Secondary Production

Marine Snow

Although chemosynthesis is an important process in the Abyssal zone, it only produces up to 25% of all organic carbon in the deep oceans (Maruyama et al., 1998). Therefore there must be another carbon source which sustains life across this large area. Much of this originates from the photic zone. Here, intense sunlight and high nutrient levels causes periods of phytoplankton bloom. As the plankton increase in abundance, some die and flocculate to form large plankton aggregates (Beaulieu & Smith, 1998). These phytodetritus particles then fall to the lower depths of the ocean. This is known as marine snow, and is an important delivery mechanism of organic carbon to the Abyssal zone. Marine snow may also consist of faecal matter from pelagic zooplankton, meiofauna and megafauna (Bishop et al., 1977). This is then consumed by suspension and deposit feeders within the Abyssal zone.

Oceanic processes leading to the production and transport of marine snow. Photo credit: American Scientist.

Another carbon source, not considered as substantial until recent years, is provided by Larvaceans. They were discovered as early as 1898, but their feeding methods remained a mystery until the 1960s when they were viewed by submersibles (Barham, 1979). These are suspension feeding tunicates, which consume the marine snow as it sinks. To feed, they produce mucus ‘houses’ through which water is pumped. As particles get caught in this house, it eventually clogs up and has to be shed. This may only take 24 hours. The discarded house collapses and sinks rapidly at a rate of ~800 m per day (Hamner & Robison, 1984).This is an extremely quickform of carbon delivery to the Abyssal zone. However, due to difficulties in collecting and observing these sinkers, it is not entirely known how substantial a carbon source these are, and their contributions to nutrient flux is not considered in oceanic carbon budgets (Robison et al., 2005).

This video shows the discarded mucus house of a Larvacean collapsing and sinking:

 Whale falls

A large, albeit localised, carbon source is the sinking of carcasses from the surface. Cetacean carcasses provide a massive source of organic matter for Abyssal fauna. The amount of carbon delivered by a 40 ton whale carcass to the Abyssal benthos is equivalent to 100-200 years’ worth of background carbon flux descending upon a one hectare area (Smith & Demopoulos, 2003).  Food sources of this magnitude are incredibly rare at these depths, so organisms must take full advantage of the opportunity. A wide variety of scavengers are attracted, including fish (e.g. hagfish, sleeper sharks, grenadier fish, eelpout, Macrourid fish), Crustaceans (e.g. Galatheid crabs, Lithodid crabs, Lyssianasid amphipods), and Echinoderms (sea cucumber Scotoplanes sp.) (Jones et al., 1998; Smith & Baco, 2003; Goffredi et al., 2004). These carcasses can be consumed rapidly by the scavengers – in a study by Jones et al. (1998), the corpse of an Atlantic white-sided dolphin was skeletonized in 5 days. However, a whale carcass may take a year to be fully consumed.

This video shows a whale fall scavenger community. Sharks, hagfish, and various other organisms can be observed feeding:

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