrogen, Losses are as Important as Loads

Many studies have shown conclusively that eutrophication in coastal systems is due in large part to increases in nutrient loading, particularly of nitrogen. Nowhere is this framework more established than in the Chesapeake Bay. But there is another side to the nitrogen equation which is critical as well: nitrogen loss from the system, in the form of denitrification, the anaerobic process of converting nitrate to nitrogen gas which is then returned to the atmosphere. A recent paleoecological study of the Chesapeake makes the point that modern eutrophication is probably the result of both increases in loading and decreases in denitrification, and stresses that both processes need to be addressed by restoration efforts in order to combat eutrophication and its effects.

Using indicators from sediment cores such as the types of pollen and diatoms present in various time horizons, as well as other historical records, the study reconstructed the history of Chesapeake Bay watershed ecology and eutrophication for the past 14,000 years. In addition to the expected increases in nitrogen input to the bay (due to increases in agriculture and increases in human population and concomitant sewage loadings), the study found that denitrification capacity has decreased in the watershed over time. As agriculture intensified in the bay’s watershed in the early twentieth century, many marshes were drained, decreasing the land available for denitrification. The capacity for denitrification declined further as increases in impervious surfaces led to drier soils and lower groundwater levels.

The study’s author observes that techniques utilized to decrease nitrogen loading to the bay have met with mixed success, and suggests that complementary approaches focused on increasing denitrification should be employed. These approaches might include enhancing existing denitrifying areas in the watershed and adding new ones by planting forest stands and adding retention ditches around agricultural fields.

Source: S Brush, G.. 2009. Historical land use, nitrogen, and coastal eutrophication: a paleoecological perspective. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9106-z).

Fluorescence and Reflectance Indices are Promising as Early Warning Signals of Effects of Herbicide on Marsh Plants

Wouldn’t it be useful to be able to use remote sensing techniques to determine whether wetland plants are stressed by pesticide contamination? A California study of sub-lethal effects of the commonly-used but little-studied herbicide diuron on cordgrass (Spartina foliosa) suggests that this approach may not be far-fetched. In both field and lab studies, the authors found evidence that some fluorescence and spectral reflectance parameters (both related to photosynthetic function) show promise as sensitive, rapid, nondestructive indicators of the response of cordgrass to diuron.

In the laboratory, the investigators exposed plants to two levels of diuron (plus a diuron-free control), and found that while the herbicide did not have an effect on the plants’ morphology, plants exposed to diuron exhibited reduced photosynthetic function, as determined by a range of fluorescence and reflectance indices. These results were verified by the fact that CO2 assimilation in the plants was also reduced. Using an index of pesticide exposure, field sites at two locations were ranked along a gradient of pesticide contamination. Leaf biomass, percent cover, and canopy height varied significantly with herbicide exposure, as did the two most sensitive fluorescence and reflectance indices used in the laboratory experiments (spectral reflective index ARI and fluorescence parameter Fo).

The responses of these two indices in the lab and the field suggest that the parameters might be used successfully as indicators of the response of S. foliosa to herbicides in the field. In particular, ARI shows promise as a sensitive indicator of pesticide-induced plant stress. Measurement of similar parameters is often already undertaken in remote sensing surveys, and so they could be used to determine sublethal stress of plants on a landscape scale, serving as an early warning that plants are being stressed by toxins in the environment.

Source: Williams, S. L., A. Carranza, J. Kunzelman, S. Datta, and K. M. Kuivila. 2009. Effects of the herbicide diuron on cordgrass (Spartina foliosa) reflectance and photosynthetic parameters. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9114-z).

For Coastal Ecosystems, Maybe You Can’t Go Home Again

Many management initiatives rely on the seemingly logical concept that if elevated nutrient loading to a coastal ecosystem results in eutrophication, reduced loads should cause the ecosystem to revert to its pre-eutrophied conditions. While this pattern has been observed in some places, case studies in many estuaries demonstrate that things are often not that simple. In many cases, nutrient trajectories were not directly reversible, and decreases in nutrient loads have not led to expected ecosystem improvements. In a review of data collected from four such case studies (Marsdiep, The Netherlands; Helgoland, Germany; Odense Fjord, Denmark; and Gulf of Riga, Latvia/Estonia), the paper’s authors argue that the failure to return to pre-eutrophied conditions result from broad changes to other aspects of the estuaries’ ecology that have occurred in the intervening decades which render an identical reverse response impossible. These changes include habitat loss, changes to biotic communities resulting from overfishing, global warming, and landscape alterations.

The authors contend that it is up to scientists to better communicate the complex nature of ecosystem interactions to policy-makers so false hopes will not be raised that nutrient reductions will automatically result in a return to some historical baseline. At the same time, nutrient reductions need to continue to be encouraged as part of more comprehensive ecosystem management programs. The authors state, “Emphasis in returning ecosystems to a particular past state, an unlikely outcome in a world of shifting baselines, should be replaced by targets ensuring the maintenance of key ecosystem functions and, thereby, the constant supply of valuable ecosystem services to society.” At the same time, research priorities must ensure that we learn more about ecosystem interactions so realistic trajectories can be predicted.

Source: Duarte, C. M., D. J. Conley, J. Carstensen, and M. Sánchez-Camacho. 2009. Return to Neverland: Shifting baselines affect eutrophication restoration targets. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9111-2).

Invasive Plants Maybe Not Such Bad News for Northern Gulf of Mexico Invertebrates

While the serious and growing phenomenon of invasive aquatic species has concerned managers and scientists for decades, some studies have shown that not all invasives lead to the downfall of an ecosystem. In some cases, the exotic species has even been shown to benefit the native ecology in some way. One case in point is a recently-published study which examined the impact of aquatic invasive plants on production of macroinvertebrates in the Mobile-Tensaw Delta (northern Gulf of Mexico) in order to draw conclusions about the impacts of invasives on the base of the estuarine food web there. Entire plants of the three species studies – the exotic plant Myriophyllum spicatum and the natives Heteranthera dubia and Vallisneria americana – were collected from April – December at four sites. Surface area, biomass, and structural complexity measurements were carried out on each plant, and all associated macrofauna species were identified, counted, and weighed.

Macroinvertebrate production was actually observed to be three times greater on the non-native M. spicatum than on either of the native SAV species. Production on M. spicatum was very high, almost three times greater than any other report in the literature for any SAV species, mostly due to the abundance of one amphipod species. The differences might have been attributable to plant surface area, which explained 98% of the variance in macrofaunal biomass: M. spicatum and V. americana had the highest surface areas, and M. spicatum was also the most complex. Further laboratory experimentation revealed that the differences were probably not attributable to invertebrate grazing on the plants. Instead, the authors believe it is probable that the high production within the more complex plant species was the result of reduced predator foraging efficiency. Further work is required in order to determine whether the apparent enhanced production might translate up the Delta food web.

Source: Chaplin, G. I., and J. F. Valentine. 2009. Macroinvertebrate production in the submerged aquatic vegetation of the Mobile-Tensaw Delta: Effects of an exotic species at the base of an estuarine food web. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9117-9).