1 What are the costs and benefits of biodiversity recovery in a highly polluted estuary? 1 2 Authors: M. Pascual * 1,2 , A. Borja * 1 , J. Franco 1 , D. Burdon 2 , J.P. Atkins 3 , M. Elliott 2 3 4 Addresses: 5 1 AZTI-Tecnalia, Marine Research Division, Herrera Kaia, Portualdea s/n, 20110 Pasaia, 6 Spain. 7 2 IECS, Institute of Estuarine and Coastal Studies, University of Hull, Hull HU6 7RX, UK. 8 3 Centre for Economic Policy, The Business School, University of Hull, Hull HU6 7RX, UK 9 10 * Corresponding authors: mpascual@azti.es, aborja@azti.es, tel: +34 946574000 11 12 Abstract: 13 Biodiversity recovery measures have often been ignored when dealing with the restoration of 14 degraded aquatic systems. Furthermore, biological valuation methods have been applied only 15 spatially in previous studies, and not jointly on a temporal and spatial scale. The intense 16 monitoring efforts carried out in a highly polluted estuary, in northern Spain (Nervión 17 estuary), allowed for the economic valuation of the costs and the biological valuation of the 18 benefits associated with a 21 years sewage scheme application. The analysis show that the 19 total amount of money invested into the sewage scheme has contributed to the estuaryｴs 20 improvement of both environmental and biological features, as well as to an increase in the 21 uses and services provided by the estuary. However, the inner and outer parts of the estuary 22 showed different responses. An understanding of the costs and trajectories of the 23 environmental recovery of degraded aquatic systems is increasingly necessary to allow policy 24 makers and regulators to formulate robust, cost-efficient and feasible management decisions. 25