Sven Blomqvist

Formation of Fe(III) oxyhydroxide colloids in freshwater and brackish seawater, with incorporation of phosphate and calcium

Abstract The formation of Fe(III) oxyhydroxide colloids by oxidation of Fe(II) and their subsequent aggregation to larger particles were studied in laboratory experiments with natural water from a freshwater lake and a brackish coastal sea. Phosphate was incorporated in the solid phase during the course of hydrolysis of iron. The resulting precipitated amorphous Fe(III) oxyhydroxide phases were of varying composition, depending primarily on the initial dissolved Fe/P molar ratio, but with little influence by salinity or concentration of calcium ions. The lower limiting Fe/P ratio found for the solid phase suggests the formation of a basic Fe(III) phosphate compound with a stoichiometric Fe/P ratio of close to two. This implies that an Fe/P stoichiometry of ≈2 ultimately limits the capacity of precipitating Fe(III) to fix dissolved phosphate at oxic/anoxic boundaries in natural waters. In contrast to phosphorus, the uptake of calcium seemed to be controlled by sorption processes at the surface of the iron-rich particles formed. This uptake was more efficient in freshwater than in brackish water, suggesting that salinity restrains the uptake of calcium by newly formed Fe(III) oxyhydroxides in natural waters. Moreover, salinity enhanced the aggregation rate of the colloids formed. The suspensions were stabilised by the presence of organic matter, although this effect was less pronounced in seawater than in freshwater. Thus, in seawater of 6 to 33 ‰S, the removal of particles was fast (removal half time 900 h). Overall, oxidation of Fe(II) and removal of Fe(III) oxyhydroxide particles were much faster in seawater than in freshwater. This more rapid turnover results in lower iron availability in coastal seawater than in freshwater, making iron more likely to become a limiting element for chemical scavenging and biologic production.

Phosphate exchange across the sediment-water interface when shifting from anoxic to oxic conditions - an experimental comparison of freshwater and brackish-marine systems

Comparative, experimental studies on sediment cores from freshwater andbrackish-marine conditions reveal major differences in the benthic exchangeof phosphate across the sediment-water interface when shifting from anoxicto oxic conditions. The flux of phosphate to the sediment during this shiftwas found to be mediated mainly by scavenging from newly formed colloidalferric oxohydroxide. The capacity of the iron-rich particles to scavengephosphorus depended on the stoichiometric ratio between dissolved iron andphosphorus built up in the supernatant water during reducing conditions. Thefreshwater system was characterized by high iron to phosphorus ratios in thedissolved phase and thus most of the phosphate was incorporated into thecolloidal iron oxohydroxide during the oxygenation. In contrast, the marinesystems reached lower iron to phosphorus ratios during the anoxic period whichresulted in less efficient phosphate scavenging. Consequently, significantamounts of phosphate remained dissolved in the marine systems after the changeto oxic conditions, possibly increasing the proportion of phosphate recycledto the euphotic zone. Manganese showed a consistent redox-dependent behaviourin all the investigated systems, but interacted neither with phosphate norwith iron.

Hydrological alterations with river damming in northern Sweden: Implications for weathering and river biogeochemistry

Hydrological alterations with river damming in northern Sweden : Implications for weathering and river biogeochemistry

An improved Kajak-type gravity core sampler for soft bottom sediments

A new, modified model of the Kajak tube core sampler is presented. The new sampler is intended for use on soft sediments, and hence of light weight (12 kg). The improved sampler offers the following advantages. (1) increased robustness, (2) simplified attachment of the coring tube to the holder and valve assembly, permitting quick exchange of sampling tubes, (3) unimpeded flow of water through the tube during slow descent, (4) a reliable, automatic mechanism closing the top of the core tube after impact, and (5) an easily dismantled supporting frame, which also protects the sampler from damage against the side of the boat in rought weather. The operation of the sampler has been observedin situ and during ordinary shipboard collection of core samples, with satisfactory results.

Heavy metal decrease in the sediments of a Baltic bay following tertiary sewage treatment

Abstract Concentrations of copper, iron, lead, and zinc were recorded in soft bottom sediments of a Baltic bay prior to, and seven and a half years after the start-up of a modern tertiary sewage treatment plant. In general, the metal levels declined. In the surficial layer, the most pronounced decreases were recorded near the point of effluent discharge. Strong linear, inter-element correlations indicate copper, iron and zinc to be chiefly of petrogenic origin. Normalizing metal concentrations to iron suggests the decline in these metals to be due primarily to dilution. Only lead showed an independent decrease in the area. In contrast to the metals, organic matter content increased after plant start-up. Our study demonstrates the utility of analysing detrital petrogenic matter when studying the metal pollution of coastal marine soft bottom sediments.

A benthic sled for sampling soft bottoms

A new benthic sled is described.Favourable features of the sled include: simple operation, broad runners, balanced towing position, simple height adjustment of the sediment cutting edge and quick exchange of the collecting bag. Metallic parts are aluminium, making the sled light (15 kg). This sled has functioned well in coastal research for more than a decade.

Influence of phosphate concentration and reaction temperature when using the molybdenum blue method for determination of phosphate in water

Abstract The formation rate in water of the blue phosphoantimonylmolybdenum complex declines both with decreasing phosphate concentration (studied range 5–500 μg PO 4 -P l −1 ) and decreasing reaction temperature (studied range + 1 to + 21°C). At + 15°C, only solutions containing ≥ 50 μg PO 4 -P l −1 reached full colour expression within 5 min, and at + 10°C all tested concentrations needed a longer reaction time. At temperatures below + 15°C, not even a reaction time of 10 min is always sufficient for complete colour development. For instance, at + 7°C, maximum absorbance level is reached in 10 min only at concentrations ≥ 250 μg PO 4 -P l −1 , whereas a solution of 5 μg PO 4 -P l −1 requires about 30 min. Slow formation rate of the coloured complex is a possible, insidious source of under-estimation, particularly when determining low concentrations of phosphate.

Sampling performance of Ekman grabs — in situ observations and design improvements

The performance of five types of Ekman grab: the Birge-Ekman, the Kebo Lab-Ekman, the Hydro-Bios-Ekman, the Winnipeg-Ekmanand the Håkanson-Ekman, were examined in situ by SCUBA diving. Trials and observations are presented, and photographic documentation provided. Each type of grab often produced unrepresentative samples due to operation deficiencies. Tilting, brimming, and entry of sediment from outside the sample chamber, as well as redistribution, resuspension and loss of enclosed matter are the main biasing factors. Since the grabs under study are models in general use, these findings suggest that caution must be exercised when evaluating studies based on Ekman grabs of the usual design. Sampling reliability can be significantly improved by furnishing the grabs with a supporting stand and a mechanism which secures the lids during ascent. Tested designs for such devices are presented. A properly operated Ekman grab of adequate design is a suitable instrument for collecting certain types of samples such as benthic macrofauna, and infauna in particular.

The Challenge of Sedimentation in the Baltic Sea

This chapter focuses on the sinking of particulate matter through the water column of the Baltic Sea. This downward flux represents a loss from the pelagic system, as well as a link between the pelagic and benthic realms, transferring matter and energy to the deep water and to the sediments. From an applied perspective, knowledge of the factors governing formation and subsequent sedimentation of the particulate matter is vital for a proper understanding of changes in the pelagic and benthic realms of the Baltic Sea. We provide here a brief overview of those processes of sedimentation that have received particular attention in Baltic studies, and address some critical current issues related to this topic, and hope to encourage further efforts in this field of Baltic Sea research.

A device for rapid sectioning of soft bottom sediment cores

A new mechanical sectioning device for transverse cutting of cylindrical soft bottom sediment cores is presented. The instrument is rapid and efficient in use, works serially when sectioning a core and permits rinsing of the sample compartment without removal from the coring tube. The device is made of plastic, is lightweight, convenient to operate even in the field and simple and cheap to manufacture.