Posts Tagged 'biogeochemistry'

Net community metabolism and seawater carbonate chemistry scale non-intuitively with coral cover

Coral cover and reef health have been declining globally as reefs face local and global stressors including higher temperature and ocean acidification (OA). Ocean warming and acidification will alter rates of benthic reef metabolism (i.e., primary production, respiration, calcification, and CaCO3 dissolution), but our understanding of community and ecosystem level responses is limited in terms of functional, spatial, and temporal scales. Furthermore, dramatic changes in coral cover and benthic metabolism could alter seawater carbonate chemistry on coral reefs, locally alleviating or exacerbating OA. This study examines how benthic metabolic rates scale with changing coral cover (0-100%), and the subsequent influence of these coral communities on seawater carbonate chemistry based on mesocosm experiments in Bermuda and Hawaii. In Bermuda, no significant differences in benthic metabolism or seawater carbonate chemistry were observed for low (40%) and high (80%) coral cover due to large variability within treatments. In contrast, significant differences were detected between treatments in Hawaii with benthic metabolic rates increasing with increasing coral cover. Observed increases in daily net community calcification and nighttime net respiration scaled proportionally with coral cover. This was not true for daytime net community organic carbon production rates, which increased the most between 0 to 20% coral cover and then less so between 20% to 100%. These differences in scaling resulted in larger diel variability in seawater carbonate chemistry as coral cover increased. To place the results of the mesocosm experiments into a broader context, in situ seawater carbon dioxide (CO2) at three reef sites in Bermuda and Hawaii were also evaluated; reefs with higher coral cover experienced a greater range of diel CO2 levels, complementing the mesocosm results. The results from this study highlight the need to consider the natural complexity of reefs and additional biological and physical factors that influence seawater carbonate chemistry on larger spatial and longer temporal scales. Coordinated efforts combining various research approaches (e.g. experiments, field studies, and models) will be required to better understand how benthic metabolism integrates across functional, spatial, and temporal scales, and for making predictions on how coral reefs will respond to climate change.

Continue reading ‘Net community metabolism and seawater carbonate chemistry scale non-intuitively with coral cover’

Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions (update)

In an experimental assessment of the potential impact of Arctic Ocean acidification on seasonal phytoplankton blooms and associated dimethyl sulfide (DMS) dynamics, we incubated water from Baffin Bay under conditions representing an acidified Arctic Ocean. Using two light regimes simulating under-ice or subsurface chlorophyll maxima (low light; low PAR and no UVB) and ice-free (high light; high PAR + UVA + UVB) conditions, water collected at 38 m was exposed over 9 days to 6 levels of decreasing pH from 8.1 to 7.2. A phytoplankton bloom dominated by the centric diatoms Chaetoceros spp. reaching up to 7.5 µg chlorophyll a L−1 took place in all experimental bags. Total dimethylsulfoniopropionate (DMSPT) and DMS concentrations reached 155 and 19 nmol L−1, respectively. The sharp increase in DMSPT and DMS concentrations coincided with the exhaustion of NO3− in most microcosms, suggesting that nutrient stress stimulated DMS(P) synthesis by the diatom community. Under both light regimes, chlorophyll a and DMS concentrations decreased linearly with increasing proton concentration at all pH levels tested. Concentrations of DMSPT also decreased but only under high light and over a smaller pH range (from 8.1 to 7.6). In contrast to nano-phytoplankton (2–20 µm), pico-phytoplankton ( ≤  2 µm) was stimulated by the decreasing pH. We furthermore observed no significant difference between the two light regimes tested in term of chlorophyll a, phytoplankton abundance and taxonomy, and DMSP and DMS net concentrations. These results show that ocean acidification could significantly decrease the algal biomass and inhibit DMS production during the seasonal phytoplankton bloom in the Arctic, with possible consequences for the regional climate.

Continue reading ‘Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions (update)’

Response of export production and dissolved oxygen concentrations in oxygen minimum zones to pCO2 and temperature stabilization scenarios in the biogeochemical model HAMOCC 2.0 (update)

Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO2 radiative forcing. Atmospheric pCO2 is increased at a rate of 1 % annually and the model is stabilized at 2 ×, 4 ×, 6  ×, and 8 × preindustrial pCO2 levels. With an increase in CO2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the OMZs within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction in POC export production, vertical POC flux, and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600–800 m. In contrast, the vertical expansion of the OMZs within the Atlantic is linked to increases POC flux as well as changes in oxygen solubility with increasing seawater temperature. Changes in total organic carbon and increase sea surface temperature (SST) also lead to the formation of a new OMZ in the western subtropical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of  ≤  50 µmol kg−1 throughout the equatorial Pacific Ocean at 4 times preindustrial pCO2. Total ocean volume with dissolved oxygen concentrations of  ≤  50 µmol kg−1 increases by 2.4, 5.0, and 10.5 % for the 2 ×, 4 ×, and 8 × CO2 simulations, respectively.

Continue reading ‘Response of export production and dissolved oxygen concentrations in oxygen minimum zones to pCO2 and temperature stabilization scenarios in the biogeochemical model HAMOCC 2.0 (update)’

Spatiotemporal assessment of CO2–carbonic acid system dynamics in a pristine coral reef ecosystem, French Frigate Shoals, Northwestern Hawaiian Islands

Observations of surface seawater fugacity of carbon dioxide (fCO2) and pH were collected over a period of several days at French Frigate Shoals (FFS) in the Northwestern Hawaiian Islands (NWHI) in order to gain an understanding of the natural spatiotemporal variability of the marine inorganic carbon system in a pristine coral reef ecosystem. These data show clear island-to-open ocean gradients in fCO2 and total alkalinity that can be measured 10–20 km offshore, indicating that metabolic processes influence the CO2–carbonic acid system over large areas of ocean surrounding FFS and by implication the islands and atolls of the NWHI. The magnitude and extent of this spatial gradient may be driven by a combination of physical and biogeochemical processes including reef water residence time, hydrodynamic forcing of currents and tidal flow, and metabolic processes that occur both on the reef and within the lagoon.

Continue reading ‘Spatiotemporal assessment of CO2–carbonic acid system dynamics in a pristine coral reef ecosystem, French Frigate Shoals, Northwestern Hawaiian Islands’

Effects of eutrophication and benthic respiration on water column carbonate chemistry in a traditional hypoxic zone in the Northern Gulf of Mexico

A simple river-ocean mixing approach has been frequently used to examine estuarine and coastal carbonate system speciation. Coastal areas receiving significant nutrient inputs, however, can have the carbonate chemistry greatly deviated from this mixing-only scheme because of disparate, but spatially coupled biogeochemical processes, i.e., intense primary production in surface waters and elevated respiration in bottom waters; the latter often leads to bottom-water hypoxia (dissolved oxygen or DO concentration < 2 mg L− 1) and acidification. As a result of land use change, riverine TA input is known to enhance coastal water buffer capacity, although this effect in eutrophic coastal water has not been systematically studied. The physical disturbances of shallow coastal waters by storms can disrupt bottom hypoxia through overturning the water column. This overturn has been proposed to exacerbate bottom water acidification, because of the different exchange rates of oxygen and CO2, which could lead to a ‘reset’ of oxygen concentration but little change in the total dissolved inorganic carbon concentration. We used data from the summer 2010 hypoxia cruise in the northern Gulf of Mexico shelf, during which a tropical depression (Bonnie) perturbed the bottom water. Carbonate buffer capacity in both surface and subsurface waters along the salinity gradient suggested that eutrophication-induced surface production and bottom respiration far outweighed the influence of river TA variation and temperature changes in determining carbonate changes on centennial time scales. We propose, based on literature-based CO2 flux reported in this area, that the benthic (both aerobic and anaerobic) respiration-produced CO2 flux (with a lesser flux of alkalinity), instead of bottom water reset by storms, could be responsible for further acidifying hypoxic bottom water in addition to water column aerobic respiration.

Continue reading ‘Effects of eutrophication and benthic respiration on water column carbonate chemistry in a traditional hypoxic zone in the Northern Gulf of Mexico’

Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability

This study used microelectrodes to record pH profiles in fresh shelf sea sediment cores collected across a range of different sediment types within the Celtic Sea. Spatial and temporal variability was captured during repeated measurements in 2014 and 2015. Concurrently recorded oxygen microelectrode profiles and other sedimentary parameters provide a detailed context for interpretation of the pH data. Clear differences in profiles were observed between sediment type, location and season. Notably, very steep pH gradients exist within the surface sediments (10–20 mm), where decreases greater than 0.5 pH units were observed. Steep gradients were particularly apparent in fine cohesive sediments, less so in permeable sandier matrices. We hypothesise that the gradients are likely caused by aerobic organic matter respiration close to the sediment–water interface or oxidation of reduced species at the base of the oxic zone (NH4+, Mn2+, Fe2+, S−). Statistical analysis suggests the variability in the depth of the pH minima is controlled spatially by the oxygen penetration depth, and seasonally by the input and remineralisation of deposited organic phytodetritus. Below the pH minima the observed pH remained consistently low to maximum electrode penetration (ca. 60 mm), indicating an absence of sub-oxic processes generating H+ or balanced removal processes within this layer. Thus, a climatology of sediment surface porewater pH is provided against which to examine biogeochemical processes. This enhances our understanding of benthic pH processes, particularly in the context of human impacts, seabed integrity, and future climate changes, providing vital information for modelling benthic response under future climate scenarios.

Continue reading ‘Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability’

Increased appendicularian zooplankton alter carbon cycling under warmer more acidified ocean conditions

Anthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications.

Continue reading ‘Increased appendicularian zooplankton alter carbon cycling under warmer more acidified ocean conditions’


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Ocean acidification in the IPCC AR5 WG II

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