Continuous fluorescent dissolved organic matter (FDOM) and water quality parameters from a YSI EXO sonde in Little Indian Slough in Padilla Bay, Washington, from July 2024 through July 2025

The Little Indian Slough Sonde dataset consists of continuous measurements collected using two alternating YSI EXO2 multiparameter sondes deployed in a tidal slough along the southwestern margin of Padilla Bay, Washington (48.457810° N, 122.471191° W). Data span July 17, 2024 through July 23, 2025, recorded at 15-minute intervals until August 1, 2024, after which measurements were collected at 10-minute intervals. Each sonde was equipped with sensors for conductivity and temperature, optical dissolved oxygen, pH and oxidation–reduction potential (ORP), total algae (chlorophyll; RFU and µg/L), fluorescent dissolved organic matter (FDOM; calibrated QSU and uncalibrated RFU), and turbidity (FNU, installed beginning July 22, 2024), and included a central wiper for automated cleaning of optical surfaces. The EXO sondes used in this study did not include a depth sensor. Two sondes with identical configurations were rotated monthly, with each instrument undergoing post-calibration checks, cleaning, storage, and pre-deployment calibration between field cycles. Several gaps occur throughout the record due to periods when the sonde was not correctly marked as deployed, battery issues, one wiper installation error that resulted in significant biofouling and exclusion of affected data, and periods of likely kelp accumulation around the deployment tube that caused optical sensor readings near zero and required exclusion of affected intervals. Additional short-term biofouling occasionally influenced chlorophyll and FDOM signals, and slight negative values in dissolved oxygen, FDOM, or other optical channels reflect normal sensor noise. The EXO Total Algae sensor also outputs a phycoerythrin channel (TAL PE RFU), but this channel was not calibrated and is not recommended for quantitative use. Turbidity values occasionally exceeded the sensor’s calibrated accuracy range (>4000 FNU) and should be interpreted with caution. Timestamp corrections were applied where necessary to ensure consistency with Pacific Standard Time. Little Indian Slough is a tidally influenced channel receiving both freshwater agricultural drainage and marine water from Padilla Bay, resulting in strong variability in salinity, dissolved oxygen, and biogeochemical conditions. The dataset is provided as a single CSV file suitable for analyses of tidal dynamics, estuarine biogeochemistry, and dissolved organic matter (DOM) variability.