Yes. EPA believes that nutrient criteria, to be effective, should address the causal and response variables in a manner that results in quantifiable measures. In general, however, EPA is receptive to specific case studies and wishes to promote flexibility as long as the goal of protection of the designated use in all waters is achieved. For example, if a state or authorized tribe demonstrates that the algae growth in certain waters and the immediate downstream waters are all limited by phosphorus, then criteria could be initially established for phosphorus and appropriate biological and physical response variables (e.g., chlorophyll a and turbidity) to protect designated uses in those waters. For large river systems that drain into estuaries far downstream, nitrogen loading limitations may be necessary to attain downstream estuarine criteria. However, if nitrogen levels in the watershed far exceed what would be considered "reference conditions" or "natural background" based on comparison to EPA's recommended criteria or other analyses, then nitrogen load reduction strategies should be employed in advance of adoption of a specific nitrogen criterion for the farfield downstream water (e.g. estuary). If a state or authorized tribe has identified nitrogen load reduction strategies at the time the nutrient plans are developed, EPA encourages them to discuss the strategies in their plans.