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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 2009


Mexican Study Uses Alternate Method to Corroborate Sediment Chronology
Fringe Benefits: To Provide Critical Services, How Big Does a Marsh Have to Be?
Climate Can Eclipse Nutrient Loading as a Major Influence on Phytoplankton Communities, Says North Carolina Study
Unintended Consequences: Dredging Removes Fringing Vegetation Along with Sediments in Brazilian Lagoon

Mexican Study Uses Alternate Method to Corroborate Sediment Chronology

Three lagoons in the southeastern Gulf of California, Ohuira, Chiricahueto, and Estero de Urias, have been designated as priority biodiversity sites by the Mexican government. As important as these sites are, they are also threatened. The region includes industrialized areas, large tracts of intensively cultivated agricultural land, and shrimp farms, all of which impact the water and sediment quality of the lagoons. In order to assess the extent and history of pollution in these bays, a traditional sediment coring method was used, but with a twist.

The study set out to determine the chronology of cadmium (Cd), copper (Cu), mercury (Hg), and lead (Pb) accumulations in sediment cores taken from the lagoons. Usually, the decay of radioactive 210Pb is used to determine the age of a sedimentary layer, and is confirmed or cross-referenced with 137Cs, which is only present in sediment time horizons during the brief period of atmospheric testing of thermonuclear weapons (from about 1950 to 1963). Because the 137Cs signal is absent from these sediments (typical of equatorial and southern hemisphere sites where 137Cs atmospheric flux was low due to distance from nuclear testing sites and lack of precipitation), the sediment profiles of the above suite of heavy metals was used instead, as their concentrations can be tied to specific environmental modifications and human activities. For example, surges in sediment concentrations of Cu and Hg, both used in agricultural applications, corresponded well with the expansion of agriculture in the region. In Estero de Urias, Hg sediment enrichment corresponded with an almost twenty-fold increase in the capacity of a nearby thermoelectric generating plant.

Peak concentrations of Hg and Pb exceed sediment quality guidelines (NOAA-SQuiRTs) in all three lagoons, Cd in two lagoons, and Cu in one. Because this contamination represents a serious threat to the biota and environmental quality of these lagoons, further monitoring is recommended. This study may serve as a model for investigations of sediment contamination in other regions where 137Cs levels are undetectable, such as in the southern hemisphere.

Source: C Ruiz-Fernandez, A., M. Frignani, C. Hillaire-Marcel, B. Ghaleb, M. D. Arvizu, J. R. Raygoza-Viera, and F. Páez-Osuna. 2009. Trace metals (Cd, Cu, Hg and Pb) accumulation recorded in the intertidal mudflat sediments of three coastal lagoons in the Gulf of California, Mexico. Estuaries and Coasts 32 (DOI 10.1007/s12237-009-9150-3).

Fringe Benefits: To Provide Critical Services, How Big Does a Marsh Have to Be?

The values of salt marshes are now well-understood: they provide nursery grounds, protect landward areas from storm surge, and are important components of estuarine food webs. Do narrow ribbons of marsh, called fringing marshes, provide the same benefits as larger “meadow marshes?” Results of a recent study indicate that, square meter for square meter, the services fringing marshes provide are generally the same as their larger counterparts.

The investigators compared a range of functions of meadow and fringing marshes (and, in some cases, areas without marsh) in New England, also examining how physical characteristics of the sites (size, elevation, surface slope, soil salinity) affect marsh function. They found that primary productivity of fringing marshes is as great as that of meadow marshes, whereas the organic content of meadow marsh soils is three times greater than that of fringing marshes. The rate of sediment deposition was highly variable but tended to be higher in fringing marshes than meadow marshes. Although meadow marsh species richness and density is greater than that of fringing marshes, the plant communities of the two types of marsh are comparable and the relative abundances of species are similar. Finally, fringing marshes are as capable of dissipating wave energy as the larger marshes, so they are likely to provide the function of protecting adjacent shorelines from the erosive force of waves.

This study is among the first to consider fringing marshes as unique habitats, distinct from the larger meadow marshes. Although most conservation efforts focus on the larger marshes, this study demonstrates that fringing marshes provide many of the same benefits, but are also in some ways ecologically distinct from the larger versions. For these reasons, further studies of these smaller marshes are warranted.

Source: Morgan, P. A., D. M. Burdick, and F. T. Short. 2009. The functions and values of fringing salt marshes in northern New England, USA. Estuaries and Coasts 32 (DOI 10.1007/s12237-009-9145-0).

Climate Can Eclipse Nutrient Loading as a Major Influence on Phytoplankton Communities, Says North Carolina Study

Estuaries are such complicated ecosystems that it is amazing that we understand as much as we do about their ecology. In addition to fluctuations in temperature, salinity, and other physical factors at a range of spatial and temporal scales, estuaries are frequently affected by anthropogenic activities ranging from habitat destruction to nutrient enrichment. Add to that list global climate change, and the result is a very complex ecosystem.

For example, it can be difficult to tease apart the effects of human-induced nutrient loadings and climatic influences on phytoplankton communities. In some places, climatic effects such as an increase in rain events seem to be growing stronger, frequently eclipsing the influences of other drivers. In the Neuse River Estuary, NC, a nitrogen-limited system, an examination of two long-term data sets revealed that freshwater inputs to the estuary (and the “new” nutrient loads they deliver) are a significant driver of the location, magnitude, and community composition of phytoplankton biomass. One such impact was observed in the wet spring of 2003, where large freshwater pulses led to marked increases in photopigment concentrations. Fast-growing cryptophytes showed an immediate, rapid response to the increased flow and nutrient enrichment of that rainy period. Then, as summer progressed and flow decreased, cyanobacteria became a more dominant component of the phytoplankton community. Similar responses were observed for events at a range of temporal scales (hurricanes, storms, etc.), but these impacts depended also on winds, mixing, salinity, and other factors.

Global climate change is likely to bring about even more variability in the timing and severity of hydrological events, which may mean an even more complex knot to untangle with respect to impacts on phytoplankton communities. As climate impacts become increasingly important, it will be critical to incorporate climate drivers such as hydrologic events and variability into eutrophication models aimed at predicting the effects of nutrient management on water quality.

Source: Paerl, H. W., K. L. Rossignol, N. S. Hall, B. L. Peierls, and M. S. Wetz. 2009. Phytoplankton community indicators of short and long-term ecological change in the anthropogenically and climatically impacted Neuse River Estuary, North Carolina, USA. Estuaries and Coasts 32 (DOI 10.1007/s12237-009-9137-0).

Unintended Consequences: Dredging Removes Fringing Vegetation Along with Sediments in Brazilian Lagoon

In addition to being introduced to shallow coastal systems via surface or ground water, nutrient loads can also come from in situ sediments with a legacy of nutrient pollution. One solution is dredging -- simply removing those sediments that serve as a source of nutrients to the system. But dredging projects must be designed carefully; at some sites dredging can do more harm than good, such as when fringing vegetation will be removed by the dredging process. Marsh vegetation has been shown to assimilate nutrients, and removal of that vegetation might, paradoxically, make the nutrient problem worse.

Such was the case in Imboassica Lake, a coastal lagoon in Brazil that receives significant sewage inputs. Nitrogen and phosphorus levels were measured in the lagoon and in the river that feeds it over the course of 13 years, twice before the dredging project and once afterward. The dredging project, implemented to reduce nutrient pollution in the lagoon, eliminated an area of fringing wetland measuring 2 km long and 7 km wide. No other nutrient reduction techniques were implemented along with the dredging. Before the dredging, nutrient concentrations were elevated upstream of the lagoon. After the dredging project, upstream nutrient concentrations declined, but N and P concentrations in the lagoon itself actually increased. The authors contend that the removal of the lagoon’s fringing wetlands led to the increase in the lagoon’s nutrients, as the vegetation’s nutrient assimilation function was removed.

Such projects must be designed carefully to avoid such unintended consequences. Ideally, dredging should be used in consort with other nutrient reduction techniques.

Source: Marotta, H., L. Bento, F. de A. Esteves, and A. Enrich-Prast. 2009. Whole ecosystem evidence of eutrophication enhancement by wetland dredging in a shallow tropical lake. Estuaries and Coasts 32 (DOI 10.1007/s12237-009-9152-1).