A comparative analysis of recruitment variability in North Atlantic flatfishes — testing the species range hypothesis

The hypothesis that recruitment variation in flatfishes should be most variable at the northern edge of the species range, least near the centre of the range, and intermediate near the southern limit was tested using stock and recruitment data generated from sequential population analysis for several different flatfish stocks involving four species (plaice Pleuronectes platessa, sole Solea vulgaris from the eastern Atlantic, American plaice Hippoglossoides platessoides, and yellowtail flounder Limanda ferruginae from the western Atlantic). Several groundfish species have been found to conform to this so-called species range hypothesis with the suggestion that density-independent processes predominate at the edges of the distributional range and density-dependent processes dominate in the centre of the range. Our results were generally inconsistent with the hypothesis: the coefficient of variation (CV) of recruitment for plaice in the eastern Atlantic was independent of latitude, the CV of recruitment for sole exhibited a dome-shaped relationship with latitude with the highest CVs occurring at the mid-point of the range, and the CV of recruitment for the western Atlantic stocks exhibited a monotonic decrease with latitude. We extended our latitudinal analyses by assessing both the degree of dependency of recruitment on spawning stock biomass and the spatial and temporal scales of variability in recruitment and pre-recruit survival for the eastern Atlantic stocks. In general our analysis revealed no evidence of a strong stock and recruitment relationship for any of the stocks examined, and previously published analyses revealed no such patterns with latitude. Analysis of both de-trended recruitment and pre-recruit survival time series over the species ranges of sole and plaice revealed strong positive correlations among adjacent stocks and inverse correlations among stocks at the extremes of the range. Recruitment variation in the flatfish stocks examined appears to be dominated by density-independent factors, operating at a local scale, on the egg and larval stages.     

Acoustic backscatter and sediment textural properties of inner shelf sands,northeastern Gulf of Mexico

The relationship between spatial variations of the properties of sea-floor sediments and acoustic backscatter from the surface of the sea floor on the continental shelf off of Panama City, Florida, USA, is investigated using surficial sediment grab samples and digital side-scan sonar data. Acoustic backscatter strength has a high, direct correlation with the mean grain size of the sediments. Acoustic backscatter strength also correlates directly with the carbonate content of the sediments, particularly in medium-and coarse-sand facies, because large, irregularly shaped, carbonate particles affect both the mean grain size of the sediments and the roughness of the surface of the sea floor.     

Spatio-temporal variation in the structure of a deep water Posidonia oceanica meadow assessed using non-destructive techniques

The Malta‐Comino Channel (Maltese islands, central Mediterranean), supports extensive meadows of the seagrass Posidonia oceanica that in some places extend to a depth of around 43 m, which is rare for this seagrass. To assess spatial and temporal variation in the state of the deeper parts of the P. oceanica meadow with time, data on the structural characteristics of the seagrass meadow at its lower bathymetric limit were collected during the summers of 2001, 2003 and 2004 from four stations (two stations within each of two sites) located at a similar depth, over a spatial extent of 500 m. Shoot density was estimated in situ, while data on plant architecture (number of leaves, mean leaf length, and epiphyte load) were successfully obtained using an underwater photographic technique that was specifically designed to avoid destructive sampling of the seagrass. Results indicated that P. oceanica shoot density was lower than that recorded from the same meadow during a study undertaken in 1995; the observed decrease was attributed to the activities of an offshore aquaculture farm that operated during the period 1995–2000 in the vicinity of the meadow. ANOVA indicated significant spatial and temporal variations in meadow structural attributes at both sites during the 3‐year study; for example, shoot density values increased overall with time at site A; a indication of potential recovery of the meadow following cessation of the aquaculture operations. Lower shoot density values recorded from site B (compared with site A) were attributed to higher epiphyte loads on the seagrass, relative to those at site A. The findings, which include new data on the structural characteristics of P. oceanica occurring at depths >40 m, are discussed with reference to the use of the non‐destructive photographic technique to monitor the state of health of deep water seagrass meadows.     

Mississippi-Alabama outer continental shelf topographic features formed during the late Pleistocene-Holocene transgression

A geophysical survey of the outer Missisippi-Alabama continental shelf revealed three types of topographic features: 1) reef-like mounds (RLM), 2) isobath-parallel ridges, and 3) shallow depressions. RLM appear to be bioherms. Their bases cluster in two isobath bands, 105 to 120 m and 74 to 82 m, implying the late Pleistocene-Holocene transgression was nonuniform. The deeper RLM probably formed during a slow sea level rise at the beginning of the deglaciation. An 11 ka BP C¹⁴ age was obtained from the summit of one, at a depth of 90 m. Depth relationships suggest the shallower RLM and most ridges formed about the time of the mid-deglacial Younger Dryas cooling episode.     

Dissolved carbohydrates in seawater. I,A precise spectrophotometric analysis for monosaccharides

A relatively precise and rapid method for the analysis of total dissolved monosaccharides at the concentrations that occur in seawater is described which uses 1-ml quantities for each analytical and control replicate. The alditols remain unchanged while the pentoses and hexoses are reduced to the alditol form by borohydride. The total alditols are then oxidized with periodate to form two moles of formaldehyde per mole of monosaccharide and the formaldehyde is determined spectrophotometrically with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH). Unlike other spectrophotometric methods, similar curves are obtained for equimolar concentrations of different carbohydrates while the differences on a weight basis are only due to the small difference in molecular weight between pentoses and hexoses. Winter—Spring samples from surface waters of lower Narragansett Bay ranged from 122 to 226 μg l^?1 with a mean of 159 μg l^?1.     

A convenient method for the determination of carbon in marine “net zooplankton”

Measurements of carbon in marine “net zooplankton” were made by use of a Leco 70 Second TC-12 Automatic Carbon Determinator. The instrument employs high-temperature (ca. 1600°C), dry-combustion and thermal-conductivity detection of the purified carbon-dioxide combustion product and oxygen carrier gas mixtures. The methodology developed in this study is convenient and rapid (ca. 70 sec per freeze-dried sample), with a mean error ($\text{±2σ/}\text{x}\text{·100}$) of ±4.4% at the 3 mg-C level and ±7.4% at the 10 mg-C level when benzoic acid is used as a standard, and ±9.2% at the 5 mg-C level and ±6.0% at the 10 mg-C level when casein is used as the standard. The technique was applied to the “splash zone” marine copepod Tigriopus californicus to obtain an average value of 38.6% C by weight. Concentrated samples of “net zooplankton” of varying size fractions were collected in the Monterey upwelling region over an eight-month period. Values up to 2000 mg-C/m² ocean surface (0–200 m) were observed in a seasonal cycle doubly peaked for some size fractions but not for others.     

Observations of the Sediment-Water Interface: Marine and Fresh Water Environments

The sediment at the interface immediately beneath the water column is distinct from deeper-lying sediments in its properties and, at least quantitatively, in the processes driving diagenesis. Progress in understanding the sediment-water interface can be based on consideration of fundamentals of biogeochemical particle / fluid interactions and on application of certain biological techniques especially suited to this challenging portion of the sediment column. This article reports results achieved by combining theoretical fundamentals and specialized experimental techniques in the study of the interface from selected depositional environments. For fine-grained and sandy deposits from fresh-water to coastal marine environments, the interface is characterized by exaggerated microrelief, great porosity, and significant biological alteration. Additional application of this research approach is poised to further our understanding of engineering, and acoustic and xenochemical responses of sedimentary materials, with special emphasis on the influence of the bio-organic phase of the interface upon its fabric and physical properties.