1 Scientific RepoRts | 5:13167 | DOi: 10.1038/srep13167 www.nature.com/scientificreports Dynamics of a deep-water seagrass population on the Great Barrier Reef: annual occurrence and response to a major dredging program paul H. York 1,* , Alex B. Carter 1 , Kathryn Chartrand 1 , tonia sankey 1 , Linda Wells 2 & Michael A. Rasheed 1,* Global seagrass research eforts have focused on shallow coastal and estuarine seagrass populations where alarming declines have been recorded. Comparatively little is known about the dynamics of deep-water seagrasses despite evidence that they form extensive meadows in some parts of the world. Deep-water seagrasses are subject to similar anthropogenic threats as shallow meadows, particularly along the Great Barrier Reef lagoon where they occur close to major population centres. We examine the dynamics of a deep-water seagrass population in the GBR over an 8 year period during which time a major capital dredging project occurred. seasonal and inter-annual changes in seagrasses were assessed as well as the impact of dredging. the seagrass population was found to occur annually, generally present between July and December each year. Extensive and persistent turbid plumes from a large dredging program over an 8 month period resulted in a failure of the seagrasses to establish in 2006, however recruitment occurred the following year and the regular annual cycle was re-established. Results show that despite considerable inter annual variability, deep-water seagrasses had a regular annual pattern of occurrence, low resistance to reduced water quality but a capacity for rapid recolonisation on the cessation of impacts. Seagrasses are highly productive, meadow-forming marine angiosperms that provide ecosystem services (e.g. nutrient cycling, sediment stabilisation, carbon sequestration and enhancement of ishery produc- tion) of great economic value 1,2 . Mapping and monitoring of seagrass systems over decadal time-scales has revealed alarming declines in global coverage, with habitat loss estimated at 110 km 2 year -1 and accel- erating 3 . he majority of the global information on seagrass change has been focused on shallow coastal populations with comparatively little known about the dynamics of deeper ofshore seagrasses (>15 m depth), except for the Mediterranean Posidonia oceanica 4 . his is despite recent information showing that deep-water seagrass meadows are extensive 5 and highly productive 6 . For seagrasses much of the loss has been attributed to the development of coastal watersheds that result in increased nutrients and declining water quality 7,8 . Seagrasses are extremely vulnerable to these changes because they are highly light dependent, requiring levels of irradiance one to two orders of magnitude higher than most marine macro-algae 9 . Global assessments of the threats to seagrasses have also identiied urban and port infrastructure development and dredging as key threats 10,11 . 1 centre for tropical Water & Aquatic ecosystem Research (tropWAteR), James cook University, cairns Qld, Australia. 2 Jacobs Group, 32 cordelia St, South Brisbane, Queensland 4101, Australia. * these authors contributed equally to this work. correspondence and requests for materials should be addressed to M.A.R. (email: michael. rasheed@jcu.edu.au) Received: 07 november 2014 Accepted: 20 July 2015 Published: 17 August 2015 opeN