Coastal marine heatwaves have destructive and lasting impacts on foundational species 13 and are increasing in frequency, duration, and magnitude. High atmospheric temperatures are 14 often associated with marine heatwaves (MHW) which are defined as 5-days of water 15 temperatures above a seasonally varying 90th percentile threshold. In this study we consider the 16 prevalence of MHW propagation into surficial sediments to cause sediment heatwaves (SHW). 17 Within a shallow, subtidal seagrass meadow in Virginia, USA, sediment temperature was 18 measured at hourly intervals at a depth of 5 cm between June 2020-October 2022 at the meadow 19 edge and central meadow interior. The observed sediment temperature, along with a 29-year 20 record of water temperature and water level was used to develop a sediment temperature model 21 for each location. Modeled sediment temperatures were used to identify sediment heatwaves that 22 may thermally stress belowground seagrass. At both meadow locations, sediment heatwave 23 frequency increased at a rate twice that of MHWs in the average global open ocean, coinciding 24 with a 172% increase in the annual number of SHW days, from 11 to 30 days year-1 between 25 1994-2022. Sediment heatwaves at both meadow locations co-occurred with a MHW 79-81% of 26 the time, with nearly all SHWs having a zero day lag. The top 10% most extreme MHWs and 27 SHWs occurred between November and April when thermal stress to seagrass was unlikely. In 28 June 2015 a SHW co-occurred with an anomalously long duration MHW that was associated 29 with a 90% decline in seagrass from this system, suggesting that SHWs may have contributed to 30 the observed seagrass loss. These results document heatwave propagation across the pelagic-31 sediment interface which likely occur broadly in shallow systems with impacts to critical coastal 32 ecosystem processes and species dynamics.
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Effects of temperature on the behaviour and metabolism of an intertidal foraminifera and consequences for benthic ecosystem functioning
Abstract Heatwaves have increased in intensity, duration and frequency over the last decades due to climate change. Intertidal species, living in a highly variable environment, are likely to be exposed to such heatwaves since they can be emerged for more than 6 h during a tidal cycle. Little is known, however, on how temperature affects species traits (e.g. locomotion and behaviour) of slow-moving organisms such as benthic foraminifera (single-celled protists), which abound in marine sediments. Here, we examine how temperature influences motion-behaviour and metabolic traits of the dominant temperate foraminifera Haynesina germanica by exposing individuals to usual (6, 12, 18, 24, 30 °C) and extreme (high; i.e. 32, 34, 36 °C) temperature regimes. Our results show that individuals reduced their activity by up to 80% under high temperature regimes whereas they remained active under the temperatures they usually experience in the field. When exposed to a hyper-thermic stress (i.e. 36 °C), all individuals remained burrowed and the photosynthetic activity of their sequestered chloroplasts significantly decreased. Recovery experiments subsequently revealed that individuals initially exposed to a high thermal regime partially recovered when the hyper-thermic stress ceased. H. germanica contribution to surface sediment reworking substantially diminished from 10 mm 3 indiv −1 day −1 (usual temperature) to 0 mm 3 indiv −1 day −1 when individuals were exposed to high temperature regimes (i.e. above 32 °C). Given their role in sediment reworking and organic matter remineralisation, our results suggest that heatwaves may have profound long-lasting effects on the functioning of intertidal muddy ecosystems and some key biogeochemical cycles.
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- Award ID(s):
- 1657887
- NSF-PAR ID:
- 10339628
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Variable temperature regimes that expose corals to sublethal heat stress have been recognized as a mechanism to increase coral thermal tolerance and lessen coral bleaching. However, there is a need to better understand which thermal regimes maximize coral stress hardening. Here, standardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (
Acropora ,Pocillopora ,Porites ) originating from six reef sites representing an increasing gradient of annual mean diel temperature fluctuations of 1–3 °C day−1. Bleaching severity and dark-acclimated photochemical yield (i.e.,F v/F m) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3 °C. The greatest thermal tolerance (i.e.,F v/F meffective dose 50) was found at the site with intermediate mean diel temperature variability (2.2 °C day−1), suggesting there is an optimal priming exposure that leads to maximal thermal tolerance. Interestingly,Acropora andPocillopora originating from the least thermally variable regimes (< 1.3 °C day−1) had lower thermal tolerance than corals from the most variable sites (> 2.8 °C day−1), whereas the opposite was true forPorites , suggesting divergent responses across taxa. Remarkably, comparisons across global studies revealed that the range in coral thermal tolerance uncovered in this study across a single reef (< 5 km) were as large as differences observed across vast latitudinal gradients (300–900 km). This finding indicates that local gene flow could improve thermal tolerance between habitats. However, as climate change continues, exposure to intensifying marine heatwaves is already compromising thermal priming as a mechanism to enhance coral thermal tolerance and bleaching resistance. -
Abstract. Although the depth of bioturbation can be estimated on the basisof ichnofabric, the timescale of sediment mixing (reworking) and irrigation(ventilation) by burrowers that affects carbonate preservation andbiogeochemical cycles is difficult to estimate in the stratigraphic record.However, pyrite linings on the interior of shells can be a signature of slowand shallow irrigation. They indicate that shells of molluscs initiallyinhabiting oxic sediment pockets were immediately and permanentlysequestered in reduced, iron-rich microenvironments within the mixed layer.Molluscan biomass-stimulated sulfate reduction and pyrite precipitation wasconfined to the location of decay under such conditions. A high abundance ofpyrite-lined shells in the stratigraphic record can thus be diagnostic oflimited exposure of organic tissues to O2 even when the seafloor isinhabited by abundant infauna disrupting and age-homogenizing sedimentaryfabric as in the present-day northern Adriatic Sea. Here, we reconstructthis sequestration pathway characterized by slow irrigation (1) by assessingpreservation and postmortem ages of pyrite-lined shells of theshallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) byevaluating whether an independently documented decline in the depth ofmixing, driven by high frequency of seasonal hypoxia during the 20thcentury, affected the frequency of pyrite-lined shells in the stratigraphicrecord of the northern Adriatic Sea. First, at prodelta sites with a highsedimentation rate, linings of pyrite framboids form rapidly in the upper5–10 cm as they already appear in the interiors of shells younger than 10 yearsand occur preferentially in well-preserved and articulated shells withperiostracum. Second, increments deposited in the early 20th centurycontain < 20 % of shells lined with pyrite at the Po prodelta and30 %–40 % at the Isonzo prodelta, whereas the late 20th centuryincrements possess 50 %–80 % of shells lined with pyrite at both locations.At sites with slow sedimentation rate, the frequency of pyrite linings islow (< 10 %–20 %). Surface sediments remained well mixed by depositand detritus feeders even in the late 20th century, thus maintainingthe suboxic zone with dissolved iron. The upcore increase in the frequencyof pyrite-lined shells thus indicates that the oxycline depth was reducedand bioirrigation rates declined during the 20th century. Wehypothesize that the permanent preservation of pyrite linings within theshells of V. gibba in the subsurface stratigraphic record was enabled by slowrecovery of infaunal communities from seasonal hypoxic events, leading tothe dominance of surficial sediment modifiers with low irrigation potential.The presence of very young and well-preserved pyrite-lined valves in theuppermost zones of the mixed layer indicates that rapid obrution by episodicsediment deposition is not needed for preservation of pyrite linings whensediment irrigation is transient and background sedimentation rates arenot low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organismsdie at their living position within the sediment. Abundance ofwell-preserved shells lined by pyrite exceeding ∼ 10 % perassemblage in apparently well-mixed sediments in the deep-time stratigraphicrecord can be an indicator of inefficient bioirrigation. Fine-grainedprodelta sediments in the northern Adriatic Sea deposited since themid-20th century, with high preservation potential of reducedmicroenvironments formed within a mixed layer, can represent taphonomic andearly diagenetic analogues of deep-time skeletal assemblages with pyritelinings.more » « less
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Abstract Global warming is resulting in unprecedented levels of coral mortality due to mass bleaching events and, more recently, marine heatwaves, where rapid increases in seawater temperature cause mortality within days. Here, we compare the response of a ubiquitous scleractinian coral,
Stylophora pistillata , from the northern Red Sea to acute (7 h) and chronic (7–11 d) thermal stress events that include temperature treatments of 27°C (i.e., the local maximum monthly mean), 29.5°C, 32°C, and 34.5°C, and assess recovery of the corals following exposure. Overall,S. pistillata exhibited remarkably similar responses to acute and chronic thermal stress, responding primarily to the temperature treatment rather than duration or heating rate. Additionally, corals displayed an exceptionally high thermal tolerance, maintaining their physiological performance and suffering little to no loss of algal symbionts or chlorophylla up to 32°C, before the host suffered from rapid tissue necrosis and mortality at 34.5°C. While there was some variability in physiological response metrics, photosynthetic efficiency measurements (i.e., maximum quantum yieldFv /Fm ) accurately reflected the overall physiological response patterns, with these measurements used to produce theFv /Fm effective dose (ED50) metric as a proxy for the thermal tolerance of corals. This approach produced similar ED50values for the acute and chronic experiments (34.47°C vs. 33.81°C), highlighting the potential for acute thermal assays with measurements ofFv /Fm as a systematic and standardized approach to quantitively compare the upper thermal limits of reef‐building corals using a portable experimental system. -
Increasing ocean temperatures threaten coral reefs globally, but corals residing in habitats that experience high thermal variability are thought to be better adapted to survive climate-induced heat stress. Here, we used long-term ecological observations and in situ temperature data from Heron Island, southern Great Barrier Reef to investigate how temperature dynamics within various thermally variable vs. thermally stable reef habitats change during a marine heatwave and the resulting consequences for coral community survival. During the heatwave, thermally variable habitats experienced larger surges in daily mean and maxima temperatures compared to stable sites, including extreme hourly incursions up to 36.5 °C. The disproportionate increase in heat stress in variable habitats corresponded with greater subsequent declines in hard coral cover, including a three-times greater decline within the thermally variable Reef Flat (70%) and Deep Lagoon (83%) than within thermally stable habitats along sheltered and exposed areas of the reef slope (0.3–19%). Interestingly, the thermally variable Reef Crest experienced comparatively small declines (26%), avoiding the most severe tidal ponding and resultant heat stress likely due to proximity to the open ocean equating to lower seawater residence times, greater mixing, and/or increased flow. These results highlight that variable thermal regimes, and any acclimatization or adaptation to elevated temperatures that may lead to, do not necessarily equate to protection against bleaching and mortality during marine heatwaves. Instead, thermally stable habitats that have greater seawater exchange with the open ocean may offer the most protection to corals during the severe marine heatwaves that accompany a changing climate.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41598-021-83311-z
