Latitudinal gradients of species richness in the deep-sea benthos of the North Atlantic

Latitudinal species diversity gradients (LSDGs) in the Northern Hemisphere are the most well established biogeographic patterns on Earth. Despite long -standing interest in LSDGs as a central problem in ecology, their explanat ion remains uncertain. In terrestrial as well as coastal and pelagic marine ecosystems, these poleward declines in diversity typically have been repre sented and interpreted in terms of species richness, the number of coexisti ng species. Newly discovered LSDGs in the bathyal (500-4,000 m) benthos of the North Atlantic may help to resolve the underlying causes of these large -scale trends because the deep sea is such a physically distinct environmen t. However, a major problem in comparing surface and deep-sea LSDGs is that the latter have been measured differently, by using species diversity indi ces that are affected by both species richness and the evenness of relative abundance. Here, we demonstrate that deep-sea isopods, gastropods, and biv alves in the North Atlantic do exhibit poleward decreases in species richne ss, just as those found in other environments. A comprehensive systematic r evision of the largest deepsea gastropod family (Turridae) has provided a u nique database on geographic distributions that is directly comparable to t hose used to document LSDGs in surface biotas. This taxon also shows a pole ward decline in the number of species. Seasonal organic enrichment from sin king phytodetritus is the most plausible ecological explanation for deep-se a LSDGs and is the environmental factor most consistently associated with d epressed diversity in a variety of bathyal habitats.