Factoring in the Influence of Tidal Range 
on Water Clarity Attainment 
Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based 
Requirements and Restoration Targets: A Second Technical Synthesis specifies that 
half the diurnal tidal range for that Chesapeake Bay Program segment should be 
added to the restoration depth Z before calculating PLW or PLL (Batiuk et al. 2000, 
page 102). These half tidal-range values, taken from tidal-range tables and averaged 
by Chesapeake Bay Program segment, were listed on page 202 of that document in 
Table D-4. However, for the purposes of testing attainment of the water clarity 
criteria, the EPA recommends using the water clarity criteria application depths 
without adding half the diurnal tidal range to it (see U.S. EPA 2003). This recom¬ 
mendation is based on the biologically-based water clarity criteria reference curves. 
The methodology followed in the derivation of those reference curves did not 
include adding the half tidal range to the restoration depth, Z (see Appendix H). The 
EPA believes it is important to maintain consistency throughout the entire set of 
procedures for determining water clarity criteria attainment. 
Using Midchannel Data to Estimate Shallow-water Conditions 
The majority of baywide, regional and local tidal Bay water quality monitoring 
programs in the past have collected data only from fixed midchannel stations. Incor¬ 
porating a rotational shallow-water monitoring into the tidal monitoring network is 
leading to the generation of shallow-water data for evaluating attainment for the 
water clarity criteria. However, given the rotational nature of this shallow-water 
monitoring network component, fixed midchannel stations are still going to be used 
in criteria assessment. It is relevant, in assessing water clarity criteria attainment, to 
note the extent to which water quality monitoring data collected from midchannel 
stations in the Chesapeake Bay and its tidal tributaries represent conditions at 
shallow-water sites where underwater bay grasses potentially occur and the water 
clarity criteria apply. 
Evaluation of Midchannel and Nearshore Data Comparability. Several 
studies have addressed the shallow-water versus midchannel sampling issue in the 
Chesapeake Bay (Stevenson et al. 1991; Batiuk et al. 1992; Ruffin 1995; Bergstrom, 
unpublished data; Parham 1996; Karrh 1999; Hunley, unpublished data). While most 
studies indicate that midchannel data can be used to describe shallow-water condi¬ 
tions, several suggest the opposite. There is no doubt that demonstrable differences 
in water quality can occur between shallow-water and midchannel stations over 
varying temporal and spatial scales, especially when bay grasses are present (Ward 
et al. 1984; Moore 1996). Other possible causes of variability between shallow-water 
and midchannel environments include localized resuspension of sediments, algal 
patchiness, point source effluents or sediment chemistry variability (Goldsborough 
and Kemp 1988; Moore 1996). 
chapter vi • Recommended Implementation Procedures 
