The role of biophysical coupling in concentrating marine organisms around shallow topographies

Rather than being evenly spread throughout the ocean, the distribution of fish and plankton biomass is generally quite patchy. In most parts of the ocean, horizontal and vertical gradients exist, such that biomass (and diversity) generally decrease as one moves further offshore or deeper in the water column. The general explanation underlying these trends is that primary and secondary production is highest near the surface and close to coastlines, where both sufficient light and nutrients are available. Beginning in the late 1960s, oceanographers began to map the distribution of biological ‘‘hot-spots’’, regions of the ocean in which production is significantly higher, and aggregations of organisms denser, than in the surrounding waters (and often most evident as higher than average fish catches). Such areas include the waters over nearshore sills and banks on the continental shelf, above and within submarine canyons, along the continental shelf-break, and above (or downstream of) shallow seamounts and oceanic islands. Despite being found in very different parts of the ocean, all can be characterized as regions in which ocean circulation interacts with an abrupt topographic feature. Detailed examination of these hot-spots has since revealed that the enhanced biological production typical of these regions is, in fact, the result of strong biophysical coupling. The transfer of oceanic energy within these shallow regions may thus represent an important (and largely unrecognized) flux affecting mesoscale patterns of production and which is fundamental for energy transfer to higher trophic levels. The specific mechanisms by which dense aggregations of higher trophic level organisms are produced and maintained, however, can be quite different.