<p>This data release contains a 17-year record (2005-2022) of discrete chlorophyll data from inland waters, collected from across the nation and territories. These data are from discrete samples (collected in the field and analyzed in the laboratory) from plankton (suspended algae) and periphyton (benthic algae) from lakes, streams, rivers, reservoirs, canals, and other sites. These data are gathered to support process and remote sensing modeling and prediction of Harmful Algal Blooms (HABs). The chlorophyll data were compiled from the Water Quality Portal (WQP) and USGS National Water Quality Lab (NWQL). <br/> <br/> Data for uncorrected chlorophyll a, corrected chlorophyll a, and pheophytin from EPA Methods 445 and 446 are included and reported, following the conventions of EPA section 445:</p> <ul> <li><b>12.1 uncorrected chlorophyll a</b> – based on the initial optical measurement of the sample, which includes both chlorophyll a and pheophytin</li> <li><b>12.2 corrected chlorophyll a</b> – based on optical measurement after acidification and calculation to estimate the amount of active chlorophyll in the original sample</li> <li><b>12.3 pheophytin</b> – based on calculations to estimate the amount of pheophytin in the original sample</li> </ul> <p>These data were harmonized for reporting units (plankton as µg/L and periphyton as mg/m2), duplicate values, collection depth, site type, negative values, and some extreme values. HABs events are associated with high values of chlorophyll, as algae may be concentrated in dense surface scums. These data may include an indicator for hydrologic event code, which may be helpful in determining HABs events. <br/><br/></p> <h4>Acknowledgements</h4> <p> This work was completed as part of two projects in the USGS Water Mission Area (WMA) Water Quality Processes Program: (1) the Improved Understanding and Prediction of Prioritized Constituents Project, an effort to advance datasets and understanding of chlorophyll, nutrient, carbon, and sediment occurrence and transport, and (2) the Proxies Project, an effort to develop estimation methods for PFAS, harmful algal blooms, and metals, at multiple spatial and temporal scales. <br/> <br/> Arar, E.J. and Collins, G.B. 1997. Method 445.0: In vitro determination of chlorophyll a and pheophytin a in marine and freshwater algae by fluorescence. EPA Office of Research and Development.</p>