Dynamics of a deep-water seagrass population on the Great Barrier Reef: annual occurrence and response to a major dredging program

Abstract

Global seagrass research efforts 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.

Blue Carbon stocks of Great Barrier Reef deep-water seagrasses

“…An argument against deep-water seagrasses in the GBR having a high OC sequestration capacity is they are smaller growth-form species with sometimes annual or ephemeral occurrence [7], and therefore have lower biomass and less capacity to trap particulate OC from the water column than larger, more persistent growth forms. However, Lavery et al [8] found that Halophila ovalis in coastal waters-a small growth-form species-had the second highest OC stock among 10 Australian species tested, including large growth forms.…”

Sediment Resuspension and Deposition on Seagrass Leaves Impedes Internal Plant Aeration and Promotes Phytotoxic H2S Intrusion

“…Silt/clay-induced H 2 S intrusion into Z. muelleri seemed tightly coupled to prolonged exposure to sediment re-suspension, such as typically found during harbor dredging activities (York et al, 2015) and resulting from river plumes (Petus et al, 2014). Leaf silt/clay-covers thus impeded the plants' performance and thereby their resilience toward H 2 S intrusion.…”

Effects of dredging on critical ecological processes for marine invertebrates, seagrasses and macroalgae, and the potential for management with environmental windows using Western Australia as a case study

“…As such, vulnerability to sedimentation and reduced light is low during the dormant seed dispersal stage during the wet season, and presents an EW to reduce the impact of dredging. In contrast, intensive dredging activities could have major impacts on this species during the dry season in this region, as has been seen for the species on the east coast of Australia (York et al 2015) when the plants rely on higher light levels to stimulate germination of the seed bank, meadow development flowering and seed production. While this window may be appropriate for colonising seagrass species the same may not hold true for opportunistic and persistent tropical species that have a less pronounced seasonality in life history and a higher reliance on the adult phase to confer their resilience to impacts.…”