was estimated to reduce the amount of light reaching leaf surfaces by an average of about 60% in the 
ocean dominated portion of the Yaquina Estuary (Zone 1). At present we are not sure how epiphyte 
load and its impact on light availability to eelgrass leaves compares to that found in other estuaries, but 
such variation will need to be considered in future efforts to derive water column light criteria for Z. 
marina. Epiphyte light reduction will be incorporated in future versions of the seagrass stress-response 
model described in Chapter 12. 
Although the effects of tides, temperature and epiphytes constitute current uncertainties in 
estimating minimum light requirements for seagrass in Yaquina Estuary, general conclusions can be 
made. Maximum depth of colonization of eelgrass in Yaquina Estuary suggests that a mean of 12.7% 
of surface illumination is required for persistence of seagrass at the deepest edge of the bed. Applying 
the median light extinction coefficient (kj) for Zone 1 (0.8 m' 1 , Chapter 9) to Equation 11.2 yields an 
estimate of percent of surface illumination at depths of 1, 2, and 3 m of 36, 20 and 9 %, respectively. 
This suggests that the use of the median kd as a criterion in Zone 1 would allow persistence of eelgrass 
to a depth between 2-3 m. Use of the median kj for Zone 2 (1.5 m' 1 ) in Equation 11.2, yields estimates 
of percent surface illumination at depths of 1 and 2 m of 22 and 5%, respectively. This suggests that 
the use of the median kj as a criterion in Zone 2 would allow persistence of eelgrass to a depth between 
1-2 m. These results are generally consistent both with empirical data on bathymetric distribution of 
eelgrass within the Yaquina Estuary and with the conclusions generated by use of the Stressor- 
Response Model (see Chapter 12). 
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