The short-term combined effects of temperature and organic matter enrichment on permeable coral reef carbonate sediment metabolism and dissolution

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Shallow, permeable calcium carbonate (CaCO 3 ) sediments make up a large proportion of the benthic cover on coral reefs and account for a large fraction of the standing stock of CaCO 3 . There have been a number of laboratory, mesocosm, and in situ studies examining shallow sediment metabolism and dissolution, but none of these have considered seasonal variability. Advective benthic chambers were used to measure in situ net community calcification (NCC) rates of CaCO 3 sediments on Heron Island, Australia (Great Barrier Reef) over an annual cycle. Sediments were, on average, net precipitating during the day and net dissolving at night throughout the year. Night dissolution rates (−NCC NIGHT ) were highest in the austral autumn and lowest in the austral winter driven by changes in respiration (R) and to a lesser extent temperature and Ω arag /pH. Similarly, precipitation during the day (+NCC DAY ) was highest in March and lowest in winter, driven primarily by benthic net primary production (NPP) and temperature. On average, sediments were net precipitating over a diel cycle (NCC 24h ) but shifted to net dissolving in July and December. This shift was largely caused by the differential effects of seasonal cycles in organic metabolism and carbonate chemistry on NCC DAY and NCC NIGHT . The results from this study highlight the large variability in sediment CaCO 3 dynamics and the need to include repeated measurements over different months and seasons, particularly in shallow reef systems that can experience large swings in light, temperature, and carbonate chemistry.

Section: Discussioncontrasting

confidence: 99%

Section: Discussionsupporting

confidence: 86%

Section: Discussionmentioning

confidence: 88%

Section: Discussionmentioning

confidence: 94%

mentioning

confidence: 99%

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Seasonal variability of calcium carbonate precipitation and dissolution in shallow coral reef sediments

Stoltenberg

Schulz

Cyronak

et al. 2019

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Habitat‐specific biogenic production and erosion influences net framework and sediment coral reef carbonate budgets

Brown

Bender-Champ

Achlatis

et al. 2020

Carbonate budgets are increasingly being used as a key metric to establish reef condition. To better understand spatial variations in framework and sediment net carbonate budgets, we quantified biogenic carbonate production, erosion, and dissolution within and between five distinct geomorphological habitats of Heron Reef on the southern Great Barrier Reef. The protected reef slope had the greatest estimated net framework carbonate budget (22.6 kgCaCO 3 m −2 yr −1 AE 2.4 SE), driven by abundant, fast-growing acroporid corals coupled with low levels of macro-and micro-bioerosion. The estimate of the exposed reef slope was significantly lower due to localized damage from a single tropical cyclone that occurred 7 years prior to this study (9.7 kgCaCO 3 m −2 yr −1 AE 2.8 SE). Within the extensive lagoon, net framework carbonate budgets ranged from 0.24 kgCaCO 3 m −2 yr −1 (AE 0.1 SE) to 3.0 kgCaCO 3 m −2 yr −1 (AE 0.7 SE). The greatest net sediment carbonate budget was estimated within the reef crest (6.0 kgCaCO 3 m −2 yr −1 AE 1.1 SE) and the lowest in the shallow lagoon (1.2 kgCaCO 3 m −2 yr −1 AE 0.2 SE). Chemical dissolution of the sediments exhibited spatial variability, with reef crest and reef flat sediments in a state of net production. Considering the area of each habitat, the net reef framework and sediment budgets across Heron Reef were 4.06 kgCaCO 3 m −2 yr −1 and 2.82 kgCaCO 3 m −2 yr −1 , respectively. The results of this study improve our understanding of spatial variability in carbonate production and bioerosion and provide a comprehensive reef-scale carbonate budget for a relatively undisturbed coral reef ecosystem.

Ocean acidification may mitigate negative effects of warming on carbon burial potential in subtidal unvegetated estuarine sediments

Simone

Schulz

Eyre

et al. 2021

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Estuarine sediments make an important contribution to the global carbon cycle, but we do not know how this will change under a future climate, which is expected to have lower pH oceans and frequent high‐temperature days. Six combinations of warming and partial pressures of CO2 (pCO2) were chosen to investigate the combined and individual effects of short‐term pressures on the diel metabolic response of shallow unvegetated sediments ex‐situ. Whereas warming significantly increased respiration, making sediments more heterotrophic, high‐pCO2 increased net primary productivity, resulting in less heterotrophic sediments. As a result, warming decreased the carbon burial potential of estuarine sediments and high‐pCO2 had the opposite effect. High‐pCO2 mitigates the negative effects of warming on benthic metabolism under the combined scenario, with carbon burial similar to that expected under high‐pCO2 conditions alone. Climate scenarios also changed the diurnal pCO2 variation, with ranges increasing by 33% with warming, and almost doubling under high‐pCO2 conditions. An additive response in pCO2 variability was observed under the combined scenario, increasing to 2.3× the current diel‐pCO2 range, highlighting the reduced buffering capacity of the water associated with a high CO2 climate. Future carbon burial and export under increased frequencies of unseasonably warm days projected for mid and end of century (30% and 50% of days‐per‐year, respectively) were estimated with and without ocean acidification. By 2100, warming alone could decrease annual estuarine sediment burial potential by 25%. However, ocean acidification could mitigate the negative effects of more frequent high‐temperature days and increase carbon burial potential over current conditions by ~18%.