Abundance of seagrass (Zostera marina L.) and macroalgae in relation to the salinity-temperature gradient in Yaquina Bay, Oregon, USA

The relative abundances of the seagrass,Zostera marina L., and associated macroalgae were examined for Yaquina Bay, Oregon, U.S.A., to investigate variability in autotroph abundance along the salinity-temperature gradient and the potential for nuisance algal blooms. Possible explanations for the patterns in autotroph abundances were explored through examination of their correlations with the physicochemical characteristics of the water column. Study sites were established in each of three zones in the estuary defined by temperature and salinity and were sampled monthly June through September 1998 and in July 1999.Z. marina and macroalgal cover andZ. marina shoot density were measured in 0.25-m² plots at each site. After cover estimates and shoot counts were made, material was harvested for determination ofZ. marina and macroalgal biomass. Water column variables were measured from stations near each study site and composited on a depth-averaged, monthly basis for each zone. BothZ. marina and green macroalgal abundance differed between sites, over the summer in 1998, and between years. Seasonal patterns were most obvious forZ. marina at the site closest to the ocean while the pattern in macroalgal abundance suggested a bloom moving up river as summer progressed. The physicochemical characteristics of the zones differed with the season and could be related to the patterns inZ. marina and macroalgal abundance. In particular, salinity was positively correlated withZ. marina abundance, while abundance of both autotrophs was related to light availability.Z. marina biomass ranged 19–109 g dry weight m^?2; green macroalgae biomass ranged 5–234 g dry weight m^?2. The biomass of the green macroalgae at several sites and dates equaled or exceed that of theZ. marina suggesting the potential for nuisance algal blooms does exist in Yaquina Bay.     

Rock-mass characterization using intrinsic and scattering attenuation estimates at frequencies from 400 to 1600 Hz

Intrinsic and scatteringS-wave quality factors (Q ) were estimated using the Multiple Lapse Time Window Analysis (MLTWA) for microseismic events (M<–1) with source-sensor distances of 45 to 120 m, associated with an excavation at 630 m depth in Strathcona Mine, Sudbury, Canada. Additional information on the rock mass was provided by underground structural mapping data. IntrinsicQ values, at 800 Hz, were on the order of 140, similar to quality factor values obtained in previous studies using Spectral Decay and Coda-Q methods (120 to 170). The scattering quality factor at this frequency was about 520. An observed frequency dependence of the scattering attenuation suggested that a decrease in the density of scatterers, with scale lengths on the order of 2 m, exists at the site. Characteristic fracture scale lengths were considered to range from 4 to 6 m as identified in the mapping data. These observations were supported by the increase in scattering found for seismic waves with frequencies less than 1000 Hz. By assuming that the identified scatters are characteristic faults, these scatterers can then be considered to increase nonsimilar behavior in source scaling. Overall, our results suggest that MLTWA provides a practical method for remotely characterizing the quality of a rock mass when visual observations are not attainable.     

Seasonal patterns of production in the sandy-beach amphipod Haustorius canadensis

The Haustorius canadensis population at Long Sands beach in southern Maine was studied over a period of 45 months. The length-weight relationship was examined over 24 months; it showed no difference due to either sex or month. Caloric content of the population displayed a seasonal cycle around a mean of 18·381 KJ gm⁻¹ dry weight, peaking in mid to late summer. The increment summation, instantaneous growth, removal summation and size-frequency methods of calculating production were used and the results compared. The increment summation and instantaneous growth methods provided the best estimates of production, averaging 98·1 gm m⁻¹ y⁻¹ and 100·2 gm m⁻¹ y⁻¹, respectively. The annual production to mean biomass ratio averaged 1·48. The population exhibited strong peaks of production; the four summer months accounting for 4·65 times the production during the remainder of the year. Most of the production was due to one-year-old individuals.     

Dual extraction ofR-mode andQ-mode factor solutions

It is mathematically possible to extract both R-mode and Q-mode factors simultaneously (RQ-mode factor analysis)by invoking the Eckhart-Young theorem. The resulting factors will be expressed in measures determined by the form of the scalings that have been applied to the original data matrix. Unless the measures for both solutions are meaningful for the problem at hand, the factor results may be misleading or uninterpretable. Correspondence analysis uses a symmetrical scaling of both rows and columns to achieve measures of proportional similarity between objects and variables. In the literature, the resulting similarity is a χ ² distance appropriate for analysis of enumerated data, the original application of correspondence analysis. Justification for the use of this measure with interval or ratio data is unconvincing, but a minor modification of the scaling procedure yields the profile similarity, which is an appropriate measure. Symmetrical scaling of rows and columns is unnecessary for RQ-mode factor analysis. If the data are scaled so the minor product W'Wis the correlation matrix, the major product WW'is expressed in the Euclidean distances between objects. Therefore, RQ-mode factor analysis can be performed so that the Rmode is a principal components solution and the Qmode is a principal coordinates solution. For applications where the magnitudes of differences are important, this approach will yield more interpretable results than will correspondence analysis.     

Geochemistry of Precambrian mafic dikes,central Bighorn Mountains,Wyoming, U.S.A.

Precambrian quartz dolerites and metadolerites of the central Bighorn Mountains form dikes that intrude a Precambrian metamorphic and igneous terrane typical of the Laramide uplifts of the middle Rocky Mountains. They have a restricted range of major- and trace-element compositions and are typical of basalts in the middle stages of tholeiitic fractionation. Fractionation in the direction of iron enrichment occurred by removal of plagioclase. Average element concentrations of the two groups are nearly identical to one another, are comparable to those in Archean metabasalts from numerous shield areas, and are intermediate between those of modern oceanic tholeiites and continental tholeiites. These average concentrations suggest a depth of magma generation and thickness of crust intermediate between those for the oceanic and continental environments.     

Screened Auger Sampling: The Technique and Two Case Studies

The screened auger is a laser-slotted, hollow-stem auger through which a representative sample of ground water is pumped from an aquifer and tested for water-quality parameters by appropriate field-screening methods. Screened auger sampling can be applied to ground water quality remedial investigations, providing:(1) a mechanism for determining a monitoring well's optimal screen placement in a contaminant plume; and (2) data to define the three-dimensional configuration of the contaminant plume.
Screened auger sampling has provided an efficient method for investigating hexavalent chromium and volatile organic compound contamination in two sandy aquifers in Cadillac, Michigan. The aquifers approach 200 feet in thickness and more than 1 square mile in area. A series of screened auger borings and monitoring wells was installed, and ground water was collected at 10-foot intervals as the boreholes were advanced to define the horizontal and vertical distribution of the contaminant plumes. The ability of the screened auger to obtain representative ground water samples was supported by the statistical comparison of field screening results and subsequent laboratory analysis of ground water from installed monitoring wells.     

Source parameters of mining-induced seismic events: An evaluation of homogeneous and inhomogeneous faulting models for assessing damage potential

Source parameter estimates based on the homogeneous and inhomogeneous source models have been examined for an anomalous sequence of seven mine-induced events located between 640 and 825 m depth at Strathcona mine, Ontario, and having magnitudes ranging betweenm _N 0.8 and 2.7. The derived Brune static stress drops were found to be similar to those observed for natural earthquakes (30 bars), whereas dynamic stress drops were found to range up to 250–300 bars. Source radii derived from Madariaga's model better fit documented evidence of underground damage. These values of source radii were similar to those observed for the inhomogeneous model. The displacement at the source, based on the observed attenuation relationship, was about 60 mm for three magnitude 2.7 events. This is in agreement with slip values calculated using peak velocities and assuming the asperity as a Brune source within itself (72 mm). By using Madariaga's model for the asperity, the slip was over 3 times larger than observed. Peak velocity and acceleration scaling relations with magnitude were investigated by incorporating available South African data, appropriately reduced to Canadian geophysical conditions. The dynamic stress drop scaled as the square root of the seismic moment, similar to reported results in the literature for crustal earthquakes. This behavior suggests that the size of the asperities responsible for the peak ground motion, with respect to the overall source size, follow distributions that may be similar over a wide range of magnitudes. Measurements of source rupture complexity (ranging from 2 to 4) were found to agree with estimates of overall source to asperity radii, suggesting, together with the observed low rupture velocities (0.3 to 0.6 ), that the sources were somewhat complex. Validation of source model appropriateness was achieved by direct comparison of the predicted ground motion level to observed underground damage in Creighton mine, located within the same regional stress and geological regime as Strathcona mine. Close to the source (<100 m), corresponding to relatively higher damage levels, a good agreement was found between the predicted peak particle velocities for the inhomogeneous model and velocities derived based on established geomechanical relationships. The similarity between asperity radii and the regions of the highest observed damage provided additional support for the use of the inhomogeneous source model in the assessment of damage potential.