Estuaries are the interface between freshwater systems and the sea, with clearly recognizable changes in the distribution of organisms along various environmental gradients from riverine to fully marine conditions. This thesis studied ecological processes affecting the distribution of marine species along estuarine gradients using both field and laboratory experiments with barnacles as a tractable model system. Elminius modestus Darwin, Semibalanus balanoides Linneaus, Chthamalus montagui Southward are the most common barnacle species found in the intertidal of British estuaries. Surveys performed in the Plym and the Yealm Estuaries revealed that E. modestus occurred furthest up estuaries and was dominant along most of their length, with the exception of sites closest to the sea; C. montagui had the most restricted degree of penetration up-estuary; and S. balanoides occurred at low abundances, with limits of penetration located between those of C. montagui and E. modestus. Transplants of adult specimens to sites along the marine-to-freshwater gradient revealed that E. modestus was better adapted to environmental conditions found in inner areas of the estuaries than C. montagui and S. balanoides. Survival of the latter two species was negatively correlated with deposition of silt and decreasing salinity. Laboratory experiments showed that, in contrast to the other two species, E. modestus was highly tolerant to burial by silt. A comparison of observed distributions along the gradients with survivability showed that C. montagui was able to survive in areas where adults were absent, indicating that early life cycle stages were a potential limiting factor to the distribution of this species. Examination of patterns of cyprid settlement and recruitment to the benthic phase reinforced the suggestion that pre-settlement processes contribute to restrictions in the distribution of both C. montagui and S. balanoides. The abundance of these species, post-settlement, was also affected by physico-chemical conditions in the upper estuary. Pre-settlement processes did not limit the distribution of E. modestus and post-settlement processes appeared to be more important in limiting its abundance at sites closest to the sea.

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