Biology of Fronts
Vertical movements that bring nutrient-rich waters into the well-lit surface layers are at the base of the biological production of marine fronts; phytoplankton show strong positive reaction to such nutrient enrichment. The high primary production generated is transferred then to higher trophic levels reaching top predators, and also benthic organisms. High nutrient supply promotes the growth of large-sized phytoplankton, and consequently the development of shorter and more efficient food webs at fronts. Moreover, large-sized phytoplankton sinks relatively fast, increasing the food supply to benthic assemblages. The largest and more stable fronts are recognized as biogeographic boundaries, and those having lesser spatial scales or persistence exert their influences at finer spatial scales (ecoregions, assemblages). In most of the cases fronts do not appear to be absolute barriers, but are leaky boundaries. The most accepted effects of fronts on biodiversity are their impacts on divergences in species composition (β-diversity; assemblages), while their effects on absolute measures of biodiversity seem to be contradictory. Fronts result typically spawning grounds for species laying planktonic eggs. They offer adequate conditions for the development of the early life stages (abundant food; suitable physical-chemical ranges), and the possibility for eggs and larvae to be retained near the front, both passively or by coupling vertical migrating behavior to frontal circulation. Adult animals migrate to take advantages of seasonal habitats; migrants could utilize fronts as marks or paths to guide them in the highly dispersive and traceless pelagic realm. Animals may respond to physical-chemical gradients and/or prey abundance to find their way along the migration routes.