Dataset: Nutrient data 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. 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. 

Samples for nutrient concentrations were collected from each carboy three times per day, at approximately 06:00, 12:00, and 18:00 local time. Samples were filtered through a 0.22 μm syringe filter into 50–mL plastic conical tubes and stored at –20^o C until analysis. Reactive nitrite (NO₂^-) and silicate (silicic acid, H₄SiO₄) concentrations were measured on a ThermoScience Genesys150 UV–Vis spectrophotometer using colorimetric methods (sulfanilamide + N–(1–naphthyl)–ethylenediamine and metol sulfite, respectively) modified from Strickland and Parsons (1972). NO₂^- + NO₃^- concentration was measured via chemiluminescent detection using a Teledyne NOx analyzer (NOxBox) according to Braman & Hendrix (1989). NO₃^- concentrations were calculated by subtracting the colorimetrically derived NO₂^- concentrations from their paired NO₂^- + NO₃^- NOxBox concentrations.