65 research outputs found

    Tidal and groundwater fluxes to a shallow, microtidal estuary : constraining inputs through field observations and hydrodynamic modeling

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    This paper is not subject to U.S. copyright. The definitive version was published in Estuaries and Coasts 35 (2012): 1285-1298, doi:10.1007/s12237-012-9515-x.Increased nutrient loading to estuaries has led to eutrophication, degraded water quality, and ecological transformations. Quantifying nutrient loads in systems with significant groundwater input can be difficult due to the challenge of measuring groundwater fluxes. We quantified tidal and freshwater fluxes over an 8-week period at the entrance of West Falmouth Harbor, Massachusetts, a eutrophic, groundwater-fed estuary. Fluxes were estimated from velocity and salinity measurements and a total exchange flow (TEF) methodology. Intermittent cross-sectional measurements of velocity and salinity were used to convert point measurements to cross-sectionally averaged values over the entire deployment (index relationships). The estimated mean freshwater flux (0.19 m3/s) for the 8-week period was mainly due to groundwater input (0.21 m3/s) with contributions from precipitation to the estuary surface (0.026 m3/s) and removal by evaporation (0.048 m3/s). Spring–neap variations in freshwater export that appeared in shorter-term averages were mostly artifacts of the index relationships. Hydrodynamic modeling with steady groundwater input demonstrated that while the TEF methodology resolves the freshwater flux signal, calibration of the index– salinity relationships during spring tide conditions only was responsible for most of the spring–neap signal. The mean freshwater flux over the entire period estimated from the combination of the index-velocity, index–salinity, and TEF calculations were consistent with the model, suggesting that this methodology is a reliable way of estimating freshwater fluxes in the estuary over timescales greater than the spring– neap cycle. Combining this type of field campaign with hydrodynamic modeling provides guidance for estimating both magnitude of groundwater input and estuarine storage of freshwater and sets the stage for robust estimation of the nutrient load in groundwater.Funding was provided by the USGS Coastal and Marine Geology Program and by National Science Foundation Award #0420575 from the Biocomplexity/Coupled Biogeochemical Cycles Program

    River Influences on Shelf Ecosystems: Introduction and Synthesis

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    River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ?shadow zone? off northern Oregon

    Adaptive Significance of the Formation of Multi-Species Fish Spawning Aggregations near Submerged Capes

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    BACKGROUND: Many fishes are known to spawn at distinct geomorphological features such as submerged capes or "promontories," and the widespread use of these sites for spawning must imply some evolutionary advantage. Spawning at these capes is thought to result in rapid offshore transport of eggs, thereby reducing predation levels and facilitating dispersal to areas of suitable habitat. METHODOLOGY/PRINCIPAL FINDINGS: To test this "off-reef transport" hypothesis, we use a hydrodynamic model and explore the effects of topography on currents at submerged capes where spawning occurs and at similar capes where spawning does not occur, along the Mesoamerican Barrier Reef. All capes modeled in this study produced eddy-shedding regimes, but specific eddy attributes differed between spawning and non-spawning sites. Eddies at spawning sites were significantly stronger than those at non-spawning sites, and upwelling and fronts were the products of the eddy formation process. Frontal zones, present particularly at the edges of eddies near the shelf, may serve to retain larvae and nutrients. Spawning site eddies were also more predictable in terms of diameter and longevity. Passive particles released at spawning and control sites were dispersed from the release site at similar rates, but particles from spawning sites were more highly aggregated in their distributions than those from control sites, and remained closer to shore at all times. CONCLUSIONS/SIGNIFICANCE: Our findings contradict previous hypotheses that cape spawning leads to high egg dispersion due to offshore transport, and that they are attractive for spawning due to high, variable currents. Rather, we show that current regimes at spawning sites are more predictable, concentrate the eggs, and keep larvae closer to shore. These attributes would confer evolutionary advantages by maintaining relatively similar recruitment patterns year after year

    OMAE2004-51377 SPACE UTILIZATION FOR OCEAN POWER GENERATION TECHNOLOGIES

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    ABSTRACT This paper concerns how much ocean space would be utilized for a 10MW or 50MW ocean power plant. The most promising emerging technologies from space utilization points-of-view (i.e., power per unit sea area, environmental aspects, navigational aspects, cost, etc.) appear to be distributed subsea systems. A case example for a 10 MW subsea ocean power project would be 20 to 100 individual moored generators, positioned in an array. The units would generate electricity independently but be interconnected to a single transmission/communications infrastructure. Along with the size of the individual moored generators (which are relatively small), the calculation procedure for ocean space utilization takes into account that: (1) there is an optimum positioning for the individual moored units so that the energy extracted from one unit does not interfere with adjacent units; (2) there is a spacing criteria both among the individual units and surrounding the entire array for environmental, operations and maintenance, and navigational issues, and; (3) there is an optimum spacing to minimize electrical transmission and communication infrastructure costs. Several case examples are presented for distributed subsea systems. These examples range from 1 MW pilot scale to 10-100 MW utility scales. It has been calculated that a 10MW distributed subsea ocean power plant would occupy 100 acres of ocean space when taking into account ocean space utilization factors identified above. For larger utility scale facilities, ocean space utilization was calculated to be approximately 50 megawatts per square mile. INTRODUCTION Renewable energy resources can be found above the sea (wind), at the sea-air surface (waves) and sub-sea (surge and current). The distributed generation approach has always been considered for ocean energy -arrays of power generation equipment each capable of independent operation and feeding into a single transmission/communications infrastructure (ref. 1) and the present offshore wind power projects in the North Sea follow this design approach (ref. 2)

    Shifts in the locus of crustal thickening during Mesoproterozoic orogenesis in the Mt Isa Terrane

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    A fundamental change from thin-skinned to thick-skinned tectonics in the Mt Isa Terrane is interpreted to reflect a westward shift in the locus of crustal thickening during Mesoproterozoic orogenesis in northeastern Australia. Geochronological constraints indicate that west-directed, thin-skinned deformation in the eastern part of the terrane occurred before ca 1540 Ma. This upper crustal system is interpreted to link with deeper crustal thickening in the Georgetown Inlier to the east, where synorogenic granites intruded at ca 1550 Ma. This early system was dissected by basement-cutting reverse faults across the entire Mt Isa Terrane after 1540 Ma. The nature of deeper crustal tectonics within the Mt Isa Terrane is determined by evaluating possible origins for the crustal structure revealed in a seismic-refraction profile. In particular, alignment of shallowly west-dipping, mid-crustal, high-velocity layers is interpreted to reflect tectonic duplication of underplated crust. This crustal stacking fault is inferred to be the controlling structure for the thick-skinned deformation associated with the Isan Orogeny
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