Posts Tagged 'biogeochemistry'

Aragonite saturation state in a tropical coastal embayment dominated by phytoplankton blooms (Guanabara Bay – Brazil)


  • The spatio-temporal variations of Ωarag were studied in a highly polluted coastal embayment.
  • High values of Ωarag were prevalent in surface waters dominated by phytoplankton blooms.
  • Lowest values of Ωarag were restricted to poorly buffered waters that receive direct effluent discharges.
  • Variations of Ωarag related to biological processes override those related to the atmospheric CO2.


The dynamics of the aragonite saturation state (Ωarag) were investigated in the eutrophic coastal waters of Guanabara Bay (RJ-Brazil). Large phytoplankton blooms stimulated by a high nutrient enrichment promoted the production of organic matter with strong uptake of dissolved inorganic carbon (DIC) in surface waters, lowering the concentrations of dissolved carbon dioxide (CO2aq), and increasing the pH, Ωarag and carbonate ion (CO32 ), especially during summer. The increase of Ωarag related to biological activity was also evident comparing the negative relationship between the Ωarag and the apparent utilization of oxygen (AOU), with a very close behavior between the slopes of the linear regression and the Redfield ratio. The lowest values of Ωarag were found at low-buffered waters in regions that receive direct discharges from domestic effluents and polluted rivers, with episodic evidences of corrosive waters (Ωarag < 1). This study showed that the eutrophication controlled the variations of Ωarag in Guanabara Bay.

Continue reading ‘Aragonite saturation state in a tropical coastal embayment dominated by phytoplankton blooms (Guanabara Bay – Brazil)’

Natural ocean acidification at Papagayo upwelling system (North Pacific Costa Rica): implications for reef development

Numerous experiments have shown that ocean acidification impedes coral calcification, but knowledge about in situ reef ecosystem response to ocean acidification is still scarce. Bahía Culebra, situated at the northern Pacific coast of Costa Rica, is a location naturally exposed to acidic conditions due to the Papagayo seasonal upwelling. We measured pH and pCO2 in situ during two non-upwelling seasons (June 2012, May–June 2013), with a high temporal resolution of every 15 and 30 min, respectively, using two Submersible Autonomous Moored Instruments (SAMI-pH, SAMI-CO2). These results were compared with published data from the upwelling season 2009. Findings revealed that the carbonate system in Bahía Culebra shows a high temporal variability. Incoming offshore waters drive inter- and intra-seasonal changes. Lowest pH (7.8) and highest pCO2 (658.3 µatm) values measured during a cold-water intrusion event in the non-upwelling season were similar to those minimum values reported from upwelling season (pH = 7.8, pCO2 = 643.5 µatm), unveiling that natural acidification occurs sporadically also in non-upwelling season. This affects the interaction of photosynthesis, respiration, calcification, and carbonate dissolution and the resulting diel cycle of pH and pCO2 in the reefs of Bahía Culebra. During non-upwelling season, the aragonite saturation state (Ωa) rises to values of > 3.3 and enhances calcification. Aragonite saturation state values during upwelling season falls below 2.5, hampering calcification and coral growth. Low reef accretion in Bahía Culebra indicates high erosion rates and that these reefs grow on the verge of their ecological tolerance. The Ωa threshold values for coral growth, derived from the correlation between Ωa and coral linear extension rates, suggest that future ocean acidification will threaten reefs in Bahía Culebra. These data contribute to build a better understanding of the carbonate system dynamics and coral reefs key response (e.g. coral growth) to natural low-pH conditions, in upwelling areas in the Eastern Tropical Pacific and beyond.

Continue reading ‘Natural ocean acidification at Papagayo upwelling system (North Pacific Costa Rica): implications for reef development’

Combined, short-term exposure to reduced seawater pH and elevated temperature induces community shifts in an intertidal meiobenthic assemblage

In future global change scenarios the surface ocean will experience continuous acidification and rising temperatures. While effects of both stressors on marine, benthic communities are fairly well studied, consequences of the interaction of both factors remain largely unknown. We performed a short-term microcosm experiment exposing a soft-bottom community from an intertidal flat in the Westerscheldt estuary to two levels of seawater pH (ambient pHT = 7.9, reduced pHT = 7.5) and temperature (10 °C ambient and 13 °C elevated temperature) in a crossed design. After 8 weeks, meiobenthic community structure and nematode staining ratios, as a proxy for mortality, were compared between treatments and structural changes were related to the prevailing abiotic conditions in the respective treatments (pore water pHT, sediment grain size, total organic matter content, total organic carbon and nitrogen content, phytopigment concentrations and carbonate concentration). Pore water pHT profiles were significantly altered by pH and temperature manipulations and the combination of elevated temperature and reduced pH intensified the already more acidic porewater below the oxic zone. Meiofauna community composition was significantly affected by the combination of reduced pH and elevated temperature resulting in increased densities of predatory Platyhelminthes, reduced densities of Copepoda and Nauplii and complete absence of Gastrotricha compared to the experimental control. Furthermore, nematode staining ratio was elevated when seawater pH was reduced pointing towards reduced degradation rates of dead nematode bodies. The observed synergistic interactions of pH and temperature on meiobenthic communities and abiotic sediment characteristics underline the importance of multistressor experiments when addressing impacts of global change on the marine environment.

Continue reading ‘Combined, short-term exposure to reduced seawater pH and elevated temperature induces community shifts in an intertidal meiobenthic assemblage’

Effects of the interaction of ocean acidification, solar radiation, and warming on biogenic dimethylated sulfur compounds cycling in the Changjiang River Estuary

Ocean acidification (OA) affects marine primary productivity and community structure, and therefore may influence the biogeochemical cycles of volatile biogenic dimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) and photochemical oxidation product dimethyl sulfoxide (DMSO). A 23-day incubation experiment on board was conducted to investigate the short-term response of biogenic sulfur compounds production and cycling to OA in the Changjiang River Estuary and further understand its effects on biogenic sulfur compounds. Result showed that phytoplankton abundance and species presented remarkable differences under three different pH levels in the late stage of the experiment. A significant reduction in chlorophyll a (Chl-a), DMS, particulate DMSP (DMSPp), and dissolved DMSO (DMSOd) concentrations was identified under high CO2 levels. Moreover, minimal change was observed in the production of dissolved DMSP (DMSPd) and particulate DMSO (DMSOp) among treatments. The ratios of DMS, total DMSP (DMSPt), and total DMSO (DMSOt) to Chl-a were also not affected by a change in pH. In addition, DMS and DMSOd were highly related to mean bacterial abundance under three pH levels. Additional incubation experiments on light and temperature showed that the influence of pH on productions of dimethylated sulfur compounds also depended on solar radiation and temperature conditions. DMS photodegradation rate increased with decreasing pH under full-spectrum natural light and UVB light. Thus, OA may lead to decreasing DMS concentrations in the surface seawater. Light and temperature conditions also play an important role in the production and cycling of biogenic sulfur compounds.

Continue reading ‘Effects of the interaction of ocean acidification, solar radiation, and warming on biogenic dimethylated sulfur compounds cycling in the Changjiang River Estuary’

Submarine groundwater discharge drives biogeochemistry in two Hawaiian reefs

Groundwater inputs are typically overlooked as drivers of environmental change in coastal reef studies. To assess the impact of groundwater discharge on reef biogeochemistry, we examined two fringing reef environments, located in Maunalua Bay on the south shore of O‘ahu, Hawai‘i, that receive large inputs of submarine groundwater discharge. We supplemented 25- and 30-d time series measurements of salinity, water temperature, pH, dissolved oxygen, and 222Rn with high-resolution 24-h nutrient, dissolved inorganic carbon (DIC), total alkalinity (TA), and δ13C–DIC measurements to evaluate both groundwater-induced and biologically-driven changes in coastal carbonate chemistry across salinity gradients. Submarine groundwater discharge at these two locations was characterized by low pHT (7.36–7.62), and variable DIC (1734–3046 μM) and TA (1716–2958 μM) content relative to ambient seawater. Groundwater-driven variability in coastal carbonate system parameters was generally on the same order of magnitude as biologically-driven variability in carbonate system parameters at our study locations. Further, our data revealed a shift in reef metabolism from net dissolution to net calcification across this groundwater-driven physicochemical gradient. At sites with high levels of groundwater exposure, net community production and calcification rates were reduced. Our findings shed light on the importance of considering groundwater inputs when examining coastal carbonate chemistry.

Continue reading ‘Submarine groundwater discharge drives biogeochemistry in two Hawaiian reefs’

Impact of climate change variables on nutrient cycling by marine microorganisms in the Southern California Bight and Ross Sea, Antarctica

Ocean environments are being impacted by climate warming, elevated carbon dioxide (CO2) levels, and shifting nutrient sources and sinks. It is essential to quantify the sensitivity of microorganisms to these effects of global change because they form the base of the marine food web and are an integral component of nutrient cycling on the planet. Their role in photosynthesis, nutrient uptake, and transfer of organic matter into higher trophic levels or to the deep ocean via the biological pump render microorganisms key in ecosystem structure and function and in regulating the global climate. The goal of this dissertation research was to determine how changing environmental conditions impact microbial communities and the rates at which they take up nutrients. Research for this dissertation took place in the Southern California Bight and in the Ross Sea, Antarctica, where fully factorial designs were used to investigate the response of microorganisms to multiple global change parameters. Nutrient uptake rates were measured using 13C and 15N stable isotopes for carbon and nitrogen substrates and 33P radioisotopes for phosphorus substrates. In the Southern California Bight, a microbial assemblage was collected and incubated in an ‘ecostat’ continuous culture system, where elevated temperature, CO2, and the dominant nitrogen substrate (nitrate or urea) in the diluent were manipulated. During this experiment uptake rates of dissolved inorganic carbon (DIC), nitrate (NO3-), and urea were determined for two microbial size classes (0.7-5.0 μm and >5.0 μm). Urea uptake rates were greater than NO3-, and uptake rates of urea and DIC for both size fractions increased at elevated temperature, while uptake rates of NO3- by smaller microorganisms increased when CO2 levels were high. In the Ross Sea, the impact of elevated temperature, CO2, and iron addition on DIC and NO3- uptake rates by two size classes (0.7-5.0 μm and >5.0 μm ) of a late-season microbial community were investigated using a semi-continuous and continuous ‘ecostat’ culturing approach. Temperature impacted the microbial community the most, significantly increasing NO3- and DIC uptake rates by larger microorganisms. The effects of iron addition were more apparent when temperature was also elevated, and CO2 did not impact rates. Bioassay experiments were also conducted in the Ross Sea to determine how increasing and decreasing the N:P supply ratio in combination with other parameters (temperature and iron) impact uptake rates of DIC, NO3-, and amino acids. Results from these experiments show that changes to the dissolved N:P supply ratio have the potential to alter nutrient uptake rates over short time scales, but that temperature elevation and iron addition have a larger impact. Additional experiments were completed on diatoms (Fragilariopsis cylindrus and Pseudo-nitzschia subcurvata) and Phaeocystis antarctica, three important phytoplankton species collected from the Ross Sea, to assess how temperature elevation and iron addition impact uptake rates of a number of inorganic and organic carbon, nitrogen, and phosphorus substrates. These culture studies generally show that when temperature is increased, diatoms are able to take up nutrients more rapidly than Phaeocystis antarctica.
Results from this dissertation show that nutrient cycles and phytoplankton communities in the Southern California Bight and the Ross Sea, Antarctica will likely be different in the future. Although all variables tested were found to exert some influence on microbial nutrient cycling, temperature elevation generally had the largest effect, increasing biomass and uptake rates, structuring the composition of the microbial community, and altering stoichiometry. This research did not include top down effects and it is limited spatially and temporally, however, it demonstrates the importance of studying different nutrient substrates and looking at multiple interactive stressors to gain a more comprehensive view of potential change.

Continue reading ‘Impact of climate change variables on nutrient cycling by marine microorganisms in the Southern California Bight and Ross Sea, Antarctica’

Potential influence of ocean acidification on deep-sea Fe–Mn nodules: results from leaching experiments

With the continuous rise in CO2 emissions, the pH of seawater may decrease extensively in the coming centuries. Deep-sea environments are more vulnerable to decreasing pH since sediments in deep oceans below the carbonate compensation depth (CCD) are often completely devoid of carbonate particles. In order to assess the potential risk of heavy metal release from deep-sea deposits, the mobility of elements from ferromanganese (Fe–Mn) nodules and pelagic clays was examined by means of leaching experiments using phosphate buffer solutions ranging in pH from 7.1 to 8.6 (NBS scale). With decreasing pH, the results showed an enhanced leaching of elements such as Li, B, Mg, Si, Sc, Sr, Ba, Tl, and U, but a reduced leaching of V, Cu, Mo, Cd, and W. Elements in leachates originate mainly from exchangeable fractions, and tend to be affected by sorption–desorption processes. Concentrations of most elements did not exceed widely used international water quality criteria, indicating that changes in pH caused by future ocean acidification may not increase the risk of heavy metal release during deep-sea nodule mining operations.

Continue reading ‘Potential influence of ocean acidification on deep-sea Fe–Mn nodules: results from leaching experiments’

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Ocean acidification in the IPCC AR5 WG II

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