BATS Validation Cruises

Following the first several years of the BATS project it was deemed necessary by the JGOFS steering committee and BATS PI’s to conduct validation cruises in the vicinity of the nominal BATS site to better understand the mesoscale and larger scale variability of the region. In particular, a focus of the BVAL cruises was to assess the spatial scale representation of the BATS and Hydrostation ‘S’ programs. Initial focus of the BVAL cruises was to investigate mesoscale variability and meridional gradients of the local region. Later, cruises focused on specific mesoscale eddies (e.g., Mcgillicuddy et al., 1998; McGillicuddy et al., 1999) and effects of tropical cyclones through the local region.

In 2000 it was deemed more important to document the larger scale changes in the North Atlantic Subtropical gyre and BVAL cruises established a transect line from ~ 35N to 19N (Bermuda to Puerto Rico) very similar to the WOCE A22 repeat hydrography line (Johnson et al., 2020). These annual Bermuda to Puerto Rico transects have been run since 2000 and target stations at every one degree of latitude and typically have been conducted in September/October of each year to capture maximal heat content in the upper ocean. However, since this timeframe coincides with high tropical cyclone activity the cruises were reluctantly (as of 2022) moved to start in June/July of each year for safety and operational reasons. In the pentad prior to 2022 every BVAL cruise was significantly impacted but multiple tropical cyclones. Parameters presented are the same as provided in the standard BATS bottle files.

Data were collected on BVAL cruises from April 1991 (BVAL cruise #50001) through June/July 2024 (BVAL cruise #50061). Research was conducted on the R/V Weatherbird II through 2005 and thereafter on the R/V Atlantic Explorer. There were numerous Chief Scientists for these cruises including Rachel Dow, Anthony Michaels, Kjell Gundersen, Rodney Johnson, Paul Lethaby, Mike Lomas, Steven Bell, Gwyn Evans, Claire Medley, and Dominic Smith.

Water sampling
Full depth water sampling and data collection at the BATS site are achieved with a total of three hydrocasts using a General Oceanics Intelligent Rosette® with an array of 24 12L water bottles and a Sea-Bird Scientific CTD system. Water samples are collected during the upcast with a 1-minute resting period between reaching the sampling depth and triggering the bottle to close. Bottom measurements/ sampling are achieved within 20 meters from the bottom, as determined using an altimeter.

Water samples are taken right after Rosette® recovery. On any cast, if only a single water bottle is collected to sample all biogeochemical parameters, then gas samples are collected first due to their exposure to air when opened. However, if enough bottles are available, two bottles can be taken for a single depth. Water is usually split between large-volume particulate samples (POCN, HPLC, POP and PSi) and all other small volume samples, including gas samples. When two bottles are taken for a single depth, particulate samples are collected first to prevent settling within the Niskin bottle. Samples are fixed or frozen once all same-sample bottles from one cast have been collected. Particulate samples are filtered as soon as collected.

Nutrients
The BATS nutrient methodology is based on the Protocols for the Joint Global Ocean Flux Study (JGOFS) Core Measurements (Intergovernmental Oceanographic Commission, 1994) which describes the method for the determination of dissolved inorganic macronutrients in seawater: nitrite (NO2 – ), nitrate + nitrite (NO3 – + NO2 – ), orthophosphate (PO4 3 – ) and reactive silicate (Si(OH)4) using Continuous Flow Analysis (CFA).

While the definition of the dissolved fraction has changed throughout the years that the BATS time series has operated, the pore size used has remained constant in order to create a comparable temporal dataset. While similar studies in oligotrophic ocean regions have opted to forego the use of nutrient filters under the assumption that the particulate nutrient pool is negligible, we continue the use of filters for the sake of continuity. Sample filtration also removes the potential for turbidity-derived uncertainties during analysis, and may aid preservation of frozen samples.

Discrete samples are collected at the Bermuda Atlantic Time-series Study (BATS) site from surface to bottom depths (∼4,200 meters). Sea water is filtered directly from the Niskin spigot using a 0.8 µm membrane to remove particulates. Collected sea water is preserved by freezing until analysis. Replicate samples are taken during each cast to ensure quality control standards are met during analytical and data processes. Dissolved inorganic nutrients are measured using a SEAL AA500 Autoanalyzer by Continuous Flow Analysis (CFA). During this process, a subset of sample is drawn and further split into four different channels driven by a peristaltic pump. The sample stream is segmented with air or nitrogen bubbles throughout the flow path to enhance the mixing of reagents with the sample. The nutrients, NO₂-, nitrate + nitrite (NO₃– + NO2-) , PO₄^3–and Si, are chemically reacted in the separate channels to produce a color change and are measured colorimetrically at different wavelengths using a flow-through colorimeter located at the end of the flow path. The light absorption by the sample-reagent mixture is proportional to the concentration of nutrient in the sample according to the principles of the Beer-Lambert Law. Raw absorbance units are converted into nutrient concentrations according to a linear calibration curve formulated from known standards.

Bacterial enumeration
In addition to the casts for shallow water, mode water, and deep water, a separate cast is deployed for the estimation of bacterial growth rates using 3H-thymidine. Heterotrophic bacteria are expected to grow and assimilate 3H-thymidine into nucleic acid material under incubation conditions.

Three replicate samples from the same depth are used as live tubes for thymidine incorporation, and are incubated for four hours. Samples used as killed controls (aka kill tubes) are treated with 100 microliters of 100% TCA (trichloroacetic acid) at the beginning of the incubation to halt biological activity. After incubation, 10 microliters (µl) from the live tubes are extracted for Specific Activities measurements and the biological activity in the live tubes is halted by adding 100% TCA.

All tubes are centrifuged at 14,000 RPM at 4°C for 7 minutes. The supernatant is discarded and DNA is extracted by adding 100% TCA; centrifuging again for 7 minutes at 4°C at 14,000 RPM; adding 80% ethanol and centrifuging once more for 7 minutes at 4°C at 14,000 RPM. The DNA in the resulting pellet is resuspended in Ultima Gold by vortexing. Samples are stored at room temperature until analysis.

Full methodology
Detailed methods are available in Knap et al. (1997).