Methodology:
Surface water temperature, salinity, chlorophyll a (chl a), silicate (SiO4), phosphate (PO4), ammonium (NH4), nitrate/nitrite (NO3/2), water clarity (Secchi depth) and light (irradiance) were collected weekly at the NBPTS location from 1959-2019.

The detailed methodology will be included in an upcoming manuscript: Thibodeau, P.S., Puggioni, G., Strock, J., Borkman, D., and Rynearson T.A., Using one of the world’s longest plankton time series to identify long-term changes in phytoplankton biomass and phenology. In prep, Target journal: Global Change Biology.

Sampling and analytical procedures:
Weekly surface water temperature and salinity were determined using a bucket sample. Total surface chl a was determined weekly at the NBPTS location from May 1968 to 2019, by filtering whole seawater over 25 mm diameter GF/F filters and measured fluorometrically using three protocols, either following Yentsch and Menzel (1963) as outlined in Li and Smayda (1998) after frozen storage at -20°C (1959-1997), or following Strickland and Parsons (1972) after frozen storage at -20°C (1997-2007), or following Graff & Rynearson (2011) with no filter storage (2008-2019). We expanded an existing, corrected chl a data set (1999-2007) (Graff & Rynearson 2011) by applying the same correction factor to historical chl a measurements (1968-1997) to account for pigment loss on frozen filters.

From 1959 to 2003, water samples for nutrients were collected with a plastic bucket and stored in 20 L polyethylene carboys until returned to the laboratory for analysis (within 90 minutes) (Furnas 1983) and then filtered through pre-rinsed GFC glass fiber filters (Furnas et al. 1976). Between 1969 and 1997, PO4, SiO4, and NO3 were determined via either manual colorimetric methods (Strickland & Parsons 1972) or automated colorimetric methods using a Technicon autoanalyzer following the methodologies of (Furnas et al. 1976; Furnas 1983) and references therein (Armstrong 1951; Grasshoff 1966; Wood et al. 1967). NH4 concentrations were determined via the Witting-Buch method (Barnes 1959) from 1972 to 1980.

Post 1980, NH4 measurements were made via automated colorimetric methods using an autoanalyzer following the methodologies of Furnas et al. (1976), Furnas (1983), and (Solórzano 1969). Since only surface NO3 data were available before 1994, and only surface NO3/2 data were available in 1995 & 1996, a correction factor of 1.04 was determined through regression analysis by comparing observed NO3/2 measurements from 2003 to 2019 with NO3 concentrations measured during the same period. Methods of analysis of PO4, SiO4, and NO3 from 1959 to 1963 were not described in the historic archives but were likely similar to methods used from 1969 to 1997 (colorimetric). From 2003 to 2019, surface nutrient samples for PO4, SiO4, NH4, and NO3/2 were collected weekly at NBPTS and kept on ice and filtered within one to three hours of collection.

Samples were run following Grasshoff (1976) for all nutrients as well as and EPA Method 353.4 for NO3/2, EPA Method 365.3 for NH4, EPA Method 365.5 and Murphy and Riley (1962) for PO4, and Parsons et al. (1984) for SiO4. Light, measured as irradiance (W m-2), was collected weekly at NBPTS from 1959 to 1996. Photosynthetically active radiation (PAR) data were obtained from the National Estuarine Research Reserve, Narragansett Bay station (41° 38.22’ N, 71° 20.34’ W) (http://cdmo.baruch.sc.edu/) to provide light data from 2003 through 2019. A conversion factor (2.1) was used to convert irradiance (W m-2) data from 1959 to 1996 into comparable data in the form of µmol m-2 s-1 (Sager & McFarlane 1997). Secchi depth (i.e., water clarity) was measured weekly at NBPTS from 1972 to 1996 and then again from Dec. 2003 through 2019.

No data were collected for the following parameters during the following years:
- Temperature: 1997-1999; 2012
- Salinity: 1964-1970; 1997-1999; 2012
- Secchi Depth: Collection did not begin until 1972, 1997-2003; 2012
- Light: 1997-2003
- Phosphate: 1964-1970; Biweekly or no sampling 1988; Biweekly sampling only 1990-1993; 1997-2003
- Ammonium: Collection did not begin until 1972; Biweekly or no sampling 1988; Biweekly sampling only 1990-1993; 1997-2003
- Nitrate/nitrite: 1963-1970; Biweekly or no sampling 1988; Biweekly sampling only 1990-1993; 1997-2003
- Silicate: 1964-1970; Biweekly or no sampling 1988; Biweekly sampling only 1990-1993; 1997-2003; 2010-2013
- Chlorophyll: Collection did not begin until 1968; 1970-1971; Biweekly sampling only 1990-1994; 1995-1999; 2012