The trend for deeper depth limits with increased water clarity is evident for all species of 
seagrasses, where the maximum colonization depth corresponded to approximately 11% of surface 
irradiance (Duarte, 1991). However, the data for Z marina presented by Duarte (1991) suggested that 
the amount of light needed to sustain this species at depth is almost double that for seagrasses in 
general (Table 11.1). Duarte (1991) went on to note that the world-wide relationship between kj and 
the maximum seagrass colonization depth (all species) was linear, and that it could be simply 
calculated as: 
where Z c is the maximum colonization depth (m). 
Duarte (1991) also noted that this result was similar to the results obtained for Z marina (Z c = 
1.62/kj and Z c = 1.53 /kj) on the Atlantic Coast of the U.S. (Dennison, 1987) and within Danish 
estuaries (Nielsen et al., 1989), respectively. The trends in Z c and kj for Z marina in the Yaquina 
Estuary are consistent with Duarte’s (1991) relationship (Figure 11.7) even though the waters in the 
Yaquina Estuary were more turbid than those reported by Duarte (1991). These kj values from Duarte 
(1991) were converted (Equation 11.2) to the percent of surface irradiance at Z c to generate the values 
which are presented in Table 11.1. Also included in Table 11.1 are the minimum light requirements 
recommended for the growth and survival of SAV in Chesapeake Bay (Batiuk et al., 2000). 
Although the mean percent of surface irradiance needed to maintain Z marina in the Yaquina 
Estuary is lower than literature values, they are within the range of published values. Literature values 
were either determined in waters which were considerably less turbid than those of the Yaquina 
Estuary (Duarte, 1991) or derived from a synthesis of literature values and area specific research 
(Batiuk et al., 2000). The Z marina values published by Duarte (1991) rely heavily on Danish studies, 
especially Nielsen et al. (1989), which account for 20 of the 29 literature values shown in Figure 11.7. 
80 
