Dataset: Perturbation experiments: Nutrients

Sampling and Analytical Methodology: 

Experiments were conducted between July 2011 and April 2013 during five research cruises to Station ALOHA (22.75°N, 158°W), the well-characterized study site of the Hawaii Ocean Time-series (HOT) program.  Sampling occurred during four HOT cruises and one Center for Microbial Oceanography: Research and Education (C-MORE) cruise (termed HOE-DYLAN 5) aboard the R/V Kilo Moana. Seawater was collected in 12 L polyvinylchloride bottles affixed to a 24-bottle rosette sampler equipped with a Sea-Bird 911+ conductivity, temperature, and depth profiler. Nine 20-L polycarbonate carboys were filled with 25 m Station ALOHA seawater pre-filtered off the rosette sampler through a Nitex mesh (pore size ~202 μm) to exclude larger zooplankton. Of these, 3 carboys received additions of nitrate (target ~2.8 μM N final concentration as NaNO₃) and three carboys received additions of ammonium (target ~2.8 μM N final concentration as NH₄Cl). All carboys, including three ‘Control’ carboys, received additions of phosphate (target ~0.2 μM P final concentration as KH₂PO₄) and silicic acid (target ~2.8 μM Si final concentration as Na₂SiO₃) to achieve a final N:P:Si stoichiometric ratio (14:1:14). Carboys were incubated for 120 to 144 hours and subsampled at approximately daily scales throughout the experiment (Table 1). All sampling was conducted before sunrise in order to allow productivity rate measurements to span the full photoperiod.

Nutrient samples were collected at each time point in 125 mL or 500 mL acid-washed high density polyethylene (HDPE) bottles and frozen upright at -20°C until analyzed. Nitrate + nitrite (N+N), phosphate (PO₄^3-), and silicic acid (Si(OH)₄) concentrations in seawater samples were determined using a 3-channel, Bran+Luebbe AA III™ continuous segmented flow autoanalyzer (Armstrong et al., 1967; Bernhardt and Wilhelms, 1967; Atlas et al., 1971). Detection limits for the instrumental settings used, defined as the lowest resolvable concentrations for each analyses, were 0.058 μM N+N, 0.014 μM PO₄^3-, and 0.013 μM Si(OH)₄. Analyses for NH₄⁺ were run on a hybrid SEAL Autoanalyzer 3 with a 2 m liquid waveguide capillary detection cell, with modified chemistry and an indo-phenol blue reaction.