Dataset: Sargassum exudation experiments: fluorescence
Deployment: HRS1608

Sargassum (urn:lsid:marinespecies.org:taxname:144132) was collected during two sampling events in July and in late September/early October 2016, described in detail in a previous study (Powers et al., 2019, Global Biogeochemical Sciences (GBC), 33(11), 1423-1439). Sargassum samples were collected using a hand-held net aboard the R/V Sharp in the Sargasso Sea during July 2016 for controlled laboratory exudation experiments. Sargassum was housed onboard in a tank (<3 days) with continuously flowing seawater and transported back to the Chesapeake Biological Laboratory (CBL) immediately upon return to another tank with filtered ambient bay water (adjusted to salinity = 35) that was circulated through a UV treatment system (Neotech Aqua Solutions, Inc.) to keep background DOC levels low. These are exudation experiments are referred to as CBL exudation experiments. Sargassum samples were also collected aboard the R/V Henry Stommel 9 km off the coast of Bermuda in late September 2016 and were transferred to outdoor tanks housed at the Bermuda Institute of Ocean Sciences, with continuously flowing seawater within 2 h of collection. These exudation experiments are referred to as BDA exudation experiments. All exudation experiments are described in detail below.

Exudation experiments

Three types of exudation experiments were conducted during this study. One set of experiments was conducted indoors at CBL (listed as “CBL” EXP1 and 2 in the excel sheet). For these experiments, Sargassum subsamples (approximately 100 g wet weight) were rinsed with 24 h UV-treated artificial seawater (Instant Ocean) and transferred to small tanks equipped with a Radion LED lamp (Eco Tech Marine) containing 7 L 24 h UV-treated artificial seawater. Lamps were set for a 14 h day/10 h night cycle and tanks were maintained at 29 °C with Eheim Jager TruTemp Quantum heaters. One tank containing no Sargassum served as a blank and was sampled for dissolved organic carbon (DOC) concentrations, total dissolved nitrogen (TDN) and optical properties. To minimize any stress from transfer, the water was drained and replaced before monitoring DOC/TDN and optical property exudation from the Sargassum samples. CBL EXP3 were the same as EXP1 and 2 with the exception that Sargassum was in mid-senescent conditions (based on visual inspection).

Another set of exudation experiments was conducted in outdoor tanks at the Bermuda Institute of Ocean Sciences (listed as “BDA” EXP1, 3 and 6 in the excel sheet). For these incubations, Sargassum was placed in small tanks containing open ocean seawater within a large tank of continuously flowing seawater. Temperature (°C) and solar intensity (Lux) in the tanks monitored with HOBO® pendant temperature/light data loggers. Temperatures ranged between 26 and 27.5 °C. Tanks were either left uncovered and exposed to full solar irradiation (listed as “UV” or “U” experiments in the excel sheet) or tanks were covered with a Plexiglas cover that had irradiation cut off at 345 nm and UVA (320 to 400 nm) transmission reduced to 65% (listed as “Plexi” or “P” experiments in the excel sheet).

The third set of experiments was performed to investigate the impacts of stress and senescent conditions on the release of DOC/TDN/optical properties by Sargassum (listed as “BDA” EXP 5 and 7 in the excel sheet). The optimal temperature range for pelagic Sargassum sp. is 24 to 30 °C (Hanisak & Samuel, 1987), so one tank was kept indoors and left at 20 °C. All other treatments were conducted outdoors. One tank contained mid-senescent Sargassum and was kept under low light (no direct sunlight) where the temperature did not change from 25 °C over the course of 12 h. The other tanks contained healthy Sargassum but were left outdoors for 12 h in direct sunlight with no temperature control. In these tanks the temperature ranged from 25 °C at the start of the experiment up to 49 °C after 12 h (Table 1). Although 49 °C is unrealistic for the open ocean, similar conditions may be present during annual Sargassum inundation events that occur in coastal environments.

For all experiments, subsamples of tank water were 0.2 µm-filtered (Whatman 25 mm GD/X syringe filters) into clean combusted (500 °C) 40 mL amber glass vials at discrete time points. Optical properties (absorbance and fluorescence spectra) of samples were either analyzed immediately or stored at 4 °C until analysis (within 1 to 10 d of collection). At the same time points, additional samples were acidified to pH 2 using concentrated HCl (Sigma Aldrich 32 %, pura) and analyzed for DOC and TDN concentrations using a Shimadzu TOC-V (see Powers et al., 2019, GBC).