Long-term variation of macrobenthos in a mesohaline region of Chesapeake Bay

Macrobenthic species abundances and physical/chemical factors known of affect them were measured in a mesohaline region of the Chesapeake Bay from 1971 to 1982. Variation in species abundance due to station differences, seasonal patterns and year-to-year fluctuations in physical/chemical factors were quantified. The major source of variation in abundance of most species was that associated with seasonal recruitment cycles. Spatial variation in abundances of dominant species was small within habitats defined by sediment characteristics. However, abundance variation among sediment types was relatively large. All species showed significant year-to-year fluctuation in abundance, but no species had systematic long-term increases or decreases in abundance during the study period. Macrobenthos populations were persistent over the 11 years within abundance boundaries defined chiefly by fluctuations in salinity and dissolved oxygen concentration. Most species responded to salinity changes, especially those of an extreme nature, with predictable increases or decreases in abundance. Abundances of all species declined rapidly under conditions of low dissolved oxygen concentration (<2 ppm). Macrobenthos with planktonic developmental stages rapidly repopulated the region following periods of population decline. Repopulation by brooding species was slower.     

Tropical soils could dominate the short-term carbon cycle feedbacks to increased global temperatures

Results of a simple model of the effects of temperature on net ecosystem production call into question the argument that the large stocks of soil carbon and greater projected warming in the boreal and tu ndra regions of the world willlead to rapid efflux of carbon from these biomes to the atmosphere. We show that low rates of carbon turnover in these regions and a relatively greater response of net primary production to changes in temperature may lead to carbon storage over some limited range of warming. In contrast, the high rates of soil respiration found in tropical ecosystems are highly sensitive to small changes in temperature, so that despite the less pronounced warming expected in equatorial regions, tropical soils are likely to release relatively large amounts of carbon to the atmosphere. Results for high-latitude biomes are highly sensitive to parameter values used, while the net efflux of carbon from the tropics appears robust.     

Flux decomposition into coherent structures

This study examines the intermittency of the momentum flux near the surface and the relation of such intermittency to coherent structures. Toward this goal, variances and covariances are decomposed into coherent structures and less coherent activity. The sampled structures are identified using the Haar transform and then decomposed into eigenvectors of the lagged covariance matrix.The methodology is applied to the momentum flux for a relatively stationary 50-h period of strong winds measured from a 45 m tower in the Lammefjord Experiment. Events of sinking motion with strong horizontal momentum account for the majority of the flux. Such sweeping motions arrive as gust microfronts. The large momentum flux is associated with strong coherent fluctuations of the longitudinal wind component and high correlation with relatively modest fluctuations of vertical motion. In the heated case (HAPEX), a phase lag between the vertical and horizontal velocity fluctuations leads to less efficient momentum transport by the main coherent structures.The event nature of the flux is used to formulate an expression for the flux error due to sampling problems. Estimation of the momentum flux requires a significantly longer record than for the heat flux. Modulation of the flux by mesoscale variations also affects the sampling strategy.     

A numerical diffusion model for continuous releases

The purpose of this study was to develop a diffusion model for a continuous point source which takes into account the increase of wind speed with height, and to compare this model with short-range diffusion experiments. The main problem was to find a good expression for the vertical diffusion coefficient. It turned out that good agreement between theory and experiment could only be obtained by introducing a settling speed W for the tracer combined with a conventional expression for the vertical diffusion (K(Z) = K ₀Z^1−p). An empirical relation was found between K ₀ and τ _vU and between W and bar σ _vU².     

The surface wind over an upwelling area near Pisco,Peru

Harmonic analysis of longshore and onshore components of surface winds over an upwelling area off the coast of Peru shows regular, nearly sinusoidal, diurnal variations in both velocity components. Spectral analysis of wind speed also indicates strong diurnal variations in kinetic energy. The data are compatible with a recent model of coastal winds proposed by Lettau and Lettau.     

Gas saturation prediction and effect of low frequencies on acoustic impedance images at Foinaven Field

Low frequencies are necessary in seismic data for proper acoustic impedance imaging and for petrophysical interpretation. Without lower frequencies, images can be distorted leading to incorrect reservoir interpretation and petrophysical predictions. As part of the Foinaven Active Reservoir Management (FARM) project, a Towed Streamer survey and an Ocean Bottom Hydrophone (OBH) survey were shot in both 1995 and 1998. The OBH surveys contain lower frequencies than the streamer surveys, providing a unique opportunity to study the effects that low frequencies have on both the acoustic impedance image along with petrophysical time‐lapse predictions. Artefacts that could easily have been interpreted as high‐resolution features in the streamer data impedance volumes can be distinguished by comparison with the impedance volumes created from the OBH surveys containing lower frequencies. In order to obtain results from the impedance volumes, impedance must be related to saturation. The mixing of exsolved gas, oil and water phases involves using the Reuss (uniform) or Voigt (patchy approximation) mixing laws. The Voigt average is easily misused by assuming that the end‐points correspond to 0% and 100% gas saturation. This implies that the patches are either 0% gas saturation or 100% gas saturation, which is never the case. Here, the distribution of gas as it comes out of solution is assumed to be uniform until the gas saturation reaches a sufficiently high value (critical gas saturation) to allow gas to flow. Therefore, at low gas saturations the distribution is uniform, but at saturations above critical, it is patchy, with patches that range from critical gas saturation to the highest gas saturation possible (1 minus residual oil and irreducible water saturation).