A study of peatland-derived dissolved organic matter from headstream to sea using multiple analytical tools.

The River Thurso, North Scotland, receives substantial terrestrial deliveries of dissolved organic matter (DOM) leached from Europe's most extensive blanket bogs. The relatively short distance between peatlands and coastal ocean offers potential for research to investigate source-to-sea processing of terrigenous dissolved organic carbon (DOC). Here, we determined DOC concentrations in the bulk (< 0.4 µm), truly dissolved (< 5 kDa), and colloidal fraction (5 kDa - 0.4 µm) as well as DOM absorbance and fluorescence spectra during two river catchment surveys and two corresponding coastal plume surveys, in early spring (1st sampling period) and late spring (2nd sampling period). DOC concentrations ranged from 79 to 3799 µM in early spring and from 115 to 5126 µM in late spring. DOM exhibited conservative mixing across the plume in both surveys, but the plume extended further offshore in the second survey due to a pulse of freshwater caused by recent rainfall. Fluorescence excitation-emission matrices (EEMs) and fluorescence indices revealed that the flushed DOM was humic-like, recently synthesized DOM. Coupled with C/N ratio analyses and molecular weight fractionation, the fluorescence indices also provided evidence for the gradual altering of DOM characteristics along the bog - headstream - loch - river continuum. The same analytical tools revealed that seasonal variations occurred within the DOM pool of marine origin, i.e., greater abundance of low-molecular weight bacterial or algal DOM in the late spring survey. The time scale of such variations relative to the flushing time of water through the aquatic continuum should be taken into account when interpreting the DOM property-salinity distributions of major river plumes.

The effects of low pH on the taste and amino acid composition of tiger shrimp.

Recent research has revealed that shrimp sensory quality may be affected by ocean acidification but we do not exactly know why. Here we conducted controlled pH exposure experiments on adult tiger shrimp, which were kept in 1000-L tanks continuously supplied with coastal seawater. We compared survival rate, carapace properties and flesh sensory properties and amino acid composition of shrimp exposed to pH 7.5 and pH 8.0 treatments for 28 days. Shrimp reared at pH 7.5 had a lower amino acid content (17.6% w/w) than those reared at pH 8.0 (19.5% w/w). Interestingly, the amino acids responsible for the umami taste, i.e. glutamate and aspartic acid, were present at significantly lower levels in the pH 7.5 than the pH 8.0 shrimp, and the pH 7.5 shrimp were also rated as less desirable in a blind quality test by 40 volunteer assessors. These results indicate that tiger shrimp may become less palatable in the future due to a lower production of some amino acids. Finally, tiger shrimp also had a lower survival rate over 28 days at pH 7.5 than at pH 8.0 (73% vs. 81%) suggesting that ocean acidification may affect both the quality and quantity of future shrimp resources.

Alteration of the Copper-Binding Capacity of Iron-Rich Humic Colloids during Transport from Peatland to Marine Waters.

Blanket bogs contain vast amounts of Sphagnum-derived organic substances which can act as powerful chelators for dissolved iron and thus enhance its export to the coastal ocean. To investigate the variations in quantity and quality of these exports, adsorptive cathodic stripping voltammetry (CSV) was used to characterize the metal binding properties of molecular weight-fractionated dissolved organic matter (MW-fractionated DOM) in the catchment and coastal plume of a small peat-draining river over a seasonal cycle. Within the plume, both iron- and copper-binding organic ligands showed a linear, conservative distribution with increasing salinity, illustrating the high stability of peatland-derived humic substances (HS). Within the catchment, humic colloids lost up to 50% of their copper-binding capacity, expressed as a molar ratio to organic carbon, after residing for 1 week or more in the main reservoir of the catchment. Immediately downstream of the reservoir, the molar ratio [L2]/[Corg], where L2 was the second strongest copper-binding ligand, was 0.75 × 10-4 when the reservoir residence time was 5 h but 0.34 × 10-4 when it was 25 days. Residence time did not affect the carbon specific iron-binding capacity of the humic substances which was [L]/[Corg] = (0.80 ± 0.20) × 10-2. Our results suggest that the loss of copper-binding capacity with increasing residence time is caused by intracolloidal interactions between iron and HS during transit from peat soil to river mouth.

Seasonal variations in surface water chemistry at disturbed and pristine peatland sites in the Flow Country of northern Scotland.

Weekly monitoring of surface water chemistry took place over a one-year period in a small boggy sub-catchment of the River Thurso, northern Scotland. Monitoring started 6 months after the felling to waste of plantation conifers. The chemistry of ground surface waters was monitored at four bog sites situated in former forestry plots as well as one control site situated in an intact bog. The chemistry of the receiving stream (Sleach Water) was monitored at seven points along a 2 km stretch. Dissolved organic carbon and metals were very significantly affected by seasonal factors. On land, seasonal variations accounted for between 35% (Al) and 80% (Fe) of the total variance in the data at the intact bog site, with similar seasonal effects observed at the impacted sites. The amplitude of the seasonal signal was generally much higher at the impacted sites than at the control site. Except for dissolved Al and Mn, the chemical composition of the stream was only marginally influenced by surface runoff from the felled plantation despite evidence of intense seasonal mobilisation of e.g. DOC, K or Fe at or near the ground surface within the felled plots. This was attributed to the presence of a buffer zone between the plantation and the stream. On the other hand, surface inputs from former forestry plots caused measurable increases in stream water [Al] and [Mn]. The likely sources of Al and Mn were the disturbance of the mineral soil that had taken place some 20 years previously as a result of forestry ground preparation and the leaching from the recently felled conifer material, respectively. Such inputs occurred in late autumn or winter for Al and in summer for Mn, thus intensifying their natural seasonal patterns in this peat draining stream.

Evidence for strong but dynamic iron-humic colloidal associations in humic-rich coastal waters.

This study investigated the physicochemical forms of dissolved iron in the coastal plume (salinity = 28-35) of a small river draining a peat-rich catchment. Speciation information was obtained through a combination of fractionation by crossflow filtration (CFF) along with voltammetric detection of either naturally occurring iron-humic complexes (July survey) or known, synthetic complexes (September survey) formed by titrating the samples with the competing ligand 2-(2-thiazolylazo)-p-cresol (TAC). The majority of colloidal iron (>5000 Da) was present as iron-humic complexes supplied by the river and showing uniform conditional stability constants throughout the plume (log K'(Fe'HS) = 11.3 ± 0.1, i.e. log K(Fe3+HS) = 21.3 ± 0.1). Noncolloidal or soluble iron was strongly complexed to ligands of marine origin with log K'(Fe'HS) = 11.9 ± 0.1. Equilibrium of the total iron pool with the added TAC ligand was achieved in all but the highest salinity sample, albeit more slowly for colloidal than for soluble iron. In addition, measurements of humic like fluorescence suggested that the conformation of colloids could change over time as a result of dissociation of the iron-humic associations. These results are consistent with the concept that iron in coastal waters is strongly but reversibly bound to humic substances and therefore may be available for complexation by siderophore-type ligands released by microorganisms.

Estimating the organic acid contribution to coastal seawater alkalinity by potentiometric titrations in a closed cell.

This paper examines the performance of a previously reported, closed cell, potentiometric titration technique [J.M. Hernández-Ayón, S.L. Belli, A. Zirino, Anal. Chim. Acta 394 (1999) 101] for the simultaneous determination of pH, total inorganic carbon (TCO2), total alkalinity (TA), and organic alkalinity (OA) in coastal seawater samples. A novel interpretation of the titration data, as recently proposed by Hernández-Ayón et al. [J.M. Hernández-Ayón, A. Zirino, A.G. Dickson, T. Camiro-Vagas, E. Valenzuela-Espinoza, Limnol. Oceanogr.: Methods 5 (2007) 225] who applied it to waters of unusually high organic matter content, was applied here to fjord surface waters collected over the duration of a phytoplankton bloom. The parameters pH and TCO2--combined with knowledge of boric, phosphate and silicate species concentrations--allowed calculation of all inorganic species that contributed to TA. This inorganic alkalinity term was then subtracted from TA to produce an estimation of OA. Although the OA values obtained were very small (2-22+/-3 micromol L(-1)), they showed a reproducible trend over time in two simultaneous experiments. The organic acids that may have contributed to OA were characterised in back titrations of acidified and CO2-stripped samples with CO2-free NaOH. Two classes of organic titratable species, with pK(a) values around 4.0+/-0.2 and 9.1+/-0.2 were detected. The first occurred in concentrations that co-varied linearly (r2=0.75) with protein-like fluorescence, indicating a marine biological source, but were only weakly correlated (r2=0.46) to OA. By contrast, Class 2 organic species were not significantly correlated to any fluorescence component of either marine or terrestrial origin but were linearly correlated to OA (r2=0.69). These new results reveal that the method proposed by Hernández-Ayón et al. [J.M. Hernández-Ayón, A. Zirino, A.G. Dickson, T. Camiro-Vagas, E. Valenzuela-Espinoza, Limnol. Oceanogr.: Methods 5 (2007) 225] for estimating OA can provide a powerful and hitherto unused tool for analysing DOM dynamics and sources in most coastal environments, i.e. as a complement to the more widely used optical tools.