Effects of size and temperature on mud crab,Panopeus herbstii,predation on hard clams,Mercenaria mercenaria

Predation byPanopeus herbstii onMercenaria mercenaria was significantly affected by temperature, and the size of predators and prey items. LargerP. herbstii opened more clams and preyed more successfully on larger clams than did smaller crabs. Increase in seed clam size and decrease in water temperature significantly reduced predation. Clam size appeared to be more important than crab satiation in reducing predation rate. Planting larger seed clams in cooler months should help to improve clam survival by reducing the impact ofP. herbstii in culture operations.     

Benthic community structure and sediment processes on an intertidal flat: results from the ECOFLAT project

Interactions between physical and biological processes in intertidal sediments have been the focus of studies in the framework of the ECOFLAT project. Process studies were focused on the Molenplaat, a 1.5 km² intertidal flat in the Westerschelde estuary, The Netherlands. Distinct spatial patterns in the biological community on the flat were found and related to patterns in bottom shear stress as derived from a hydrodynamic model. Based on these survey results five stations were selected with contrasting sediment composition and benthic communities.Long-term net sedimentation of mainly sand was deduced from radionuclide profiles. Sedimentation of mud on the surface of part of the intertidal flat was shown to be a seasonally cyclic process, with a magnitude sufficient to significantly affect the mud balance of the estuary. Measurements of erodability as a function of tidal current showed a biological control on this variable. Benthic microalgae stabilise the sediment surface, whereas the benthic macrofauna has a destabilising effect.Food web studies show that the contrasting biological communities (microbenthos, meiobenthos, macrobenthos) at muddy and sandy sites have substantial differences in the quantitative aspects of their food web. Turnover of microalgae is much lower at muddy than at sandy sites, and this is mostly related to a lower grazing pressure. It is hypothesised that high mud content decreases the availability of benthic microalgae to grazers.We demonstrate a positive feedback between benthic microalgae and mud sedimentation which may lead to the existence of different stable states. This may explain the existence of relatively sharp boundaries and strong community contrasts in the presence of weak and smooth environmental gradients.     

Effects of oilfield brine effluent on sediments and benthic organisms in trinity bay,Texas

Field studies have established the concentrations of naphthalenes in bay sediments and water in the vicinity of an oil separator platform and their effects on the benthic fauna. Fifteen stations were occupied monthly, from July, 1974 to December, 1975, along three transects extending from the separator platform outfall outwards for a distance of 4·0 to 5·6 km. A lesser number of stations were occupied from April, 1974 to June, 1974. Bottom sediments at each station were analysed for total naphthalenes content and for number of species and individuals. All stations were located in 2 to 3 m of water. The outfall was located 1 m off the bay bottom.There was a definite correlation between sediment naphthalenes concentration and number of species and individuals. As expected, the first station, located 15 m from the outfall, had the highest concentration of naphthalenes of all stations sampled. The naphthalenes levels dropped sharply from the outfall to the stations located 75 m from the platform where levels were about 20–50% of those found 15 m from the outfall. Naphthalenes concentrations then decreased gradually to near background levels at stations farther out. Hydrocarbon concentrations in bottom water 15 m from the outfall were three orders of magnitude lower than those in the full strength effluent, but sediments 15 m from the outfall had hydrocarbon concentrations four times as great as in the full strength effluent. There were approximately four orders of magnitude more hydrocarbons in the sediment than in the overlying water.The bay bottom was almost completely devoid of organisms within 15 m of the effluent outfall. Stations located 150 m from the outfall had severely depressed benthic faunas but not to the extent of stations nearer the outfall. Stations located 455 m from the platform were unaffected. Both numbers of species and individuals increased with distance from the platform and reached a peak at the first station medial to the control on each transect (685 to 1675 m from the platform) and then dropped at the control station. Physical environmental factors such as temperature, salinity, water depth and sediment type were essentially the same at all stations.The temporary use of a second outfall located 275 m from the main platform outfall resulted in a rapid build up of naphthalenes in surrounding sediments which persisted for at least six months following the termination of use of the second outfall. The benthic fauna was also severely depressed in the vicinity of the second outfall. The use of multiple outfalls, located some distance apart, appears to be more harmful than the use of a single outfall.Trinity Bay, Texas, the site of this investigation, has a mean depth of 2·5 m. The bay water is highly turbid due to the presence of a high concentration of clay-sized particulate material. The brine outfall was located approximately 1 m above the bay bottom. These special conditions undoubtedly contributed significantly to the observed impact of the brine. Therefore, extrapolations from the results of this study to offshore oil production and brine disposal should be made with extreme caution.     

The distribution and abundance of pelagic gammarid amphipods on Georges Bank and Nantucket Shoals

In this study, long-term, broad-scale zooplankton survey data were used to estimate the temporal and spatial distribution and abundance of gammarid amphipods present in the water column on Georges Bank. Delta-mean abundances computed from 10 years of data showed that gammarid amphipod abundances peaked in summer and again in fall. The amphipods were also most numerous in water less than 50 m deep. The statistical tests employed revealed no conclusive evidence for diurnal vertical migration. Interannual delta-mean abundances fluctuated approximately 5-fold between 1977 and 1986, ranging from 217 to 1181 amphipods 100 m⁻³. Peak amphipod biomass occurred in July and was estimated to be 2.8 kcal m⁻². Using production-to-biomass ratios from the literature, mean annual production of amphipods in these samples was estimated to be between 1.6 and 9.8 kcal m⁻². Production in shallow areas was especially high, 22 kcal m⁻² year⁻¹.     

Depositional history of the Lagniappe Delta,northern Gulf of Mexico

The northern Gulf of Mexico continental shelf is characterized by superimposing deltas. One such delta, informally named Lagniappe, extends east of the Mississippi Delta from mid-shelf to the continental slope. This late Wisconsinan delta is adjacent to, but not associated with the Mississippi Delta complex: the fluvial source was probably the ancient Pearl and/or Mobile Rivers. The fluvially dominated Lagniappe Delta is characterized by complex sigmoid-oblique seismic-reflection patterns, indicating delta switching of high-energy sand-prone facies to low-energy facies. The areal distribution and sediment thickness of the delta were partially controlled by two diapirs.     

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using co-spectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle.

A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m²/s) at incident wave frequency.

Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m²/s) associated with bound long waves and stronger onshore transport (co-spectral peak = 0.006 kg/m²/s) at incident wave frequencies. Conversely, co-spectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m²/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m²/s) occurring at incident wave frequencies.     

Novel prebiotic model systems: interactions of nucleosides and nucleotides with aqueous micellar sodium dodecanoate

The partition coefficients, K-values, of adenosine, guanosine, cytidine, uridine, 5¹-adenylic acid (AMP), 5¹-guanylic acid (GMP), 5¹-cytidylic acid (CMP), and 5¹-uridylic acid (UMP) between water and aqueous sodium dodecanoate has been determined by gel filtration on Sephadex G-25. Below the critical micelle concentration of the surfactant, K-values for all nucleotides and nucleosides are similar. Differential uptake of the four nucleotides and nucleosides is observed, however, by micellar sodium dodecanoate. K-values for adenosine, AMP; guanosine, GMP; cytidine, CMP; and uridine, UMP are 6.16·10³, 9.52·10³; 12.0·10³, 14.6·10³; 19.6·10³, 19.6·10³, 34.2·10³ and 40.3·10³. Similar K-values for corresponding pairs of nucleotides and nucleosides imply that the predominant interaction is between the pyrimidine or purine rings of the substrates and the charged Stern layer of the surfactant. Prebiotic significance of these results is discussed.     

Interpretation of the 6818.9-Å methane line in terms of inhomogeneous scattering models for Uranus and Neptune

We have obtained high-resolution spectra of Uranus and Neptune in the methane transition near 6800 Å, and in particular, the 6818.9Å feature. Calculated equivalent widths for this line using recently proposed models of the atmospheres of these two planets indicate that the C/H ratio is greater than or equal to 5 × 10^?3 below the CH₄ saturation level. This value is 12 times the solar mixing ratio. The half-widths of the computed line profiles are in agreement with the observed half-widths. Therefore, it is unnecessary to introduce an unidentified constituent with an abundance comparable to H₂, postulated recently by Belton and Hayes, and by Bergstrahl, to account for the observed line broadening.     

Optically stimulated luminescence age of the Old Cedar midden, St. Joseph Peninsula State Park, Florida

We report optically stimulated luminescence (OSL) dating results from the Old Cedar midden in St. Joseph Peninsula State Park, located between the Gulf of Mexico and St. Joseph Bay near Port St. Joe, FL, USA. The Old Cedar site (8GU85) is located on top of a relict beach ridge which is actively eroding into St. Joseph Bay. Old Cedar is noted for its conch shell tool assemblage, otherwise rare at northwest Florida archaeological sites, and is believed to have been utilized during the Late Woodland Weeden Island and possibly the Fort Walton periods [Benchley, E.D., Bense, J.A., 2001. Archaeology and history of St. Joseph Peninsula State Park: Phase I investigations. Report of Investigations, No. 89, University of West Florida Archaeology Institute]. After removing surficial erosion deposits, we extracted OSL core samples from both the midden layer and the underlying beach ridge. The resulting OSL age is compared with the age of another beach ridge on St. Joseph Peninsula. We hope that this study will aid in the investigation and conservation of Old Cedar specifically and other Weeden Island sites in the St. Joseph Bay area.