11.3 Temporal Seagrass Patterns 
Aerial photos of the Yaquina Estuary suggest that there is little year to year variability in Z. 
marina coverage from 1997 to the present. For example, Figure 11.4 shows details of the spatial 
distribution of a large Z marina meadow and a narrow fringing bed, both of which are in a portion of 
the ocean dominated area of the estuary. Although there are apparent differences in these seagrass 
coverages across years, most of these differences are likely within classification error limits resulting 
from differences in ambient lighting conditions, the presence/absence of small amounts macroalgae, 
and subtle differences in how photos were interpreted. For the fringing seagrass bed in Figure 11.4, 
there is a possible relationship between seagrass temporal variability and the formation of intertidal 
drainage channels. Fringing seagrass beds which grow on steeply sloped sites are often less 
aggregated and tend to form into elongated and complicated shapes (Fonseca et al., 1983; Fonseca and 
Kenworthy, 1987; Frederiksen et al., 2004) thus providing more bed edges where erosion may be more 
effective in dislodging shoots. Erosion and strong physical disturbance events have often been 
observed in these marginal seagrass areas of the Yaquina Estuary where tidal drainage channel changes 
and storm events have either eroded Z marina bed margins or deposited large woody debris on top of 
them (Boese and Robbins, in prep.). Episodic events such as these have been implicated in other 
studies (e.g. Krause-Jensen et al., 2003) as factors which alter shallow water seagrass populations. 
Thus, it is likely that the Z marina losses observed at this marginal seagrass habitat area of the 
Yaquina Estuary were due to natural rather than anthropogenic stressors. Overall the result of our 
aerial surveys, when coupled with ancillary published (Boese et al., 2003; Young et al., 1999) and 
WED unpublished data, indicate that over the past decade the spatial distribution of Z marina within 
the Yaquina Estuary has been stable. 
The oldest known spatial coverage data for Z marina in the Yaquina Estuary were published in 
the Oregon Estuary Plan Book (Cortright et al., 1987). This coverage was based on aerial photographs 
that were taken in the mid 1970’s. A comparison of this historical coverage (Figure 11.5) to the 
present Z marina distribution (Figure 11.1 and Figure 11.4) suggests an overall loss of seagrass in the 
Yaquina Estuary. However, there is no indication of what was meant by “seagrass bed” in terms of 
percent cover criteria that were used to delineate areas where Z marina was present or absent 
(Cortright et al., 1987). In general, where seagrass habitat is shown on the Oregon Plan Book map, 
some seagrass is found in that general location either in recent photographs or has been observed as 
less than 10% cover during recent ground truthing surveys. Considering the differences in 
i r 
75 
