Changes in disturbance regimes due to climate change are increasingly challenging the capacity of ecosystems to absorb recurrent shocks and reassemble afterwards, escalating the risk of widespread ...ecological collapse of current ecosystems and the emergence of novel assemblages
. In marine systems, the production of larvae and recruitment of functionally important species are fundamental processes for rebuilding depleted adult populations, maintaining resilience and avoiding regime shifts in the face of rising environmental pressures
. Here we document a regional-scale shift in stock-recruitment relationships of corals along the Great Barrier Reef-the world's largest coral reef system-following unprecedented back-to-back mass bleaching events caused by global warming. As a consequence of mass mortality of adult brood stock in 2016 and 2017 owing to heat stress
, the amount of larval recruitment declined in 2018 by 89% compared to historical levels. For the first time, brooding pocilloporids replaced spawning acroporids as the dominant taxon in the depleted recruitment pool. The collapse in stock-recruitment relationships indicates that the low resistance of adult brood stocks to repeated episodes of coral bleaching is inexorably tied to an impaired capacity for recovery, which highlights the multifaceted processes that underlie the global decline of coral reefs. The extent to which the Great Barrier Reef will be able to recover from the collapse in stock-recruitment relationships remains uncertain, given the projected increased frequency of extreme climate events over the next two decades
.
Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience ...of ecosystems and the people who depend on them
. Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 2016
, corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3-4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world's largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems
, by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.
Nature-based coastal defense schemes commonly value bivalve reefs for i) reducing coastal erosion in the intertidal and for ii) forming fringing reefs near salt marsh edges to protect them against ...lateral retreat. The capacity for a reef to reduce erosion increases at a higher position in the tidal frame as the lower over-lying water level magnifies the influence of the reef on wave attenuation. Unfortunately, ecological constraints on reef development typically limit their practical application in coastal protection schemes to the lower intertidal, as bivalves grow best with long inundation times. In micro-tidal areas this is a lesser problem, given the close proximity of lower and upper intertidal ecosystems in space. By contrast, in meso- and macro-tidal estuaries, bivalve reefs tend to form hundreds of meters away from existing marshes, nullifying any wave-protective benefits. In this study, we produce evidence that with the assistance of management measures, widespread reef formation is possible on open mudflats, including bordering the marsh edge in meso- and macro-tidal estuaries, where natural reef formation is normally strongly limited.
In four locations throughout the meso- to macro-tidal Dutch Scheldt estuary, we observed the presence of individuals of two major intertidal reef-forming bivalves, Pacific oysters (Crassostrea gigas) and blue mussels (Mytilus edulis), within low-lying Spartina anglica-dominated marshes. As these communities lie well outside of the expected range of reef formation, this observation suggests the existence of mechanisms that extend the habitable range of these bivalves. In a series of field experiments, we first demonstrate how the stabilization of shell-substrate within the marsh promotes successful establishment and adult survival. Secondly, by placing artificial stable substrate in transects from the subtidal up to the marsh edge, we demonstrate that bivalve establishment is possible throughout a much larger range of the intertidal than where natural reefs occur. The effectiveness of stable substrate in stimulating bivalve establishment is likely a consequence of bridging size-dependent thresholds that limit the effective range for natural reef formation on tidal flats. The success of this approach is tempered by a consistent decrease in reef size and growth at higher elevations, suggesting that the optimal reef position for utility in coastal defense lies at an intermediate tidal position, well above the observed range of natural occurrence, but below the maximum achievable upper limit of reef formation. Together this work provides a pathway forward concerning how artificial reefs may be fostered to increase their utility as a nature-based flood defense measure.
•Mussels and oysters found to occur in upper intertidal underneath salt marsh canopy.•Shell-substrate stabilization in marsh may benefit juvenile bivalve establishment.•High oyster survival in marsh due to reduction in hydrodynamic disturbance•Artificial structures mimicking marsh-effects create reefs throughout intertidal.•Reduced reef size in high intertidal signals trade-off in coastal protection design.
By adding coral genes to a nonmineralizing relative, team aims to show how reefs are built
By adding coral genes to a nonmineralizing relative, team aims to show how reefs are built
Biotic connectivity between ecosystems can provide major transport of organic matter and nutrients, influencing ecosystem structure and productivity
, yet the implications are poorly understood owing ...to human disruptions of natural flows
. When abundant, seabirds feeding in the open ocean transport large quantities of nutrients onto islands, enhancing the productivity of island fauna and flora
. Whether leaching of these nutrients back into the sea influences the productivity, structure and functioning of adjacent coral reef ecosystems is not known. Here we address this question using a rare natural experiment in the Chagos Archipelago, in which some islands are rat-infested and others are rat-free. We found that seabird densities and nitrogen deposition rates are 760 and 251 times higher, respectively, on islands where humans have not introduced rats. Consequently, rat-free islands had substantially higher nitrogen stable isotope (δ
N) values in soils and shrubs, reflecting pelagic nutrient sources. These higher values of δ
N were also apparent in macroalgae, filter-feeding sponges, turf algae and fish on adjacent coral reefs. Herbivorous damselfish on reefs adjacent to the rat-free islands grew faster, and fish communities had higher biomass across trophic feeding groups, with 48% greater overall biomass. Rates of two critical ecosystem functions, grazing and bioerosion, were 3.2 and 3.8 times higher, respectively, adjacent to rat-free islands. Collectively, these results reveal how rat introductions disrupt nutrient flows among pelagic, island and coral reef ecosystems. Thus, rat eradication on oceanic islands should be a high conservation priority as it is likely to benefit terrestrial ecosystems and enhance coral reef productivity and functioning by restoring seabird-derived nutrient subsidies from large areas of ocean.
Declining coral cover and loss of structural complexity are widely reported on today's coral reefs. While coral loss frequently triggers changes in coral reef fish assemblage structure, the ...ecosystem‐scale consequences of these changes are poorly known.
Here we evaluate how four metrics of energy flow and storage that underscore a critical coral reef function, consumer biomass production, respond to severe coral loss on a coral reef in the northern Great Barrier Reef, Australia.
We compared fish and benthic surveys at Lizard Island from 2003 to 2004 with surveys in 2018 using an individual‐level modelling approach that integrates growth and mortality coefficients to estimate community‐level standing biomass, productivity, consumed biomass and turnover.
In the study period, coral cover declined by 72%–83% in forereef zones while turf cover increased by 18%–100% across all zones. Reef fish assemblages, in turn, responded with a 71% increase in standing biomass, 41% in productivity and 37% in consumed biomass, mainly driven by nominally‐herbivorous fishes (Labridae—Scarini, Acanthuridae and Siganidae). By contrast, biomass turnover rates declined by 19%.
Our findings suggest that coral loss can drive energetic shifts on coral reefs, leading to more productive, but slower paced reef fish assemblages. Although the observed build‐up of biomass may appear positive, the decreased turnover rates indicate that the system is unable to maintain biomass replacement levels. This suggests that the enhanced productivity that accompanied coral loss may be driven by storage effects from the somatic growth of individuals already present, questioning the temporal stability of these changes to coral reef ecosystem functioning.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
Ocean acidification (OA) reduces the concentration of seawater carbonate ions that stony corals need to produce their calcium carbonate skeletons and is considered a significant threat to the ...functional integrity of coral reef ecosystems. However, detection and attribution of OA impact on corals in nature are confounded by concurrent environmental changes, including ocean warming. Here we use a numerical model to isolate the effects of OA and temperature and show that OA alone has caused 13 ± 3% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950. This OA‐induced thinning of coral skeletons, also evident in Porites from the South China Sea but not in the central Pacific, reflects enhanced acidification of reef water relative to the surrounding open ocean. Our finding reinforces concerns that even corals that might survive multiple heatwaves are structurally weakened and increasingly vulnerable to the compounding effects of climate change
Plain Language Summary
Measurable anthropogenic‐induced acidification of the oceans (OA) has occurred over the last four decades. But its impact on coral reef ecosystems, such as coral calcification, has yet to be unambiguously demonstrated. This problem with detection and attribution of OA impacts is due, in large part, to the fact that multiple co‐varying environmental and biological factors influence coral growth at the same time, and our inability to deconvolve them. Here, we use a numerical model of coral growth to isolate the contributions of ocean acidification to long coral growth timeseries generated on multiple Indo‐Pacific reefs over the 20th century. We show that ocean acidification has had a significant negative impact on skeletal growth of a keystone reef‐building genus across the Great Barrier Reef and in the South China Sea, where the rate of reef acidification outpaces that of the surrounding open ocean. Conversely, the OA‐induced thinning of coral skeletons observed on these reefs has not yet affected corals in the central Pacific, where the rates of reef acidification have been lower. Nevertheless, as ocean acidification accelerates over the next few decades, even these reefs will be affected, resulting in a measurable weakening of coral reef structures across the global tropics.
Key Points
Numerical model of coral growth isolates the respective impacts of ocean acidification and ocean warming on coral growth
Ocean acidification has caused ~13% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950
OA‐induced thinning of coral skeletons reflects enhanced acidification of reef water relative to the surrounding open ocean
Investing in what matters most Steven Mana’oakamai Johnson
Science (American Association for the Advancement of Science),
04/2022, Volume:
376, Issue:
6589
Journal Article
Peer reviewed
Faced with a family crisis, a marine scientist finds parallels with Earth’s imperiled coral reefs
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