Environmental consequences of tidal power in a hypertidal muddy regime: the Severn estuary (UK)

Abstract

Muddy hypertidal regimes, such as the Severn Estuary in the UK, are especially difficult for plants and animals. The difficulties stem from the semi-diurnal and semi-lunar energy fluctuations. On spring tides entrained fine sediment induces elevated suspended sediment concentrations such that phtosynthesis is inhibited. On neap tides much of the entrained fine sediment is deposited on the subtidal bed over periods of several days to form ephemeral dense layers, which reach in excess of 100 g/l and rapidly become anaerobic on stagnation. Such occasional bed faunas as develop are characterised by very large numbers of immature individuals of a few species. One of the few organisms able to cope with the extreme conditions is the siliceous reef-building worm Sabellaria. Arising from the long term supres-sion in its calcareous fauna, erosion and winnowing of these Holocene clays fails to give rise to lag shell deposits, called chenier ridges, found elsewhere in eroding muddy intertidal systems. A tidal power barrage would shift the regime from hypertidal to macrotidal decrease in turbidity would permit photosynthesis and phytoplankton growth, so stimulating the higher food chain. Ironically, perhaps, cleaning up the sewage discharges in the estuary, in the absence of barrage construction, would lead to a wading bird crash whereas barrage construction would lead to an improved carrying capacity.