Fate of primary production by macroalgae

Rocky shores as nursery grounds

Rocky shores as feeding areas for birds and mammals

One of the defining characteristics of a rocky shore is that it exists at the interface between marine and terrestrial ecosystems. Consequently, there are strong interactions between the communities of the shore, land and sea. Coasts, including rocky shores, are highly productive (Hawkins et al. 1992). Shallow water allows the penetration of light which promotes primary production in macroalgae and the microbial film which coats the rock. Rocky shores also benefit from primary production in phytoplankton which are washed over the shore with each tide. The sea also brings detritus in a range of particle sizes. Whole kelp plants and other algae washed up on the shore are eaten by littorinids, crustaceans and limpets. Banks of mixed seaweeds can accumulate on the strandline, where they support populations of invertebrate scavengers of both terrestrial and marine origin. Small detrital fragments and phytoplankton are consumed by filter feeders such as barnacles and mussels. Particulate organic matter in this form is the main route of material flowing up the detrital food chain.

Macroalgae exude considerable amounts of dissolved organic carbon which are taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1996). On exposed shores, grazers feed mainly on the microbial film. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores also make a contribution to the food of many marine species through the production of planktonic larvae and propagules which supply essential biochemicals to pelagic food chains.

The rocky subtidal is an important nursery area for many commercially important species of fish including herring and gadoids. These can migrate into the intertidal zone at high tide but there is little evidence of any dependence on littoral communities. However, fish and crustaceans, migrating into the intertidal to feed as the tide rises, are important predators of rocky shore species. Corkwing wrasse, which are important to the aquaculture industry as a cleaner species rely heavily on the intertidal. Juveniles are commonly found in rockpools.

Shore birds also feed on the rocky shore (Feare and Summers, 1985). The invertebrates attracted to seaweed on the strandline are a particularly important food source. Rich pickings can also be had under macroalgae canopies. Otters often use rocky shores and will feed on animals such as shore crabs which, in turn, feed on rocky shore species. Opportunistic mammalian species such as rats, rabbits and even sheep will forage on available rocky shores.

Next Section                     References