Tidal and ocean-current power: Impact on the biosphere

What happened to an estuary after engineers dammed it

What this tab covers Effects on human, animal, plant, fungal and microbial life and on whole ecosystems: in the short term (construction and early years), medium term (operating life) and long term (decommissioning and legacy).

Barrages transform estuaries: La Rance lost most marine life during construction and took about ten years to reach a new, different balance. Tidal stream turbines have small footprints, and no harm has yet been shown, but collision risk remains the main unresolved question.

Ecological recovery time at La Rance after closure about 10 years⁴ Bird species at La Rance about 120, unchanged⁴ Life-cycle emissions (ocean energy) 17 g CO2e/kWh (range 5.6-28)⁶

Effects over time

Life affected	Short term	Medium term	Long term
Humans³⁴	Barrage construction closes estuaries to shipping and fishing.	La Rance carries a road that saves up to 60,000 vehicles a day a 30 km detour; new scallop and oyster fisheries.	Barrages also act as flood defences; silting needs management.
Animals⁴¹⁰	Closure and salinity swings at La Rance wiped out marine fauna.	Sand-eels and plaice disappeared, sea bass and cuttlefish returned; bird species stayed at about 120. Marine mammals seem to avoid turbine rotors; salmon smolts may pass through and be disoriented.	Fish passage through barrage turbines and sluices continues for the life of the plant.
Plants⁴	Marine flora disappeared during La Rance's closure.	Recolonisation within about ten years.	Little evidence of effect.
Fungi and soil life⁴	Little evidence of effect.	Silt deposits increased in the low intertidal zone at La Rance, changing sediment habitats.	Little evidence of effect.
Ecosystems³⁴¹⁰	Barrages reduce tidal range and the volume of water entering the estuary.	La Rance mean water level rose about 2.5 m; Sihwa improved water quality in a formerly closed sea-arm. Small tidal stream arrays have no detectable effect on currents.	Progressive silting of the basin; effects of large stream arrays are only modelled.

How the effects are reduced

For barrages: building on existing dams (as at Sihwa), fish-friendly turbines and sluice operation, and lagoons that leave the estuary open. For tidal stream: avoiding key feeding and migration areas, slow-turning rotors, monitoring with sonar and cameras, and consenting arrays step by step so that collision data build up before arrays grow.

Sources

Tidal Energy: Technology Brief IRENA, 2014
La Rance Tidal Barrage (Annex IV site page) PNNL Tethys, 2024
Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters, Table A.III.2 IPCC AR5 Working Group III, 2014
OES-Environmental 2024 State of the Science Report, Chapter 11: Summary and Path Forward PNNL / Ocean Energy Systems, 2024