OS21C-1189
Seasonal Distributions of Inorganic and Organic Nutrients on the Louisiana Continental Shelf and Implications for Nutrient Limitation and Hypoxia Formation
It is of paramount importance to understand the factors controlling phytoplankton biomass when designing management programs to mitigate eutrophication. Nutrients delivered by the Mississippi and Atchafalaya Rivers to the Louisiana continental shelf derive from the Mississippi watershed, covering nearly 50% of the continental United States, and cause local eutrophication resulting in an annual summertime bottom water hypoxic zone that can be as large as Massachusetts. Data on organic and particulate nutrient distributions on the Louisiana shelf are limited to a few studies and a small number of samples. To provide a more complete picture of nutrient biogeochemistry in the northern Gulf of Mexico, we measured dissolved inorganic, dissolved organic, and particulate carbon, nitrogen and phosphorus concentrations along the Louisiana continental shelf between the Mississippi and Atchafalaya Rivers during three cruises in March, May and July 2004. Mean inorganic nitrogen to phosphorus ratios were highest in May (60) but were still high (>32) in March and July. Total nitrogen to total phosphorus ratios were even higher, averaging about 100 in March and May and around 50 in July. Dissolved organic nitrogen was higher than dissolved inorganic nitrogen in higher salinity waters, but was lower near the two rivers. Dissolved organic phosphorus was lower than inorganic phosphorus at most stations and does not appear to alleviate inorganic P stress. The combination of low P concentrations, high inorganic N:P ratios, and even higher total N:P ratios indicated P limitation of phytoplankton biomass on the Louisiana shelf during the spring and early summer of 2004. This P limitation occurs during the period of hypoxia formation as a result of excess N loads delivered by the rivers, indicating that controls of both N and P are necessary to reduce the size of the hypoxia. A recent update to the Action Plan for mitigating hypoxia in this area aims at concurrent N and P reductions, but actions remain voluntary, making it unclear that eutrophication will be relieved any time soon.
OS21C-1190
Sources and Cycling of CDOM and DOC in the Gulf of Mexico
The distribution and cycling of colored dissolved organic matter (CDOM) is of importance to the study of biogeochemical cycling of carbon and other elements in the ocean as well as to ocean optics due to its strong absorption of UV and short wavelength visible light. The absorption properties of gelbstoff make it photo-chemically active and also make it a controlling factor in the quality and quantity of light available for photosynthesis in the coastal ocean. Much of the variability of CDOM distribution in the ocean occurs nearshore, due to differences in riverine CDOM concentrations and discharge. CDOM and DOC sometimes show a correlation in regions where DOC concentrations are controlled by freshwater discharge. The Gulf of Mexico plays a key role in the North American Carbon cycle, draining more than 60% of the U.S. and more than 40% of Mexico, with freshwater input from 33 major river systems as well as from submarine groundwater discharge. Over the eight year period between 1998 and 2005, we have collected more than 1,000 CDOM samples 34 cruises in the region between the Mississippi River and the Florida Keys. We have also sampled 10 major river systems over multi-year periods. The major factors controlling CDOM distributions in the Gulf of Mexico are river of origin, hydrographic factors such as floods and droughts, and circulation on the shelf. Rivers in southern Florida have higher CDOM concentrations than those in the west central and southwest regions, which are higher than those found in the Mississippi River water. Dry seasons and drought years are characterized by high fluorescence efficiencies due to photobleaching. High CDOM, high salinity water masses are produced in the hypersaline waters of Florida Bay and from coastal upwelling of near-bottom waters along the shelf in the region of Charlotte Harbor. This paper will present a compilation and summary of CDOM and DOC distributions and analysis of the key controlling factors for the eastern Gulf of Mexico.
OS21C-1191
Export of Dissolved Lignin from Coastal Wetlands to the Louisiana Shelf
Here we report on spatial and temporal changes in the concentration and composition of dissolved lignin- phenols in surface and bottom waters off the Louisiana coast (USA). Samples were collected at 7 stations on 2 cruises (April, and July, 2008) along a transect that spanned from inside Terrebonne Bay, Louisiana (12 m water depth) to the outer-most station on the inner Louisiana shelf (21 m water depth). The highest average concentration of dissolved organic carbon (DOC) and dissolved lignin, during both cruises, occurred at the interface between Terrebonne Bay and the inner shelf. Average DOC and dissolved lignin concentrations were significantly higher in April than in July across most stations. Based on hydrologic data, these higher concentrations clearly reflect a combined mixing of DOM from plume waters to the west and local marsh inputs. The cinnamyl/vanillyl (C/V) and syringyl/vanillyl (S/V) ratios indicated that the predominant source of lignin was from non-woody angiosperms - likely the dominant species of wetland plants Spartina alterniflora and S. patens (Spartina spp.) that border the entire bay. The high vanillic acid to vanillin (Ad/Al)v ratios for all stations were typical of that found near estuarine boundaries, where biologically- and photochemically-mediated lignin decay processes are important. This preliminary data indicates that wetlands provide another source of dissolved organic matter (DOM) to the Louisiana shelf that likely contributes to microbial food resources and hence hypoxia, especially in the context of the instability and extensive erosion of these marshes over the past ca. 50 years. This has important implications for the current management plan to reduce hypoxia in the GOM, particularly in those regions that extend west of the nutrient-rich highly productive near-field zones of Atchafalaya-Mississippi river plumes.
OS21C-1192
Estimating Dissolved Organic Nitrogen from CDOM on the West Florida Shelf
Ecosystem models to predict and quantify primary production on the West Florida Shelf require the best available hydrographic and biogeochemical data on varying temporal and spatial scales. Although it is the physical parameters and hydrography that help determine the presence and distribution of primary producers, ultimately it is the nutrient availability that regulates growth rate, biomass and the duration of a phytoplankton bloom. Recent advances in field sensors have allowed for high-resolution measurements of inorganic nutrients, such as phosphate and nitrate, which have improved the community's understanding of the seasonality, cycling and fate of inorganic N and P in the coastal ocean. Measurements of organic nutrients, however, pose a larger problem as there currently exists no means of obtaining Dissolved Organic Nitrogen (DON) concentrations in the ocean beyond labor- intensive and expensive discrete sampling methodology via ship surveys. This results in data gaps due to course sampling practices, both in time and space. Discussed here will be the implementation of a new resource-efficient strategy for estimating organic nitrogen in the ocean that supplements measurements from survey cruises. This paper will demonstrate that space-based Colored Dissolve Organic Matter (CDOM) measurements of ocean color and in field CDOM fluorescence measurements can be used to estimate DON distributions on the West Florida Shelf, a river-dominated margin in the Gulf of Mexico.
OS21C-1193
The Role of Organic Carbon in Absorption Properties of Louisiana Shelf Suspended Particulate Matter
River-derived suspended particulate matter (SPM) in the northern Gulf of Mexico is an important vector for the delivery of particulate organic carbon (POC) to the Louisiana shelf region. Optical proxies for SPM and POC, such as in situ and remotely-sensed absorption, allow the higher resolution and autonomous measurements necessary for study of carbon cycling in spatially- and temporally-variable river-influenced coastal regions. In this study, the relationship between absorption by particles (300-800nm) and their organic carbon content is investigated. The mass-specific absorption properties of a variety of suspended and sedimentary particulate matter samples collected from the sediments and water column of the Atchafalaya River and the surrounding inner continental shelf were determined in aqueous suspension from 300-800nm on a spectrophotometer equipped with an integrating sphere. POC was then oxidatively removed from the samples using a variety of chemical and photochemical techniques, and absorption properties remeasured. Prior to oxidation, samples exhibited similar mass-specific absorption spectra, which is consistent with the comparable iron and organic carbon contents of marine sediments in the region. After organic carbon removal, samples were bleached at visible wavelengths. Mass-specific absorption consistently decreased, with spectral changes on the order of 1 m2 per g organic carbon removed. Changes in the ultraviolet region of the spectrum were larger and more variable, with increases and decreases on the order of 0.01-1 m2 per g sediment. Part of the variability in the observed mass-specific UV absorption was specific to heat-driven oxidation methods. Changes in UV absorption also appeared consistent with simultaneous changes in mineral composition, crystallinity, and organic carbon content and quality.
OS21C-1194
Characterization of the Effects of Microbial Processing in Gulf of Mexico Coastal Sands on the Composition of Dissolved Organic Matter Using Ultrahigh Resolution Mass Spectrometry
Permeable coastal sands along the Gulf coast of Florida filter large volumes of water containing terrestrially derived humic materials, but there is very limited data on the biogeochemical and microbial processes affecting Dissolved Organic Matter (DOM) turnover during filtration. DOM undergoes changes as it moves from fresh water sources through estuaries and into the coastal zone. We have used ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) to define the exact molecular changes associated with DOM filtration through these sands. High field (9.4 Tesla) FT-ICR MS can resolve greater than 10,000 peaks and provide elemental composition (CcHhOoNnSs) for individual compounds found in complex DOM mixtures. Microbially dense sand cores were obtained from off the Gulf coast side of St. George Island located at Apalachicola Bay near the mouth of the Apalachicola River in Northern Florida. Humic-rich river water and water spiked with "fresh" DOM isolated from diatom growth cultures were passed through the sand columns with flow rates comparable to those in sandy sediments. DOM was extracted from these two water samples before and after passage through the columns using polymer Solid Phase Extraction (SPE) cartridges and then analyzed with a 9.4 T FT-ICR mass spectrometer. Negative ion mode ESI FT-ICR mass spectra of DOM before and after passage through the sand columns produced about 8,000 resolved peaks providing close to 6,400 unambiguous molecular formula assignments containing C, H, O, N, and S based on mass accuracy and homologous series. Using molecular analysis techniques such as Kendrick plots, van Krevelen diagrams, and Double Bond Equivalency (DBE) frequencies, we were able to conclude that "fresh" DOM isolated from algal growth chambers was much more reactive than older riverine DOM flowing into the bay.
OS21C-1195
Identification of saline fluids in the northern Gulf of Mexico: Sr and B isotopic evidences
Surface and depth profile seawater specimens were collected from several transect stations to study fluid sources and mixing processes near the Mississippi River estuary in the northern Gulf of Mexico (GOM). Potential fluid sources in the region include riverine inputs of fresh water and anthropogenic content, as well as the widely distributed the submarine seepages. Several end-member fluids with distinct chemical compositions were identified, two brines, seawater and freshwater. One of the brines is composed of high δ11B, low 87Sr/86Sr, possibly related to halite/anhydrite dissolution. Precipitation of K- and Mg- sulfate minerals affect significantly the δ11B in brine, as well as 87Sr/86Sr and Br/Cl by sylvite dissolution. The fresh water end-member contains low δ11B and high 87Sr/86Sr, accompanied with low major element. In addition, other source of submarine discharge freshwater was detected at several depth intervals, possible indication of submarine groundwater discharge.
OS21C-1196
High Frequency and Multi-parameter Observation of Land-Sea Connection at the Aransas Pass Tidal Inlet, South Texas in Summer 2008
Understanding the nature of water exchange and material transport processes at tidal inlets is critical in improving our knowledge of land-sea connection and exchange processes. High-frequency multi-parameter water property measurement was conducted over a month period during mid-June to mid-July in 2008 at the UT Marine Science Institute pier at Port Aransas, Texas throughout 12-m water column. The pier is at the Aransas Pass tidal inlet, which is a major water and property exchange pathway in South Texas between several local bays and the Gulf of Mexico. Unlike the summer 2007 when a large-scale freshwater discharge event occurred, the summer 2008 during the observation period was relatively dry in general. Offshore influence was more pronounced this year than 2007 with multiple days of higher salinity water (higher than 36 psu) dominating over tidal cycles. The offshore influence was also marked by lower oxygen and chlorophyll concentrations. The lower oxygen content water with higher salinity seems to be connected to low-oxygen bottom water on near shore shelf area. Additional instrument mooring data during hurricane Dolly will also be presented along with the current meter and tide gauge information. Comparison of the data with that observed from nearby Mission-Aransas National Estuarine Research Reserve SWMP stations will be presented as well. Continuous water column measurements at a local inlet show a potential to quantify water property flux and to detect episodic events in the coastal environment.