Posts Tagged 'biogeochemistry'

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’

Export flux, biogeochemical effects, and the fate of a terrestrial carbonate system: from Changjiang (Yangtze River) Estuary to the East China Sea

Seasonal variations in the transports of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) from the Lower Changjiang (Yangtze) River/Estuary to the East China Sea (ECS) were investigated based on a series of field surveys in 2015–2017, including monthly samplings at Datong Station and seasonal mapping cruises in the Changjiang Estuary and the adjacent northwestern ECS. In comparison with historical data sets, the Changjiang TAlk flux varied around a nearly stable average over the past 55 years. This is much different from some American rivers, where TAlk export fluxes increased for a century long. To assess effects of riverine carbonate inputs on coastal carbonate chemistry, we compared several cases showing freshwater‐dilution‐induced decline in coastal aragonite saturation state (Ωarag), including rainwater dilution and riverine water dilution. Without riverine carbonate inputs, the effect of a unit of salinity decrease (due to rainwater dilution) on Ωarag was expected to be counteracted by a DIC removal of 10 μmol kg–1 relative to the baseline value along relevant conservative mixing line, when coastal Ωarag was close to a critical value of 1.5. Considering terrestrial carbonate inputs from Changjiang, however, the freshwater‐dilution‐induced coastal Ωarag suppression decreased by 12%. Our data also showed that, more than 10% of wet‐season DIC flux discharged from the Changjiang Estuary was sequestered by biological activities in nearshore areas, while the TAlk flux was rarely affected. This biological alteration effectively transformed the terrestrial carbonate system from a feature of DIC:TAlk >1.0 to the usual seawater feature of DIC:TAlk <0.9.

Continue reading ‘Export flux, biogeochemical effects, and the fate of a terrestrial carbonate system: from Changjiang (Yangtze River) Estuary to the East China Sea’

Response of N2O production rate to ocean acidification in the western North Pacific

Ocean acidification, induced by the increase in anthropogenic CO2 emissions, has a profound impact on marine organisms and biogeochemical processes. The response of marine microbial activities to ocean acidification might play a crucial role in the future evolution of air–sea fluxes of biogenic gases such as nitrous oxide (N2O), a strong GHG and the dominant stratospheric ozone-depleting substance. Here, we examine the response of N2O production from nitrification to acidification in a series of incubation experiments conducted in subtropical and subarctic western North Pacific. The experiments show that when pH was reduced, the N2O production rate during nitrification measured at subarctic stations increased significantly while nitrification rates remained stable or decreased. Contrary to previous findings, these results suggest that the effect of ocean acidification on N2O production during nitrification and nitrification rates are probably uncoupled. Collectively, these results suggest that if seawater pH continues to decline at the same rate, ocean acidification could increase marine N2O production during nitrification in the subarctic North Pacific by 185 to 491% by the end of the century.

Continue reading ‘Response of N2O production rate to ocean acidification in the western North Pacific’

High‐frequency CO2‐system variability over the winter‐to‐spring transition in a coastal plain estuary

Understanding the vulnerability of estuarine ecosystems to anthropogenic impacts requires a quantitative assessment of the dynamic drivers of change to the carbonate (CO2) system. Here we present new high‐frequency pH data from a moored sensor. These data are combined with discrete observations to create continuous time series of total inorganic carbon (TCO2), CO2 partial pressure (pCO2) and carbonate saturation state. We present two deployments over the winter‐to‐spring transition in the lower York River (where it meets the Chesapeake Bay mainstem) in 2016/17 and 2017/18. TCO2 budgets with daily resolution are constructed and contributions from circulation, air‐sea CO2 exchange, and biology are quantified. We find that TCO2 is most strongly influenced by circulation and biological processes; pCO2 and pH also respond strongly to changes in temperature. The system transitions from autotrophic to heterotrophic conditions multiple times during both deployments; the conventional view of a spring bloom and subsequent summer production followed by autumn and winter respiration may not apply to this region. Despite the dominance of respiration in winter and early spring, surface waters were undersaturated with respect to atmospheric CO2 for the majority of both deployments with mean fluxes ranging from ‐9 to ‐5 mmol C m‐2 d‐1. Deployments a year apart indicate that the seasonal transition in the CO2‐system differs significantly from one year to the next and highlights the necessity of sustained monitoring in dynamic nearshore environments.

Continue reading ‘High‐frequency CO2‐system variability over the winter‐to‐spring transition in a coastal plain estuary’

Carbon and phosphorus processing in a carbonate karst aquifer and delivery to the coastal ocean

In siliciclastic systems, submarine groundwater discharge (SGD) provides a fraction of freshwater and nutrients delivered to coastal waters, but in many carbonate karst terrains SGD represents the predominant source of terrestrial water and solutes. Water compositions may be modified by reactions in subterranean estuaries, altering chemical fluxes via SGD. In carbonate settings, feedbacks between organic carbon remineralization and calcium carbonate mineral (CaCO3) dissolution and precipitation may alter carbon dioxide (CO2) and phosphorus (P) concentrations and fluxes associated with SGD. To assess these effects, we sampled water from multiple submarine springs along the east coast of the Yucatan peninsula, as well as inland cenotes, and a coastal groundwater well. We measured ammonium (NH4), phosphate (PO4), major element, dissolved inorganic and organic carbon concentrations (DIC and DOC), fluorescent characteristics of colored dissolved organic matter (CDOM) and modeled calcite saturation indices (SIcal) and the partial pressure of dissolved CO2 (PCO2). These data indicate that reactions along a hypothetical flow path to the coast control the composition of terrestrial fresh water entering the subterranean estuary. Non-conservative mixing between brackish groundwater and lagoon water reflect changes to groundwater compositions within the subterranean estuary from CO2 produced during organic matter remineralization and CaCO3 dissolution. Although both organic matter remineralization and carbonate dissolution should liberate P, molar N:P ratios in spring discharge are higher than the Redfield Ratio of 16:1, suggesting sequestration of remineralized P through sorption to carbonate minerals within the STE. This result indicates that SGD, the primary source of water and nutrient to this coastal zone, results in P limitation in coastal water and is a source of CO2 despite buffering by CaCO3 dissolution. This result also emphasizes the importance of biogeochemical reactions within subterranean estuaries for estimates of SGD solute delivery to the oceans and impacts to the coastal carbon cycle.

Continue reading ‘Carbon and phosphorus processing in a carbonate karst aquifer and delivery to the coastal ocean’

Spatial patterns in aragonite saturation for the north central California shelf

Ocean acidification is exacerbated along the California shelf due to the upwelling of deep CO2 rich waters. This process of upwelling is driven by along-shore winds, which vary in strength by season. We present the relationship between along-shore wind and aragonite undersaturation utilizing an empirical formula to determine aragonite saturation from salinity, temperature, and dissolved oxygen. Our models show that stronger along-shore winds are correlated with a higher percentage of the water column undersaturated in aragonite. In addition, pteropod and juvenile krill density decrease in upwelled water which is cold, salty, and low in aragonite. With a predicted increase in along-shore winds, California shelf waters will become more undersaturated in aragonite and lead to a decrease in pteropod and krill density.

Continue reading ‘Spatial patterns in aragonite saturation for the north central California shelf’

Short-term effects of artificial reef construction on surface sediment and seawater properties in Daya Bay, China

The degradation and modification of habitats are dominant drivers of biodiversity fluctuation, which is increasingly threatened in marine ecosystems (especially coral reefs) and can be mitigated by the construction of artificial reefs (ARs). Environmental indices are considered excellent indicators of how disturbances affect organisms, but the effects of AR construction on nearby sediment and seawater environments remain unclear. In the current study, changes in the properties of surface sediment and seawater (surface and bottom) for 2 years following AR construction were investigated in Daya Bay, China. The reef habitat and the nearby nonreef habitat were sampled. The results indicated that surface sediment organic matter significantly increased but sediment texture did not change following AR construction, regardless of habitat. The pH, dissolved oxygen, and salinity significantly declined, but chemical oxygen demand, inorganic nitrogen, and suspended particulate organic matter significantly increased in both surface and bottom seawater, regardless of habitat; temperature, nitrogen/phosphorus ratio, and available phosphate were not consistently altered by AR construction. Considering the intimate relationships between abiotic factors and organisms, these results indicate that long-term assessment of multiple environmental properties is needed to evaluate and predict the effects of AR construction on marine wildlife.

Continue reading ‘Short-term effects of artificial reef construction on surface sediment and seawater properties in Daya Bay, China’

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Ocean acidification in the IPCC AR5 WG II

OUP book