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SAMPLING TIMES Most sampling occurred between the hours of 9:00 and 15:00, and are denoted in the DateTime as 12:00. Some sampling occurred outside of these hours, including some overnight sampling; these instances are noted by the actual sampling time within the DateTime column. For more information about nighttime sampling, see Doubek et. al. 2018.

SAMPLE COLLECTION AND EQUIPMENT Total nitrogen (TN) and total phosphorus (TP) Unfiltered water samples were collected at specified depths for each reservoir using a 4L Van Dorn water sampler (Wildco, Yulee, Florida, USA). Sample timestamps for samples collected between approximately 9 a.m. and 3 p.m. were standardized to noon. Samples collected for projects that required sample collection at specific times of day were standardized to the top of the hour during which they were collected (i.e., a sample timestamp of 16:00 indicates a sample taken between 4 and 5 p.m.). Samples were stored in acid-washed 125 mL polypropylene bottles and frozen within 12 hours. Samples were generally analyzed within one year of collection date. Soluble reactive phosphorus (SRP), nitrate (NO3), ammonium (NH4), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) Water samples were collected at specified depths for each reservoir using a Van Dorn water sampler and were filtered with a 0.7 um glass fiber filter (Thomas Scientific GF/F) before being stored in acid-washed 125 mL polypropylene bottles. Sample timestamps for samples collected between approximately 9 a.m. and 3 p.m. were standardized to noon. Samples collected for projects that required sample collection at specific times of day were standardized to the top of the hour during which they were collected (i.e., a sample timestamp of 16:00 indicates a sample taken between 4 and 5 p.m.). Samples were frozen within 12 hours. Samples were generally analyzed within six months of collection date.

CHEMICAL ANALYSES AND EQUIPMENT

TN and TP Samples were digested with alkaline persulfate (Patton and Kryskalla 2003) and then analyzed colorimetrically using flow injection analysis (APHA 2005). TN was analyzed using the cadmium reduction method (APHA 1998) and TP was analyzed using the ascorbic acid method (Murphy and Riley 1962) on a Lachat Instruments XYZ Autosampler ASX 520 Series and QuikChem Series 8500 (Lachat ASX 520 Series, Lachat Instruments, Loveland, Colorado, USA).

SRP, NO3, NH4 Samples were analyzed colorimetrically using flow injection analysis (APHA 2005). SRP was analyzed using the ascorbic acid method (Murphy and Riley 1962), NO3 was analyzed using the cadmium reduction method (APHA 1998), and NH4 was analyzed using the Berthelot Reaction method (Solorzano 1969, APHA 2005) with a common modification as to the source of the hypochlorite ion as described in Zhang et al. 1997 on a Lachat Instruments XYZ Autosampler ASX 520 Series and QuikChem Series 8500 (Lachat ASX 520 Series, Lachat Instruments, Loveland, Colorado, USA).

DOC and DIC DOC was analyzed using the persulfate catalytic method (Brenton and Arnett 1993) on a TOCA 1010 from OI Analytical from 2013-2016 (OI Analytical 1010 Total Organic Carbon Analyzer with 1051 autosampler, College Station, TX USA) and on a Vario TOC Cube from Elementar from 2016-2017 (vario TOC cube, Elementar Analysensysteme GmbH, Hanau, Germany). Carbon in samples is oxidized to carbon dioxide (CO 2 ) either by reaction with acid or by catalyzed combustion at 850 C. Resulting carbon dioxide is detected by nondispersive infrared (NDIR) spectrometry.

DN Sample is combusted using the Vario TOC Cube from Elemantar at 850 degrees C. Total bound nitrogen in the combustion product is converted to nitrogen monoxide (NO) by oxidative pyrolysis then reacts with electrolyte in the electrochemical cell producing a measurable current.

References APHA. 2012. Standard methods for the examination of water and wastewater. 22nd edn. Washington, DC: American Public Health Association, American Water Works Association, Water Environment Federation. Brenton R, Arnett T. 1993. Method of analysis by the U.S. Geological Survey National Water Quality Laboratory - Determination of dissolved organic carbon by UV-promoted persulfate oxidation and infrared spectrometry. Denver, CO: U.S. Geological Survey. Murphy J, Riley JP. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal Chim Acta 27:31.6. Patton CJ, Kryskalla JR. 2003. Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory-Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water. Denver, CO: U.S. Geological Survey. Revesz KM, Doctor DH. 2014. Automated determination of the stable carbon isotopic composition (_13C) of total dissolved inorganic carbon (DIC) and total nonpurgeable dissolved organic carbon (DOC) in aqueous samples: RSIL lab codes 1851 and 1852: U.S. Geological Survey Techniques and Methods, book 10, chap. C20, 38 p., http:// dx.doi.org/10.3133/tm10C20. Solorzano, L. 1969. Determination of ammonia in natural water by the phenolhypochlorite method. Limnol Oceanogr 14:799-801. USEPA. 2004. RSKSOP-175 STANDARD OPERATING PROCEDURE Sample Preparation and Calculations for Dissolved Gas Analysis in Water Samples Using a GC Headspace Equilibration Technique, Revision No.2 http://www.epa.gov/region1/info/testmethods/pdfs/RSKsop175v2.pdf Retrieved 20APR2015.

Zhang, J.Z., Orter, P., Fisher, Ch. J. and Moore, L.D. 1997. Determination of ammonia in estuarine and coastal waters by gas segmented flow colorimetric analysis. Methods for determination of chemical substances in marine and estuarine environmental matrices. 2nd ed. EPA/7664-41-7.