Environmental Science & Technology Article Figure 3. Corrected ChlRS-a boxplots for all coastal segments between 1997 and 2009 with the minimum and maximum (black dots), the 10th and 90th (whiskers), the 25th and 75th (boxes) percentiles. determination of the cumulative distribution functions and calculation of criteria. Chl-a and KdPAR observations within the 3 NM boundary represented 6 and 16%, respectively, of the total available data for satellite match-ups. The relationship between Chl-a and ChlRS-a using all the Chl-a observations was stronger (R2 = 0.81, RMSE = 0.24, p < 0.01, N = 1,941) with a higher slope (1.18). Using all the data for KdPAR resulted in a slope (0.76, R2 = 0.46, RMSE = 0.14, p < 0.01, N = 429) similar to that obtained within the 3 NM limit. Data from outside a jurisdictional boundary could be used for criteria development if there was no expectation that water quality varied outside the boundary. However, in this case, we expect differences associated with the transition from optically complex (Case II) to open ocean (Case I) waters,23 which is reflected in the apparent differences in the slopes discussed above. As a result, it was determined that only data within the 3 NM boundary would be used in this approach. Figures 2 and S2 of the SI indicate the value of collecting additional field observations within the 3 NM boundary to improve the regressions. In addition, observations from under-sampled areas of Florida’s coastal waters, such as AC and FP would help ensure that the relationships were applicable to all of Florida’s coastal waters. CDOM and bottom reflectance were identified as possible interferences affecting the relationship between Chl-a and ChlRS-a in coastal waters. These interferences were additive to the ChlRS-a value within each coastal segment and set the lower limit for the ChlRS-a response. However, the proposed approach used nonparametric statistics and focused on the upper quartiles, so the interferences by bottom reflectance and CDOM were unlikely to affect derivation of numeric criteria. Although we distinguish a difference in nomenclature between water column chlorophyll-a (Chl-a) and the satellite response (ChlRS-a), which was affected by these interferences, we suggest that the latter was a suitable measure for use in numeric water quality criteria development and subsequent assessments. Bottom reflectance was reduced by incorporating the stray light contamination flag, which identified near shore bins where reflected light from land enters the satellite’s field of view. To identify interference of bottom reflectance on ChlRS-a, the distributions of bathymetry and PAR integrated satellite penetration depth were examined within each coastal segment (Figure S3 of the SI). Differences in penetration depths were expected between the single OC4 bands (443, 490, 510, and 555) and the PAR integrated response since the PAR spectrum changes with depth in the water column due to different absorption rates at each wavelength. The red wavelengths attenuate rapidly with depth and the blue wavelengths penetrate deeper into the water column.24 PAR integrated satellite penetration depth was used here as a general indicator of the penetration depth from the four OC4 bands. Mean penetration depth was consistently shallower than median water depth within each FP coastal segment. The deepest penetration depths (90th percentile) were rarely greater than the shallowest water depths (10th percentile) in the FP. Similarly, AC exhibited little overlap between the deepest penetration depths and shallowest water depths. In the WFS mean penetration depth was deeper than the median water depth in 10% of the coastal segments. Coastal segments with median bathymetry shallower than 25 m exhibited bottom reflectance interference (Figure S4 of the SI) described by an exponential decay function (ChlRS-a = 1.17*exp(−0.14x), where x is the depth in meters; R2 = 0.62, p < 0.01, N = 76). Seagrass also impacts ChlRS-a, representing a special case of bottom reflectance. Although seagrass were not present in coastal waters in the FP, AC, and most of the WFS, they occur in the northern WFS between Cedar Key and Anclote Bay, and 919 dx.doi.org/10.1021/es2014105 | Environ. Sci. Technol. 2012, 46, 916−922