determination of whether conditions in Zone 1 are ocean derived, but these techniques have not yet 
been validated. 
In addition to patterns associated with nutrients, there are strong zonal patterns in turbidity, 
TSS and water clarity within Yaquina Estuary, with increasing turbidity and light attenuation with 
distance from the mouth of the estuary. The lower depth limit of Z. marina habitat becomes shallower 
with distance from the mouth of the estuary, suggesting that light conditions may be influencing the 
distribution of Z. marina particularly within Zone 2 of the estuary. Median values of TSS are not 
considered at this time for inclusion in the list of potential water quality indicators. Comparison of 
spatial and temporal patterns of TSS with other estuaries in Oregon showed inconsistencies for reasons 
that are not clear at present. 
Chlorophyll a levels within the Yaquina Estuary are typically low (median of 2-5 pg f 1 ), and 
would be considered in the Tow’ category when used as an indicator within the NOAA eutrophication 
framework (Bricker et al., 1999) and in the ‘good’ category using the West Coast criteria for water 
quality parameters from the National Coastal Condition Report (US EPA, 2004a). The present Oregon 
criterion (15 pg l' 1 ) is rarely exceeded except in the tidal fresh region (upper Zone 2) where the 
criterion is exceeded frequently during May-August. 
There do not appear to have been major long-term changes in either water column nutrients or 
chlorophyll a within the Yaquina Estuary. Although the Yaquina Estuary experiences dense 
macroalgal blooms during the dry season (particularly in the lower estuary), we do not believe that 
these blooms have increased in frequency, duration or intensity, nor are they likely to be a product of 
cultural eutrophication. Modeling combined with determination of natural abundance, stable isotope 
patterns demonstrated that these macroalgal blooms are primarily fueled by oceanic nitrogen. We 
therefore conclude that green macroalgae biomass is not a useful indicator of cultural eutrophication in 
Yaquina Estuary. 
Comparison of recent and historic Z. marina distributions suggests that there have not been any 
major changes in the last 30 years. The trend analyses did reveal that there was a significant increasing 
trend in DO levels in Zone 2 during the interval of 1960-1984. Review of watershed history suggests 
that current anthropogenic impacts are probably less than they were historically (particularly during 
1960’s-1980’s). 
Assessment of lower depth limits for eelgrass within the Yaquina Estuary allowed estimation of 
the minimum light requirements for sustaining seagrass. The mean light requirement was compared to 
98 
