Sampling and analytical procedures:

Surface water from the “Microbes in the Coastal Region of Orange County” (MICRO) time series station at Newport Pier (33 36.37’ N, -117 55.87’ W ) was collected weekly, in the morning, from 11 Jan 2012 to 26 December 2018. One liter polycarbonate bottles rinsed three times prior to sampling were filled for quantification of particulate organic matter and nutrient concentrations (three samples each from two bottles for a total of six replicates). Temperature, salinity, and chlorophyll a were continuously monitored using an automated shore station mounted next to the sampling site (www.sccoos.org).

Triplicate 300 ml samples for POC/PON or POP from each bottle were filtered within an hour of collection through pre-combusted (500 °C, 5 h) 25 mm GF/F filters (Whatman, MA). Each filter was rinsed with Milli-Q water before being fitted in order to remove potential P residues. The filtrate from the initial filtration was collected and used for macronutrient quantification. The filtrate was filtered through a 0.2 μm syringe filter into a 50 ml tube. Triplicates were collected for both macronutrient and stored at −20 °C.

Nitrate and phosphate samples were collected in prewashed 50 mL Falcon tubes and filtered through a 0.2 μm syringe filter and stored at −20 °C until further analysis. Soluble reactive phosphorus (SRP) concentrations were determined using the magnesium-induced co-precipitation (MAGIC) protocol and calculated against a potassium monobasic phosphate standard. (Karl and Tien, 1992; Lomas et al., 2010). Nitrate samples were treated with a solution of ethylenediaminetetraacetate and passed through a column of copperized cadmium fillings (Knap et al., 1993). Measurements were conducted using the same standards and protocols throughout the time series.

After thawing, POC/PON filters were allowed to dry overnight at 65 °C before being packed into a 30 mm tin capsule (CE Elantech, Lakewood, New Jersey). Samples were then analyzed for C and N content on the same FlashEA 1112 nitrogen and carbon analyzer (Thermo Scientific, Waltham, Massachusetts), following the Sharp (1974) protocol. POC and PON concentrations were calibrated using known quantities of atropine.

For published methodologies, see Allison et al. (2012), Fagan et al. (2019) and Martiny et al., (2015).