OS23G-01 INVITED
CO2 Dynamics and Community Metabolism in the Mississippi River Plume
Dissolved inorganic carbon (DIC), total alkalinity (TAlk), pH, DO, and nutrients were measured in the Mississippi River plume during five cruises in spring, summer and fall. In contrast to other large rivers, both DIC and TAlk were higher in the river than in seawater and were removed along the mixing gradient between river and coastal water. In the intermediate salinity regions, the strong DIC removal was accompanied by strong nutrient removal, high dissolved oxygen (DO), high pH, and high chlorophyll a concentration. Net community production (NCP) rate estimated based on DIC and TAlk removal peaked during summer, and was among the highest observed for large river plumes. In summer and fall, 60-80% of the nutrient load from the Mississippi River was consumed in the plume, while only 30% of nutrient load was consumed in spring as the result of higher discharge, higher nutrient load, and lower productivity. Spatial and temporal changes in carbon and nutrient dynamics and NCP as well as a comparison with other large river plumes suggest that nutrient supply, light availability, temperature, and water residence time are the most important factors regulating NCP in this large river plume. Budget analysis suggests that the Mississippi River plume produces sufficient amount of labile organic carbon during March-June that meets the demand for developing the summer hypoxia of the northern Gulf of Mexico. CaCO3 precipitation is also likely, but the mechanism is still not clear.
OS23G-02 INVITED
Gulf Coast Atmospheric Inflow: A Key Element of the North American CO2 Budget
Quantitative estimation of surface sources and sinks of carbon from atmospheric trace gas transport
inversions has emerged as an important methodology for understanding regional biogeochemistry. To this
end, the North America Carbon Program includes a significant effort to build and operate a network of
instrumented towers measuring continuous atmospheric CO2 mixing ratio in situ. We have used these
data to optimize a coupled model of land-atmosphere CO2 exchange over most of North America, and
find a land sink of about 0.6 GtC yr-1 over the eastern third of the USA, which is in broad agreement
with other recent studies. Our estimate of the terrestrial carbon budget is quite sensitive to the CO2
mixing ratio of lateral inflow air into the region. We used a global transport model to quantify the degree to
which lateral inflow CO2 is determined by distant (Eastern Hemisphere) emissions vs. proximal (Western
Hemisphere) emissions. We find that during summer months as much as 90% of the variance of
atmospheric CO2 along the coast of the Gulf of Mexico is driven by sources and sinks in the Eastern
Hemisphere. These variations must be prescribed to the regional inversions form a global model, but are
almost completely unconstrained by observations given the current network of CO2 stations. Adding
even a single continuous monitoring tower near the Gulf Coast would substantially reduce the uncertainty of
the carbon budget of eastern North America. The juxtaposition of atmospheric inflow, Mississippi River
outflow of dissolved and particulate carbon, and the changing biogeochemistry of the Gulf waters themselves
highlights this region as a linchpin in terms of better understanding the continental carbon cycle.
http://biocycle.atmos.colostate.edu
OS23G-03
A Seasonal CO2 Sink Associated with the Mississippi and Atchafalaya Rivers
Recent studies in the northern Gulf of Mexico and elsewhere have demonstrated that enhanced biological production in large river plumes may influence surface pCO2 levels resulting in a net surface influx of atmospheric CO2. However, such systems deliver large amounts of terrestrial carbon into continental margin waters, and hence, the potential for large and variable signals in carbon flux exist in these regions. Our prior findings for the Mississippi River plume suggest that the late spring and early summer is a period of lower surface pCO2 corresponding to a strong biological pump and autotrophic fixation of inorganic carbon. Here, we examine an extended set of observations for the northern Gulf of Mexico that includes regions influenced by both the Mississippi and Atchafalaya rivers. A combination of satellite and ship-based observations were used to examine variability in surface pCO2 and air-sea flux of carbon dioxide. Ship-based observations revealed regions of low surface pCO2 in June 2006 following a period of moderately high river flow. Low surface pCO2 was observed in the both the Mississippi and Atchafalaya river outflow regions. These regions corresponded to high chlorophyll concentrations as revealed by ocean color satellite imagery. In contrast in September 2006, surface pCO2 was uniformly higher with the exception of a localized area around the Mississippi River plume. Our results reinforce earlier findings that large river dominated margins exhibit highly variable air-sea fluxes of CO2, but may serve as a sink for CO2 on a seasonal basis.
OS23G-04
Nutrient Enrichment Effects on Benthic Biodiversity by the Mississippi River and Submarine Canyon of the Northern Gulf of Mexico
Biodiversity is measured by (1) α diversity: number of species in relation to a standardized number of individual within a define habitat; (2) β diversity: compositional change or turnover of species between two or more spatial units; and (3) γ diversity: total number of species in a large geographic area. The pattern of biodiversity is usually driven by various physico-chemical conditions. In the deep sea, a cross-isobath parabolic diversity pattern has been well-documented for benthic macrofauna and the cause has been attributed to a dynamic equilibrium between population growth and competition exclusion along a gradient of declining food resources with depth (Rex 1981). Both nutrient-enriched (dominated by opportunistic species) and oligotrophic conditions (slow growth rate) could depress diversity, while the highest diversity can be reached by competitive equilibrium within communities at intermediate resource conditions. In the Gulf of Mexico (GoM), the discharge of Mississippi River can enhance the organic flux to the seafloor adjacent to the mouth of Mississippi River and Mississippi Canyon. The goal of this study was to test Rex's (1981) dynamic equilibrium model between depth-transects that were exposed to different levels of organic enrichment. Four treatments contrasted along the upper slope (250m to 1500m) included (1) Mississippi Canyon (active canyon), (2) De Soto Canyon (inactive canyon), (3) central slope transect (in proximity to Mississippi Canyon), and (4) the west and east slope transects (away from the influence of the Mississippi River). SeaWifs satellite data confirmed that the head of Mississippi Canyon experience highest surface primary production and export POC flux. The lowest α diversity of benthic macrofauna (collecting between 2000 and 2002) was observed at the head of the Mississippi Canyon where γ diversity was relatively high. This suggested that the canyon head was dominated by opportunistic species due the high POC flux but were still able to maintain a large number of species due the physical complexity of the canyon. The change of β diversity was nominal within the Mississippi and De Soto Canyon transects, suggesting that the faunal composition was more homogenous within the canyon than outside of canyon.
OS23G-05
Watershed Export and Estuarine Ecosystem Response to Pulsed Inputs of Nitrogen to South Texas Bays
Precipitation and associated watershed export from land to sea along the south Texas coast is highly variable within and between years. Past studies in this region have emphasized the effects of variable salinity on estuarine ecosystems, and in particular the response of benthic communities. This presentation will focus on (1) patterns of nitrogen export from watersheds to coastal waters of south Texas and (2) nitrogen dynamics in coastal waters following major watershed export events. Recent work in the Guadalupe, San Antonio, Mission, and Aransas river watersheds shows a strong positive correlation between river discharge and organic nitrogen concentrations in river water. At the same time, nitrate concentrations in river water are diluted as discharge increases. Although variable, ammonium concentrations in river water do not correlate strongly with discharge. Results from Copano Bay (which receives direct inputs from the Mission and Aransas rivers) following major watershed export events in July 2007 exemplify the estuarine ecosystem response to such inputs. Salinity in Copano Bay dropped from 12 to 2 psu within 5 days of the first event and did not reach pre-event salinity again until December. Nitrate and ammonium concentrations in the bay increased substantially with each watershed export event, but showed different patterns of subsequent recovery. Nitrate decreased rapidly while ammonium was more variable and decreased more slowly. Dissolved organic nitrogen decreased immediately after the first watershed export event and then increased to concentrations greater than pre-event values within a week. Although the entire bay showed a large salinity response, variations in nitrogen concentrations associated with the storm events decreased with increasing distance from the river mouth. With changes in the magnitude and timing of precipitation events in the south western US predicted over the coming decades, understanding linkages between pulsed watershed inputs and estuarine ecosystem responses is increasingly important with respect to management of south Texas estuaries. These systems also provide a good model for thinking about the role of storm events in watershed-estuary systems more generally.
OS23G-06
Size and Chemical Affinity Fractionated Dissolved Cadmium, Copper and Nickel in Gulf of Mexico Surface Waters
Dissolved trace metals Cd, Cu, and Ni in the surface waters of Gulf of Mexico exhibit distinct chemical reactivity and physical size distributions when using cross-flow ultrafiltration and ion exchange methods during a field survey conducted in May 2006. Variations of total dissolved metal concentrations in surface waters were found across the salinity gradient, which ranged as follows; Cd: 87-187 pM; Cu: 1.4-18.3nM; and Ni: 2.6-18.8nM. Dissolved Cd was predominantly present as a truly dissolved (97%) and cationic-labile fraction (94%) in the surface waters. The anionic-organic metal fractions accounted for just 3กำ1 % on average for Cd, 24% for Cu, and 9% for Ni. The dissolved inert metal fractions, on average, were 31% of total dissolved Cu and 29% of total dissolved Ni concentrations. Small but noticeable amounts (6%) of dissolved inert Cd fractions were also present. Some fractions of the total dissolved Cu (17%) and Ni (8%) could be adsorbed by both cation and anion exchange resins, suggesting binding to zwitterionic molecules. Despite evidence that partitioning among chemically and physically defined species is dynamic, mixing between freshwater and seawater end-members across the Mississippi River plume produced linear mixing curves, while trace metal concentrations determined within warm core and cold core rings in the Gulf of Mexico maintained significantly different concentrations and species distributions.
OS23G-07
A Semantic Approach for Knowledge Discovery to Help Mitigate Habitat Loss in the Gulf of Mexico
Noesis is a meta-search engine and a resource aggregator that uses domain ontologies to provide scoped
search capabilities. Ontologies enable Noesis to help users refine their searches for information on the open
web and in hidden web locations such as data catalogues with standardized, but discipline specific
vocabularies. Through its ontologies Noesis provides a guided refinement of search queries which produces
complete and accurate searches while reducing the user's burden to experiment with different search strings.
All search results are organized by categories (e. g. all results from Google are grouped together) which
may be selected or omitted according to the desire of the user.
During the past two years ontologies were developed for sea grasses in the Gulf of Mexico and were used to
support a habitat restoration demonstration project. Currently these ontologies are being augmented to
address the special characteristics of mangroves. These new ontologies will extend the demonstration project
to broader regions of the Gulf including protected mangrove locations in coastal Mexico. Noesis contributes
to the decision making process by producing a comprehensive list of relevant resources based on the
semantic information contained in the ontologies.
Ontologies are organized in a tree like taxonomies, where the child nodes represent the Specializations and
the parent nodes represent the Generalizations of a node or concept. Specializations can be used to provide
more detailed search, while generalizations are used to make the search broader. Ontologies are also used
to link two syntactically different terms to one semantic concept (synonyms). Appending a synonym to the
query expands the search, thus providing better search coverage. Every concept has a set of properties that
are neither in the same inheritance hierarchy (Specializations / Generalizations) nor equivalent (synonyms).
These are called Related Concepts and they are captured in the ontology through property relationships. By
using Related Concepts users can search for resources with respect to a particular property. Noesis
automatically generates searches that include all of these capabilities, removing the burden from the user
and producing broader and more accurate search results.
This presentation will demonstrate the features of Noesis and describe its application to habitat studies in the
Gulf of Mexico.
http://www.gomrc.org/