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PLoS One
2020 Jan 01;1510:e0239136. doi: 10.1371/journal.pone.0239136.
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Remnant kelp bed refugia and future phase-shifts under ocean acidification.
Ling SD
,
Cornwall CE
,
Tilbrook B
,
Hurd CL
.
Abstract
Ocean warming, ocean acidification and overfishing are major threats to the structure and function of marine ecosystems. Driven by increasing anthropogenic emissions of CO2, ocean warming is leading to global redistribution of marine biota and altered ecosystem dynamics, while ocean acidification threatens the ability of calcifying marine organisms to form skeletons due to decline in saturation state of carbonate Ω and pH. In Tasmania, the interaction between overfishing of sea urchin predators and rapid ocean warming has caused a phase-shift from productive kelp beds to overgrazed sea urchin barren grounds, however potential impacts of ocean acidification on this system have not been considered despite this threat for marine ecosystems globally. Here we use automated loggers and point measures of pH, spanning kelp beds and barren grounds, to reveal that kelp beds have the capacity to locally ameliorate effects of ocean acidification, via photosynthetic drawdown of CO2, compared to unvegetated barren grounds. Based on meta-analysis of anticipated declines in physiological performance of grazing urchins to decreasing pH and assumptions of nil adaptation, future projection of OA across kelp-barrens transition zones reveals that kelp beds could act as important pH refugia, with urchins potentially becoming increasingly challenged at distances >40 m from kelp beds. Using spatially explicit simulation of physicochemical feedbacks between grazing urchins and their kelp prey, we show a stable mosaicked expression of kelp patches to emerge on barren grounds. Depending on the adaptative capacity of sea urchins, future declines in pH appear poised to further alter phase-shift dynamics for reef communities; thus, assessing change in spatial-patterning of reef-scapes may indicate cascading ecological impacts of ocean acidification.
Fig 1. Marine climate change trends in eastern Tasmania.(A) Location of study sites at St. Helens in Tasmania, south-eastern Australia; long-term oceanographic station offshore from Maria Island also shown. (B) 70 year trend in August sea temperature in eastern Tasmania as sampled at the Maria Is. oceanographic station (1944–2014), horizontal line is lower temperature threshold for successful larval development of Centrostephanus rodgersii (inset) as spawned during August [28]. (C) Trend in ocean pH as sampled every 2 hours from April 2011 –April 2015 at the Maria Island National Reference Station, which is part of long-term pH decline for this region. Data for (B) and (C) courtesy of the Integrated Marine Observing System/ CSIRO.
Fig 2. Sampling of pH in kelp beds and on sea urchin barrens ground at St. Helens, northeast Tasmania.Automated SeaPhOx sampling within kelp beds (A and C) and on barrens ground (B); panel (D) shows the sea urchin, Centrostephanus rodgersii, grazing at the edge of a kelp bed (Photographic credit: A, C, D Scott Ling; B Christopher Cornwall).
Fig 3. Kelp bed elevation of local pH relative to barrens ground.(A) pH traces from automated loggers in kelp beds (green) and urchin barrens (grey) at St. Helens Island (SHI); for shallow kelp beds (LHS plot, from 29th to 31st Oct 2013; see legend), and for deeper (16 m depth) kelp beds and barrens ground (RHS plot, from 5th June to 7th Aug 2014); temperature traces in kelp beds and barrens (navy blue and red, respectively) and open ocean pH (blue) are also shown, see legend. (B) Composite plot of mean diurnal pH for shallow kelp beds (Oct 2013), deep kelp beds (June-Aug 2014), regional open ocean surface pH (June-Aug 2014 at Maria Island (MI)), and urchin barrens (June-Aug 2014); see S3 Fig for seaonality in regional open ocean surface pH and temperature in S1 File.
Fig 4. Spatial patterns in pH across kelp bed/ barren ground interfaces.(A) pH within shallow kelp beds and on sea urchin barrens in intermediate and deep reefs at St. Helens Island during Aug 2014, data are means of n = 6 replicate bottle samples (note that barrens are not currently found on shallow reefs at this site and at this depth generally in eastern Tasmania); (B) pH within kelp beds, barrens edge and barrens interior at St. Helens Island and Sloop Rock (St. Helens, north eastern Tasmania) during Oct 2013, data are means of n = 6 replicate bottle samples. Different letters indicate significantly different pH between habitats (see S2 Table for analyses of variance in S1 File).
Fig 5. Modelling future phase-shifts under different scenarios of adaptation by sea urchins to ocean acidification.(A) Spatial dynamics of pH proximate to kelp beds in 2014 (means ± SE, n = 4 per sample position) and projected under OA business as usual (RCP8.5) for years 2050 and 2100; dashed horizontal lines indicate thresholds for compromised performance of sea urchins at 25% (light grey) and 50% (black) (S5 Fig in S1 File); numbers in parentheses indicate proportional contribution by each life-history stage to the compounded decline in sea urchin performance across life-history stages. (B) Conceptualization of present-day (i) and future reef-scapes under OA (green cells “0” = kelp; pink cells “1” = urchin barren); (i) maintenance of present-day configuration due to adaptation by urchins, i.e. extensive barrens and narrow fringing kelp beds in the shallows; (ii) emergent mosaicked kelp/ barrens future simulated by cellular automaton of barrens persistence within 40 m of kelp refuge with nil adaptation by urchins (from 4A.); (iii) extensive kelp beds due to nil adaptation by urchins and insufficient kelp refuge from OA. Simulated reef-scapes run alongshore in the direction of the y-axis; and offshore in the direction of the x-axis; and are a total of 640 m by 640 m with each cell 40 m by 40 m. Inset image shows urchin at the base of a remnant kelp patch at St. Helens Island study site.
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