Coral bleaching events continue to drive the degradation of coral reefs
worldwide, causing a shift in the benthic community from coral- to algae-dominated ecosystems. Critically, this shift may ...decrease the capacity of
degraded coral reef communities to maintain net positive accretion during
warming-driven stress events (e.g., reef-wide coral bleaching). Here we
measured rates of net ecosystem calcification (NEC) and net ecosystem
production (NEP) on a degraded coral reef lagoon community (coral cover
< 10 % and algae cover > 20 %) during a reef-wide
bleaching event in February 2020 at Heron Island on the Great Barrier
Reef. We found that during this bleaching event, rates of NEP and NEC across
replicate transects remained positive and did not change in response to
bleaching. Repeated benthic surveys over a period of 20 d indicated an
increase in the percent area of bleached coral tissue, corroborated by
relatively low Symbiodiniaceae densities (∼ 0.6 × 106 cm−2) and dark-adapted photosynthetic yields in photosystem II of
corals (∼ 0.5) sampled along each transect over this period.
Given that a clear decline in coral health was not reflected in the overall
NEC estimates, it is possible that elevated temperatures in the water column
that compromise coral health enhanced the thermodynamic favorability for
calcification in other ahermatypic benthic calcifiers. These data suggest
that positive NEC on degraded reefs may not equate to the net positive
accretion of a complex, three-dimensional reef structure in a future, warmer
ocean. Critically, our study highlights that if coral cover continues to
decline as predicted, NEC may no longer be an appropriate proxy for reef
growth as the proportion of the NEC signal owed to ahermatypic calcification
increases and coral dominance on the reef decreases.
The degree of philopatry in two new Black‐legged Kittiwake Rissa tridactyla colonies in northeast England was studied for 36 and 16 successive years, respectively. There was a tendency in both ...colonies for the number of recruits to the breeding group to increase with the age (and size) of the colony, but by an average of only one additional recruit each year. In the two new colonies, it was 7 and 9 years respectively before the first young birds bred in their natal colony. By that time, over 100 immigrants had recruited and bred in each colony. Thereafter, new philopatric birds were recorded annually. But these formed only an average of 23% of the recruits during 36 years (1955–90) at North Shields, and 4.2% over 16 years (1991–2006) at Coquet Island. In every year at both colonies the number of new immigrants exceeded the number of philopatric recruits. Males formed 82% of philopatric birds. The low degree of philopatry in the Kittiwake will result in appreciable gene flow between colonies and this is consistent with little geographic variation in the species. Each immigrant has to make a choice of which colony to enter, but the criteria for this selection are not understood. It is argued that many measures of philopatry in birds are biased towards high values because of the greater ease in identifying philopatric individuals. The generalization that there is a high degree of philopatry in colonial seabird species is not justified, as there is considerable variation between species, and possibly between colonies. More estimates based on unbiased data are needed.
Rates of gross primary production (GPP), respiration (R), and net calcification (Gnet) in coral reef sediments are expected to change in response to global warming (and the consequent increase in sea ...surface temperature) and coastal eutrophication (and the subsequent increase in the concentration of organic matter, OM, being filtered by permeable coral reef carbonate sediments). To date, no studies have examined the combined effect of seawater warming and OM enrichment on coral reef carbonate sediment metabolism and dissolution. This study used 22 h in situ benthic chamber incubations to examine the combined effect of temperature (T) and OM, in the form of coral mucus and phytodetritus, on GPP, R, and Gnet in the permeable coral reef carbonate sediments of Heron Island lagoon, Australia. Compared to control incubations, both warming (+2.4 °C) and OM increased R and GPP. Under warmed conditions, R (Q10 = 10.7) was enhanced to a greater extent than GPP (Q10 = 7.3), resulting in a shift to net heterotrophy and net dissolution. Under both phytodetritus and coral mucus treatments, GPP was enhanced to a greater extent than R, resulting in a net increase in GPP / R and Gnet. The combined effect of warming and OM enhanced R and GPP, but the net effect on GPP / R and Gnet was not significantly different from control incubations. These findings show that a shift to net heterotrophy and dissolution due to short-term increases in seawater warming may be countered by a net increase GPP / R and Gnet due to short-term increases in nutrient release from OM.
The design of a pilot symbol for orthogonal frequency-division multiplexing packet detection and synchronization is presented. Algorithms are described for packet detection and for synchronization ...using maximum likelihood estimation of channel frequency offset, phase offset for coherent detection, and subsample timing offset. The efficacy of the detection and synchronization algorithms is demonstrated experimentally.
Ocean acidification (OA) and organic matter (OM) enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a ...future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22‐h incubations in flume aquaria with captive sediment communities to measure the combined effect of elevated pCO2 (representing Ocean Acidification) and particulate organic carbon (representing coastal eutrophication) on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA (pCO2 ~ 1,000 μatm) and OM enrichment (~ +40 μmol C L−1) significantly decreased rates of sediment Gnet by 1.16 and 0.18 mmol CaCO3 m−2 h−1, respectively, but the mechanism behind this decrease differed. The OA‐mediated transition to net dissolution was physiochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Ωarg) of the overlying water column and the physical factors governing the pore water exchange rate with this overlying water column. In contrast, the OM‐mediated decline in Gnet was due to a decline in the overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (−0.09 mmol CaCO3 m−2 h−1 relative to OA alone), but this decrease did not significantly differ from the individual effect of either stressor. In this study OA was the primary driver of future carbonate sediment dissolution, but longer‐term experiments with chronic organic matter enrichment are required.
Key Points
Ocean acidification transitioned the sediments from net calcifying to net dissolving through geochemical pathways
Organic matter enrichment enhanced rates of carbonate dissolution at night through biological pathways
The response of dissolution to a combination of both stressors was additive, but this result did not significantly differ from the effect of OA alone
The visualization of chemical processes that occur in the solid-state is key to the design of new functional materials. One of the challenges in these studies is to monitor the processes across a ...range of timescales in real-time. Here, we present a pump-multiprobe single-crystal X-ray diffraction (SCXRD) technique for studying photoexcited solid-state species with millisecond-to-minute lifetimes. We excite using pulsed LEDs and synchronise to a gated X-ray detector to collect 3D structures with sub-second time resolution while maximising photo-conversion and minimising beam damage. Our implementation provides complete control of the pump-multiprobe sequencing and can access a range of timescales using the same setup. Using LEDs allows variation of the intensity and pulse width and ensures uniform illumination of the crystal, spreading the energy load in time and space. We demonstrate our method by studying the variable-temperature kinetics of photo-activated linkage isomerism in Pd(Bu
dien)(NO
)BPh
single-crystals. We further show that our method extends to following indicative Bragg reflections with a continuous readout Timepix3 detector chip. Our approach is applicable to a range of physical and biological processes that occur on millisecond and slower timescales, which cannot be studied using existing techniques.
The threat represented by ocean acidification (OA) for coral reefs has received considerable attention because of the sensitivity of calcifiers to changing seawater carbonate chemistry. However, most ...studies have focused on the organismic response of calcification to OA, and only a few have addressed community-level effects, or investigated parameters other than calcification, such as photosynthesis. Light (photosynthetically active radiation, PAR) is a driver of biological processes on coral reefs, and the possibility that these processes might be perturbed by OA has important implications for community function. Here we investigate how CO2 enrichment affects the relationships between PAR and community net O2 production (Pnet), and between PAR and community net calcification (Gnet), using experiments on three coral communities constructed to match (i) the back reef of Mo'orea, French Polynesia, (ii) the fore reef of Mo'orea, and (iii) the back reef of O'ahu, Hawaii. The results were used to test the hypothesis that OA affects the relationship between Pnet and Gnet. For the three communities tested, pCO2 did not affect the Pnet–PAR relationship, but it affected the intercept of the hyperbolic tangent curve fitting the Gnet–PAR relationship for both reef communities in Mo'orea (but not in O'ahu). For the three communities, the slopes of the linear relationships between Pnet and Gnet were not affected by OA, although the intercepts were depressed by the inhibitory effect of high pCO2 on Gnet. Our result indicates that OA can modify the balance between net calcification and net photosynthesis of reef communities by depressing community calcification, but without affecting community photosynthesis.
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