Architecture of the deep critical zone in the Río Icacos watershed (Luquillo Critical Zone Observatory,Puerto Rico) inferred from drilling and ground penetrating radar (GPR)

In the critical zone, surficial bedrock interactions result in the formation of a mantle of chemically‐ and physically‐altered material defined here as regolith. In the watershed of the Río Icacos, an upland river draining the Luquillo Mountains in tropical Puerto Rico, we explored the influence of lithology (quartz diorite versus hornfels‐facies volcaniclastic rock) on weathering. Regolith profiles were studied by drilling boreholes and imaging the subsurface using ground penetrating radar (GPR). Overall, the regolith structure is not laterally continuous but rather is punctuated by zones of deep fractures that host in situ weathering, corestones, and colluvial material. GPR images of these vertical zones show reflectors at 15–20 m depth. Thus, the architecture of the critical zone in the upper Luquillo Mountains is highly dependent on lithology and its influence on fracture development. At the highest elevations where hornfels overlies quartz diorite, positive feedbacks occur when the water table drops so that oxidative weathering of biotite in the more felsic rock creates microfractures and allows deeper infiltration of meteoric waters. Such exposure results in some of the fastest weathering rocks in the world and may contribute to formation of the knickpoint in the Río Icacos watershed. This work represents the first study combining GPR and drilling to look at the structure of the deep critical zone and demonstrates: (1) the importance of combining direct methods (such as drilling) with indirect methods (such as GPR) to understand the architecture of the critical zone in tropical systems; (2) the interplay of the surficial stress regime, lithology and climate in dictating the architecture of weathering. Copyright © 2016 John Wiley & Sons, Ltd.     

Radiocarbon deficiencies of US Gulf Coast lakes compromise paleo-hurricane records

Radiocarbon assays of sediments from Lake Shelby, US Gulf Coast, exhibit substantial ¹⁴C deficiencies of 9.3% and 4.7% for the limnic sapropel and associated fresh-water clams Rangia cuneata, respectively. Measured radiocarbon dates from the sapropel and clams require corrections of 785 ± 80 and 365 ± 90 ¹⁴C yr (1σ), respectively, in order to achieve consistency with the radiocarbon time scale. Lake sediments of the US Gulf Coast serve as unique repositories of pre-historic hurricane strikes. Previously unrecognized radiocarbon deficiencies likely render compromised chronologies of the paleo-hurricane records, and erroneous estimates of return period and landfall probability that are derived from the storm chronologies. The recalculated severe hurricane chronologies suggest that the annual landfall probabilities of the last millennium on the US Gulf Coast are equal to, or higher than, the preceding three millennia.     

Exploring how alternative mapping approaches influence fireshed assessment and human community exposure to wildfire

Attaining fire-adapted human communities has become a key focus of collaborative planning on landscapes across the western United States and elsewhere. The coupling of fire simulation with GIS has expanded the analytical base to support such planning efforts, particularly through the “fireshed” concept that identifies areas where wildfires could ignite and reach a human community. Previous research has identified mismatches in scale between localized community wildfire planning and the broader fireshed considering patterns of wildfire activity across landscapes. Here we expand upon this work by investigating the degree to which alternative geospatial characterizations of human communities could influence assessment of community exposure and characterization of the fireshed. We use three methods of mapping human communities (point, raster, and polygon) and develop three fireshed metrics (size, number of fires reaching houses, and number of houses exposed), and apply this analytical framework on a 2.3 million ha case study landscape encompassing the Sierra National Forest in California, USA. We simulated fire occurrence and growth using FSim for 10,000 iterations (fire seasons) at 180-m resolution. The simulation resulted in 3.9 large fires per million ha per year, with a mean size of 3432 ha. Results exhibit similarities and differences in how exposure is quantified, specifically indicating that polygons representing recognized community boundaries led to the lowest exposure levels. These results highlight how choice of the mapping approach could lead to misestimating the scope of the problem or targeting mitigation efforts in the wrong areas, and underscore the importance of clarity and spatial fidelity in geospatial data representing communities at risk.     

A transport‐distance approach to scaling erosion rates: 3. Evaluating scaling characteristics of Mahleran

In the two previous papers of this series, we demonstrated how a novel approach to erosion modelling (Mahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) provided distinct advantages in terms of process representation and explicit scaling characteristics when compared with existing models. A first evaluation furthermore demonstrated the ability of the model to reproduce spatial and temporal patterns of erosion and their particle‐size characteristics on a large rainfall‐simulation plot. In this paper, we carry out a more detailed evaluation of the model using monitored erosion events on plots of different size. The evaluation uses four plots of 21·01, 115·94, 56·84 and 302·19 m², with lengths of 4·12, 14·48, 18·95 and 27·78 m, respectively, on similar soils to the rainfall‐simulation plot, for which runoff and erosion were monitored under natural rainfall. Although the model produces the correct ranking of the magnitude of erosion events, it performs less well in reproducing the absolute values and particle‐size distributions of the eroded sediment. The implications of these results are evaluated in terms of requirements for process understanding and data for parameterization of improved soil‐erosion models. We suggest that there are major weaknesses in the current understanding and data underpinning existing models. Consequently, a more holistic re‐evaluation is required that produces functional relationships for different processes that are mutually consistent, and that have appropriate parameterization data to support their use in a wide range of environmental conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km² since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m⁻² d⁻¹ of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.     

Linking juvenile fish and their habitats: An example from Narragansett Bay, Rhode Island

We used two methods and existing field survey data to link juvenile fish and their habitats. The first method used seine survey data collected monthly from July to October 1988–1996 at fixed stations in Narragansett Bay, Rhode Island. Thirteen fish species making up 1% or more of the catch were analyzed by principal components analysis for two time periods: July–August and September–October. The stations were then plotted by their principal component scores to identify station groupings and habitat types. The second method used environmental data collected in July and August 1996 at the established survey stations in a principal components analysis. The stations and 13 most abundant species were plotted by principal components scores resulting from the environmental data. For the environmental data, the first two principal components explained 59% of the variance. The first principal component described the amount of energy shaping the habitat and was positively correlated with salinity, dissolved oxygen, current flow, and slope, and negatively correlated with silt. The second component was positively correlated with depth and silt, and negatively correlated with dissolved oxygen. The environmental data grouped the stations according to their distance from the ocean and three habitat types emerged. The uppermost station was a silty barren having low salinities and dissolved oxygen. Three other stations grouped together as low energy, protected habitats with sandy substrates. Lower bay stations had higher salinities, higher dissolved oxygen, higher flow rates, greater slopes, and larger size substrates, mostly cobble and gravel. Results from the fish data grouped the stations similarly. Combining results from both datasets revealed the uppermost station had the highest catches, most species, and greatest number of winter flounder (Pseudopleuronectes americanus) juveniles. Plots of winter flounder catches with principal component scores from the environmental data indicated the winter flounder distribution in the bay has shrunk from baywide to mostly the upper estuary near their primary spawning grounds. Results illustrate the value of coupling historic fish survey data with environmental measurements for identifying previously undervalued habitats important to fish.     

Influence of climate change on interannual variation in contaminant body burden in Gulf of Mexico oysters

As part of NOAA's National Status and Trends Mussel Watch Program, oysters were sampled along the Gulf of Mexico coast each winter from 1986 to 1992 and analyzed for trace metal, polynuclear aromatic hydrocarbon (PAH), and pesticide body burden. We identified contaminant variables for which large-scale climate processes played an important role in establishing the interannual variation in body burden by examining cases where body burdens rose or fell more or less in unison over broad geographic regions and distinguished these concordant changes from cases where bays varied independently. Of the 11 metals analyzed, nine had scales of concordancy 100 km. Ni and Se, however, had among the largest scales of concordancy in the study, 1200 km. That is, oyster body burdens in bays as far apart as 1200 km tended to rise and fall in unison from one year to the next. Interannual variations in body burden of organic contaminants had a much stronger regional component. All but two of the 11 organic contaminants had scales of concordancy of 200 km or greater and six exceeded 400 km. Concordancy was strongest either in the southern, northwestern, or north-central Gulf, depending upon the contaminant. For all contaminants, bays tended to vary independently in the northeastern Gulf. For three contaminants, total chlordanes, dieldrin and Cd, regional concordancy may originate from a widespread decrease in use and, therefore, input. These contaminants declined nearly monotonically over the 7 years. For others, including Zn and many of the PAHs and pesticides, the El Niño/Southern Oscillation (ENSO) cycle may be important in establishing the interannual variability in contaminant body burden: (1) ENSO has the geographic scale required; (2) a subtropical influence is required to explain the similarity in interannual variation between south Texas and southern Florida; (3) ENSO-related climate responses in the Gulf follow the southwestern/northeastern trend that would establish the northwestern Gulf focus in concordancy so prominent in many of the contaminants; and (4) deviations in yearly mean body burden from the Gulf-wide mean track the Southern Oscillation Index in some cases.