When the first Z. marina patch was seen on the monitor the pole was quickly thrust into the 
sediment to stop the momentum of the boat. A GPS reading was taken and a lead line was used 
to record the depth to the closest centimeter. In shallower waters, seagrass blades were clearly 
apparent on the waters surface and could easily be approached by foot. Personnel walked from 
shore along the transect to the deepest Z. marina patch, recorded a GPS location, measured the 
depth with a lead line and collected other water quality parameters. 
Tidal corrections for Lower Depth Limits of Z. marina 
Tidal corrections were applied to account for variations in tide elevation at time of lower 
limit depth observations, with corrected lower depth limits expressed as depth below mean lower 
low water (MLLW). Tidal predictions were used to make these tidal corrections. Tidal 
predictions are least accurate during storms and extreme low and high tides. Review of weather 
and tidal conditions during time periods when the lower depth limit of Z. marina was measured, 
suggests that conditions were relatively calm during the sampling and not collected during 
extreme tides. In addition, the difference between predicted and observed tidal heights at the 
South Beach tide gauge was less than 0.15 m on all data collection days. Tidal predictions that 
take into account variations in amplitude and phase lags of tides are available for four locations 
(South Beach, Yaquina, Winant, and Toledo) in the Yaquina Estuary (http://co- 
ops.nos.noaa.gov/tides05/tab2wclb.html# 132). Tidal heights relative to MLLW were calculated 
using WXTIDE 32 for each depth site using the time of data collection and the nearest tide 
prediction location. The maximum distance along the river any given depth station from a tide 
station was 4.5 km. The largest source of error in the tidal corrections results from not having 
predicted tides available for all locations along the longitudinal axis of the estuary and having to 
use the closest tide prediction station. To estimate the error associated with the tidal correction, 
we compared the differences in tidal corrections between the two stations that are located 
upstream and down stream of the observations. The error associated with the tidal correction is 
estimated to be 0.1-0.2 m. The error associated with the tidal correction increases with distance 
from the mouth of the estuary, being a minimum of 0.1 m in the lower estuary and as high as 0.2 
m near Toledo. 
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