Award: OCE-1558957

Chemical processes on the Louisiana Shelf are strongly affected by the freshwater discharge from the Mississippi River and its distributaries. Diatoms are a major primary producer group in this system, and there have been well-documented changes to the nutrient concentrations (e.g. reduced dissolved silicate) in the Mississippi River which may directly affect diatoms' productivity and growth. Prior studies have suggested that the anthropogenic factors affecting this region have led to systematic shifts in the phytoplankton (microscopic primary producers) away from diatom-dominated assemblages toward flagellates, especially harmful algal bloom species. Such a shift has potentially altered how the base of the food web operates. Despite the decade-old recommendations for the study of Silicon as a potential factor regulating diatom production in this region, there have been no studies to date which addressed this issue comprehensively. Using the Louisiana Shelf as a model system, this project leveraged water-column and sediment sampling approaches to examine how biotic and abiotic processes affect the Silicon cycle at the land-sea interface. Specifically, we examined 1) whether diatom growth was limited by dissolved silicate concentrations, 2) if diatoms' silicon demand could be met by recycled (vs. riverine) silicon sources and 3) the strength of silicon sequestration in the sediments. Field surveys were conducted during the Coastal LouisianA Silicon Cycling (CLASiC) cruises in 27 August - 6 September 2016 and 4-13 May 2017; vessel time aboard the R/V Pelican was supported by this project. These periods corresponded to low and high Mississippi River discharge conditions. Diatom productivity was high during the CLASiC cruises. Euphotic zone integrated biogenic silica production (a proxy for diatom productivity) ranged two orders of magnitude, from <0.5 to ~70 mmol Si m-2 d-1 among cruises. Rates were strongly coupled to irradiance; average rates in the upper euphotic zone were >20-fold higher than at the base of the euphotic zone. The concentration of dissolved silicate limited the rate of biogenic silica production (i.e. kinetic limitation) in ~40% and ~75% of samples during low and high-river flow conditions, respectively. At times, the extent of kinetic limitation was high enough to infer that the diatom assemblage may have been growth limited by the low dissolved silicate concentration. There was large spatial and temporal variability in benthic silicate fluxes ranging between -4 to 15 mmol Si m-2 d-1. At times, the quantity of dissolved silicate from benthic flux could have sustained a high proportion of diatom silicon demand if the euphotic zone penetrated the main halocline. The general trend observed in this study was that fluxes decreased from east to west. The largest flux estimates occurred in proximity to the river delta. Overall, flux estimates from in situ lander deployments were consistently greater than ex situ laboratory incubations by factor of 1.5-5.5, probably due to better representation of biological factors such as macrofaunal irrigation within the benthic lander. Regional eutrophication, which previous studies has shown to be concurrent with increased diatom preservation and biogenic silica accumulation in local sediments, may have facilitated additional sequestration of both sedimentary silicon and cations within early diagenetic products. This hypothesis is supported by sediment core data demonstrating that the quantity of sediment silica from early diagenetic products, relative to biogenic silica, is about 50% higher than other regions studied to date. Additionally, silicon stable isotope composition of various sedimentary silica pools demonstrated that biogenic silica serves as a substrate for these early diagenetic products and this biogenic silica was isotopically heavy, inferring it originated from diatoms that settled from the overlying surface waters. Collectively, this project has demonstrated that despite decreasing dissolved silicate loads from the Mississippi River, and kinetic limitation of biogenic silica production by suboptimal silicate, regional diatoms are highly productive relative to many deep-water systems studied to date. A significant quantity of production could be supported from recycled sedimentary silicate sources depending on the strength of water column stratification. Additionally, diatom shells promote postmortem silica sequestration by acting as a template for early diagenetic products. Given the strong anthropogenic footprint in this region, these conditions may be like other land-sea regions transformed by human-induced changes. This project helped train students and interns. Two Ph.D. students were supported at the University of South Alabama, one M.S. student at Louisiana State University, and provided sampling opportunities for three post-doctoral scholars and four graduate students not supported financially by this project. Three Research Experience for Undergraduate projects at the Dauphin Island Sea Lab were supported by this work in addition to four undergraduate and postgraduate interns in the PI's laboratory. Data are available through the Biological & Chemical Oceanography Data Management Office under project 712667. Last Modified: 06/11/2020 Submitted by: Jeffrey W Krause