Dataset: Nitrogen and carbon uptake rates for mesocosm incubation experiment simulating a phytoplankton bloom in Chesapeake Bay August 2021

The mesocosm experiment was performed in August 2021 on the R/V Hugh Sharp, cruise HRS2110, at a station near the mouth of Chesapeake Bay. Surface water (2–5 m) was collected from the study site (37.27^o N, 76.09^o W), located near the mouth of the bay. Incubation medium was prepared by pumping surface water (~5 m) directly from the sample site through a series of nylon mesh and glass fiber filters, ending with a 0.3 μm filter, using a double diaphragm pump into three 24-L translucent polycarbonate (PC) carboys. Surface water inoculum was collected using a rosette system with 12–L Niskin bottles and a CTD profiler from 2–4 m depth and pre–filtered through 210 μm nylon mesh before being added to the mesocosms to produce a 10 % inoculation.

NaNO₃, NaH₂PO₄, and Na₂SiO₃ solutions were added to each carboy to achieve final concentrations of approximately 40 μM, 5 μM, and 50 μM, respectively. Carboys were incubated for eight days in an on–deck water bath, using a seawater flow–through system drawn from surface water and a plastic screen shade covering to keep incubation temperature and light similar to in situ conditions. Continuous light and temperature measurements were recorded using two Onset HOBO Pendant Temperature/Light data loggers suspended ~10 cm below the surface of the on–deck water bath.

Sub–incubations to measure nitrogen and carbon uptake rates using ¹⁵N–NO₃^- and ¹³C–HCO₃^- were carried out once per day. Sample water (150–200 mL) was aliquoted into PC bottles for triplicate sub–incubations for each carboy (9 sub–incubations per tracer per day). ¹⁵N–NaNO₃^- (1.5–2.67 mL of a 0.3 mM solution) and ¹³C–NaHCO₃^- (1–1.3 mL of a 30 mM solution) were added to each PC bottle to attain isotopic enrichments of ~8–93 % and ~10 %, respectively. Bottles were then placed into mesh bags to simulate surface water light intensities and incubated for 4 h (approximately 10:00 to 14:00 local time) in a similar water bath system as the carboys. Sub–incubations were terminated by filtration onto pre–combusted 0.3 μm 25 mm GF–75 filters. Filters were stored individually at -20^o C until analysis.

POM filters were fume acidified for 4–6 h in a desiccator with concentrated HCl, packed into tin capsules, and measured using a Sercon ANCA–SL Elemental Analyzer and a Europa 20/20 Isotope Ratio Mass Spectrometer (EA–IRMS). Transport rates (⍴) were calculated according to equations modified from Dugdale and Goering (1967).