Triplicate 5 mL samples were preserved for flow cytometry with 0.5% glutaraldehyde (final concentration), incubated at 4°C for 10 min and frozen (-80°C) until analysis (within 2-3 weeks; Kemp et al. 1993). To calculate phytoplankton group abundances, 200 µl aliquots of fixed sample were added to a 96-well plate and run on a Guava flow cytometer (Millipore). Filtered seawater (0.45 µm) was run as a blank and instrument-specific beads were used to calibrate the cytometer. Samples were analyzed at low flow rate (0.24 µl s-1) for 3 min. Three major phytoplankton groups were distinguishable based on plots of forward scatter vs. orange (phycoerythrin-containing, Synechococcus spp.) or red (pico- and nanoeukaryotes) fluorescence signals (Worden and Binder 2003). 

Samples for enumerating bacteria were stained prior to running on the Guava in 0.5% v/v SybrGreen I DNA stain for 1 hour at room temperature in the dark.

Mesocosm treatment for all HHQ experiments was as follows:
Redfield: N:P added in a 16:1 ratio during the first 3 days of the experiment, no shading

HHQ treatments here are as follows:
High HHQ - 100 ng mL-1 (410 uM) added to triplicate 5L bottles.
DMSO control - equivalent (v:v) DMSO added to triplicate 5L bottles.

 All bottles were incubated for 24h in a flow-through tank, that was shaded to mimic in situ conditions. Chlorophyll samples were taken at T0 and T24 for all experiments.

Data were processed in Excel with statistics run in Excel, R, or Matlab.