We report an assessment for determining the contribution by diatoms to community productivity and respiration within a coastal benthic ecosystem with multiple autotrophs. During summer, cores of open sediment and seagrass habitat were collected from a lagoon within the Northern Gulf of Mexico. Cores were maintained in an outdoor mesocosm. Germanic acid, an inhibitor of diatom cell division, was added to half the cores and quantification of production and respiration was done. Inhibition of diatoms reduced benthic productivity within the seagrass habitat. 71 to 83% of production was attributable to diatoms and this contribution moved the benthic system into net autotrophy. Diatom contribution to production in other habitat-community components was more variable (varied from 0 to 86%). Findings underscore the ecological importance of diatoms as producers in seagrass beds, the role of seagrasses in maintaining productivity, and infer that diatoms may have similar contributions in other aquatic vegetated habitats.

Table of Contents

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

Three repeated experimental trials were done in summer months. Thirty-two cores (27 cm diameter, 14 cm depth) were collected from 50 m2 area of seagrass bed at 1 m depth on: June 28, July 12 and July 26, 2017 for trials 1-3, respectively. On each date, 16 cores were collected from seagrass habitat in pairs. Another 16 cores were collected from open sediment (OS) habitat. Extracted, paired cores were placed upright into an open-top plastic tub (49 x 33 x 42 cm) to produce eight tubs of each habitat.

Tubs were transported to Dauphin Island Sea Lab (~30-minute drive) filled with seawater (to core depth of 16 cm) pumped from Mobile Bay (20 km, east of site) and arranged in four blocks within an outdoor mesocosm. Each block contained two tubs of each habitat. After two days, a diatom-specific inhibitor (3 µM solution of germanic acid, i.e. Ge treatment) was randomly added to water, i.e. two tubs per block, one of each habitat type. Germanium (Ge) at high Ge/Si ratios (> 0.01) prevents formation of siliceous cell wall (Azam and Chisholm 1976). We added 3 µM solution and allowed two days for Ge incorporation.

Metabolism measurements:

Two days after, we quantified productivity and respiration from changes in oxygen content within 2-3 hour incubations of chambers and bottles following methods in Anton et al. (2009). Oxygen content was measured with a meter (HQ30d, Hach, Loveland, Colorado, USA) and, this was initial oxygen content for both chamber and bottle incubation. After incubation, we measured final oxygen content in bottles and chambers.

To compare rates between treatments, net community production (NCP) and respiration were assessed in mg O2 m–2 h–1 and mg O2 L-1 h-1 for benthic and water-column (WC) communities, respectively. Equations were:

WC NCP = (Fcb – Icb) t-1 (1)

WC respiration = (Fdb – Idb) t-1 (2)

Benthic NCP = [(Fcc – Icc) – (Fcb – Icb)] V t-1 A-1 (3)

Benthic respiration = [(Fdc – Idc) – (Fdb – Idb)] V t-1 A-1 (4)

where capital letters are for initial (I) or final (F) oxygen content (mg L-1) for clear (c) and dark (d) incubations (first letter in subscript) in chambers (c) or bottles (b) (second letter in subscript); t is incubation time (h), V is volume (L) and A is area (m2) of chamber. Gross primary productivity (GPP) was calculated as sum between NCP and absolute respiration for each tub.

To compare GPP between communities, control values were expressed in mg O2 m-2 h-1 after WC metrics of NCP and respiration were integrated over a 1 m depth (x 1000 L). System GPP was obtained by summing WC and benthic GPP.

Environmental measurements:

Salinity and temperature were measured at time of oxygen measurements using the same meter. Surface photosynthetic active radiation (PAR) (from environmental station 30°15.075' N, -88°04.670' E Dauphin Island, Alabama, USA; http://arcos.disl.org) was averaged over incubation duration and integrated over a 48 hour-period prior to incubations (Photosynthetic Photon Flux Density, PPFD). 48 hours reflects a short-term measure of light history.

Statistical analyses:

A series of two-way ANOVAs with trial and treatment as fixed factors were used to test for differences in producer biomass in both habitats and were used to test for differences in rates with and without diatom metabolism. Differences in rates were attributed to diatom metabolism and percent contribution to GPP was calculated based off mean GPP for each trial (n=4) with Ge rate as a proportion of control rate, expressed as a change from 100%.

 

Excel, Sigma Plot

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. Added ND as a missing data identifier.
* removed all spaces in headers and replaced with underscores
* removed all units from headers
* converted dates to ISO Format yyyy-mm-dd
* set Types for each data column 

Coverage: Northern Gulf of Mexico, specifically the Louisiana Shelf region dominated by the discharge of the Mississippi River on the western side of the delta

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.