Posts Tagged 'chemistry'

Water motion and vegetation control the pH dynamics in seagrass-dominated bays

Global oceanic pH is lowering, which is causing great concern for the natural functioning of marine ecosystems. Current pH predictions are based on open ocean models; however, coastal zones are dynamic systems with seawater pH fluctuating temporally and spatially. To understand how coastal ecosystems will respond in the future, we first need to quantify the extent that local processes influence the pH of coastal zones. With this study, we show that over a single diurnal cycle, the total pH can fluctuate up to 0.2 units in a shallow seagrass-dominated bay, driven by the photosynthesis and respiration of the vegetation. However, these biologically controlled pH fluctuations vary significantly over small distances. Monitoring conducted at neighboring sites with contrasting hydrodynamic regimes highlights how water motion controls the extent that the local pH is altered by the metabolism of vegetation. The interactive effects of hydrodynamics and vegetation were further investigated with an in situ experiment, where the hydrodynamics were constrained and thus the local water residence time was increased, displaying the counteractive effect of hydrodynamics on the pH change caused by vegetation. With this research, we provide detailed in situ evidence of the spatial variation of pH within marine ecosystems, highlighting the need to include hydrodynamic conditions when assessing the pH-effects of vegetation, and identifying potential high-pH refuges in a future low pH ocean.

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Carbon dynamics in a marsh-influenced marine-dominated ecosystem

A combination of global climate change, local anthropogenic pressures, and naturally occurring processes have impacted biogeochemical cycling in coastal systems. Here, a coastal estuarine ecosystem in North Carolina is studied in order to determine spatial relations, seasonal changes, and overall fluxes of carbon, as well as the influences of these factors on the biogeochemistry of the system as a whole. Partial pressure of carbon dioxide (pCO2), percent dissolved oxygen (DO), particulate organic carbon (POC), total dissolved inorganic carbon (DIC), total alkalinity (TA), and carbon isotopes of organic and inorganic carbon—amongst additional data—were collected from numerous study locations in the Cape Lookout region of North Carolina in April 2017, October 2017, April 2018, June 2018, and October 2018. Carbon isotopes of POC ranging between -30 and -17.79‰ coupled with a decreasing trend in C/N values moving down-estuary indicate that the organic carbon in the system is mainly sourced from upland vascular plant and agricultural inputs, with a small influence from in-estuary Spartina marsh grasses. The majority of the estuary was oversaturated with CO2 compared to the atmosphere during all seasons, with the marsh-creek Smyrna Creek consistently exhibiting the most extreme pCO2 values, peaking at 14606 µatm in the head of the creek in June 2018. Some estuarine sites were occasionally undersaturated in CO2, likely from local phytoplankton blooms occurring during spring and summer. Carbon flux from these three creeks into Jarrett Bay is evident, as is further flux of CO2 through the sound and out into the ocean where the CO2-saturated estuarine waters combine with the less CO2-rich marine waters to produce ocean values of ~625 µatm. TA values throughout the system range from 1872–2342 µmol kg-1, excluding Smyrna and Williston marsh-creeks which exhibited anomalous TA in several different seasons. Omitting these two creeks, the remainder of the system shows an increasing spatial TA trend moving down estuary over the salinity gradient with the lowest values in Jarrett Bay and the highest values in the ocean. Due to seasonal mixing trends, DIC concentration increased down-estuary in the Summer and Spring and decreased over the salinity gradient in the Fall; however, the head of Smyrna Creek typically exhibited notably high DIC compared to the rest of the system, as CO2 is the main contributor to DIC within the salt marsh. Plotting DIC against TA indicates that inorganic carbon likely originates from a combination of sulfate reduction, denitrification, CO2 invasion, and aerobic respiration. Calculations of air–sea CO2 flux indicate that the estuarine waters as a whole are a significant source of CO2 to the atmosphere with an average air–sea CO2 flux of 13.4 mmol m-2 day-1.

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Episodic Arctic CO2 limitation in the west Svalbard shelf

The European Sector of the Arctic Ocean is characterized by low CO2 concentrations in seawater during spring and summer, largely due to strong biological uptake driven by extensive plankton blooms in spring. The spring plankton bloom is eventually terminated by nutrient depletion and grazing. However, low CO2 concentrations in seawater and low atmospheric resupply of CO2 can cause episodes during which the phytoplankton growth is limited by CO2. Here, we show that gross primary production (GPP) of Arctic plankton communities increases from 32 to 72% on average with CO2 additions in spring. Enhanced GPP with CO2 additions occur during episodes of high productivity, low CO2 concentration and in the presence of dissolved inorganic nutrients. However, during summer the addition of CO2 supresses planktonic Arctic GPP. Events of CO2 limitation in spring may contribute to the termination of the Arctic spring plankton blooms. The stimulation of GPP by CO2 during the spring bloom provides a biotic feedback loop that might influence the global role played by the Arctic Ocean as a CO2 sink in the future.

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High-frequency variability of CO2 in Grand Passage, Bay of Fundy, Nova Scotia (update)

Assessing changes in the marine carbon cycle arising from anthropogenic CO2 emissions requires a detailed understanding of the carbonate system’s natural variability. Coastal ecosystems vary over short spatial and temporal scales, so their dynamics are not well described by long-term and broad regional averages. A year-long time series of pCO2, temperature, salinity, and currents is used to quantify the high-frequency variability of the carbonate system at the mouth of the Bay of Fundy, Nova Scotia. The seasonal cycle of pCO2 is modulated by a diel cycle that is larger in summer than in winter and a tidal contribution that is primarily M2, with amplitude roughly half that of the diel cycle throughout the year. The interaction between tidal currents and carbonate system variables leads to lateral transport by tidal pumping, which moves alkalinity and dissolved inorganic carbon (DIC) out of the bay, opposite to the mean flow in the region, and constitutes a new feature of how this strongly tidal region connects to the larger Gulf of Maine and northwest Atlantic carbon system. These results suggest that tidal pumping could substantially modulate the coastal ocean’s response to global ocean acidification in any region with large tides and spatial variation in biological activity, requiring that high-frequency variability be accounted for in assessments of carbon budgets of coastal regions.

Continue reading ‘High-frequency variability of CO2 in Grand Passage, Bay of Fundy, Nova Scotia (update)’

The spatiotemporal variability of pH in waters of the Black Sea

Based on archival data of the Institute of Natural and Technical Systems for the period from 1956 to 2010, the large-scale structure, seasonal variability, and long-term trends of pH variations in the upper 150-m layer of the deep-water part of the Black Sea were analyzed. The spatial climatic inhomogeneities of pH in the surface layer of the open part of the Black Sea are about 0.06 pH units; the magnitude of the average seasonal cycle is 0.05 pH units. In the deep-water part of the sea, low values of pH are observed in the vicinity of the centers of cyclonic gyres, where more acidic waters move upward. A long-term increase in the acidity of waters of the surface layer is observed. Thetrend of pH in this layer is generally conditioned by the increase in the CO2 content in the lower troposphere and absorption of part of the excess CO2 by sea water. The increase in the acidity manifests itself in the decrease in pH (at a level of –0.06 pH units per 50 years) in waters of the surface layer. This is close to the estimations for other regions of the World Ocean. However, the intermediate waters are characterized by a negative pH trend, the absolute value of which exceeds more than fivefold the pH trend on the surface. A likely reason for the intense decrease in pH here is the intensification long-term vertical circulation in the intermediate layer of the Black Sea waters, resulting in the ascension of more acidic waters with a typical velocity of ~1 m per year. This can lead to more intense acidification of surface waters in the sea over the next 10 years.

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Physical and biogeochemical controls on pH dynamics in the northern Gulf of Mexico during summer hypoxia

High accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico (nGOM) in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017; Müller & Rehder, 2018), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005±0.016, n=550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The non‐conservative pH changes in both surface and bottom waters correlated well with the biologically‐induced changes in DIC, i.e., per 100 μmol kg‐1 biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO2 invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.

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Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms

Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (δ11B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Ωsw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Ωcf), and hence their potential to acclimate to changing Ωsw. We show that lifelong exposure to low Ωsw did not enable the corals to acclimate and reach similar Ωcf as corals grown under ambient conditions. The lower Ωcf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Ωsw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.

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Ocean acidification in the IPCC AR5 WG II

OUP book