Characteristics of seasonal and spatial variations of primary production over the southeastern Bering Sea shelf

In situ primary production data collected during 1978–1981 period and 1997–2000 period were combined to improve understanding of seasonal and spatial distribution of primary production in the southeastern Bering Sea. Mean daily primary production rates showed an apparent seasonal cycle with high rates in May and low rates in summer over the entire shelf of the southeastern Bering Sea except for oceanic region due to lack of data. There was also an increasing trend of primary production rates in the fall over the inner shelf and the middle shelf. There was a decreasing trend of primary production rates between late April and mid-May over the inner shelf while there was an abrupt increase between late April and mid-May over the middle shelf and the outer shelf. In the shelf break region, there was an increasing pattern in late May. These suggest that there was a gradual progression of the development of the spring phytoplankton bloom from the inner shelf toward the shelf break region. There was also a latitudinal variability of primary production rate over the middle shelf, probably due to either spatial variations of the seasonal advance and retreat of sea ice or horizontal advection of saline water in the bottom layer. Annual rates of primary production across the southeastern Bering Sea shelf were 121, 150, 145, 110, and 84 g C m⁻² yr⁻¹ in the inner shelf, the middle shelf, the outer shelf, the shelf break, and oceanic region, respectively. High annual rates of primary production over the inner shelf can be attributed to continuous summer production based on regenerated nitrogen and/or a continuous supply of nitrogen at the inner front region, and to fall production. There were some possibilities of underestimation of annual primary production over the entire shelf due to lack of measurement in early spring and fall, which may be more apparent over the shelf break and oceanic region than the inner shelf, the middle, and the outer shelf. This study suggests that the response of primary production by climate change in the southeastern Bering Sea shelf can be misunderstood without proper temporal and seasonal measurement.     

Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate

 We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice age general circulation model simulations that include water source tracers and water isotope tracers. The continental air temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant dynamical and radiative feedbacks that might amplify ice age cycles. The isotopes in precipitation generally follow the temperature response at higher latitudes, but regional δ¹⁸O/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, ¹⁸O/¹⁶O ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling of SSTs produce much lower tropical continental δ¹⁸O values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents, and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean. Received: 26 July 1999 / Accepted: 10 July 2000     

Response of estuarine marsh vegetation to interannual variations in precipitation

The response of deltaic emergent marsh vegetation to increases in precipitation was examined over a 14-mo period at three sites in the lower Nueces Estuary in south Texas. At all three sites, significant changes in plant biomass, percent cover, and allocation of aboveground and belowground tissues were associated with more than double the rainfall in late winter and early spring 1992 compared to the previous year and the 50-yr average for this region. Water column salinities, which ranged from 10‰ to 35‰ at all three sites in 1991, remained below 10‰ through August 1992. Significant changes in marsh vegetative structure included decreases in bare space, increases in the percent cover and aboveground biomass of a relatively less salt tolerant halophytes (Borrichia frutescens), and significant increases in root:shoot ratios inB. frutescens, Batis maritima andSuaeda linearis (inSalicornia virginica root:shoot ratios decreased significantly). Higher precipitation generally led to an overall increase in the biomass of most marsh perennials, but these increases were not statistically significant. For one species,Lycium carolinianum, additional rainfall extended its growing season through August 1992, 2 mo longer than in the previous year. The expansion (+58%) ofB. frutescens at one site was also coincident with the significant loss ofB. maritima, whose cover decreased nearly 20%. In an ecological context, these responses suggest that precipitation events in arid environments may be considered a major physical disturbance that can result in large changes in the composition and relative abundance of emergent vascular plants over a relatively short period. The long-term significance of these changes is unknown and demonstrates the value of ecological studies that are conducted over several years for a more complete understanding of the dynamic processes that regulate marsh productivity.     

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

Structure of the crust in the conterminous United States

On the basis of the newest interpretations of all sufficiently long seismic profiles, a contour map of the Mohorovi?i? discontinuity for the conterminous United States is compiled. Differences from earlier interpretations are discussed.     

Complications for traditional land consolidation in Central Europe

An integral part of post-socialist transformation under the Washington Consensus has been the privatisation of previously state owned and/or co-operative farms. In many instances, there have been attempts at ‘turning back the clock’ to pre-War conditions, including land ownership structures. Frequently, the result has been the division of large, economically efficient units into numerous, small and economically barely viable private plots. Following accession to the European Union, these re-constituted peasant-scale structures pose a major challenge to the Union’s agricultural policy. As this paper demonstrates, a simple transfer of western European practices, such as land consolidation, is not possible. The particular legacies of the communist system have given land ownership particular values, legitimacy and personal identity, and emotional bonds. Simple technocratic solutions, as applied in the West earlier, are thus not feasible. Consequently, the paper points out, new forms and instruments of tackling disadvantageous agricultural structures need to be found.     

Three-dimensional simulations of air flow and momentum transfer in partially harvested forests

In order to predict wind loading on trees (canopy height h) in partially harvested forests, it is necessary to characterize air flow and momentum transfer in progressively more complex patterns where groups of trees (or aggregates) are retained. In this study, we used large-eddy simulation to explore the effects of aggregate size, inter-aggregate spacing, and the ratio between the aggregate size and inter-aggregate spacing on air flow and momentum transfer. Forty-five grid points across an aggregate were needed to achieve an adequate level of turbulence. Using grid sizes of h/15 throughout was too viscous for the smaller aggregates. Vertical and horizontal flow deflection by the leading aggregates sheltered some of the downstream aggregates to varying degrees where turbulence increased for subsequent rows. The number of rows of protected aggregates decreased as aggregate dimensions and the space between aggregates increased. A theoretical treatment of time-dependent wind is presented for the lead aggregate and a simulation case is presented for the case of a gust of reduced wind passing through the aggregate pattern. The leading aggregate responded with decreasing moment for decreasing ambient wind speed as predicted by theory. However, downwind aggregates experienced substantial increases in bending moment. The overall results of the disruptive aspects of time dependence agrees with arguments regarding the role of irrotational (potential) flow to this problem. Our treatment of retention pattern design is only a first step and further research suggestions are presented.     

Minerals produced during cooling and hydrothermal alteration of ash flow tuff from Yellowstone drill hole Y-5

A rhyolitic ash-flow tuff in a hydrothermally active area within the Yellowstone caldera was drilled in 1967, and cores were studied to determine the nature and distribution of primary and secondary mineral phases. The rocks have undergone a complex history of crystallization and hydrothermal alteration since their emplacement 600,000 years ago. During cooling from magmatic temperatures, the glassy groundmass underwent either devitrification to alkali feldspar + α-cristobalite ± tridymite or granophyric crystallization to alkali feldspar + quartz. Associated with the zones of granophyric crystallization are prismatic quartz crystals in cavities similar to those termed miarolitic in plutonic rocks. Vapor-phase alkali feldspar, tridymite, magnetite, and sporadic α-cristobalite were deposited in cavities and in void spaces of pumice fragments. Subsequently, some of the vapor-phase alkali feldspar crystals were replaced by microcrystalline quartz, and the vapor-phase minerals were frosted by a coating of saccharoidal quartz.Hydrothermal minerals occur primarily as linings and fillings of cavities and fractures and as altered mafic phenocrysts. Chalcedony is the dominant mineral related to the present hydrothermal regime and occurs as microcrystalline material mixed with various amounts of hematite and goethite. The chalcedony displays intricate layering and was apparently deposited as opal from silica-rich water. Hematite and goethite also replace both mafic phenocrysts and vapor-phase magnetite. Other conspicuous hydrothermal minerals include montmorillonite, pyrite, mordenite, calcite, and fluorite. Clinoptilolite, erionite, illite, kaolinite, and manganese oxides are sporadic. The hydrothermal minerals show little correlation with temperature, but bladed calcite is restricted to a zone of boiling in the tuff and clearly was deposited when CO₂ was lost during boiling.Fractures and breccias filled with chalcedony are common throughout Y-5 and may have been produced by rapid disruption of rock caused by sudden decrease of fluid pressure in fractures, most likely a result of fracturing during resurgent doming in this part of the Yellowstone caldera. The chalcedony probably was deposited as opal or β-cristobalite from a pre-existing silica floc that moved rapidly into the fractures and breccias immediately after the sudden pressure drop.