In order to achieve and maintain the water quality conditions necessary to protect the aquaticliving resources of the Chesapeake Bay and its tidal tributaries, EPA Region III has developed theRegional Criteria Guidance. The EPA is issuing this guidance in accordance with Section 117(b) ofthe Clean Water Act and in accordance with the water quality standards regulations (40 CFR PartB1). This document presents EPAs regionally-based nutrient and sediment enrichment criteriaexpressed as dissolved oxygen, water clarity and chlorophyll a criteria, applicable to the ChesapeakeBay and its tidal tributaries. This guidance is intended to assist the Chesapeake Bay states,Maryland, Virginia and Delaware, and the District of Columbia in adopting revised water qualitystandards to address nutrient and sediment-based pollution in the Chesapeake Bay and its tidaltributaries. EPA Region III has identified and described five habitats (or designated uses) that, when adequately protected, will ensure the protection of the living resources of the Chesapeake Bay and tidal tributaries. Those five uses (summarized below and described in detail in Appendix A and B) provide the context in which EPA Region III derived adequately protective Chesapeake Bay water quality criteria for dissolved oxygen, water clarity and chlorophyll a, which are the subject of the Regional Criteria Guidance. Accurate delineation of where to apply these tidal water designated uses is critical to the Chesapeake Bay water quality criteria (U.S. Environmental Protection Agency 2003a). The Chesapeake Bay dissolved oxygen criteria vary significantly across the five refined tidalwater designated uses to fully reflect the wide array of species living in these different Bay habitats,as reflected in Table 1. The water clarity criteria reflect the different light requirements for underwater plant communities that inhabit low salinity versus higher salinity shallow water habitats throughout the Bay and its tidal tributaries. Table 2 provides an overview of the recommended light requirementsfor water clarity criteria. The EPA is providing the states with a recommended narrative chlorophyll a criteria applicable to all Chesapeake Bay and tidal tributary waters (Table 3). The EPA encourages thestates to adopt numerical chlorophyll a criteria for application to those tidal waters where algal related designated use impairments are likely to persist even after attainment of the applicable dissolved oxygen and water clarity criteria. The Regional Criteria Guidance contains technical information to support quantitative interpretation of the narrative chlorophyll a criteria.