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.
Highlights
More than a billion people live within 50 km of the coast worldwide, exerting substantial pressure on critical ecosystems including seagrasses, coral reefs, mangroves and saltmarshes [1,2,3]
To quantify changes in seagrass cover in meadows dominated by Posidonia australis between 2009 and 2014, aerial imagery was obtained from Nearmap Australia for three sites within each of five highly impacted estuaries: Lake Macquarie, Pittwater, Sydney Harbour, Botany Bay and Port Hacking (Fig 1)
The total area of P. australis within these five meadows was estimated at 5.6 km2 by [25], making up 25.3% of total seagrass species area across all estuaries on the east coast of Australia
Summary
More than a billion people live within 50 km of the coast worldwide, exerting substantial pressure on critical ecosystems including seagrasses, coral reefs, mangroves and saltmarshes [1,2,3]. Seagrasses are amongst the most valuable and threatened ecosystems, together with coral reefs and rainforests [4,5,6], but since 1990, seagrasses have declined globally at the rate of 7% per year [7] These declines have led to a range of conservation efforts [8] including: extinction risk assessment [3], investment in improved water quality [9,10], redesigned boating infrastructure [11], regulation of damaging fishing activities [12] and direct restoration efforts [13]. Development of commercially-available databases of aerial imagery for multiple purposes have since reduced the need for chartering custom flights, reducing the cost of obtaining imagery and enabling the mapping of fine resolution changes in seagrass meadows with less resource investment
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