Human impact on the silica cycle : reduction of dissolved silica inputs into the ocean as a result of the increasing impervious cover

Frequent harmful algal blooms in coastal waters have been linked to increasing nitrogen (N) and phosphorous (P) loadings. Recent studies, however, have shown that dissolved silica (DSi) depletion in natural waters can be an important if not the most important factor that triggers these events. Long term hydrologic and water quality data give signs of significant human impact on the silicon cycle. More specifically, by altering the hydrology of the land, humans may have reduced the amount of DSi that reaches the oceans through freshwater streams. This study examined the hypothesis that a watershed with more impervious cover discharges less DSi per unit watershed than a more undisturbed watershed. DSi discharge data were collected from 2 different freshwater streams with watersheds of different % impervious cover during 5 non-rain and 4 rain events. The stream with higher impervious cover discharged higher DSi per unit watershed during non-rain events. During intense rain events the more impervious watershed rapidly released stormwater as low-DSi runoff while the less impervious watershed released less runoff and more DSi per unit watershed. During low intensity rain events the less impervious watershed released no runoff while DSi discharge increased. The more impervious watershed released runoff even during the lightest event. Using the CN method developed by the Soil Conservation Service, it was found that a more impervious watershed not only produced more runoff than a less impervious watershed, but it also produced runoff more often. Higher volume of runoff can cause short term DSi dilution during rain events as well as long-term reduction of DSi inputs to coastal waters. According to the CN method a long-term reduction of DSi loads is taking place in response to increasing impervious cover. Since diatom primary production is possibly the most important link of the marine food chain, and since diatom growth is DSi limited, reduction of the coastal oceans’ silica budget may have negative impacts on all levels of the food chain

Detecting the calcium carbonate saturation state under the stress of ocean acidification using saturometry technique

CO2-induced ocean acidification lowers the degree of carbonate saturation state of the seawater, which affects calcification of marine organisms and influences the marine carbonate cycle, thus have negative impacts on the entire marine biogeochemical system. This study seeks to develop a rapid technique to detect carbonate saturation state of seawater based on conductivity changes. A series of batch and flow-through experiments were conducted using various CaCO3 materials. Results show that the conductivity ratios of seawater with and without carbonate addition increase generally with decreasing carbonate saturation states (Ω). The relationship between conductivity ratio and log10Ω apparently follows a linear trend when Ω < 1. It suggests that conductivity measurements can be used to indicate carbonate saturation state of seawater. It is expected to be deployed on CTD instrument to produce depth profiles of seawater carbonate saturation state and will be of great help to future studies on ocean acidification

The measurement of pH in saline and hypersaline media at sub-zero temperatures::Characterization of Tris buffers

The pH on the total proton scale of the Tris-HCl buffer system (pHTris) was characterized rigorously with the electrochemical Harned cell in salinity (S) 35 synthetic seawater and S = 45–100 synthetic seawater-derived brines at 25 and 0 °C, as well as at the freezing point of the synthetic solutions (−1.93 °C at S = 35 to −6 °C at S = 100). The electrochemical characterization of the common equimolal Tris buffer [RTris = mTris/mTris‐H+ = 1.0, with mTris = mTris‐H+ = 0.04 mol kgH2O‐1 = molality of the conjugate acid-base pair of 2-amino-2-hydroxymethyl-1,3-propanediol (Tris)] yielded pHTris values which increased with increasing salinity and decreasing temperature. The electrochemical characterization of a non-equimolal Tris buffer variant (RTris = 0.5, with mTris = 0.02 mol kgH2O‐1 and mTris−H+ = 0.04 mol kgH2O‐1) yielded pHTris values that were consistently less alkaline by 0.3 pH unit than those of the equimolal Tris buffer. This is in agreement with the values derived from the stoichiometric equilibrium of the Tris-H+ dissociation reaction, described by the Henderson – Hasselbalch equation, pHTris = pKTris⁎ + logRTris, with pKTris⁎ = stoichiometric equilibrium dissociation constant of Tris-H+, equivalent to equimolal pHTris. This consistency allows reliable use of other RTris variants of the Tris-HCl buffer system within the experimental conditions reported here. The results of this study will facilitate the pH measurement in saline and hypersaline systems at below-zero temperatures, such as sea ice brines

Characterization of meta-Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures

Accurate pH measurements in polar waters and sea ice brines require pH indicator dyes characterized at near-zero and below-zero temperatures and high salinities. We present experimentally determined physical and chemical characteristics of purified meta-Cresol Purple (mCP) pH indicator dye suitable for pH measurements in seawater and conservative seawater-derived brines at salinities (S) between 35 and 100 and temperatures (T) between their freezing point and 298.15 K (25 °C). Within this temperature and salinity range, using purified mCP and a novel thermostated spectrophotometric device, the pH on the total scale (pHT) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as pHT=−log(kT2e2)+log(R−e11−Re3e2) Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and − log(kT2e2) of purified mCP is described by the following functions: e1 = −0.004363 + 3.598 × 10−5T, e3/e2 = −0.016224 + 2.42851 × 10−4T + 5.05663 × 10−5(S − 35), and − log(kT2e2) = −319.8369 + 0.688159 S −0.00018374 S2 + (10508.724 − 32.9599 S + 0.059082S2) T−1 + (55.54253 − 0.101639 S) ln T −0.08112151T. This work takes the characterisation of mCP beyond the currently available ranges of 278.15 K ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40 in natural seawater, thereby allowing high quality pHT measurements in polar systems

Measuring pH in the Arctic Ocean: Colorimetric method or SeaFET?

The suitability of the colorimetric method in a custom-made instrumental set-up and the commercial potentiometric SeaFET®electrode sensor to measure pH in surface oceanic water in the Arctic was investigated during the Chinese Arctic Research Expedition (CHINARE) in summer 2014. The instruments were set up in parallel on the on-board underway seawater supply for 65 days, enabling comparison in various conditions in the Arctic Ocean from the Chukchi Sea to the ice-covered high latitudes (81°N) and the open-water North-western Pacific Ocean. Overall, the instruments yielded pH datasets of similar high quality (method uncertainty ). Detailed comparison with the parallel colorimetric pH measurements indicated that the measurements with the SeaFET external electrode in the low salinity ice-covered area were problematical and that the internal reference electrode failed after almost 2 months of cruise. Reasons for discrepancies between the data from the two instruments and recommendations for the use of either instrument for pH measurements in the Arctic are discussed. Finally, the investigation yielded a reliable high-resolution pH dataset in surface waters along a transect from the Pacific to the Arctic Ocean. Large pH variations were observed in the ice-free Arctic surface waters, with pH ranging between 7.98 and 8.49. The highest pH values were observed at the ice edge, whereas a relatively invariable pH () was measured in under-ice seawater in the ice-covered Arctic Ocean. The high resolution surface seawater pH dataset obtained here could be used as reference to detect the on-going acidification rate in the Pacific Arctic

Norwegian Sea net community production estimated from O2 and prototype CO2 optode measurements on a Seaglider

We report on a pilot study using a CO2 optode deployed on a Seaglider in the Norwegian Sea from March to October 2014. The optode measurements required drift and lag correction and in situ calibration using discrete wa ter samples collected in the vicinity. We found that the op tode signal correlated better with the concentration of CO2, c(CO2), than with its partial pressure, p(CO2). Using the calibrated c(CO2) and a regional parameterisation of to tal alkalinity (AT) as a function of temperature and salin ity, we calculated total dissolved inorganic carbon content, c(DIC), which had a standard deviation of 11 μmol kg-2 compared with in situ measurements. The glider was also equipped with an oxygen (O2) optode. The O2 optode was drift corrected and calibrated using a c(O2) climatology for deep samples. The calibrated data enabled the calcu lation of DIC-and O2-based net community production, N(DIC) and N(O2). To derive N, DIC and O2 inventory changes over time were combined with estimates of air sea gas exchange, diapycnal mixing and entrainment of deeper waters. Glider-based observations captured two periods of increased Chl a inventory in late spring (May) and a second one in summer (June). For the May period, we found N(DIC) = (21±5) mmol m-2 d-1 , N(O2) = (94± 16) mmol m-2 d-1 and an (uncalibrated) Chl a peak con centration of craw(Chl a) = 3 mg m-3. During the June pe riod, craw(Chl a) increased to a summer maximum of 4 mg m-3 , associated with N(DIC) = (85±5) mmol m-2 d-1 and N(O2) = (126±25) mmol m-2 d -1. The high-resolution dataset allowed for quantification of the changes in N be fore, during and after the periods of increased Chl a inven tory. After the May period, the remineralisation of the mate rial produced during the period of increased Chl a inventory decreased N(DIC) to (-3 ± 5) mmol m-2 d-1 and N(O2) to (0 ± 2) mmol m-2 d-1 . The survey area was a source of O2 and a sink of CO2 for most of the summer. The deployment captured two different surface waters influenced by the Nor wegian Atlantic Current (NwAC) and the Norwegian Coastal Current (NCC). The NCC was characterised by lower c(O2) and c (DIC) than the NwAC, as well as lower N(O2) and craw(Chl a) but higher N(DIC). Our results show the poten tial of glider data to simultaneously capture time-and depth resolved variability in DIC and O2 concentrations

Quantification of a subsea CO2 release with lab-on-chip sensors measuring benthic gradients

We present a novel approach to detecting and quantifying a subsea release of CO2 from within North Sea sediments, which mimicked a leak from a subsea CO2 reservoir. Autonomous lab-on-chip sensors performed in situ measurements of pH at two heights above the seafloor. During the 11 day experiment the rate of CO2 release was gradually increased. Whenever the currents carried the CO2-enriched water towards the sensors, the sensors measured a decrease in pH, with a strong vertical gradient within a metre of the seafloor. At the highest release rate, a decrease of over 0.6 pH units was observed 17 cm above the seafloor compared to background measurements. The sensor data was combined with hydrodynamic measurements to quantify the amount of CO2 escaping the sediments using an advective mass transport model. On average, we directly detected 43 ± 8% of the released CO2 in the water column. Accounting for the incomplete carbonate equilibration process increases this estimate to up to 61 ± 10%. This technique can provide long-term in situ monitoring of offshore CO2 reservoirs and hence provides a tool to support climate change mitigation activities. It could also be applied to characterising plumes and quantifying other natural or anthropogenic fluxes of dissolved solutes

The stoichiometric dissociation constants of carbonic acid in seawater brines from 298 to 267 K

The stoichiometric dissociation constants of carbonic acid (K1C∗ and K2C∗) were determined by measurement of all four measurable parameters of the carbonate system (total alkalinity, total dissolved inorganic carbon, pH on the total proton scale, and CO2 fugacity) in natural seawater and seawater-derived brines, with a major ion composition equivalent to that of Reference Seawater, to practical salinity (SP) 100 and from 25 °C to the freezing point of these solutions and −6 °C temperature minimum. These values, reported in the total proton scale, provide the first such determinations at below-zero temperatures and for SP > 50. The temperature (T, in Kelvin) and SP dependence of the current pK1C∗ and pK2C∗ (as negative common logarithms) within the salinity and temperature ranges of this study (33 ≤ SP ≤ 100, −6 °C ≤ t ≤ 25 °C) is described by the following best-fit equations: pK1C∗ = −176.48 + 6.14528 SP0.5 − 0.127714 SP + 7.396 × 10−5 SP2 + (9914.37 − 622.886 SP0.5 + 29.714 SP) T−1 + (26.05129 − 0.666812 SP0.5 ) lnT (σ = 0.011, n = 62), and pK2C∗ = −323.52692 + 27.557655 SP0.5 + 0.154922 SP − 2.48396 × 10−4 SP2 + (14763.287 − 1014.819 SP0.5 − 14.35223 SP) T−1 + (50.385807 − 4.4630415 SP0.5 ) lnT (σ = 0.020, n = 62). These functions are suitable for application to investigations of the carbonate system of internal sea ice brines with a conservative major ion composition relative to that of Reference Seawater and within the temperature and salinity ranges of this study

Detection and quantification of CO2 seepage in seawater using the stoichiometric Cseep method:Results from a recent subsea CO2 release experiment in the North Sea

Carbon Capture and Storage (CCS) is a potential significant mitigation strategy to combat climate change and ocean acidification. The technology is well understood but its current implementation must be scaled up nearly by a hundredfold to become an effective tool that helps meet mitigation targets. Regulations require monitoring and verification at storage sites, and reliable monitoring strategies for detection and quantification of seepage of the stored carbon need to be developed. The Cseep method was developed for reliable determination of CO2 seepage signal in seawater by estimating and filtering out natural variations in dissolved inorganic carbon (C). In this work, we analysed data from the first-ever subsea CO2 release experiment performed in the north-western North Sea by the EU STEMM−CCS project. We successfully demonstrated the ability of the Cseep method to (i) predict natural C variations around the Goldeneye site over seasonal to interannual time scales; (ii) establish a process-based baseline C concentration with minimal variability; (iii) determine CO2 seepage detection threshold (DT) to reliably differentiate released−CO2 signal from natural variability and quantify released−CO2 dissolved in the sampled seawater. DT values were around 20 % of the natural C variations indicating high sensitivity of the method. Moreover, with the availability of DT value, the identification of released−CO2 required no pre-knowledge of seepage occurrence, but we used additional available information to assess the confidence of the results. Overall, the Cseep method features high sensitivity, automation suitability, and represents a powerful future monitoring tool both for large and confined marine areas