Prediction of annual average sedimentation rates in an estuary using numerical models with verification against core data – Mahurangi Estuary, New Zealand

Modelling sedimentation rates within an estuary over the time scale of years to decades is a difficult undertaking. The complex nature of sediment transport and the compounding errors associated with making predictions over longer time-scales introduce a high degree of uncertainty when predicting the fate of catchment-derived sediments. In this paper a methodology is presented that links catchment and estuary models that simulate the runoff of sediment from catchments and its subsequent dispersal within the estuary to provide estimates of annual sedimentation rates within the estuary. The models are calibrated against short-term field data and the methodology is validated against sedimentation rates obtained from sediment cores.The catchment of the Mahurangi Estuary delivers between 3800 and 39,000 tonnes/yr with an average load of just over 15,000 tonnes/yr being delivered to the estuary. Data from sediment cores show that over 80% of this load is deposited within the estuary resulting in sedimentation rates of 25 mm/yr in the upper estuary and less than 5 mm/yr in the lower sections of the estuary. The methodology predicts these rates of sedimentation across a range of sub environments within the estuary.     

Biophysical models of larval dispersal in the Benguela Current ecosystem

We synthesise and update results from the suite of biophysical, larval-dispersal models developed in the Benguela Current ecosystem. Biophysical models of larval dispersal use outputs of physical hydrodynamic models as inputs to individual-based models in which biological processes acting during the larval life are included. In the Benguela, such models were first applied to simulate the dispersal of anchovy Engraulis encrasicolus and sardine Sardinops sagax ichthyoplankton, and more recently of the early life stages of chokka-squid Loligo reynaudii and Cape hakes Merluccius spp. We identify how the models have helped advance understanding of key processes for these species. We then discuss which aspects of the early life of marine species in the Benguela Current ecosystem are still not well understood and could benefit from new modelling studies.     

Identifying optimal sites for natural recovery and restoration of impacted biogenic habitats in a special area of conservation using hydrodynamic and habitat suitability modelling

Selection of sites for successful restoration of impacted shellfish populations depends on understanding the dispersion capability and habitat requirements of the species involved. In Strangford Lough, Northern Ireland, the horse mussel (Modiolus modiolus) biogenic reefs cover only a fraction of their historical range with the remaining reefs badly damaged and requiring restoration. Previous experimental trials suggest that translocation of horse mussels accelerates reef recovery and has therefore been proposed as a suitable restoration technique. We used a series of coupled hydrodynamic and particle dispersal models to assess larval dispersion from remnant and translocated populations to identify suitable areas for adult live M. modiolus translocation in Strangford Lough, Northern Ireland. A maximum entropy model (MAXENT) was used to identify if dispersing larvae could reach habitat suitable for adult M. modiolus. From these we predicted if translocated mussels will reseed themselves or be able to act as larval sources for nearby reefs. The dispersal models showed that the remnant M. modiolus populations are largely self-recruiting with little connectivity between them. The majority of larvae settled near the sources and movement was largely dependent on the tides and not influenced by wind or waves. Higher reef elevation resulted in larvae being able to disperse further away from the release point. However, larval numbers away from the source population are likely to be too low for successful recruitment. There was also little connectivity between the Irish Sea and Strangford Lough as any larvae entering the Lough remained predominantly in the Strangford Narrows. The areas covered by these self-seeding populations are suitable for M. modiolus translocation according to the MAXENT model. As a result of this work and in conjunction with other field work we propose a combination of total protection of all remaining larval sources and small scale translocations onto suitable substrata in each of the identified self-recruiting areas.