Posts Tagged 'South Pacific'

Controls on carbon, nutrient and sediment cycling in a large, semi‐arid estuarine system; Princess Charlotte Bay, Australia

Semi‐arid estuaries are characterized by pronounced seasonal variability, and a functional understanding of these systems requires constraint of coupled biogeochemical processes and relevant temporal and spatial scales. Here we integrate two years of spatial surveys and time‐series measurements to quantify physical, chemical and biological drivers in the largest estuarine system in the Great Barrier Reef region. During wet season, freshwater inputs of nutrients and sediment to estuaries were dominated by flood pulses, whereas carbonate input was also influenced by groundwater discharge. This carbonate input counteracted the minimum buffering zone that would otherwise occur at low salinities, thereby decreasing system‐wide air‐water CO2 fluxes. Sediment resuspension was a major control on the transformation and transport of material over tidal and seasonal scales. During wet season, tidal resuspension of benthic algae in nearshore mixing zones acted as an autotrophic filter, removing most bioavailable nutrients from the brackish plume. During dry season, upstream transport combined with hypersaline conditions trapped material in upper estuaries where denitrification and net heterotrophy were high. However, the role of sediment transport varied depending on tidal asymmetry and density‐driven circulation. Estuarine regions with large intertidal areas were dominated by salt‐flat erosion, which showed a diagenetic signature associated with mid‐Holocene swamp sediments. Tidal resuspension of these organic‐rich sediments appeared to be the dominant control on biogeochemical cycling in coastal waters. This study demonstrates that a holistic understanding of coastal ecosystem connectivity and function requires resolution of both along‐axis and water‐column gradients as well as a range of timescales from tidal to geological cycles.

Continue reading ‘Controls on carbon, nutrient and sediment cycling in a large, semi‐arid estuarine system; Princess Charlotte Bay, Australia’

Estimating relative immediacy of water-related challenges in Small Island Developing States (SIDS) of the Pacific Ocean using AHP modeling

We outline nine water-related challenges faced by the Small Island Developing States (SIDS) of the Pacific Ocean and map them with relevant sustainable development goals (SDGs). The challenges thus identified have been modeled using analytical hierarchy process (AHP) to find out their priority weights. Based on this weightage, the relative immediacy of each of these water-related challenges have been calculated, and classified as high, medium, and low. The findings reveal that the most immediate challenge in terms of their relative immediacy weightage is the ‘rising sea level’. This is followed by ‘low water quality and its availability’, and ‘spread of water-borne and vector-borne diseases’. Other challenges analyzed in this study pertains to overfishing and exploitation of exclusive economic zones; soil erosion and coastal inundation; increase in incidences of natural disasters; coral reef damage and increased ocean acidification; climate refugee; and changing precipitation pattern. This study would be instrumental for policy makers and inter-governmental organizations in directing the resource allocation for adaptation and mitigation efforts in the small islands.

Continue reading ‘Estimating relative immediacy of water-related challenges in Small Island Developing States (SIDS) of the Pacific Ocean using AHP modeling’

Varying reproductive success under ocean warming and acidification across giant kelp (Macrocystis pyrifera) populations

Highlights

• Reproductive response to temperature and pH varied across populations.

• In high temperatures, only low-latitude California populations produced sporophytes.

• In low pH, all but two populations produced more eggs compared to ambient pH.

• Multiple eggs per female was the norm, not the exception.

• Gametophytes from all populations persisted in all treatments.

Abstract

Understanding how climate change may influence ecosystems depends substantially on its effects on foundation species, such as the ecologically important giant kelp (Macrocystis pyrifera). Despite its broad distribution along strong temperature and pH gradients and strong barriers to dispersal, the potential for local adaptation to climate change variables among kelp populations remains poorly understood. We assessed this potential by exposing giant kelp early life stages from genetically disparate populations in Chile and California to current and projected temperature and pH levels in common garden experiments. We observed high resistance at the haploid life stage to elevated temperatures with developmental failure appearing at the egg and sporophyte production stages among Chilean and high-latitude California populations, suggesting a greater vulnerability to climate- or ENSO-driven warming events. Additionally, populations that experience low pH events via strong upwelling, internal waves, or estuarine processes, produced more eggs per female under experimental low-pH conditions, which could increase fertilization success. These results enhance our ability to predict population extinctions and ecosystem range shifts under projected declines in ocean pH and increases in ocean temperature.

Continue reading ‘Varying reproductive success under ocean warming and acidification across giant kelp (Macrocystis pyrifera) populations’

Independent effects of ocean warming versus acidification on the growth, survivorship and physiology of two Acropora corals

Climate change is the greatest threat to coral reef ecosystems. Importantly, gradual changes in seawater chemistry compounds upon increasing temperatures leading to declines in calcification and survivorship of reef-building corals. To assess relative versus synergistic effects of warming versus ocean acidification, Acropora muricata and Acropora hyacinthus were subjected to three temperature treatments (26 °C, 28.5 °C, 31 °C) crossed with three levels of pCO2 (410 μatm, 652 μatm, 934 μatm), representing current, mid and end-of-century scenarios for 12 weeks. Temperature increased gradually in the tanks from 26 °C to target temperatures over 5 weeks. Once stress was evident in the 31 °C (+ 2.5 °C above historical summer max) tanks, water temperature was decreased to normal summertime levels (29 °C) to assess recovery. pCO2 was gradually changed from control values (410 μatm) to target values over a 3 week period where they remained constant until the end of the experiment at 12 weeks. Temperature stress (31 °C) significantly impacted survivorship (90–95% decline), and over the long-term, there was a 50–90% decline in calcification across both coral species. Negative effects of mid and end-of-century pCO2 were largely independent of temperature and caused moderate reductions (36–74%) in calcification rates compared to temperature, over the long-term. Corals that survived temperature stress had higher lipid and protein content, showing that enhanced physiological condition provides an increased capacity to tolerate adverse temperatures. This study demonstrates that given the mortality rates in response to + 2.5 °C temperature stress, warming oceans (as opposed to ocean acidification) throughout the remainder of this century poses the greatest threat to reef-building corals.

Continue reading ‘Independent effects of ocean warming versus acidification on the growth, survivorship and physiology of two Acropora corals’

Variability in the benefits of ocean acidification to photosynthetic rates of macroalgae without CO2-concentrating mechanisms

Increasing concentrations of surface-seawater carbon dioxide (CO2) (ocean acidification) could favour seaweed species that currently are limited for dissolved inorganic carbon (DIC). Among them, those that are unable to use CO2-concentrating mechanisms (CCMs) to actively uptake bicarbonate (HCO3–) across the plasmalemma are most likely to benefit. Here, we assess how the DIC uptake and photosynthetic rates of three rhodophytes without CCMs respond to four seawater CO2 concentrations representing pre-industrial (280 μatm), present-day (400 μatm), representative concentration pathway (RCP) emissions scenario 8.5 2050 (650 μatm) and RCP 8.5 2100 (1000 μatm). We demonstrated that the photosynthetic rates of only one species increase between the preindustrial and end-of-century scenarios, but because of differing photosynthetic quotients (DIC taken up relative to O2 evolved), all three increase their DIC uptake rates from pre-industrial or present-day scenarios to the end-of-century scenario. These variable, but generally beneficial, responses highlight that not all species without CCMs will respond to ocean acidification uniformly. This supports past assessments that, on average, this group will likely benefit from the impacts of ocean acidification. However, more concerted efforts are now required to assess whether similar benefits to photosynthetic rates and DIC uptake are also observed in chlorophytes and ochrophytes without CCMs.

Continue reading ‘Variability in the benefits of ocean acidification to photosynthetic rates of macroalgae without CO2-concentrating mechanisms’

Dynamics of the carbonate system across the Peruvian oxygen minimum zone

The oxygen minimum zone (OMZ) of Peru is recognized as a source of CO2 to the atmosphere due to upwelling that brings water with high concentrations of dissolved inorganic carbon (DIC) to the surface. However, the influence of OMZ dynamics on the carbonate system remains poorly understood given a lack of direct observations. This study examines the influence of a coastal Eastern South Pacific OMZ on carbonate system dynamics based on a multidisciplinary cruise that took place in 2014. During the cruise, onboard DIC and pH measurements were used to estimate pCO2 and to calculate the calcium carbonate saturation state (Ω aragonite and calcite). South of Chimbote (9°S), water stratification decreased and both the oxycline and carbocline moved from 150 m depth to 20–50 m below the surface. The aragonite saturation depth was observed to be close to 50 m. However, values <1.2 were detected close to 20 m along with low pH (minimum of 7.5), high pCO2 (maximum 1,250 μatm), and high DIC concentrations (maximum 2,300 μmol kg−1). These chemical characteristics are shown to be associated with Equatorial Subsurface Water (ESSW). Large spatial variability in surface values was also found. Part of this variability can be attributed to the influence of mesoscale eddies, which can modify the distribution of biogeochemical variables, such as the aragonite saturation horizon, in response to shallower (cyclonic eddies) or deeper (anticyclonic eddies) thermoclines. The analysis of a 21-year (1993–2014) data set of mean sea surface level anomalies (SSHa) derived from altimetry data indicated that a large variance associated with interannual timescales was present near the coast. However, 2014 was characterized by weak Kelvin activity, and physical forcing was more associated with eddy activity. Mesoscale activity modulates the position of the upper boundary of ESSW, which is associated with high DIC and influences the carbocline and aragonite saturation depths. Weighing the relative importance of each individual signal results in a better understanding of the biogeochemical processes present in the area.

Continue reading ‘Dynamics of the carbonate system across the Peruvian oxygen minimum zone’

Upwelling amplifies ocean acidification on the East Australian shelf: implications for marine ecosystems

Frequent upwelling of deep, cold water, rich in dissolved inorganic nutrients and carbon dioxide but low in oxygen concentrations and pH, is well documented in eastern boundary systems. As a consequence, waters in vast areas of the continental shelf can turn corrosive to the mineral aragonite, vital to a number of marine organisms. This phenomenon is projected to become more severe with ongoing ocean acidification. Although upwelling is also known to occur in western boundary systems, the impact on present day aragonite saturation state (Ωarag) is virtually unknown, let alone for the decades to come. Here we identified 32 events during 18 weeks of continuous measurements in Cape Byron Marine Park, Australia, with prolonged drops in ocean temperature of up to 5°C, oxygen concentrations by 34%, pH by 0.12 and Ωarag by 0.9 in a matter of hours. Temperature, salinity and oxygen saturation during these events hint at a water mass from 200 to 250 m depth off the Central East Australian shelf. Extrapolating present day upwelling to a preindustrial setting shows that ongoing ocean acidification has already lead to the crossing of a number of biological and geochemical Ωarag thresholds. The future intensity of these events critically depends on carbon dioxide emission scenario, and might be even more pronounced in the Great Barrier Reef where current day shelf associated waters carry a stronger deep water signal (based on oxygen levels) than at the study location. Finally, the proposed use of artificially upwelled water to cool increasingly temperature-stressed coral reef communities will need to take its unique carbonate chemistry properties into account.

Continue reading ‘Upwelling amplifies ocean acidification on the East Australian shelf: implications for marine ecosystems’


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Ocean acidification in the IPCC AR5 WG II

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