Posts Tagged 'chemistry'

Estimates of water-column nutrient concentrations and carbonate system parameters in the global ocean: a novel approach based on neural networks

 

A neural network-based method (CANYON: CArbonate system and Nutrients concentration from hYdrological properties and Oxygen using a Neural-network) was developed to estimate water-column (i.e., from surface to 8,000 m depth) biogeochemically relevant variables in the Global Ocean. These are the concentrations of three nutrients [nitrate (NO3−), phosphate (PO43−), and silicate (Si(OH)4)] and four carbonate system parameters [total alkalinity (AT), dissolved inorganic carbon (CT), pH (pHT), and partial pressure of CO2 (pCO2)], which are estimated from concurrent in situ measurements of temperature, salinity, hydrostatic pressure, and oxygen (O2) together with sampling latitude, longitude, and date. Seven neural-networks were developed using the GLODAPv2 database, which is largely representative of the diversity of open-ocean conditions, hence making CANYON potentially applicable to most oceanic environments. For each variable, CANYON was trained using 80 % randomly chosen data from the whole database (after eight 10° × 10° zones removed providing an “independent data-set” for additional validation), the remaining 20 % data were used for the neural-network test of validation. Overall, CANYON retrieved the variables with high accuracies (RMSE): 1.04 μmol kg−1 (NO3−), 0.074 μmol kg−1 (PO43−), 3.2 μmol kg−1 (Si(OH)4), 0.020 (pHT), 9 μmol kg−1 (AT), 11 μmol kg−1 (CT) and 7.6 % (pCO2) (30 μatm at 400 μatm). This was confirmed for the eight independent zones not included in the training process. CANYON was also applied to the Hawaiian Time Series site to produce a 22 years long simulated time series for the above seven variables. Comparison of modeled and measured data was also very satisfactory (RMSE in the order of magnitude of RMSE from validation test). CANYON is thus a promising method to derive distributions of key biogeochemical variables. It could be used for a variety of global and regional applications ranging from data quality control to the production of datasets of variables required for initialization and validation of biogeochemical models that are difficult to obtain. In particular, combining the increased coverage of the global Biogeochemical-Argo program, where O2 is one of the core variables now very accurately measured, with the CANYON approach offers the fascinating perspective of obtaining large-scale estimates of key biogeochemical variables with unprecedented spatial and temporal resolutions. The Matlab and R codes of the proposed algorithms are provided as Supplementary Material.

Continue reading ‘Estimates of water-column nutrient concentrations and carbonate system parameters in the global ocean: a novel approach based on neural networks’

Short-term variability of aragonite saturation state in the central Mid-Atlantic Bight

The uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a decrease in seawater aragonite saturation state (Ωarag), which affects the health of carbonate-bearing organisms and the marine ecosystem. A substantial short-term variability of surface water Ωarag, with an increase of up to 0.32, was observed in the central Mid-Atlantic Bight off the Delaware and the Chesapeake Bays over a short period of 10 days in summer 2015. High-frequency underway measurements for temperature, salinity, percentage saturation of dissolved oxygen, oxygen to argon ratio, pH, fCO2, and measurements based on discrete samples for pH, dissolved inorganic carbon and total alkalinity are used to investigate how physical and biogeochemical processes contribute to the changes of Ωarag. Quantitative analyses show that physical advection and mixing processes are the dominant forces for higher Ωarag in slope waters while biological carbon removal and CO2 degassing contribute to increased Ωarag in shelf waters.

Continue reading ‘Short-term variability of aragonite saturation state in the central Mid-Atlantic Bight’

Ocean acidification in the Baltic Sea : implications for the bivalve Macoma balthica

The Baltic Sea is one of the most human-impacted sea areas in the world and its ecosystems are exposed to a variety of stressors of anthropogenic origin. Large changes in the environmental conditions, species and communities of the Baltic Sea are predicted to occur due to global climate change, but the extent and magnitude of the future changes are challenging to estimate due to the multiple stressors simultaneously impacting the system. As an additional threat, future ocean acidification will play a role in modifying the environmental conditions, and these CO2-induced changes are predicted to be fast in the Baltic Sea. This is especially of concern for the species-poor, but functionally essential benthic communities where key species such as bivalve Macoma balthica live at the limits of their tolerance range, and are already regularly disturbed by environmental stressors such as hypoxia. Currently, only very limited knowledge about the effects of future ocean acidification exists for this species.

The overall aim of my thesis was to develop an understanding of the effects of CO2 increase on the vulnerability of Baltic Sea key species, and how this is related to other effects of climate change, e.g. an increase in bottom-water hypoxia. Specifically, I investigated how different life stages of the infaunal bivalve M. balthica could be affected by future ocean acidification. Survival, growth, behaviour and physiological responses were assessed in a combination of laboratory and mesocosm experiments by exposing different life stages of M. balthica to different pH levels over different time periods depending on the life stage in question. While some life stage-based differences in vulnerability and survival were found, the results indicate that reduced pH has a negative effect on all life stages. In larval M. balthica, even a slight pH decrease was found to cause significant negative changes during that delicate life stage, both by slowing growth and by decreasing survival. Other observed impacts included delayed settling of the post-larvae and increasing energetic demand of adult bivalves.

The results suggest consistent negative effects at all life stages with potential major implications for the resilience of M. Balthica populations, which are currently under threat from a range of anthropogenic stressors such as increasing hypoxia. The kind of experimental studies conducted in this thesis are useful for pinpointing mechanisms, but they are always simplifications of reality, however, and are usually conducted over time scales that are short in relation to the time scales over which ocean acidification is affecting populations, communities and ecosystems. To fully understand and to be able to estimate how the complex ecosystems are about to change in the future, incorporating more of the biotic interactions, impacting stressors and relevant environmental conditions are needed for increasing the level of realism in the experiments.

Continue reading ‘Ocean acidification in the Baltic Sea : implications for the bivalve Macoma balthica’

Coastal ocean acidification and increasing total alkalinity in the northwestern Mediterranean Sea

Coastal time series of ocean carbonate chemistry are critical for understanding how global anthropogenic change manifests in near-shore ecosystems. Yet, they are few and have low temporal resolution. At the time series station Point B in the northwestern Mediterranean Sea, seawater was sampled weekly from 2007 through 2015, at 1 and 50 m, and analyzed for total dissolved inorganic carbon (CT) and total alkalinity (AT). Parameters of the carbonate system such as pH (pHT, total hydrogen ion scale) were calculated and a deconvolution analysis was performed to identify drivers of change. The rate of surface ocean acidification was −0.0028 ± 0.0003 units pHT yr−1. This rate is larger than previously identified open-ocean trends due to rapid warming that occurred over the study period (0.072 ± 0.022 °C yr−1). The total pHT change over the study period was of similar magnitude as the diel pHT variability at this site. The acidification trend can be attributed to atmospheric carbon dioxide (CO2) forcing (59 %, 2.08 ± 0.01 ppm CO2 yr−1) and warming (41 %). Similar trends were observed at 50 m but rates were generally slower. At 1 m depth, the increase in atmospheric CO2 accounted for approximately 40 % of the observed increase in CT (2.97 ± 0.20 µmol kg−1 yr−1). The remaining increase in CT may have been driven by the same unidentified process that caused an increase in AT (2.08 ± 0.19 µmol kg−1 yr−1). Based on the analysis of monthly trends, synchronous increases in CT and AT were fastest in the spring–summer transition. The driving process of the interannual increase in AT has a seasonal and shallow component, which may indicate riverine or groundwater influence. This study exemplifies the importance of understanding changes in coastal carbonate chemistry through the lens of biogeochemical cycling at the land–sea interface. This is the first coastal acidification time series providing multiyear data at high temporal resolution. The data confirm rapid warming in the Mediterranean Sea and demonstrate coastal acidification with a synchronous increase in total alkalinity.

Continue reading ‘Coastal ocean acidification and increasing total alkalinity in the northwestern Mediterranean Sea’

Deepwater carbonate ion concentrations in the western tropical Pacific since 250 ka: Evidence for oceanic carbon storage and global climate influence

We present new “size-normalized weight” (SNW)-Δ[CO32−] core-top calibrations for three planktonic foraminiferal species and assess their reliability as a paleo-alkalinity proxy. SNWs of Globigerina sacculifer and Neogloboquadrina dutertrei can be used to reconstruct past deep Pacific [CO32−], whereas SNWs of Pulleniatina obliquiloculata are controlled by additional environmental factors. Based on this methodological advance, we reconstruct SNW-based deepwater [CO32−] for core WP7 from the western tropical Pacific since 250 ka. Secular variation in the SNW proxy documents little change in deep Pacific [CO32−] between the Last Glacial Maximum and the Holocene. Further back in time, deepwater [CO32−] shows long-term increases from marine isotope stage (MIS) 5e to MIS 3 and from early MIS 7 to late MIS 6, consistent with the “coral reef hypothesis” that the deep Pacific Ocean carbonate system responded to declining shelf carbonate production during these two intervals. During deglaciations, we have evidence of [CO32−] peaks coincident with Terminations 2 and 3, which suggests that a breakdown of oceanic vertical stratification drove a net transfer of CO2 from the ocean to the atmosphere, causing spikes in carbonate preservation (i.e., the “deglacial ventilation hypothesis”). During MIS 4, a transient decline in SNW-based [CO32−], along with other reported [CO32−] and/or dissolution records, implies that increased deep-ocean carbon storage resulted in a global carbonate dissolution event. These findings provide new insights into the role of the deep Pacific in the global carbon cycle during the late Quaternary.

Continue reading ‘Deepwater carbonate ion concentrations in the western tropical Pacific since 250 ka: Evidence for oceanic carbon storage and global climate influence’

The regulation of coralline algal physiology, an in-situ study of Corallina officinalis (Corallinales, Rhodophyta)

Calcified macroalgae are critical components of marine ecosystems worldwide, but face considerable threat both from climate change (increasing water temperatures) and ocean acidification (decreasing ocean pH and carbonate saturation). It is thus fundamental to constrain the relationships between key abiotic stressors and the physiological processes that govern coralline algal growth and survival. Here we characterize the complex relationships between the abiotic environment of rock pool habitats, and the physiology of the geniculate red coralline alga, Corallina officinalis (Corallinales, Rhodophyta). Paired assessment of irradiance, water temperature and carbonate chemistry, with C. officinalis net production (NP), respiration (R) and net calcification (NG) was performed in a south-west UK field site, at multiple temporal scales (seasonal, diurnal and tidal). Strong seasonality was observed in NP and night-time R, with a Pmax of 22.35 μmol DIC gDW−1 h−1, Ek of 300 μmol photons m−2 s−1 and R of 3.29 μmol DIC gDW−1 −1 determined across the complete annual cycle. NP showed a significant exponential relationship with irradiance (R2 = 0.67), although was temperature dependent given ambient irradiance > Ek for the majority of the annual cycle. Over tidal emersion periods, dynamics in NP highlighted the ability of C. officinalis to acquire inorganic carbon despite significant fluctuations in carbonate chemistry. Across all data, NG was highly predictable (R2 = 0.80) by irradiance, water temperature and carbonate chemistry, providing a NGmax of 3.94  μmol CaCO3 gDW−1 h−1, and Ek of 113 μmol photons m−2 s−1. Light-NG showed strong seasonality and significant coupling to NP (R2 = 0.65), as opposed to rock pool water carbonate saturation. In contrast, the direction of dark-NG (dissolution vs. precipitation) was strongly related to carbonate saturation, mimicking abiotic precipitation dynamics. Data demonstrated that C. officinalis is adapted to both long-term (seasonal) and short-term (tidal) variability in environmental stressors, although the balance between metabolic processes and the external environment may be significantly impacted by future climate change.

Continue reading ‘The regulation of coralline algal physiology, an in-situ study of Corallina officinalis (Corallinales, Rhodophyta)’

Species composition of microzooplankton Tintinnid from the coastal waters of Digha, Bay of Bengal

Tintinnid species distribution and hydrography were studied in the coastal waters of Digha during winter (November 2015) and summer (March 2016) seasons. Surface water samples were collected from 11 different stations from 0 to 10 km offshore with the help of a mechanized trawler. Parameters like tintinnid species enumeration, zooplankton biomass, phytoplankton concentration (total chlorophyll) and abundance, sea surface temperature (SST), pH, transparency, salinity, dissolved oxygen (DO), total phosphate, silicate and nitrate were analysed. A total of 20 different tintinnid species (16 agglomerated +4 non-agglomerated) belonging to 6 genera were recorded from the study area with seasonal variation in tintinnid diversity, i.e. higher in summer (total 2745 individual/l) compared to winter (total 1191 individual/l). Tintinnopsis was the most dominant genus during both the seasons, i.e. 2100 individual/l in summer and 727 individual/l in winter, contributing about 76 and 61% population for the respective seasons. The correlation between species and water quality parameters showed that Tintinnopsis sp. abundance was significantly regulated by nitrate concentration, salinity, dissolved oxygen, water transparency and pH. However, the mentioned hydrological parameters were not the only factors regulating the tintinnid abundance. Tintinnid abundance was also found to be positively related with transparency (r = 0.732) and salinity (r = 0.524) and moderately related with dissolved oxygen (r = 0.488) whereas strong negative relation (at p ≤ 0.05) was established between tintinnid abundance with nitrate (r = −0.681) and pH (r = −0.561). Bray-Curtis cluster analysis of tintinnid species showed more than 60% similarity. Shannon’s diversity index (H′), Simpson’s evenness index (D) and Margalef’s species richness index were found to be higher in summer, i.e. 1.61, 0.729 and 1.612, compared to the winter season, i.e. 1.139, 0.597 and 1.268. k-dominance curve showed maximum abundance of Tintinnopsis baltica in winter and Tintinnopsis gracilis in summer. Principal component analysis (PCA) was analysed to find out the environmental variables affecting different tintinnid species diversity. A significant spatiotemporal variation in Tintinnid population distribution was observed from two-way ANOVA. The results reflect significant seasonal (F = 840.0), spatial (F = 47.3) and interactive variation (F = 71.2) among the ciliate microzooplankton at n = 66, p ≤ 0.001. High chlorophyll content and phytoplankton population in summer indicated that tintinnid diversity in the season was positively influenced by producer community in coastal waters of Digha.

Continue reading ‘Species composition of microzooplankton Tintinnid from the coastal waters of Digha, Bay of Bengal’


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