Dispersal kernels provide a useful way to quantify the average spatial distribution of propagules originating from a given point in space. Consequently, dispersal kernels have been used in analytical and numerical studies of short-and long-distance dispersal of marine invertebrates and fish with pelagic larval stages. In most cases, the shape of dispersal kernels is pre-determined and parameterised with knowledge of larval duration or mean current velocities homogeneously across space. Here, the characteristics of planktonic larval dispersal for near-shore species in a realistic coastal ocean flow are investigated through the use of a numerical ocean model of a section of the central Chilean coast. The 3-dimensional primitive equation model was forced by 4 yr of observed winds from Las Cruces. Planktonic larval dispersal was simulated by advecting passive drifters using the evolving model velocity field. No a priori assumptions were made about diffusion-advection statistics. Drifters were released daily from regularly spaced locations along the coast and were considered to have settled if found within 1 km of the coast 30 d after release. Observed dispersal kernels were then calculated for each release location, and their variability in space and time was examined. This variability was found to be substantial over spatial scales less than a typical larval-advection scale, and, as a result, a spatially and temporally averaged dispersal kernel was inadequate as a global model of settlement. Large along-shore variation in the shape of dispersal kernels led to significant variation in the spatial pattern of connectivity among local sites, with some acting as net sources and some as net sinks within scales of 10s of kilometres. These results are linked to the alongshore and seasonal variability in ocean circulation, in particular close to shore. Both local and global dispersal kernels were found to be non-Gaussian, with their distribution related to that of the ocean velocity field. It is concluded that, in realistic flows with complicated coastal geometry, considerable departure from the expected Gaussian dispersal kernels based on homogeneous flow conditions can lead to complex spatial patterns of connectivity and successful settlement along a relatively simple but real coastline.
Over the past decade, evidence of abrupt latitudinal changes in the dynamics, structure and genetic variability of intertidal and subtidal benthic communities along central-northern Chile has been found consistently at 30–32°S. Changes in the advective and thermal environment in nearshore waters have been inferred from ecological patterns, since analyses of in situ physical data have thus far been missing. Here we analyze a unique set of shoreline temperature data, gathered over 4–10 years at 15 sites between 28–35°S, and combine it with satellite-derived winds and sea surface temperatures to investigate the latitudinal transition in nearshore oceanographic conditions suggested by recent ecological studies. Our results show a marked transition in thermal conditions at 30–31°S, superimposed on a broad latitudinal trend, and small-scale structures associated with cape-and-bay topography. The seasonal cycle dominated temperature variability throughout the region, but its relative importance decreased abruptly south of 30–31°S, as variability at synoptic and intra-seasonal scales became more important. The response of shoreline temperatures to meridional wind stress also changed abruptly at the transition, leading to a sharp drop in the occurrence of low-temperature waters at northern sites, and a concurrent decrease in corticated algal biomass. Together, these results suggest a limitation of nitrate availability in nearshore waters north of the transition. The localized alongshore change results from the interaction of latitudinal trends (e.g., wind stress, surface warming, inertial period) with a major headland-bay system (Punta Lengua de Vaca at 30.25°S), which juxtaposes a southern stretch of coast characterized by upwelling with a northern stretch of coast characterized by warm surface waters and stratification. This transition likely generates a number of latitude-dependent controls on ecological processes in the nearshore that can explain species-specific effects, and add strength to the suggestion of an oceanography-driven, major spatial transition in coastal communities at 30–31°S.
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