associated kj values monthly (Dennison et al., 1993). For Chesapeake Bay, proposed water column 
light requirements vary by estuarine salinity classification with higher light requirements suggested for 
polyhaline and mesohaline zones (>22% of surface irradiance) than for tidal fresh and oligohaline 
zones (>13% of surface irradiance). These zonal differences are in part due to the different species of 
SAV which are typically found in the different salinity zones (Batiuk, 1992). It is also important to 
recognize that the Chesapeake Bay values are designed to be protective of multiple SAV species, and 
not just Z. marina. The zonal irradiance values were based upon previously published kj values of 2.0 
m' 1 for tidal fresh and oligohaline sections and 1.5 nf 1 for polyhaline and mesohaline sections (Batiuk 
et al., 1992). The proposed irradiance criteria were adjusted for the amount of light absorbed by 
epiphytes encrusting SAV leaf surfaces and reported as the Percent of Light at the Leaf surface or PPL. 
These minimum PPL values were 9 and 15 % respectively for the two salinity groupings (Batiuk et al. 
2000). These proposed criteria were also applicable only to the SAV growing season (typically spring 
though fall). 
In contrast, light requirements for Z marina in Puget Sound were reported as integrated light 
intensity levels (Thom et al., 1998). These were estimated using maximum seagrass depth measures, 
kj values and production-irradiance (P vs. I) relationships. Based on this methodology Thom et al., 
(1998) suggested that to maintain the greatest densities of Z. marina, ~300 pmoles m" s' (3 moles m‘~ 
d' 1 ) were required for at least three hours daily during the growing season. Thom et al. (1998) went on 
to suggest that for Z marina to minimally persist would require mid-day minimum irradiance values at 
the maximum depth limit to be approximately 150 pmoles nf" s' during the year. These same values 
are also suggested as minimum requirements for outer coast PNW estuaries like Willapa Bay and Coos 
Bay (R. Thom, Pacific Northwest Environmental Laboratory, pers. comm.). Assuming that mid-day 
surface irradiance is in the range of 1000-2000 pmoles m' 2 sec' 1 , the minimum light requirement 
corresponds to approximately 15-30% of surface irradiance, which is consistent with other published 
criteria values and with the present study (Table 11.1). Additional verification of these minimum light 
requirements within the Yaquina Estuary is currently in progress (WED unpublished data). The mean 
daily irradiance value was approximately 3.8 moles m' 2 d 1 at a single lower margin site for Z marina 
in the Yaquina Estuary (WED unpublished data). Although this value exceeds the Thom et al. (1998) 
- 2-1 
criteria, irradiance values were highly variable, ranging from 0.5 to 7 moles m d , with extended 
periods of apparently inadequate lighting at depth from October to December (WED unpublished 
11 
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