Import and export of biomass and energy

Nursery, shelter and food provision

Some benthic communities are net producers of energy, others are net consumers (Hartnoll, 1983b), and so to a degree the latter must be considered dependant on the former. The infralittoral kelp-dominated biotopes are some of the most profligate net producers (Miller et al., 1971), with production levels of 40 000 kJ m-2 yr-1, of which over 90% is exported. They produce far more energy by primary production than their resident herbivores can eat, and the rest is exported to fuel other ecosystems. Sheltered rocky shores, salt marshes, sea grass beds, and planktonic communities are all regions of high, and usually surplus, primary productivity.

Other biotopes lack a significant plant component, so cannot synthesise their own energy supply, and must import it. Sandy and muddy substrata, both intertidal and subtidal, are largely of this type in north temperate regions - though intertidal and shallow sediments may have significant production by benthic diatoms . Exposed rocky shores support only limited macroalgal growth. Subtidal rock surfaces with restricted light supply - the circalittoral - are basically animal dominated communities.

So the CFT communities depend primarily on food from outside - where does it come from? Since the primary consumers of the community are filter feeders, the food arrives via the water column, and more exposed conditions mean that the food supply is more regularly replenished by water movement. The suspended food is a mixture of living and dead organic matter. The living matter is the plankton, a mixture of small floating plants and animals. The dead material originates from many sources, part from the plankton, part from other benthic communities. Fragmented algae from shallower communities will contribute to this pool of energy.

Thus anything which affects primary production in the water column or in shallow-water ecosystems may ultimately affect the CFT: the impact of pollution or eutrophication, for instance, may well extend to these deeper areas. However, although not primary producers, CFT communities are important secondary producers. They accumulate and concentrate the primary production from a large water mass, and make this readily available to higher trophic levels.

The energy flow is not entirely one way though - the CFT will re-export some of the energy it has acquired. Some will be as detritus in the form of faeces, dead bodies, etc. Some will be as reproductive products, shed into the water, and contributing to the biomass of the plankton. A substantial proportion of the plankton consists of the larval stages of benthic species - the meroplankton. And some will be exported by visiting carnivores who come to feed on an abundant food source.

Whilst most of the CFT species spend their larval life in the plankton, there are a few planktonic species which spend their early stages within the CFT biotopes. This is true, in rather different degrees, of the hydroids and the jellyfish.

We have seen in section III.B that hydroids are common and conspicuous members of CFT communities, but the attached hydroids are only the juvenile stages. The sexually reproducing mature stages are small medusae which are released into the plankton, where they reproduce to produce larvae which settle again. In contrast, for jellyfish the large adult medusae in the plankton are the prominent phase. The juveniles stages live attached to rocks as an inconspicuous scyphistoma stage in which the jellyfish overwinters. In spring this buds off a series of juvenile medusae, or ephyrae, which grow rapidly in the plankton to form the adult. Numbers of jellyfish are unpredictable, and dense aggregations are of environmental concern: factors affecting their juvenile stages could be of relevance to management.

A further way in which CFT communities interact with others is by the provision of food and/or temporary shelter to mobile species which are not permanent CFT fauna. Shelter is important to juvenile fish, which can find refuge (and food) amongst the dense turf of sessile species. A food source is provided to large mobile crustaceans and fish which are attracted by the rich and stationary food supply available on circalittoral rock. It is for this reason that anglers will fish in CFT biotopes, posing a possible impact threat (see below), and generating management implications.

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