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Research about United States, Mexico

Source-linked reports with geographic coverage including United States, Mexico.

23 records · Page 2Linked to original sources
USGS2003

Vertical structure of the phytoplankton community associated with a coastal plume in the Gulf of Mexico

Low salinity plumes of coastal origin are occasionally found far offshore, where they display a distinct color signature detectable by satellites. The impact of such plumes on carbon fixation and phytoplankton community structure in vertical profiles and on basin wide scales is poorly understood. On a research cruise in June 1999, ocean-color satellite-images (Sea-viewing Wide Field-of-view Sensor, SeaWiFS) were used in locating a Mississippi River plume in the eastern Gulf of Mexico . Profiles sampled within and outside of the plume were analyzed using flow cytometry, HPLC pigment analysis and primary production using 14 C incorporation. Additionally, RubisCO large subunit (rbcL) gene expression was measured by hybridization of extracted RNA using 3 full-length RNA gene probes specific for individual phytoplankton clades. We also used a combination of RT-PCR/PCR and TA cloning in order to generate cDNA and DNA rbcL clone libraries from samples taken in the plume . Primary productivity was greatest in the low salinity surface layer of the plume . The plume was also associated with high Synechococcus counts and a strong peak in Form IA rbcL expression. Form IB rbcL (green algal) mRNA was abundant at the subsurface chlorophyll maximum (SCM), whereas Form ID rbcL (chromophytic) expression showed little vertical structure . Phylogenetic analysis of cDNA libraries demonstrated the presence of Form IA rbcL Synechococcus phylotypes in the plume . Below the plume , 2 spatially separated and genetically distinct rbcL clades of Prochlorococcus were observed. This indicated the presence of the high- and low-light adapted clades of Prochlorococcus. A large and very diverse clade of Prymnesiophytes was distributed throughout the water column, whereas a clade of closely related prasinophytes may have dominated at the SCM. These data indicate that the Mississippi river plume may dramatically alter the surface picoplankton composition of the Gulf of Mexico , with Synechococcus displacing Prochlorococcus in the surface waters.

Gulf of Mexico↗
USGS2000

Cassin's Sparrow (Aimophila cassinii) status assessment and conservation plan

Executive Summary Cassin’s Sparrow ( Aimophila cassinii ) is a grassland species endemic to the southwestern U.S. and northern Mexico. Its behavior and ecology have been shaped by these arid ecosystems and the health of its populations is dependent on the availability of grasslands that contain a shrub component. Populations of many grassland birds have experienced dramatic declines due to the loss and deterioration of grassland habitats, and there has been concern about the trends in Cassin’s Sparrow populations. Without better information about population trends, ecology, and effects of management activities, and without a stronger emphasis on grassland management, Cassin’s Sparrows and other grassland species may continue to experience declines. Little information exists about historic Cassin’s Sparrow population levels, and current population estimates for states in the breeding range are unknown. Significant survey-wide declines are documented by the Breeding Bird Survey (BBS) between 1966 and 1996. However, closer examination of these data suggest that the survey-wide trends are driven by population declines in the Edwards Plateau and South Texas Brushlands in Texas. No other areas within the core of its range or on the periphery show consistent, significant trends. In fact, there is consensus that in the core of its range in New Mexico Cassin’s Sparrow remains the most abundant breeding bird in grasslands with a shrub component. Understanding of population patterns in this species is complicated by large yearly fluctuations in distribution and numbers at any particular location, apparently in response to precipitation. This is particularly obvious in the periphery of its range. These dramatic fluctuations have hampered a clear assessment of population status, and our understanding has been confounded by the fact that the timing of BBS data collection does not always coincide with the peak breeding season of this species. The lack of information about the Cassin’s Sparrow’s status, ecology, and response to management activities, and the apparent declines indicated by BBS data, have been cause for concern. Habitat disturbance and degradation, mainly due to grazing and rangeland management practices such as shrub control, fire management, and planting of exotics, pose threats to Cassin’s Sparrow populations, as does habitat conversion through suburban development and agriculture. There is evidence that heavy grazing negatively affects populations, but the information is primarily available from the southwestern portion of its range (Arizona) and may not be applicable throughout the Cassin’s Sparrow’s entire range. Any management practice that results in complete removal of the shrub component, or loss of grass cover and an increase in shrub density beyond a threshold preferred by the species, also poses a threat. Lack of standardized or coordinated studies of Cassin’s Sparrow ecology and response to management practices in different geographic portions of its range has made it difficult to assess its status and make specific management recommendations. Limited data are available from the wintering grounds, and from its range (both breeding and wintering) in Mexico. The greatest needs are for determining of the causes of significant declines where they occur, determining of the effects of various management activities on Cassin’s Sparrow throughout its range, improved assessments of population and trends, and a better understanding of the annual population and distribution dynamics of this species, which shows such dramatic annual distributional fluctuations.

Wyoming, Nebraska, Kansas, Oklahoma,Texas, New Mex↗
USGS1981

Pacific Coast coccolith stratigraphy between Point Conception and Cabo Corrientes, Deep Sea Drilling Project Leg 63

DSDP Leg 63 recovered Neogene coccoliths from south of Point Conception, California (Site 467), to north of Cabo Corrientes, Mexico (Site 473), as part of a traverse exploring the sedimentary record of the California Current along the Pacific Coast. A combination of temperate and tropical zonations were used to date Leg 63 coccolith assemblages because of intergradations of warm and cool species along the transect. Variations in the abundance and the presence and absence of Discoaster species are useful in recognizing warm-cool intergradations. New taxa described in this report include Discoaster okadai n. sp., D. sanmiguelensis n. sp., Helicosphaera californiana n. sp., H. crouchii n. sp., H. gertae n. sp., and H. vedderi n. sp.

California↗
USGS1961

Origin of the Gulf of California

The probable cumulative Late Cretaceous and Cenozoic right-lateral strike-slip displacement along the San Andreas fault in central California is 350 miles. The San Andreas and the allied faults into which it branches southward trend longitudinally into the Gulf of California , and the seismicity of the region indicates that the fault system follows the length of the Gulf and enters the Pacific basin south of Baja California . Crustal structure of most of the Gulf is of oceanic type, so that an origin by structural depression of continental rocks is not possible. Tectonic styles north and south of Los Angeles differ greatly. To the north, the Coast Ranges expose thick Upper Cretaceous and Cenozoic sedimentary rocks that were deposited in local basins and deformed tightly and repeatedly. To the south, in the Peninsular Ranges and Baja California , correlative rocks are thin and show little compressive deformation. The California batholith of mid-Cretaceous age and allied crystalline rocks form the basement of Baja California , southwestern Arizona, and northwestern Sonora and probably extend along the coast of mainland Mexico; the Gulf apparently bisects the crystalline belt longitudinally. These features suggest that Baja California initially lay 300 miles to the southeast, against the continental-margin bulge of Jalisco. The Gulf of California may be a pull-apart feature caused by strike-slip displacement plus up to 100 miles of cross-strike separation of the continental plate, subcontinental materials having welled up into the rift gap. The strike-slip motion has a tensional component across the continental margin south of Los Angeles but a compressional component to the north.

Gulf of California↗