Spatial and temporal variation in subtidal molluscan diversity amongst temperate estuarine habitats

Effective management of marine ecosystems is enhanced when detailed information on biodiversity is available. Key information to underpin management actions and conservation planning includes relationships between species assemblages and environmental gradients, and information on species distributions. We conducted a subtidal biodiversity assessment of surface‐dwelling subtidal molluscs in eight a priori defined habitat types using underwater visual censuses to quantitatively explore relationships between molluscan assemblages, and their correlation with benthic habitats and abiotic variables. In addition, variations in diversity were examined for two key habitat types (areas dominated by Dendronephthya australis and by filter feeders) over a period of 15 months to examine temporal change. We found that molluscs form distinct assemblages within subtidal habitats, but that assemblages within key habitats show inherent temporal variability. Regional (gamma) diversity of molluscs was found to result from a combination of: (i) within habitat alpha diversity, which increased with habitat complexity; (ii) between habitat beta diversity, with significant differences in molluscan assemblages amongst habitats with differing benthic growth, substrate type, and depth; and (iii) temporal beta diversity, with significant changes detected in molluscan assemblages over time. The results demonstrate how habitats and abiotic variables (principally depth and substrate type) combine to contribute to molluscan biodiversity in temperate estuaries, and illustrate the value of these factors as surrogates for surface‐dwelling subtidal molluscs in conservation planning.     

A Hybrid Finite-Difference and Analytic Element Groundwater Model

Regional finite-difference models tend to have large cell sizes, often on the order of 1–2 km on a side. Although the regional flow patterns in deeper formations may be adequately represented by such a model, the intricate surface water and groundwater interactions in the shallower layers are not. Several stream reaches and nearby wells may occur in a single cell, precluding any meaningful modeling of the surface water and groundwater interactions between the individual features. We propose to replace the upper MODFLOW layer or layers, in which the surface water and groundwater interactions occur, by an analytic element model (GFLOW) that does not employ a model grid; instead, it represents wells and surface waters directly by the use of point-sinks and line-sinks. For many practical cases it suffices to provide GFLOW with the vertical leakage rates calculated in the original coarse MODFLOW model in order to obtain a good representation of surface water and groundwater interactions. However, when the combined transmissivities in the deeper (MODFLOW) layers dominate, the accuracy of the GFLOW solution diminishes. For those cases, an iterative coupling procedure, whereby the leakages between the GFLOW and MODFLOW model are updated, appreciably improves the overall solution, albeit at considerable computational cost. The coupled GFLOW–MODFLOW model is applicable to relatively large areas, in many cases to the entire model domain, thus forming an attractive alternative to local grid refinement or inset models.     

Observations of asymmetry in contrasting wave‐ and tidally‐dominated environments within a mesotidal basin: implications for estuarine morphological evolution

Tides are often considered to be the dominant hydrodynamic process within mesotidal estuaries although waves can also have a large influence on intertidal erosion rates. Here, we use a combination of hydrodynamic measurements and sediment deposition records to determine the conditions under which observed waves are ‘morphologically significant’, in which case they influence tidal and suspended sediment flux asymmetry and subsequently infilling over geomorphological timescales. Morphological significant conditions were evaluated using data from contrasting arms in a dendritic mesotidal estuary, in which the orientation of the arms relative to the prevailing wind results in a marked difference in wave conditions, deposition rates and morphology. By defining the morphological significance of waves as a product of the magnitude of bed shear stress and frequency of occurrence, even small (but frequently occurring) winds are shown to be capable of generating waves that are morphologically significant given sufficient fetch. In the arm in which fetch length is restricted, only stronger but rare storm events can influence sediment flux and therefore tides are more morphologically significant over longer timescales. Water depth within this mesotidal estuary is shown to be a critical parameter in controlling morphological significance; the rapid attenuation of short period waves with depth results in contrasting patterns of erosion occurring during neaps and accretion during springs. Copyright © 2016 John Wiley & Sons, Ltd.     

Settlement and Recruitment Patterns of the Soft-Shell Clam, <Emphasis Type="Italic">Mya arenaria</Emphasis>, on the Northern Shore of the Bay of Fundy,Canada

To examine the roles of settlement and early postsettlement processes in the recruitment of the soft-shell clam, Mya arenaria, abundance of recent settlers and juveniles was monitored over two field seasons at four locations on the northern shore of the Bay of Fundy in New Brunswick, Canada. Results showed great spatial and temporal variability in patterns of settlement. M. arenaria appeared to become highly vulnerable to postsettlement processes at a shell length of approximately 2 mm. Postsettlement processes drastically altered patterns of settlement less than 1 year after they were established. Results suggest that local factors at specific sites within the Bay of Fundy, such as hydrodynamics, larval behavior, and early postsettlement events, likely control the abundance of juvenile clams. Additionally, postsettlement events are extremely important in shaping M. arenaria populations in this area. Very few mature adult clams greater than 50 mm in shell length were found at any sampling sites, and no relationship was found between abundance of setters and density of juveniles and adults.     

Modeling Bedload Transport of Juvenile Bivalves: Predicted Changes in Distribution and Scale of Postlarval Dispersal

The potential consequences of bedload transport of postlarvae for patterns of distribution of marine invertebrates were explored by developing a bedload transport model for juvenile bivalves in a small estuary in New Jersey, USA. A simple numerical model of tidal current hydrodynamics was developed based on field measurements of shear stresses near the bottom. Burrowing behavior of bivalves was incorporated into the model of bedload transport by using estimates of entrainment rates of Gemma gemma and Mya arenaria in a laboratory flume, and jump lengths of the bivalves were estimated by methods previously developed for noncohesive particles. Based on the flood domination and strong gradient of shear stresses in the Navesink estuary, our model predicted that juvenile bivalves would accumulate in the center of the estuary, traveling up to several kilometers over 30 days. Field distributions of juvenile bivalves were consistent with the model predictions for other species of bivalves but not for G. gemma, for which field distributions of both <500- and >500-μm individuals were concentrated in the eastern end of the estuary. Differences between the bedload model and G. gemma distributions suggest that spatial variation in burrowing behavior or biological interactions are playing an important role in maintaining distribution patterns of this species in spite of high levels of bedload transport. This modeling approach is applicable to other juvenile benthic invertebrates that disperse as bedload and is a useful model against which to compare field observations of rates of transport and patterns of distribution and abundance.     

Relating groundwater to seasonal wetlands in southeastern Wisconsin, USA

Historically, drier types of wetlands have been difficult to characterize and are not well researched. Nonetheless, they are considered to reflect the precipitation history with little, if any, regard for possible relation to groundwater. Two seasonal coastal wetland types (wet prairie, sedge meadow) were investigated during three growing seasons at three sites in the Lake Michigan Basin, Wisconsin, USA. The six seasonal wetlands were characterized using standard soil and vegetation techniques and groundwater measurements from the shallow and deep systems. They all met wetland hydrology criteria (e.g., water within 30 cm of land surface for 5% of the growing season) during the early portion of the growing season despite the lack of appreciable regional groundwater discharge into the wetland root zones. Although root-zone duration analyses did not fit a lognormal distribution previously noted in groundwater-dominated wetlands, they were able to discriminate between the plant communities and showed that wet prairie communities had shorter durations of continuous soil saturation than sedge meadow communities. These results demonstrate that the relative rates of groundwater outflows can be important for wetland hydrology and resulting wetland type. Thus, regional stresses to the shallow groundwater system such as pumping or low Great Lake levels can be expected to affect even drier wetland types.     

Securing the Neoliberal City: Discourses of Creativity and Priority Neighborhoods in Toronto,Canada

This paper examines the convergence between the discourse of the creative city and the discourse on priority neighborhoods within urban policy imaginaries in Toronto, Canada. In particular, it examines the development of a number of arts programs targeted at low-income neighborhoods in the city. The twin objectives of these programs are to (a) foster creative and entrepreneurial subjectivities among “at-risk” youth, and (b) reduce the risk of violence that is presumed to be associated with youths living in poor neighborhoods. The paper analyzes how these two discourses are intertwined in a neoliberal politics devoted to enhancing regional quality-of-life competitiveness and to branding Toronto as an attractive creative-class destination. [Key words: creative city, poverty, low-income neighborhoods, neoliberalism, Toronto.]     

Growth fold controls on carbonate distribution in mixed foreland basins: insights from the Amiran foreland basin (NW Zagros,Iran) and stratigraphic numerical modelling

The evolution from Late Cretaceous to early Eocene of the well dated Amiran foreland basin in the NW Iranian Zagros Mountains is studied based on the reconstruction of successive thickness, palaeobathymetry and subsidence maps. These maps show the progressive forelandwards migration of the mixed carbonate‐siliciclastic system associated with a decrease in creation of accommodation. Carbonate facies variations across the basin suggest a structural control on the carbonate distribution in the Amiran foreland basin, which has been used as initial constraint to study the control exerted by syndepositional folding in basin architecture and evolution by means of stratigraphic numerical modelling. Modelled results show that shallow bathymetries on top of growing folds enhance carbonate production and basin compartmentalization. As a consequence, coarse clastics become restricted to the internal parts of the basin and only the fine sediments can by‐pass the bathymetric highs generated by folding. Additionally, the development of extensive carbonate platforms on top of the anticlines favours the basinwards migration of the depositional system, which progrades farther with higher fold uplift rates. In this scenario, build‐ups on top of anticlines record its growth and can be used as a dating method. Extrapolation of presented modelling results into the Amiran foreland basin is in agreement with an early folding stage in the SE Lurestan area, between the Khorramabad and Kabir Kuh anticlines. This folding stage would enhance the development of carbonate platforms on top of the anticlines, the south‐westward migration of the system and eventually, the complete filling of the basin north of the Chenareh anticline at the end of the Cuisian. Incremental thickness maps are consistent with a thin (0.4–2 km) ophiolite complex in the source area of the Amiran basin.