Newport 
C c\> 
0.0 - 0.5ra \fu3v 
IkX^" 
<0.0m MLLW 
■:'r' 
Zone 2: 
iW4f 
Riverine 
Dominated 
Zone I: 
Ma line 
Sea grass Bed 
Dominated 
x£ 
_ 
Figure 11.5 Historical distribution (mid 1970’s) of Z. marina from the Oregon Estuary Plan Book 
(Cortright et al., 1987). 
11.4 Water Clarity and Seagrass Lower Depth Limit 
11.4.1 Background 
The depth distribution of seagrasses has been shown to be dependent upon light penetration, 
with coastal seagrasses in general extending to depths receiving, on average, —11% of the irradiance at 
the water’s surface (Duarte, 1991). If the maximum depth that seagrasses grow in an estuary is a result 
of water clarity alone, then the maximum depth to which seagrass grows might be used as an 
integrative water quality assessment measure (Dennison et al., 1993), and has been suggested for use 
as a monitoring tool (Sewell et al., 2001; Vimstein et al., 2002). Additionally, understanding the 
minimal light requirements for seagrasses is necessary for preservation of existing seagrass meadows 
and for restoration purposes (Batiuk et al., 2000; Dennison et al., 1993; Fonseca et al., 1998). 
Light criteria have been proposed as part of the guidelines for restoring and maintaining Z. 
marina habitat in Chesapeake Bay (Batiuk et al., 2000). However, applying these values to the U.S. 
Pacific Coast is problematic due to differences in tidal amplitude that tend to narrow the depth range of 
' !■ 
77 
