MODEL COMPONENTS
As with all models, CLUE is based on assumptions. We must make assumptions about the sources of nitrogen, the relative contributions of different sources of nitrogen to the receiving water, the transformations of nitrogen in the system and other variables, because all the components of the system cannot be measured simultaneously or definitively. Scientifically-sound assumptions are based on observation, experiment, and a reiterated process of testing the model output (and its underlying assumptions) against real-world data.
CLUE is a process-based, empirical model in which the relationships among its important variables are derived from independent observations. It represents the best synthesis of how we think the various components of land use, nitrogen loading, and ecosystem response actually fit together. Like any scientific model, CLUE is not perfect, but we feel that its generality is useful in exploring what sort of estuarine responses are likely if certain land use decisions are followed.
CLUE is comprised of two major, linked model sections: 1) a nitrogen loading model section, and 2) an ecosystem response model section.
NITROGEN LOADING MODEL SECTION
The nitrogen loading model was developed and tested originally in Waquoit Bay on Cape Cod. This model calculates the total amount of nitrogen deposited in the watershed that arrives at the shore of the estuary, by summing the sources of nitrogen and subtracting any losses that occur during transport. The N-loading model has successfully estimated nitrogen inputs into estuaries, as determined by field measurements and other supporting data.
Some of the components the N-loading model are:
The output of the nitrogen loading model is an estimate of the amount of nitrogen entering the receiving waters either as a mass loading value or a concentration. This value is not of use by itself because the amount of nitrogen that is beneficial or harmful to an embayment is site-specific. Generally, a given nitrogen load entering a smaller estuary will have a greater effect than the same amount entering a larger estuary. Also, nitrogen entering a well-flushed estuary may quickly disperse to the open sea and therefore contribute little to eutrophication of the estuary. On the other hand, a poorly-flushed embayment - one in which tidal exchange is limited - may retain its nitrogen load for a long enough time to contribute to eutrophication and disruption of the system. Thus, two other factors are considered when the model calculates the amount of nitrogen available for biological uptake in the estuary:
ECOSYSTEM RESPONSE MODEL SECTION
The ecosystem response model takes output from the nitrogen loading model and estimates the levels of four key indicators of estuarine health and habitat quality in response to the nitrogen loading: algal blooms, eelgrass, susceptibility to low dissolved oxygen conditions, and fish populations. The estuarine response model estimates water column chlorophyll concentration (a surrogate for phytoplankton algal abundance) based on relationships of algal productivity to nitrogen availability, light, temperature, mixing, and grazing by herbivores. The model estimates susceptibility to anoxia by calculating the number of days that oxygen demand is likely to exceed supply, based on average light incidence, temperature, and modeled system respiration. For a given embayment, the ecosystem response model estimates the areal extent and density of eelgrass under pristine and eutrophic conditions, using such factors as light availability, temperature, and water depth. Finally, the model estimates finfish abundance and diversity as a function of eelgrass density.