Implementation of an ecosystem model for assessment of management policy based on sustainability criteria

Abstract

We implemented the 1-D lake ecosystem model DYRESM-CAEDYM (DYCD) to the subtropical meso-eutrophic Lake Kinneret, in order to determine an acceptable management policy based on sustainability criteria. The sustainability criteria were defined based on the Lake Kinneret system of water quality indices (WQI) and composite water quality index (CWQI) developed for the lake. The CWQI was evaluated by number of parameters that are monitored routinely at the lake and provide an overall picture of many of the ecosystem processes. Both the WQI and CWQI were applied in conjunction with output from 10 yr model scenarios, which included a wide range of nutrient loads and lake levels. In both cases, the scenarios covered a range wider than that historically observed in the lake. The scenarios included both simple cases of changes to one of the management forcing variables (e.g. lake level) and more complex cases that modified multiple forcing variables in tandem. We used the hydroecological model DYCD and ran it at WLs ranging between -209 to -218 m under varying nutrient loads, 0.56 to 56.6 gNm- 2 y -1 and 0.039 to 3.9 gPm- 2 y -1 . For levels ranging between -209 to -213.4m simulation results allowed us to assess the direct relationships between nutrient loads and CWQI. The graphical solution of these relationships represents a polygon of permissible ranges and critical values of nutrient loads allowing conservation of the lake WQ at each WL. We used the combination of the model scenario results and the quantified WQ output to define a 3D (TN, TP and water level) space of acceptable management measures. The space defines the magnitude of management measures that can be performed on the lake ecosystem while sustaining the ecosystem. The results suggest a wider than expected range of management measures that can be used in the lake ecosystem, but the reaction of the ecosystem to the measures is not linear. As a result, reducing management measures does not always lead to improved sustainability. This approach is unique, and the first example of implementation of a management tool that integrates nutrient loads and WL through a WQ system. This approach could be implemented in other lakes around the world that suffer from deterioration in water quality as a result of changes in water level and nutrients loads. The application of a process based model, such as DYCD, has provided a means for examining the impact of external forcing on the lake ecosystem, beyond the observed historical conditions, and over extended periods of time. Our study demonstrates the major advantage associated with the use of process based ecosystem models as a tool for research and management.