Nekton of new seagrass habitats colonizing a subsided salt marsh in Galveston Bay, Texas

Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core⁻¹ thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change.     

The Effect of the MAPS Weather Model on GPS-Determined Ellipsoidal Heights

We investigated a current numerical weather model, known as MAPS (Mesoscale Analysis and Prediction System), to determine if it could precisely define the behavior of GPS signals in the tropospere, ultimately leading to improved GPS-determined ellipsoidal heights. MAPS is the research version of the Rapid Update Cycle (RUC2) generated by NOAA's Forecast System Laboratory. MAPS is generated on an hourly basis and provides coverage in the contiguous United States at a 40-km grid spacing. We processed numerous subsets of GPS data collected over a months-long period on 23 static baselines ranging in length from 62 to 304 km. The GPS data were processed in 1/2-hr, 1-hr, 2-hr, and 4-hr session lengths. The primary effort was to compare the precision of heights obtained using a commonly adopted seasonal weather model with the precision of heights obtained using the MAPS weather model. Our analysis shows that the current version of MAPS can lead to improvement in GPS height precision when session lengths are shorter than two hours. For sessions longer than two hours, comparably precise heights may be obtained using a less accurate seasonal model by introducing appropriate nuisance parameters into the height estimation process. ? 2001 John Wiley & Sons, Inc.     

A kinematic model of ridge-transform geometry evolution

Spreading ridge-transform geometries will remain stable so long as accretion is symmetric. Asymmetric accretion, however, will cause lengthening or shortening of transforms and, in extreme cases, may result in zero-offset transforms (ZOTs) and very-long-offset transforms (VLOTs) such as the Ninetyeast and Chagos transforms. We use a simple kinematic model to examine the effects of various parameters on the evolution of zero-offset transforms and very-long-offset transforms. Starting with the transform length spectrum found along the Mid-Atlantic Ridge distributed in a randomly determined ridge-transform configuration, we allow for asymmetric accretion along ridge segments, assuming that individual ridge segments act independently. We analyze the effects of initial configuration, degree of asymmetry, and degree of bias in asymmetry on the generation of very-long-offset and zero-offset transforms. Finally, we examine the effect of these parameters on the possible steady-state nature of the transform length spectra. This model predicts that zero-offset transforms can be generated with a minimum of asymmetry, and that bias in asymmetry and initial ridge-transform-ridge configuration have no effect on generation of ZOTs. Similarly, random variations in spreading asymmetry have difficulty generating significant increases in transform length, so VLOTs may be manifestations of dynamic processes. Of the parameters tested, only lack of memory of zero-offset transforms has any effect on transform length distribution, and therefore, the transform length spectrum remains steady-state if ZOTs have some degree of memory.     

Pito Rift: How a large-offset rift propagates

The Pito Rift area is the site of actively deforming oceanic lithosphere that has been primarily under extension for at least the past million years, based on kinematic reconstructions. The major morphologic features, Pito Deep and Pito Seamount, are aligned toward the Euler pole for relative motion between the Easter and Nazca plates. SeaMARC II side-scan and bathymetry data indicate that there are two general modes of faulting currently active in the Pito Rift area. One is associated with incipient rifting of old (3 Ma) Nazca lithosphere by large NW-SE normal faults, and the other is associated with a broad area of right-lateral transform shear between the Nazca and Easter plates. This transform shear is distributed over a broad region because of the northward growth of the East Rift and parallel tectonic rifting within the Pito Rift area. The majority of the Pito Rift area is composed of preexisting blocks of Nazca plate that are back-tilted away from Pito Deep and strike perpendicular to present and previous relative plate motions. This observation suggests that block-faulting and back-tilting are the primary mechanisms responsible for the distributed lithospheric extension, in agreement with gravity and magnetic analyses (Martinez et al., this issue).The only recent volcanic flows observed in side-scan data are from the Pito Seamount area and to the outside of the outer pseudofault of the East Rift. The significance of the young flows near the outer pseudofault is not understood. We interpret the flows extending northwest from the Pito Seamount as representing a newly formed seafloor spreading axis within the Pito Rift area. Gravity and magnetic analyses (Martinez et al., this issue) together with SeaMARC II bathymetry and side-scan data support this interpretation.Based on the tectonic evolution of the Easter microplate, we propose an evolutionary model for the formation of the Pito Rift area, where new tectonic grabens form immediately west of the previous graben and with slightly more counterclockwise orientation. The duration and history of tectonic activity for each graben are not well constrained.     

Managing cropland and rangeland for climate mitigation: an expert elicitation on soil carbon in California

Understanding the magnitude of and uncertainty around soil carbon flux (SCF) is important in light of California’s efforts to increase SCF (from the atmosphere to soils) for climate change mitigation. SCF depends, to a great extent, on how soils are managed. Here, we summarize the results of an elicitation of soil science and carbon cycle experts aiming to characterize understanding of current SCF in California’s cropland and rangeland, and how it may respond to alternative management practices over time. We considered four cropland management practices—biochar, compost, cover crops, and no-till—and two rangeland management practices, compost and high-impact grazing. Results across all management practices reveal underlying uncertainties as well as very modest opportunities for soil carbon management to contribute meaningfully to California’s climate mitigation. Under median scenarios, experts expect all the surveyed management practices to reverse SCF from negative to positive, with direct carbon additions via biochar and compost offering the best potential for boosting the soil carbon pool.     

Application of remote sensing techniques for hydrologic investigations of upper yamuna catchment

The demands of water in various spheres of life, and its insufficient supply to fulfill these demands, specially in the developing countries, has become day by day a great challange to the hydrologist. In such a situation it becomes imperative to search a suitable methodology which can be used efficiently to predict the stream flows using minimum ground data, time and funds. Recently Remote Sensing Techniques have become a more popular and suitable tool for such studies. Keeping in view this aspect of problem, an analysis of Upper Yamuna Catchment, using Landsat imagery, was carried out to obtain the catchment characteristics which affect stream flows. Landsat imagery in Bands 5 and 7 on 1:1 million scale were analysed to get landuse and vegetal cover classification in categories (1) thick forest (ii) thin forest (iii) bare land and cultivation and (iv) snow cover. Using this data, alongwith rainfall data from 20 raingauge stations and stream flow data at Dakpather and Lakhwar, the stream flows were estimated by using Lumped system and Distributed system Models based on Rational Method. The computed 10 day period stream flows of Tons Sub-Basin were compared with observed surface flows with the help of hydrographs, flow comparison graphs and average monsoon period and monthly flow values. Using the coefficients evaluated from Tons sub-basin, the stream flow of Yamuna sub-basin were computed and compared with the observed surface flows.