Archive for August, 2020

Ocean acidification has impacted coral growth on the Great Barrier Reef

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 equatorial 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.

Continue reading ‘Ocean acidification has impacted coral growth on the Great Barrier Reef’

Ocean acidification effects on calcification and dissolution in tropical reef macroalgae

Net calcification rates for coral reef and other calcifiers have been shown to decline as ocean acidification (OA) occurs. However, the role of calcium carbonate dissolution in lowering net calcification rates is unclear. The objective of this study was to distinguish OA effects on calcification and dissolution rates in dominant calcifying macroalgae of the Florida Reef Tract, including two rhodophytes (Neogoniolithon strictum, Jania adhaerens) and two chlorophytes (Halimeda scabra, Udotea luna). Two experiments were conducted: (1) to assess the difference in gross (45Ca uptake) versus net (total alkalinity anomaly) calcification rates in the light/dark and (2) to determine dark dissolution (45CaCO3), using pH levels predicted for the year 2100 and ambient pH. At low pH in the light, all species maintained gross calcification rates and most sustained net calcification rates relative to controls. Net calcification rates in the dark were ~84% lower than in the light. In contrast to the light, all species had lower net calcification rates in the dark at low pH with chlorophytes exhibiting net dissolution. These data are supported by the relationship (R2 = 0.82) between increasing total alkalinity and loss of 45Ca from pre-labelled 45CaCO3 thalli at low pH in the dark. Dark dissolution of 45CaCO3-labelled thalli was ~18% higher in chlorophytes than rhodophytes at ambient pH, and ~ twofold higher at low pH. Only Udotea, which exhibited dissolution in the light, also had lower daily calcification rates integrated over 24 h. Thus, if tropical macroalgae can maintain high calcification rates in the light, lower net calcification rates in the dark from dissolution may not compromise daily calcification rates. However, if organismal dissolution in the dark is additive to sedimentary carbonate losses, reef dissolution may be amplified under OA and contribute to erosion of the Florida Reef Tract and other reefs that exhibit net dissolution.

Continue reading ‘Ocean acidification effects on calcification and dissolution in tropical reef macroalgae’

Environmental controls on pteropod ecology and physiology along the Western Antarctic Peninsula

Pteropods (pelagic snails) are ubiquitous zooplankton in the Southern Ocean and abundant along the Western Antarctic Peninsula (WAP), one of the most rapidly warming regions on the planet. They are important prey for higher trophic levels, grazers of phytoplankton, and contribute to particulate organic and inorganic carbon export. Pteropods are heralded as bioindicators of ecosystem health due to the vulnerability of their aragonitic shells under ocean acidification conditions, which could greatly affect their abundances in the future. Despite their importance within Antarctic food webs, few studies have analyzed the effects of climate change on pteropod physiology and biogeography in the Southern Ocean. I utilized zooplankton net tows and sediment trap samples collected as part of the Palmer Antarctica Long Term Ecological Research (PAL LTER) program to determine long-term changes in pteropod biogeography and phenology (life history). I also conducted shipboard experiments on PAL LTER research cruises to analyze the effects of shifting temperature and food conditions on pteropod metabolism. Lastly, to examine WAP pteropod feeding ecology, I utilized high-throughput sequencing techniques and analyzed pteropod gut contents at an unprecedented taxonomic resolution. Pteropod populations along the WAP from 1993-2017 either remained stable (shelled pteropods) or increased (non-shelled pteropods) and were most strongly controlled by La Niña conditions the year prior, which led to warmer, ice-free waters. There was a weak relationship between pteropod abundance and carbonate chemistry, and no detectable long-term trend in carbonate chemistry parameters (i.e., aragonite saturation), thus ocean acidification is not presently a factor influencing WAP pteropod abundance. More open-water areas the year prior also increased growth rates of the shelled pteropod, Limacina helicina antarctica, and caused earlier time of appearance in the PAL LTER sediment trap. There was considerable interannual variability in the time of appearance of a new pteropod cohort, which ranged from year day 22 to 255, but no long-term, directional change in time of appearance or growth rate. The effects of warming seawater temperatures and shifting food availability on L. h. antarctica metabolism revealed that highest respiration and usually highest excretion rates occurred under higher temperatures, but the effect of food concentration was more limited. The proportion of dissolved organic matter to total organic and inorganic dissolved constituents was high and the metabolic ratios of C, N, and P were all below the canonical Redfield ratio, which can directly affect phytoplankton growth and bacterial production in the WAP. Analysis of L. h. antarctica gut contents revealed its microbiome for the first time with Mollicutes bacteria the most abundant prokaryote. Pteropods were mainly herbivorous in summer, consuming predominantly diatoms but also supplementing their diet with microzooplankton such as ciliates. My dissertation shows that pteropods along the WAP are sensitive to changes in the environment from daily to interannual time scales. These insights into the metabolic and ecologic responses of pteropods to ocean variability increase our understanding of the role of zooplankton in biogeochemical cycles and help predict future responses to climate change.

Continue reading ‘Environmental controls on pteropod ecology and physiology along the Western Antarctic Peninsula’

Ocean acidification causing coral ‘osteoporosis’ on iconic reefs

Scientists have long suspected that ocean acidification is affecting corals’ ability to build their skeletons, but it has been challenging to isolate its effect from that of simultaneous warming ocean temperatures, which also influence coral growth. New research from the Woods Hole Oceanographic Institution (WHOI) reveals the distinct impact that ocean acidification is having on coral growth on some of the world’s iconic reefs.

In a paper published Aug. 27, 2020, in the journal Geophysical Research Letters, researchers show a significant reduction in the density of coral skeleton along much of the Great Barrier Reef — the world’s largest coral reef system — and also on two reefs in the South China Sea, which they attribute largely to the increasing acidity of the waters surrounding these reefs since 1950.

“This is the first unambiguous detection and attribution of ocean acidification’s impact on coral growth,” says lead author and WHOI scientist Weifu Guo. “Our study presents strong evidence that 20th century ocean acidification, exacerbated by reef biogeochemical processes, had measurable effects on the growth of a keystone reef-building coral species across the Great Barrier Reef and in the South China Sea. These effects will likely accelerate as ocean acidification progresses over the next several decades.”

Continue reading ‘Ocean acidification causing coral ‘osteoporosis’ on iconic reefs’

Inclusion of uncertainty in the calcium-salinity relationship improves estimates of ocean acidification monitoring data quality

Highlights

• Marine carbonate system program seacarb was extended to include [Ca2+] uncertainty.

• The effect of deviations from the global [Ca2+]-salinity relationship on saturation state (Ω) was quantified.

• Variability associated with coastal systems can reduce data quality, exceeding monitoring guidelines.

• Direct measurement of [Ca2+] will increase ocean acidification monitoring data quality in coastal regions.

Abstract

The effects of Ocean Acidification (OA) and the resulting decrease of CaCO3 saturation state (Ω) on marine organisms and biogeochemistry are observed through regionally dispersed monitoring programs. The standard of data collected by these programs is assessed based on the computed propagated uncertainties of [CO32−], with data quality regulated by thresholds defined by the Global Ocean Acidification Observing Network (GOA-ON). While these thresholds account for the adoption of lower-cost methods and technologies for carbonate parameter analysis (e.g. pH and total alkalinity), the impacts of salinity measurement and calcium concentration uncertainty on data quality are poorly understood. Currently, the publicly available marine carbonate chemistry uncertainty packages do propagate salinity uncertainty, but do not include [Ca2+] uncertainty. In this study, the uncertainty propagation methods in the R-based seacarb package were extended to include [Ca2+] uncertainty, and subsequently employed to examine the effects of uncertainty in salinity and [Ca2+] on carbonate system calculations. The results indicate that underestimation of uncertainty in [Ca2+] is of primary concern in variable coastal waters, where relatively small (<4%) deviations from the global [Ca2+]-salinity relationship leads to GOA-ON's quality standards being exceeded. In contrast, the uncertainty in salinity has a relatively minor impact on uncertainty in [CO32−] and Ω. Given the importance of Ω and its sensitivity to [Ca2+], coastal OA monitoring programs should consider whether their region conforms with the global [Ca2+]-salinity relationship, and if uncertain should directly measure [Ca2+] when calculating Ω.

Continue reading ‘Inclusion of uncertainty in the calcium-salinity relationship improves estimates of ocean acidification monitoring data quality’

Monitoring the Gulf of Maine

The Gulf of Maine is an incredibly useful body of water to study when it comes to exploring the effects of climate change, says Eugen Cotei ’21. “That’s because the waters of the Gulf have warmed faster than 99 percent of the ocean.”

Cotei has been working as a research fellow this summer helping Associate Professor of Earth and Oceanographic Science Michèle LaVigne analyze the conditions in this rapidly warming corner of the Atlantic Ocean. “My goal over the summer has been to validate data collected by the pier sensor at the Coastal Studies Center,” he explained.

“The sensor measures variables like pH, carbon dioxide levels, and dissolved oxygen concentrations in the seawater, all of which are incredibly important in understanding how the dynamic system operates, especially in the context of our changing climate and coastal acidification.” Based at the Schiller Coastal Studies Center less than thirteen miles from campus, Cotei has been collecting water samples from Harpswell Sound at regular intervals during low and high tide. These samples are analyzed and compared with the sensor data, and in theory should be able to validate those data.

Continue reading ‘Monitoring the Gulf of Maine’

The impact of intertidal areas on the carbonate system of the southern North Sea (update)

The coastal ocean is strongly affected by ocean acidification because of its shallow water depths, low volume, and the closeness to terrestrial dynamics. Earlier observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) in the southern part of the North Sea, a northwest European shelf sea, revealed lower acidification effects than expected. It has been assumed that anaerobic degradation and subsequent TA release in the adjacent back-barrier tidal areas (Wadden Sea) in summertime is responsible for this phenomenon. In this study the exchange rates of TA and DIC between the Wadden Sea tidal basins and the North Sea and the consequences for the carbonate system in the German Bight are estimated using a 3D ecosystem model. The aim of this study is to differentiate the various sources contributing to observed high summer TA in the southern North Sea. Measured TA and DIC in the Wadden Sea are considered as model boundary conditions. This procedure acknowledges the dynamic behaviour of the Wadden Sea as an area of effective production and decomposition of organic material. According to the modelling results, 39 Gmol TA yr−1 were exported from the Wadden Sea into the North Sea, which is less than a previous estimate but within a comparable range. The interannual variabilities in TA and DIC, mainly driven by hydrodynamic conditions, were examined for the years 2001–2009. Dynamics in the carbonate system are found to be related to specific weather conditions. The results suggest that the Wadden Sea is an important driver for the carbonate system in the southern North Sea. On average 41 % of TA inventory changes in the German Bight were caused by riverine input, 37 % by net transport from adjacent North Sea sectors, 16 % by Wadden Sea export, and 6 % were caused by internal net production of TA. The dominant role of river input for the TA inventory disappears when focusing on TA concentration changes due to the corresponding freshwater fluxes diluting the marine TA concentrations. The ratio of exported TA versus DIC reflects the dominant underlying biogeochemical processes in the Wadden Sea. Whereas aerobic degradation of organic matter played a key role in the North Frisian Wadden Sea during all seasons of the year, anaerobic degradation of organic matter dominated in the East Frisian Wadden Sea. Despite the scarcity of high-resolution field data, it is shown that anaerobic degradation in the Wadden Sea is one of the main contributors of elevated summer TA values in the southern North Sea.

Continue reading ‘The impact of intertidal areas on the carbonate system of the southern North Sea (update)’

Physiological responses of Skeletonema costatum to the interactions of seawater acidification and combination of photoperiod and temperature

Ocean acidification (OA), which is a major environmental change caused by increasing atmospheric CO2, has considerable influences on marine phytoplankton. But few studies have investigated interactions of OA and seasonal changes in temperature and photoperiod on marine diatoms. In the present study, a marine diatom Skeletonema costatum was cultured under two different CO2 levels (LC, 400 μatm; HC, 1000 μatm) and three different combinations of temperature and photoperiod length (8:16 L:D with 5 ℃, 12:12 L:D with 15 ℃, 16:8 L:D with 25 ℃), simulating different seasons in typical temperate oceans, to investigate the combined effects of these factors. The results showed that specific growth rate of S. costatum increased with increasing temperature and daylength. However, OA showed contrasting effects on growth and photosynthesis under different combinations of temperature and daylength: while positive effects of OA were observed under spring and autumn conditions, it significantly decreased growth (11 %) and photosynthesis (21 %) in winter. In addition, low temperature and short daylength decreased the proteins of PSII (D1, CP47 and RubcL) at ambient pCO2 level, while OA alleviated the negative effect. These data indicated that future ocean acidification may show differential effects on diatoms in different cluster of other factors.

Continue reading ‘Physiological responses of Skeletonema costatum to the interactions of seawater acidification and combination of photoperiod and temperature’

Impact of dust enrichment on Mediterranean plankton communities under present and future conditions of pH and temperature: an experimental overview

In Low Nutrient Low Chlorophyll areas, such as the Mediterranean Sea, atmospheric fluxes represent a considerable external source of nutrients likely supporting primary production especially during stratification periods. These areas are expected to expand in the future due to lower nutrient supply from sub-surface waters caused by enhanced stratification, likely further increasing the role of atmospheric deposition as a source of new nutrients to surface waters. Yet, whether plankton communities will react differently to dust deposition in a warmer and acidified environment remains an open question. The impact of dust deposition both in present and future climate conditions was assessed through three perturbation experiments in the open Mediterranean Sea. Climate reactors (300 L) were filled with surface water collected in the Tyrrhenian Sea, Ionian Sea and in the Algerian basin during a cruise conducted in May/June 2017 in the frame of the PEACETIME project. The experimental protocol comprised two unmodified control tanks, two tanks enriched with a Saharan dust analog and two tanks enriched with the dust analog and maintained under warmer (+3 °C) and acidified (−0.3 pH unit) conditions. Samples for the analysis of an extensive number of biogeochemical parameters and processes were taken over the duration of the experiments (3–4 d). Here, we present the general setup of the experiments and the impacts of dust seeding and/or future climate change scenario on nutrients and biological stocks. Dust addition led to a rapid and maximum input of nitrate whereas phosphate release from the dust analog was much smaller. Our results showed that the impacts of Saharan dust deposition in three different basins of the open Northwestern Mediterranean Sea are at least as strong as those observed previously in coastal waters. However, interestingly, the effects of dust deposition on biological stocks were highly different between the three investigated stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community. Finally, ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage with all groups positively impacted by warming and acidification, suggesting an exacerbation of effects from atmospheric dust deposition in the future.

Continue reading ‘Impact of dust enrichment on Mediterranean plankton communities under present and future conditions of pH and temperature: an experimental overview’

Impact of climate change on the primary production and related biogeochemical cycles in the coastal and sea ice zone of the Southern Ocean

Highlights

• Changes in primary production in the Southern Ocean are analyzed in a comprehensive manner.

• General production increase is the sign of climate change in the Southern Ocean.

• Associated biogeochemical variables show regionally heterogenous signals of climate change.

• Sea ice affecting iron supply and stratification is a primary factor for CCSZ production change.

• Production change in SIZ is sensitive to change in iron supply arising from various mechanisms.

Abstract

Climate change in the Southern Hemisphere has exerted impact on the primary production in the Southern Ocean (SO). Using a recently released reanalysis dataset on global biogeochemistry, a comprehensive analysis was conducted on the complex biogeochemical seasonal cycle and the impact of climate change with a focus in areas within the meridional excursion of the sea ice boundary—coastal and continental shelf zone (CCSZ) and seasonal sea ice zone (SIZ). The seasonal cycles of primary production and related nutrients are closely linked with the seasonal changes in sea ice and sea surface temperatures. As sea ice retreats and allows energy and gas exchange across the sea surface, phytoplankton growth is initiated, consuming accumulated nutrients within the shallow depth of ~40 m. The seasonal evolutions of physical, biological and chemical variables show both spatial and temporal consistency with each other. Climate change has altered the timing and amplitude of the seasonal cycle. While primary production has generally increased along with an intensified uptake of CO2, some areas show a reduction in production (e.g., Prydz Bay, eastern Indian Ocean). In the CCSZ, increased iron utilization and light availability allowed production to be increased. However, the mechanism by which these factors are altered varies from one location to another, including changes in sea ice cover, surface stratification, and downwelling/upwelling. In the SIZ, where iron is generally a limiting factor, iron supply is a key driver of changes in primary production regardless of other nutrients. There is a clear influence of climatic change on the biogeochemical cycle although the signal is still weak.

Continue reading ‘Impact of climate change on the primary production and related biogeochemical cycles in the coastal and sea ice zone of the Southern Ocean’


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