Dispersal Modeling of Fish Early Life Stages: Sensitivity with Application to Atlantic Cod in the Western Gulf of Maine

As an initial step in establishing mechanistic relationships between environmental variability and recruitment in Atlantic cod Gadhus morhua along the coast of the western Gulf of Maine, we assessed transport success of larvae from major spawning grounds to nursery areas with particle tracking using the unstructured grid model FVCOM (finite volume coastal ocean model). In coastal areas, dispersal of early planktonic life stages of fish and invertebrate species is highly dependent on the regional dynamics and its variability, which has to be captured by our models. With state-of-the-art forcing for the year 1995, we evaluate the sensitivity of particle dispersal to the timing and location of spawning, the spatial and temporal resolution of the model, and the vertical mixing scheme. A 3 d frequency for the release of particles is necessary to capture the effect of the circulation variability into an averaged dispersal pattern of the spawning season. The analysis of sensitivity to model setup showed that a higher resolution mesh, tidal forcing, and current variability do not change the general pattern of connectivity, but do tend to increase within-site retention. Our results indicate strong downstream connectivity among spawning grounds and higher chances for successful transport from spawning areas closer to the coast. The model run for January egg release indicates 1 to 19 % within-spawning ground retention of initial particles, which may be sufficient to sustain local populations. A systematic sensitivity analysis still needs to be conducted to determine the minimum mesh and forcing resolution that adequately resolves the complex dynamics of the western Gulf of Maine. Other sources of variability, i.e. large-scale upstream forcing and the biological environment, also need to be considered in future studies of the interannual variability in transport and survival of the early life stages of cod.

Topographic Control of Regional Accumulation Rate Variability at South Pole and Implications for Ice-Core Interpretation

Snow-accumulation rates are known to be sensitive to local changes in ice-sheet surface slope because of the effect of katabatic winds. These topographic effects can be preserved in ice cores that are collected at non-ice-divide locations. The trajectory of an ice-core site at South Pole is reconstructed using measurements of ice-sheet motion to show that snow was probably deposited at places of different surface slope during the past 1000 years. Recent accumulation rates, derived from shallow firn cores, vary along this trajectory according to surface topography, so that on a relatively steep flank mean annual accumulation is similar to 18% smaller than on a nearby topographic depression. These modern accumulation rates are used to reinterpret the cause of accumulation rate variability with time in the long ice-core record as an ice-dynamics effect and not a climate-change signal. The results highlight the importance of conducting ancillary ice-dynamics measurements as part of ice-coring programs so that topographic effects can be deconvolved from potential climate signals.