Pelagicbenthic coupling export of benthic material to the benthos

Since photosynthetic primary production in the open ocean occurs only in the epipelagic zone, (he mesopelagic and abyssal zones are almost totally dependent upon down-transport of nutrients, with the limited exception of hydrotherma! vent environments (Honjo 1980; Deuser et al. 1981: Lampitt 1985). Benthic biomass decreases with depth and distance from the coastal areas (Mcnzies et al. 1973; WolH" 1977). There are three major processes by which nutrients are transported into abyssal waters. To some extent, nutrients entrained in surface waters can be down-drafted in currents and aquatic storms of all scales. Gravitational transport of organic matter is probably most important. This comes in two forms: the steady rain of small particles consisting of faecal pellets, dead and dying cells, etc. called pelagic "snow" (Silver el al. (1994), and the occasional phytoplankton blooms (Pfannkuche 1993) and sinking carcasses of large marine organisms, such as whales, large fishes (and unfortunate sailors). Overall biomass in the deep ocean is probably a direct function of the absolute quantity of nutrient transported from above. Biodiversity, however, will be influenced by the spatial and temporal distribution of nutrients, which appear as small-scale disturbances to the system (Grassle and Morse-Porteous 1987; Grassle ยก989).

The reconfiguration of the pelagic food webs through species shifts, and perhaps more importantly shifts in size distribution of organisms, may come about through the loss of specific key epipelagic taxa which will result in a concomitant change in the organic material reaching the benthos (Peinert el at. 1989). The reduction in pelagic megafauna has compromised deadfall to the deep. This deadfall serves both as a source of nutrients for benthk species (mentioned above), and as an important stepping stone for dispersal of species associated with hydrotherma! vents. Dead whale carcasses have been found with communities more typically found on hydrothermal vents (Smith et al. 1989), potentially linking vent communities hundreds of miles apart. The role of some organisms in the rate of movement of nutrients through the open ocean is now coming under investigation. The death and subsequent sinking of large organisms from the epipelagic systems is one effective link between the deep-sea benthic communities and the productive surface waters (Smith et al. 1989; Bennett et al. 1994). Altering the number and distribution of these (primarily) vertebrates will have profound effects on the dynamics of benthic and infaunal communities.

A recent study found that salps (Urochordata) also couple epipelagic productivity to the abyss by converting small nanoplankton (e.g. cyanobac-teria) into larger concentrated packages of nutrients (i.e. fecal pellets) which make it to the bottom more quickly. Changes in the proportion of salp-like organisms will undoubtedly affect ecosystem processes in the deep-sea benthic communities. Not only would the patchiness of nutrients be altered.

but the quantity of nutrients actually arriving at the bottom would be altered, i.e. the smallest plankton may not penetrate through the thcrmo-cline, and thus remain in the pelagic realm. It has also been suggested that Rhizosolenid diatoms may also play a similar role in affecting the flux of nutrients from the epipelagic regions to the deep-sea benthos (Villareal et al. 1993; Hayward 1994). Interestingly, mass sinking of such large diatoms may bring moderate to large-scale nutrient flux to the benthic systems.

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