Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Hydrocasts were conducted at identified stations. A SeaBird CTD and rosette system, owned and maintained by Louisiana Universities Marine Consortium (LUMCON), operating institution for the R/V Pelican, was used for sampling. Calibration information can be found associated with the CTD data. Unless otherwise stated, samples used for rate measurements were collected based on the percent irradiance relative to that just below the surface.

Water was sampled from Niskin bottles and pooled into 10 L acid-cleaned carboys. For inorganic nutrients, water was filtered using 0.6 μm pore size polycarbonate membrane and immediately frozen until analysis. Filtered water was analyzed for total dissolved nitrogen (TDN), nitrate+nitrite (NO3 + NO2), nitrite, soluble reactive phosphate (SRP), and ammonium (NH4) colorimetrically using Skalar autoanalyzer (Dzwonkowski et al. 2017), and for dissolved silicic acid (Si(OH)4) using a manual colorimetric method (Krause et al. 2009). Water for Chlorophyll a was filtered through a 0.45 μm-pore 47mm diameter HAWP Millipore filter, immediately frozen, and analyzed on shore (<2 weeks) using an acetone extraction/acidification method (Lomas et al. 2019). For biogenic silica analysis, seawater was filtered through a 1.2 μm-pore polycarbonate filter (47 mm diameter) and frozen immediately. On shore, filters were dried and analyzed using a sodium-carbonate time course digestion, to correct for lithogenic silica interference, in polymethylpentene tubes (Pickering et al. in review). In 2016, NaOH digestions were also done for biogenic silica, followed by an HF digestion to quantify lithogenic silica as in Krause et al. (2009). Diatom abundance was quantified by fixing samples with Bouin's solution; in the laboratory, cells were settled in a chamber and enumerated (Utermöhl 1958).

Diatom rate processes were quantified using a radioisotope (32Si) and fluorescent dye (PDMPO) tracers.  Sample bottles were incubated for 12 or 24 hours in acrylic incubators cooled with continually flowing surface water under a series of neutral density screens to simulate light levels at the depth of collection (i.e. see above). Measurement for ambient (Amb) conditions (i.e. no enrichment of Si(OH)4) and enhanced (Enh) conditions (i.e. +20 µM enrichment of Si(OH)4) were made at most stations and depths using 32Si. The gross rate of biogenic silica production was measured using the radioisotope tracer 32Si with high specific activity (>40 kBq µg Si-1) as described in Krause et al. (2011). For PDMPO uptake, dye was added to samples at ambient or enriched Si(OH)4, incubated in the same conditions as the 32Si samples, and processed as in McNair et al. (2015). The net rate of biogenic silica production was calculated from biogenic silica standing stock at the time of sampling and after a 24-hour incubation, under the same conditions as the 32Si and PDMPO, as described in Krause et al. (2010).

Data were processed in Microsoft Excel.

BCO-DMO Data Manager Processing Notes:
* added a conventional header with dataset name, PI name, version date
* modified parameter names to conform with BCO-DMO naming conventions
* blank values in this dataset are displayed as "nd" for "no data."  nd is the default missing data identifier in the BCO-DMO system.
* removed all spaces in headers and replaced with underscores
* removed all units from headers
* converted dates to ISO Format yyyy-mm-dd
* merged Date_Zulu and Time_Zulu to create ISO_DateTime_UTC and then removed Zulu columns
* set Types for each data column
* merged the CLASiC 2016 and 2017 Water Column data files into one dataset
* replaced - and , in Rosette_Bottle column with ;

NSF Award Abstract:
The Louisiana Shelf system in the northern Gulf of Mexico is fed by the Mississippi River and its many tributaries which contribute large quantities of nutrients from agricultural fertilizer to the region. Input of these nutrients, especially nitrogen, has led to eutrophication. Eutrophication is the process wherein a body of water such as the Louisiana Shelf becomes enriched in dissolved nutrients that increase phytoplankton growth which eventually leads to decreased oxygen levels in bottom waters. This has certainly been observed in this area, and diatoms, a phytoplankton which represents the base of the food chain, have shown variable silicon/nitrogen (Si/N) ratios. Because diatoms create their shells from silicon, their growth is controlled not only by nitrogen inputs but the availability of silicon. Lower Si/N ratios are showing that silicon may be playing an increasingly important role in regulating diatom production in the system. For this reason, a scientist from the University of South Alabama will determine the biogeochemical processes controlling changes in Si/N ratios in the Louisiana Shelf system. One graduate student on their way to a doctorate degree and three undergraduate students will be supported and trained as part of this project. Also, four scholarships for low-income, high school students from Title 1 schools will get to participate in a month-long summer Marine Science course at the Dauphin Island Sea Laboratory and be included in the research project. The study has significant societal benefits given this is an area where $2.4 trillion gross domestic product revenue is tied up in coastal resources. Since diatoms are at the base of the food chain that is the biotic control on said coastal resources, the growth of diatoms in response to eutrophication is important to study.

Eutrophication of the Mississippi River and its tributaries has the potential to alter the biological landscape of the Louisiana Shelf system in the northern Gulf of Mexico by influencing the Si/N ratios below those that are optimal for diatom growth. A scientist from the University of South Alabama believes the observed changes in the Si/N ratio may indicate silicon now plays an important role in regulating diatom production in the system. As such, understanding the biotic and abiotic processes controlling the silicon cycle is crucial because diatoms dominate at the base of the food chain in this highly productive region. The study will focus on following issues: (1) the importance of recycled silicon sources on diatom production; (2) can heavily-silicified diatoms adapt to changing Si/N ratios more effectively than lightly-silicified diatoms; and (3) the role of reverse weathering in sequestering silicon thereby reducing diffusive pore-water transport. To attain these goals, a new analytical approach, the PDMPO method (compound 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylamino-carbamoyl)methoxy)phenyl)oxazole) that quantitatively measures taxa-specific silica production would be used.