Dataset: Flow cytometric counts of picoeukaryotes, Synechococcus, and beads using natural waters from the Gulf of Maine during Jul-Aug 2019 and Jun-Jul 2021

The natural waters experiment used the experimental format established in culture experiments 1 and 2 (See Related Datasets section below), and was then adapted for use in natural waters. For detailed methods, refer to Archer et al. (2022) in the section 'Picophytoplankton Rates in Natural Waters'. 

From 16th July to 15th August, 2019 (days of the year 197 to 227) and 8th June to 26th July 2021 (days 165 to 207), a series of saturation experiments were performed that focused on determining the growth rates and grazing mortality of populations of picoeukaryote and Syncechococcus species. Gulf of Maine coastal seawater was collected from the Damariscotta River Estuary at Bigelow Laboratory’s dock off East Boothbay, Maine, USA (43.8604° N, 69.5781° W). Water for all experiments was collected within one hour of high tide at 1 meter depth using a 5 liter Niskin bottle and was gravity filtered through 200 µm mesh to remove zooplankton. Several Niskin casts of seawater were combined in an acid washed carboy before being siphoned into 600 ml polycarbonate bottles. For each saturation experiment, 12 to 14 bottles were used to generate a range of levels of surrogate prey addition. Fluorescent polystyrene microspheres of 2 µm in diameter, treated as for the laboratory experiments, were added to the bottles either as duplicate treatments or in a continuous series of abundance that spanned the range from 0 to 1.7 x 106 beads ml-1 on day 197 to 0 to 3.8 x 106 beads ml-1 on day 226. 

Bottles were incubated for 24 hours in a flow-through incubator on the dock under a nylon mesh that removed ~40 % of the surface photosynthetic active radiation (PAR). In an initial test of the flow-through incubator, five 600 ml bottles were filled with seawater from the same carboy and with no further treatment, were incubated for 24 hours. The counts of the t0 initial (time zero) abundance of picoeukaryotes from the five bottles showed a coefficient of variation of 1.3%, that increased in the T24 counts to only 4.1%, indicating consistent growth among the 5 replicate bottles. Following all experiments, the seawater to which beads had been added was filtered through a 0.45 µm capsule filter to recover the beads before discarding the water. 

Flow Cytometric Measurements
Particles were excited with a 488 nm blue excitation laser (100 mW). Data acquisition was triggered on forward scatter (FSC). Signals were recorded from detectors with bandpass filters for right angle light scatter and fluorescence emission in red (692 nm/80 nm band pass) indicative of chlorophyll a, orange for phycoerythrin (593/52 nm), and green (525/35 nm). To ensure accurate calibration of the flow cytometer, ZE5 QC beads (Bio-Rad, Hercules, CA, USA) were run daily.