Coastal Geoscience, Grimston,

Melton Mowbray LE14 3BY, United Kingdom

e-mail: r.arthurton@btinternet.com

Sediments play key roles in the development of coral reef related coastal ecosystems. Some of these sediments are formed by a combination of biological and physical processes within the ecosystems themselves. Others are sourced from river basins or the erosion of coastal hinterland and transported to the reef related ecosystems by coastal water currents and, in some cases, wind. The relative importance of these sources varies from place to place, depending, for example, on the proximity of the reef site to significant river discharges. It also varies over time, for example, as the direction of longshore sediment transport on beaches reflects the changing wind patterns of the monsoon seasons through changing wave impacts. The natural processes of sediment production, transportation and deposition are affected in various ways by human intervention, whether within the ecosystems themselves or externally, even remotely.

The production of sediment in the reef related environment, other than the reworking of pre-existing sediments, depends on the maintenance of the biota that extracts calcium carbonate from seawater. Of these biota, the constructional components of the forereef and the calcareous algae of the backreef are particularly important. Some of the sediments thus formed are lost from the forereef to deep water, others accrete on the reef bar and on the backreef where, importantly, they form the substrate for algae and seagrass.

Any human activities that adversely affect the health of these primary sediment producers should be discouraged. At the local level, these include the use of explosives in fisheries, uncontrolled ecotourism, and the indiscriminate discharge of waste waters including sewage, swimming pool flushing and industrial effluents to backreef environments. At the regional level, the main concern for the health of these biota is the impact of waters with high levels of suspended sediment and, in some cases, high levels of nutrients and/or pesticides derived from river discharge. In such circumstances policies for coastal management need to embrace the development interests within the wider river basin catchment.

Despite its negative impacts, the discharge of sediment from rivers to the coastal environment has resource benefits that need to be safeguarded. Quartz grains derived from the hinterland form most of the white sand beaches, beach plain deposits and associated dunes of the eastern African shore. Only in a few places are present-day beaches composed of calcium carbonate sand. Whatever their origins and composition, beach sands represent a valuable resource, whether for tourism, recreation or simply for sea defence. In many cases beaches are subject to natural erosion. This may be part of a seasonal fluctuation or progressive wasting over the long-term. Where human activities such as sand mining or the construction of coastal defences contribute to beach erosion they should be regulated where possible. Access to alternative sources of construction materials should be considered. The scope of ‘protection’ in protected areas should extend to the non-living as well as the living resources.

The effects on the reef-related ecosystems of eastern Africa of possible relative sea-level rise over the short term are speculative. A fundamental question is whether upward growth of fringing and patch reefs would keep pace with sea-level rise. Assuming such a capability for fringing reefs, it is suggested that lagoonal conditions with calcareous algae and coral mounds would become widespread, though existing beach deposits might become progressively vulnerable to erosion, posing a threat to some tourism investment.