Seagrass on the brink: Decline of threatened seagrass Posidonia australis continues following protection

Abstract

Seagrasses are in decline globally due to sustained pressure from coastal development, water quality declines and the ongoing threat from climate change. The result of this decline has been a change in coastal productivity, a reduction in critical fisheries habitat and increased erosion. Attempts to slow this decline have included legislative protection of habitat and direct restoration efforts. Monitoring the success of these approaches requires tracking changes in the abundance of seagrasses, but such monitoring is frequently conducted at either too coarse a spatial scale, or too infrequently to adequately detect changes within individual meadows. Here, we used high resolution aerial imagery to quantify the change in meadows dominated by Posidonia australis over five years at 14 sites in five estuaries in south-eastern Australia. Australia has some of the world's most diverse and extensive seagrass meadows, but the widely distributed P. australis has a slow growth rate, recovers poorly after disturbance, and suffers runaway attrition if the conditions for recovery are not met. In 2010, after declines of 12–57% between the 1940s and 1980s, P. australis was listed as a threatened ecological community in New South Wales. We quantified changes in area at fine spatial scales and, where loss was observed, describe the general patterns of temporal decline within each meadow. Our results demonstrate that seagrass meadows dominated by P. australis underwent declines of ~ 2–40% total area at 11 out of 14 study sites between 2009 and 2014. In the iconic Sydney Harbour, our analyses suggest that P. australis meadows are declining at an average rate greater than 10% yr-1, exceeding the global rate of seagrass decline. Highlighting these alarming declines across the study region should serve as means to prioritise management action and review the effectiveness of legislative listing as a method to limit impacts at an ecosystem level.

Seagrass canopies and the performance of acoustic telemetry: implications for the interpretation of fish movements

“…), a species endemic to temperate Australia that forms large meadows within JBMP. P. australis is a long leaved, slowgrowing seagrass of high conservation significance due to population declines and has been listed as endangered at six locations in NSW [28,50].…”

“…The spatial distributions of numerous fish species captured in both recreational and commercial fisheries are linked to seagrass meadows as fish use the habitat for foraging, shelter or as nurseries [13,[24][25][26]. Seagrass meadows, however, are under increasing pressure from anthropogenic activities and have been declining at alarming rates [27,28]. Protecting seagrass meadows is therefore a focus of conservation strategies and fisheries management [29], making them an important system in which to study the movement and behaviour of organisms.…”

Positive Ecological Interactions and the Success of Seagrass Restoration

“…They provide habitat for multiple life stages of many commercially-and recreationally-important fishes, shellfish, and crustaceans, improve water quality, sequester carbon, stabilize sediment, and reduce coastal erosion (Nagelkerken et al, 2000;Jackson et al, 2001;Heck et al, 2003;Orth et al, 2006;Fourqurean et al, 2012;Duarte et al, 2013;James et al, 2019;Lefcheck et al, 2019). However, the total area covered by seagrass is estimated to have declined by 30-60%, including total loss in some places (Evans et al, 2018). Losses of seagrasses have been caused by anthropogenic influences including direct removal during coastal development (e.g., harbors, marinas, and channels), destructive fishing methods (such as trawling), run-off of nutrients and other pollutants from land-based sources, and climate change (Short and Wyllie-Echeverria, 1996;Orth et al, 2006;Hughes et al, 2013;He and Silliman, 2019).…”

Carbon Budget for a Large Drowned River Valley Estuary Adjacent to an Emerging Megacity (Sydney Harbour)

“…The shores of the upper estuaries are lined with areas of vegetated habitat, including mangroves with saltmarsh with seagrass beds found mainly in the shallow bays of the lower estuary. Due to anthropogenic perturbations an increase in organic matter loadings and turbidity have contributed to a 40% decline in the seagrass meadows from 2010 to 2014 within the Sydney Harbour Estuary (Evans et al, ).…”