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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.

July 2004


Three Models Are Better Than One For Examining Gulf "Dead Zone"
Looong-term Sea Surface Temperature Record Available for New England
Priority-setting for Nutrient Management: Case Study in Waquoit Bay
What is the Fish-Killing Culprit in Dead-end Canals?

Three Models Are Better Than One For Examining Gulf "Dead Zone"

The coastal Gulf of Mexico's spring and summer hypoxic "dead zone" has been growing, from 8,300 km2 in 1985-1992 to 16,000 km2 in 1993-2001, as has the body of research and management initiatives designed to address it. One such initiative is a 2001 Federal-State-Tribal Action Plan which calls for a 30% reduction in nitrogen (N) load to reduce the hypoxic area to less than 5000 km2 by 2015. A recent study used the results from three different mathematical models to evaluate whether this goal is achievable, to estimate when the large-scale hypoxia began, and to quantify the relationship between N loading and Gulf hypoxia. The results from these three very different models agree that the Gulf's serious hypoxia problems probably began after the mid-1970s and that the Action Plan goal for reduction of hypoxic area may not be reached with the proposed 30% N reduction.

The simplest of the models is a one-dimensional model used to predict the area of summertime hypoxia from Mississippi and Atchafalaya River N loads. Runs of this model suggest that extensive hypoxic regions probably did not occur prior to the mid-1970s and that N reductions of 40 to 45% are needed to reach the Action Plan goal during most years. A more complex, two-layer, time-dependent model of oxygen dynamics at one particular location off the Louisiana coast suggests that summer oxygen concentrations at this site were fairly consistent between 1955 and 1969 and that hypoxic events did not likely occur before 1975. Running simulations with this model where N loads were decreased by 30%, as called for in the Action Plan, only resulted in a decrease in number of hypoxic years during the simulation (1955 - 2000) from 19 to 12. Finally, a three-dimensional food web, nutrient and oxygen dynamics model for the Louisiana inner shelf was used to predict that a 30-50% decrease in N load would result in a 35-50% increase in bottom-water oxygen concentration.

The authors include a "climate caveat": Because management actions will be implemented, and have their resultant effects, over multiple decades, the effects of climate change will have to be taken into account as managers work to remedy the Gulf's hypoxia problems.

Source: Scavia, D., D. Justic and V. J. Bierman, Jr. 2004. Reducing hypoxia in the Gulf of Mexico: Advice from three models. Estuaries 27(3): 419-425. (View Abstract)

Looong-term Sea Surface Temperature Record Available for New England

To coastal managers and scientists, valuable ocean treasure often refers not to Spanish doubloons but to newly-discovered data sets measuring historical environmental conditions. A recent Estuaries paper reveals the availability of such a treasure: perhaps the longest nearly continuous coastal sea surface temperature record in North America. The data set, for Great Harbor in Woods Hole, MA, begins in 1886 and runs to the present with remarkably few gaps. Although methods of data collection varied in the time of day, location in the water column, and equipment used (thermometer readings were replaced with thermistors in 1997), intercalibration is provided that demonstrates that the data integrity was not compromised by these differences.

In addition to informing the research and management communities about the availability and quality of this data set, the authors examined the data for trends that might be reflective of global climate change. They found that the record exhibits a distinct inflection point around 1970 with a significant increase in mean annual temperature from 1970 through 2002, while temperatures prior to 1946 fluctuated but did not increase or decrease significantly. The temperature increase observed in these data is larger than recent estimates of world-wide sea surface warming ascribed to global climate change, and is more pronounced than changes observed in areas of the southern Atlantic coast, such as Key West, FL.

Long-term records such as this one are extremely valuable for describing or even predicting changes in biological conditions. Climate change in New England is believed to play a role in changes in fish populations and winter and spring phytoplankton blooms, ctenophore population explosions, and declines in eelgrass. This temperature record will continue to prove invaluable as scientists attempt to unravel these relationships.

Source: Nixon, S. W., S. Granger, B. A. Buckley, M. Lamont and B. Rowell. 2004. A one hundred and seventeen year coastal water temperature record from Woods Hole, Massachusetts. Estuaries 27(3): 397-404. (View Abstract)

Priority-setting for Nutrient Management: Case Study in Waquoit Bay

For any given estuarine system, there are as many ways of attempting to control excess nitrogen loads as there are sources of nitrogen. How should managers prioritize management strategies to combat the widespread and ecologically devastating problems caused by nutrient over-enrichment? A recent study introduces a method for evaluating effectiveness of various nutrient reduction strategies, using Waquoit Bay, MA as an example, that will allow management actions to be prioritized based on their effectiveness and feasibility.

The investigators used two mathematical models of nutrient loading, both easily generalizable to other systems, to quantify the predicted outcomes of various management actions, ranging from improving septic system performance to conserving existing wetlands to exterminating waterfowl. The management actions chosen for analysis are specific to this particular estuary, as they address the main nutrient sources to Waquoit Bay, but the process for choosing them is simple enough to be emulated in nearly any coastal system. The effectiveness of each reduction strategy was evaluated by determining whether the predicted N reduction would be larger than the uncertainty in the models (error in the two models used is 12% and 8.1%). Feasibility of each option, based on the technological difficulty of implementing the option and its likely socio-political-economic implications, was evaluated in addition to N removal effectiveness.

For Waquoit Bay, the most effective N reduction actions were determined to be decreasing wastewater input through improved septic performance, altering zoning regulations, and preserving and conserving natural vegetated areas, salt marshes and ponds. This method could be a valuable, widely applicable tool for choosing nutrient management strategies, although extensive data on N loads and land use patterns is needed to use it.

Source: Bowen, J. L. and I. Valiela. 2004. Nitrogen loads to estuaries: Using loading models to assess the effectiveness of management options to restore estuarine water quality. Estuaries 27(3): 482-500. (View Abstract)

What is the Fish-Killing Culprit in Dead-end Canals?

Why are fish kills so common in the summertime in shallow dead-end canals? The answer is well known: excess nutrient loadings and restricted circulation lead to stratified conditions and low dissolved oxygen. However, a recent study of Torquay Canal in Rehoboth Bay, one of the Delaware Inland Bays, draws attention to another potential culprit: lethal levels of hydrogen sulfide.

Torquay Canal is typical of many dead-end canals in residential areas: Shallow except for a few deep holes remaining from dredging projects, it has a sill at its entrance, and tidal exchange and vertical mixing of the water column are restricted. As a result, hypoxia and anoxia frequently occur in the summer months. These investigators examined water quality parameters in the Canal in order to characterize the development of anoxia there and to determine the cause of the canal's summer fish kills. They found that while the canal was mostly well-mixed, the deeper pits were often anoxic in the summertime, and contained high concentrations of toxic hydrogen sulfide. On at least one occasion, the anoxic water and high hydrogen sulfide concentrations were mixed throughout the water column following a storm, shortly after which a major fish kill occurred.

The investigators conclude that the high concentrations of hydrogen sulfide may be at least partially responsible for the observed fish kills. They also suggest some methods for remedying the problems of Torquay Canal which could be applied to similar systems: sills and deep depressions should be avoided or removed. Filling the depressions with iron-rich fill sediment might help even more by acting as a sponge for the hydrogen sulfide, sequestering it as pyrite. One common method that was not successful here: Aerators installed to reduce stratification and alleviate low dissolved oxygen had no significant effect on either.

Source: Luther, G. W. III, S. Ma, R. Trouwborst, B. Glazer, M. Blickley, R. W. Scarborough and M. G. Mensinger. 2004. The roles of anoxia, H2S and storm events in fish kills of dead end canals of Delaware Inland Bays. Estuaries 27(3): 551-560. (View Abstract)