CESN Main Page

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.

August 2005


Modern Genetic Techniques Aid in Managing an Ancient Fish
Golf Course Swales May Reduce Water Hazards
Seagrass Mapping Down Under: Challenges of New Technology
A Caution on the Use of Elemental and Stable Isotope Ratios: Do You Know Your Species' Natural Variability?

Modern Genetic Techniques Aid in Managing an Ancient Fish

Armor-plated and whiskery, sturgeon look more like dinosaurs than fish, and some species have become as rare as they are ancient. Shortnose sturgeon, whose historic range extends from the St. Johns River, Florida to the St. John River, Canada, have been on the Federal Endangered Species List since 1967. Managing shortnose sturgeon turns out to be tricky: Although the National Marine Fisheries Service team tasked with shepherding their recovery recognizes 19 separate population segments, the species has actually been managed coast-wide as a single unit for lack of conclusive evidence supporting the existence of these separate stocks. A recent study set out to provide some of that evidence, characterizing the population structure of shortnose sturgeon throughout their range.

Analysis of DNA from specimens collected from 11 of the 19 putative population segments was used to construct a "family tree" indicating the relatedness among sturgeon in different river systems and the degree of gene flow among them. The results confirm that most of the NMFS Recovery Team's stated populations are, in fact, genetically distinct units. Genetic diversity of the species coast-wide was generally high, and most of the rivers tested contained genetically distinct populations. An interesting exception is the similarity between the Chesapeake and Delaware Bay populations, which the investigators attribute to dispersal of fish from the Delaware into the Chesapeake through the C&D Canal. At least 9 populations, corresponding to individual rivers or sets of neighboring rivers, were identified in this analysis.

These results support the Recovery Team's contention that individual management plans should be tailored to the population trends of each shortnose stock. While some of the populations are actually on the rise, and might even be considered for delisting, other populations need more help. The high degree of genetic isolation of populations observed in this study indicates that repopulation of rivers with extirpated or extremely small populations via migration from nearby rivers is highly unlikely.

Source: Wirgin, I., C. Grunwald, E. Carlson, J. Stabile, D. L. Peterson and J. Waldman. 2005. Range-wide population structure of shortnose sturgeon Acipenser brevirostrum based on sequence analysis of the mitochondrial DNA control region. Estuaries 28(3): 406-421. (View Abstract)

Golf Course Swales May Reduce Water Hazards

The beautiful manicured greens of golf courses, an increasingly familiar sight in many places in the U.S., usually come at a price to the environment. The fertilizers applied to the greens and the wastewater used for their irrigation can increase the loading of excess nutrients, specifically nitrate, to nearby waterways. Nowhere is this potential conflict more obvious than in the golfers' paradise of Hilton Head Island, S.C., home to more than 50 golf courses, many of which are adjacent to picturesque tidal creeks unique to the Southeastern U.S. One of Hilton Head's golf courses was the site of a comparative study of nitrogen processing in four types of environments: a grassy golf course swale, a marsh adjacent to the golf course, a relatively undeveloped marsh site, and a freshwater site in an undeveloped forest area. The study measured background concentrations of nutrients in the sediments and overlying waters at each site as well as the conversion of nitrate to either N2 gas or ammonium. Surface water nitrate levels were highest by far at the golf course swale site, where the highest potential denitrification (conversion to N2 gas) rates were also observed (up to 88% conversion). These high rates of denitrification in the swale suggest that the swale may successfully "filter out" some of the excess nitrogen before it is washed into the adjacent marsh.

The study also found that more denitrification took place at the sites with more organic carbon in the sediments. If development affects organic carbon, for example, if native plants and sediments are altered due to changes in land use, denitrification rates are therefore likely to be changed, influencing the susceptibility of "downstream" areas to the negative impacts of excess nitrogen loading.

Source: Tuerk, K.J.S. and C.M. Aelion. 2005. Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast. Estuaries 28(3): 364-372. (View Abstract)

Seagrass Mapping Down Under: Challenges of New Technology

Tracking temporal trends in estuarine seagrass beds has traditionally used the most appealingly basic approach imaginable: comparing pictures (usually aerial photos) taken of the site of interest at different times. Although this approach seems straightforward, improvements in analytical tools available to interpret the photos actually pose a problem. Are observed changes in seagrass beds real, or an artifact of the different techniques used to examine the photos?

In the late 1970s and early 80s a study of seagrass beds in Australian estuaries was conducted using analog mapping of aerial photos (referred to as the Camera lucida method or CL). The analysis of these photos was conducted by hand-drawing polygons on the photos to circumscribe patches of vegetation and measuring the areas of the polygons. Four of these estuaries were remapped in 2003 using GIS techniques to analyze the photos, and a team of researchers became interested in whether the change in methodology introduced error to the analysis. They also wanted to determine whether "interoperator" error plays a role in this type of study, when different individuals conduct the analyses in different years or in different parts of the study in the same year.

They found that while intraoperator error was consistently very low, in one case interoperator error accounted for a 130% difference in estimates of sea grass area. When comparing the two analytical techniques by using the new GIS approach to examine the older photos, more error was revealed. The CL approach tended to result in higher estimates of seagrass area. In three of the estuaries studied the difference was moderate (8-20%) but in the fourth it was much larger (243%). These errors had a substantial impact on trend analysis for the seagrass beds and therefore on perceived management status. In one estuary the CL method showed a catastrophic 65% decline in seagrass bed area but use of GIS on all photos showed a modest 14.7% decline.

As methods continue to improve and GIS becomes more available to all coastal managers, it makes sense to conduct at least simple assessments of interoperator and systemic error before assuming that GIS-analyzed data can be accurately compared to a baseline determined the "old-fashioned" way.

Source: Meehan, A.J., R.J. Williams and F.A. Watford. 2005. Detecting trends in seagrass abundance using aerial photograph interpretation: Problems arising with the evolution of mapping methods. Estuaries 28(3): 462-472. (View Abstract)

A Caution on the Use of Elemental and Stable Isotope Ratios: Do You Know Your Species' Natural Variability?

Simply by examining the ratios of carbon, nitrogen and phosphorus, and of the stable isotopes of N and C, in marine plants (and other organisms), scientists can determine nutrient sources, sinks and cycling in a particular ecosystem, study the travel of energy through food webs, and ask whether sewage-derived pollution is present. However, this popular and versatile tool is only as useful as an investigator's understanding of the natural temporal and spatial variability of the ratios of interest. An intensive study of these ratios in turtlegrass (Thalassia testudinum) in the Florida Keys reveals that the variability may be higher than initially thought.

Ratios were measured in Thalassia samples taken over 8 years at stations throughout the Florida Keys National Marine Sanctuary. The C:N, C:P, ?13C and ?15N values were quite variable in both space and time, and in fact expanded the worldwide range of ratio values previously measured for this species. Interestingly, Thalassia samples collected in nearshore areas at sites close to sources of sewage pollution were not enriched in the heavy N isotope as expected. This result indicates that for this species there are no unambiguous threshold values of ?15N that indicate the presence of sewage-derived N.

This study warns that elemental and isotope ratio data for this species could be easily misinterpreted, especially if analyses are based on a single set of samples. Particularly problematic is the use of the ratios to determine sources of nitrogen and biogeochemical processing of C, N and P. "Due diligence" must be undertaken to ensure that variability in the ratios of interest are well understood before major conclusions, especially with respect to N sources, are drawn.

Source: Fourqurean, J. W., S. P. Escorcia, W. T. Anderson and J.C. Zieman. 2005. Spatial and seasonal variability in elemental content, d13C, and d15N of Thalassia testudinum from South Florida and its implications for ecosystem studies. Estuaries 28(3): 447-461. (View Abstract)