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Dispersal has profound influences on population dynamics and is a key process maintaining spatial and temporal patterns. For many benthic marine invertebrates dispersal occurs primarily during the planktonic larval stages. It is now widely recognised that post-larval and juvenile stages of benthic invertebrate species can also exhibit high rates of dispersal. In particular, post-settlement dispersal has been demonstrated for many bivalve species. Despite this appreciation, no studies to date have analysed the direct dispersal rates and the spatial distribution of dispersing individuals in situ. We used a fluorescent stain for marking bivalves in vivo and a mark-and-recapture methodology to investigate dispersal patterns of post-larval and juvenile bivalves on a sandflat. Wave-induced energy dissipation on the seafloor was measured using a DOBIE wave gauge. Tracer sediment and bedload transport was used as a template for bivalve movement. The experiment was conducted over a short-time span (60 h) and encompassed spatial scales relevant to many sampling designs and manipulative experiments. Our results show that juvenile bivalves dispersed over scales of meters within one tidal cycle. Modelling the half-life of juvenile bivalve retention using radioactive decay equations provided insight into the local persistence of individuals. These models indicate a 50% turnover within an area of 0.25 m 2 for post-larval (<1 mm) bivalves within the first 17.4 h, whereas juvenile (1-4 mm) bivalves persist longer with a 50% turnover after 31.5 h. Considering the very calm hydrodynamic conditions during the experiment, these dispersal rates are remarkable. Bivalve dispersal was decoupled from sediment bedload transport, illustrating the importance of active dispersal behaviour under the prevailing hydrodynamic conditions. Our results suggest that dispersal is potentially more important than mortality for the population dynamics of juvenile bivalves over small and meso spatial-time scales.KEY WORDS: Bivalve dispersal · Post-settlement transport · Wind waves · Disturbance · Scale · Sediment bedload transport · Intertidal sandflats Resale or republication not permitted without written consent of the publisher

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[1] We employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions. Results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated, a priori unpredictable, behavior. This happens because the system responds to a change in wave characteristics by attempting to self-organize into a patterned seabed of different geometry and spacing. The new wavelength might not have enough time to emerge before a new change in wave characteristics occurs, leading to less regular seabed configurations. The new seabed configuration is also highly dependent on the preexisting morphology, which further limits the possibility of predicting future behavior. For the same reasons, variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms. Spatial variability in grain size distribution and different types of net sediment aggradation/degradation can also result in the development of sorted bed forms characterized by a less regular shape. Numerical simulations qualitatively agree with observed geometry (spacing and height) of sorted bed forms. Also in agreement with observations is that at shallower depths, sorted bed forms are more likely to be affected by changes in the forcing conditions, which might also explain why, in shallow waters, sorted bed forms are described as ephemeral features. Finally, simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms. Instead, variations in sorted bed form characteristics may result from variations in local hydrodynamic and/or sedimentary conditions.

The Tertiary sediments in the North Wanganui Basin are referred to 16 formations that range in age from Oligocene to Pliocene and have a combined maximum thickness of about 7500 m. Marginal marine and shallow marine sediments dominate the sequence. Bulk variations in lithology, thickness, sedimentation rate, carbonate content, texture, mineralogy, and other properties for the stratigraphic succession are traced on sedimentary logs. The resulting curves are explained in terms of the relative importance of tectonic activity in controlling sediment properties and sedimentation patterns in the region.Contemporaneous basement faulting has variably influenced the distribution and character of each of the Tertiary formations. From the sedimentary logs the relative intensity of tectonic movements with time in the North Wanganui Basin and adjacent areas is suggested to be as follows: Oligocene (38-24 m.y. )-very low; early Lower Miocene (24-20 m.y.)-high; late lower Miocene to Middle Miocene (20-11 m.y.)-low to moderate; Upper Miocene (11-5 m.y.)-high; Pliocene (5-2 m.y.)-very high.

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