At each plot, a percent cover value within a 0.25 nr quadrant was visually estimated for SAV 
and macroalgae. For much of the data, a frequency-of-occurrence value was also measured by 
recording which class of cover occurs directly beneath each of 25 point intercepts formed by two 
orthogonal sets of string intercepts. The purpose of such frequency-of-occurrence measurements 
was to provide a quality check on the estimated percent cover values. Pilot studies comparing 
the linear regression of the estimated percent cover values against the measured percentage 
frequency of occurrence values have yielded r 2 values of 0.91 - 0.97. For this current study only 
the green macroalgae data was used for analysis. 
Samples of the alga taxa was collected from a 0.05 nr area of the 0.25 nr quadrant at two 
of the three replicates sites along the station line. After cleaning, the alga was identified by 
qualified individuals. Alga keys and identification aides were available for cross-referencing. 
The alga biomass was determined by drying the alga at 80°C until dry. A sample was considered 
dry when there was no further weight change due to loss of water after additional oven drying. 
Returning a subset of sample to the oven after recording the initial dry weight was also a 
measure of repeatability. The final dry weighs for the 0.05 m“ sample was converted into gdw 
nf 2 . 
An alternative method was used to determine macroalgae biomass in the 2002 field 
season. A volumetric biomass estimate was collected according to the graduated cylinder 
method as described in Robbins and Boese 2002. The macroalgae was collected from a 0.25 nr 
quadrant and placed into the 2000 ml cylinder. The algae were pressed with a plunger to remove 
excess water before recording the algal volume in ml. This quick field method was determined 
to be an accurate surrogate for biomass dry weight determination with a linear relationship 
yielding a r 2 value of 0.78 to 0.88 for macroalgae species (Robbins and Boese 2002). 
Zostera Marina Lower Depth Limit 
The lower depth margin of Z marina was determined by georeferencing the position 
where the deepest seagrass was encountered. Transects were randomly selected in distinct Z. 
marina beds (identified from aerial photography) and were approached either by boat or by foot 
depending on the water depth. Sampling was conducted on the lowest tides possible to increase 
the accuracy of locating plants growing at the lowest depth limit. In the deeper systems an 
underwater video camera was mounted on a long PVC pole linked to a video monitor on deck. 
145 
n 
