Models

The Long Island Sound Study (LISS) has developed computer models of the Sound to help understand the causes of hypoxia and identify what can be done about it. Designated by the LISS as the priority water quality problem in the Sound, hypoxia is the term applied to low dissolved oxygen conditions. It results from a combination of natural and human induced events.

Human activities overfertilize the Sound with nitrogen from sources such as sewage treatment plant effluent, storm water runoff and atmospheric deposition. This fuels the growth of an overabundance of single-celled algae. Ultimately these microscopic plants sink to the bottom and decay, depleting the oxygen in the bottom waters in the process. In late summer, the Sound naturally divides itself into surface and bottom layers due to density differences caused by temperature and salinity variations. This keeps the relatively well-oxygenated surface waters from mixing with the oxygen-depleted bottom waters, with potentially serious consequences for marine life in the lower zone.

Why did the Long Island Sound Study decide to develop computer models?
The physical, chemical and biological factors involved in hypoxia are extremely complex. To understand hypoxia and to recommend management actions that would be both effective and cost-efficient, the LISS was convinced that a scientific approach utilizing modeling was needed to help sort out the complex system the Sound represents.

A scientific model uses mathematics to recreate or simulate a real world situation. In this case, complex and interrelated components of Long Island Sound are simulated through mathematics in a computer, quantifying the physical, chemical and biological processes. These include the various aspects of water circulation, the nitrogen stimulation of algal growth, and the impact of the algae on dissolved oxygen levels. Investigators can then test a range of environmental management options and evaluate their potential success before being applied, both in terms of benefit to the ecosystem and in cost effectiveness. Modeling can provide guidance to the LISS on the relative importance of nitrogen inputs from different geographic areas. Based on their impact on dissolved oxygen levels, the areas can be prioritized in seeking nitrogen load reductions.

What are the LISS models?
The goal of the LISS modeling effort has been to develop a three-dimensional, time-variable, coupled hydrodynamic/water quality model of Long Island Sound. This required the development of two separate, but linked or coupled submodels: a water quality model and a hydrodynamic or water circulation model. The water quality model has been developed by engineers from HydroQual, Inc., and the hydrodynamic model by scientists from the National Oceanic and Atmospheric Administration (NOAA).

The use of the term three-dimensional indicates that the model predicts water movements and water quality in north-south, east-west directions and surface to bottom, as well. Time variable refers to the model's ability to deal with changes over time and throughout the seasons of the year.

What geographic area was modeled?
The area modeled extends from the Battery at the southern tip of Manhattan on its western boundary to Block Island Sound on its eastern boundary. The models incorporate north-south detail between Throgs Neck and the boundary at Block Island Sound and can therefore distinguish dissolved oxygen differences between the Connecticut shoreline and the Long Island shoreline. The models are not meant, however, to address water quality issues in near-shore areas such as the mouths of the various Connecticut rivers and harbors or the various coves and bays along the Long Island shoreline. Localized water quality questions are best answered through monitoring and modeling programs specifically focusing on the individual locales.

What is the period of time modeled?
The models are calibrated to and verified by extensive field observations of water quality for the time period from April 1988 through July 1989. The water quality model uses information on nitrogen inputs based on monthly estimates of point sources (sewage treatment plants), tributary sources (the various Connecticut Rivers), nonpoint sources (combined sewer overflows and separate sewer flows), and atmospheric deposition. It can also account for the differences in dissolved oxygen that occur within the day due to changing light conditions. The hydrodynamic model is driven by variations in temperature, salinity and current velocities that may change on an hourly basis.

How are the LIS models determined to be correct (or accurate)?
Building the LISS models first required the collection of data to quantify conditions in the Sound and pollutant loadings to it. An intensive field program to collect data for the models was conducted in April 1988 through September 1989. This field program involved a number of government agencies and researchers from the University of Connecticut and the State University of New York at Stony Brook and resulted in the most comprehensive data set ever collected in Long Island Sound. It included some 80 monitoring sites in 1988 and 30 sites in 1989.

This information was then factored into the model framework and the model adjusted or "calibrated" for accuracy. Judging whether a model is correct or accurate is made by comparing the output from the model to field observations and measurements. This process is termed verification. If the model can generally reproduce the important features of the Sound, over time and geographic ranges, it is judged to be accurately calibrated and suitable for use as a management tool.

The hydrodynamic and water quality models were constructed and calibrated using different kinds of data. For the hydrodynamic model, the data comparisons include tide, salinity, temperature and current. In the case of the water quality model, such comparisons include dissolved oxygen, algal biomass and nitrogen.

As an integral part of the modeling program, the LISS established an independent peer review group of modeling experts to evaluate model performance and provide advice and counsel.

Why did the LISS use a phased approach to model development?
Because of the length of time required to complete the hydrodynamic model, the LISS decided to first develop and work with a simpler, two-dimensional water quality model to gain preliminary insights into the hypoxia problem. This model approximated the biological and chemical processes of the Sound but used simplified water current data, simulating the movement of the Sound's waters in only two dimensions (east-west and surface to bottom in 2 layers). It is, therefore, referred to as a two-dimensional water quality model, or LIS 2.0.

Despite its limitations, LIS 2.0 provided a level of detail that allowed the LISS to draw some preliminary, yet clear conclusions about hypoxia in the Sound, its causes, and its solutions. It showed that substantially reducing nitrogen loadings to the Sound from sources in its watershed could have positive results. Dissolved oxygen in the bottom waters during late summer could be increased to levels that would significantly reduce the probability and frequency of severe hypoxia and reduce the area affected.

As a result, the LISS decided to also implement a phased approach to nitrogen reduction. LIS 2.0 was used to identify preliminary nitrogen reduction actions while development of the hydrodynamic model progressed. These LIS 2.0-based actions are described in the 1990 Status Report and Interim Actions for Hypoxia Management and in the 1994 Comprehensive Conservation and Management Plan.

In July 1993, the NOAA hydrodynamic model was approved by the evaluation group, and the output from that model has now been coupled to the water quality model. This allows the model to more precisely compute the transport of nitrogen and other water quality parameters, the resulting algae growth and the dissolved oxygen changes in the Sound. The coupled hydrodynamic/water quality model uses tide and current measurements to simulate the water's circulation in three dimensions (east-west, north-south, surface to bottom in 7 layers). It is therefore referred to as a three-dimensional water quality model, or LIS 3.0.

How does LIS 3.0 differ from the previous LIS 2.0 model?
LIS 3.0 is a more comprehensive and complete model than LIS 2.0. It directly couples the water quality model to the three-dimensional, hydrodynamic model of the Sound. With its increased spatial and temporal precision, LIS 3.0 can simulate the relative impact of nitrogen sources originating in specific geographic areas to the hypoxia problem. The LIS 3.0 model can produce, with a higher level of confidence, the kind of information needed to assist the environmental managers in ensuring that the most cost-effective nitrogen control options are pursued.

Is this modeling approach unique to Long Island Sound?
No. The modeling framework employed in the Long Island Sound Study is quite similar to that used for Chesapeake Bay and in current studies of Boston Harbor, Massachusetts Bay and the Delaware Estuary.

How will LIS 3.0 be used to guide management actions?
Although the LIS 2.0 model has been a valuable analytical tool in the initial LISS examination of hypoxia, it could not precisely identify the impact of nitrogen sources originating in different geographic areas around the watershed on dissolved oxygen conditions. The LIS 3.0 model provides the link between the sources and the impacts of nitrogen. It will identify the movement of nitrogen with the water, the growth of algae stimulated by the nitrogen as related to geographic sources, and the ultimate late summer drain on oxygen attributed to these sources. The model will tell us the relevance of the nitrogen originating from specific, geographic management zones established around the Sound. The LISS will then be able to assign management priorities with more certainty and develop the most beneficial and cost-effective nitrogen load reduction targets by zone.

The LISS has identified 15 such management zones, generally based on natural drainage basin boundaries and political jurisdictions. Within each zone, the options and costs for reducing nitrogen loads from all point and nonpoint sources are identified. The LIS 3.0 model will be used to test the effect reducing nitrogen from each zone has on dissolved oxygen conditions. This will allow the LISS to identify the most beneficial nitrogen load reduction targets for each management zone. The states and local governments will then propose the most cost effective mix of specific point and nonpoint source reduction actions to achieve the targets within each zone.

The LIS 3.0 model provides managers with a solid foundation for forming effective nitrogen control plans with increased confidence that the desired water quality improvements will result. It is clearly the tool to guide implementation of the hypoxia management plan for Long Island Sound.

The U.S. Environmental Protection Agency has established LISS Offices in Connecticut and New York to oversee implementation of the management plan and to provide a vehicle for public involvement in the protection of the Sound.

For more information contact:
U.S. EPA Long Island Sound Office
Stamford Govt. Ctr.
888 Washington Blvd.
Stamford, CT 06904-2152
(203)977-1541

Marine Sciences Rsrch. Ctr.
State Univ. of New York
Stony Brook, NY 11794-5000
(516)632-7666

This publication was produced by the Connecticut Sea Grant Marine Advisory Program, the University of Connecticut Cooperative Extension System and the New York Sea Grant Extension Program for the Long Island Sound Study. Cooperating agencies in the Study include the U.S. Environmental Protection Agency, the Connecticut Department of Environmental Protection and the New York State Department of Environmental Conservation. Written by Joseph E. Blumberg, 9/94.

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