Victor W. Truesdale

Iodine in inshore and off-shore marine waters

Abstract The temporal variation of iodate and total iodine in the Menai Straits and Irish Sea is discussed together with station data for the Atlantic Ocean. Surprisingly little temporal variation was found in either iodate or total iodine even though seasonal nutrient cycling occurred within these water bodies. The iodate concentrations of the temperate Atlantic Ocean, Irish Sea and Menai Straits surface waters were significantly different. These results, together with the known hydrology of the Menai Straits, make it likely that iodate-reducing substances present in terrestrial run-off will be responsible for relatively low iodate concentrations in shallow-seawaters, such as the Irish Sea. However, it is suggested that the low iodate concentrations of tropical surface waters are more likely to have a biological origin. The possibility that the marine iodine system is open to a thermo-dynamical interpretation, as some workers have suggested, is refuted in the case of temperate surface waters as well as deep ocean waters (>200 m). It is shown that existing analyses indicate that below 200 m at oceanic stations, iodate does not vary significantly with depth.

‘Reactive’ and ‘unreactive’ iodine in seawater — A possible indication of an organically bound iodine fraction

An iodine component in seawater, which is ‘unreactive’ to the total inorganic method described by Truesdale and Spencer (1974), has been discovered. The component is measured as the increase in ‘reactive’ iodine that accompanies irradiation of seawater with high-intensity UV light. The highest concentration recorded for an inshore water was 5.2 ± 0.5 μg/l. As some known organic-iodine compounds behave in a similar manner to UV irradiation, it is suggested that the ‘unreactive’ iodine is organically bound iodine.

The meridional distribution of dissolved iodine in near-surface waters of the Atlantic Ocean

Abstract Dissolved total iodine and iodate have been measured along an approximately north-east, south-west transect between the British Isles and Falkland Isles. Total iodine showed very little variation between the surface and 200 m. For the surface samples the coefficient of variation was 3.5%. Iodate concentrations varied between 0.30 and 0.45 μM, with the lowest occuring at the equatorial surface. These results together with an earlier similar set for the Pacific Ocean are used to argue that, in general, iodate reduction can be modelled separately from iodine uptake–regeneration. Correlation between iodate and nitrate concentrations was high ( r 2 >0.93) in the permanently stratified waters of the sub-tropics and tropics. However, upwelling off Cape Verde shows that these correlations are the result of hydrographic coupling rather than direct chemical coupling of the two systems.

Biogenic silica dissolution in seawater — in vitro chemical kinetics

Abstract The experimental and mechanistic approaches available for the study of the kinetics of in vitro biogenic silica dissolution are synthesised to provide rigorous guidance for future investigation. Batch and flow-through reactors are discussed, as are the four mechanisms which have been proposed previously within the marine domain. In recognition of success with similar problems in mineral weathering and hydro-metallurgy, the applicability of the Activated Complex Theory is also considered. Full mathematical derivations are presented for the mechanisms before they are evaluated according to contemporary needs. At each stage the problems inherent in fitting mathematical models to experimental data are highlighted. The decrease in reaction rate with time observed in batch reactors is discussed in terms of both the approach towards saturation and decrease in surface area of the dissolving frustules. In appreciating this distinction, optimum combinations of experimental design are suggested for mimicking silica dissolution in either the water column or sediment. The role of organic frustule components, micro-organisms and faecal pellets in the dissolution process are discussed.

The automatic determination of iodate- and total-iodine in seawater

The development of procedures for the determination of iodate- and total-iodine content of seawater, which use a Technicon Auto-Analyser, is described. In both procedures the appropriate iodine species is first converted to iodate-iodine. Then, this is reacted with acid and excess iodide to give the iodonium ion, I3−, which is detected spectrophotometrically. In the total-iodine procedure the pre-oxidation is accomplished using bromine water. In the iodate procedure a pre-oxidation step using iodine-water can be included. It is anticipated that this will be used to test for the presence of naturally occurring reducing agents in seawaters, which by their action on iodonium ions could lead to an underestimate in iodate concentration. Seawaters, particularly coastal and surface oceanic ones, are known to contain iodine-reducing substances. Therefore, the validity of results obtained through the iodometric method for iodate must remain in some doubt until these tests have been made. The use of this method on anoxic waters which contain sulphides appears to be a prime example of where caution should be observed. The iodate procedure, both with and without pre-oxidation, has been tested on approximately 50 samples of waters from the Eastern Pacific Ocean; these waters did not appear to contain significant amounts of reducing agents. In a similar study, it was found that there was no significant difference between the results obtained by the new total-iodine procedure and an earlier automatic catalytic one.

Iodate reduction by Isochrysis galbana is relatively insensitive to de-activation of nitrate reductase activity: are phytoplankton really responsible for iodate reduction in seawater?

Abstract The reduction of iodate to iodide in laboratory cultures of the marine microalga Isochrysis galbana was found to be essentially independent on the presence or absence of nitrate reductase activity. This places serious doubt on the hypothesis that iodate is inadvertently reduced by NR, when it is mistaken for nitrate. NR activity was de-activated by growing the algae with tungsten instead of molybdenum, and supplying ammonium-N. An alternative mechanism is proposed linking iodate to photo-inhibition in phytoplankton. When observed, iodide production was found to be close to linear with time. A preliminary fit of the Michaelis–Menten model to the graph of iodide production versus iodate substrate concentration is promising, yielding V max and K m (with 95% confidence limits) of 62 (±21) nM day −1 and 87 (±80) μM, respectively. Iodide production was only observed when the iodate concentration was higher than that found naturally. This was also found to be the case with cultures of Skelotenema costatum . Further, no significant iodide production was observed from 0.5 μM iodate in cultures of Cyclotella cryptica and Synechococcus sp. At the higher concentrations of iodate and after rationalisation to the prevailing concentrations of chlorophyll a and iodate, iodide production was typically 0.003 nM-I − μg chlorophyll a −1 day −1 μM −1 -IO 3 − . At most, these dense cultures were only able to reduce 0.3% of the prevailing iodate, and it is argued that this is too low to explain the 0.2 μM reduction commonly experienced across the temperate continental shelf or within the photic zone of the tropical water column.

The distribution of iodine in the Baltic Sea during summer

Iodate and total inorganic iodine concentrations have been measured in near surface and intermediate waters (0–200 m) of the Baltic Sea, south of latitude 59°N, during August 1999. Additionally, organic-iodine was measured at seven selected stations in August 2000. Overall, iodines behaviour is not conservative and is consistent with its pattern of biogeochemical cycling observed elsewhere in the oceans coupled with the particular hydrographic characteristics of this fjord. Thus, total inorganic iodine concentrations rationalised to salinity 35 were higher (∼0.60 μM) in waters on the bottom or at about 200 m, above the Deeps, presumably as a result of re-mineralisation. Meanwhile, iodate was more concentrated near the surface than on the bottom, suggesting iodate reduction in the near bottom waters. Despite the overall non-conservative pattern, in much of the water body total inorganic iodine appeared to mix conservatively between water outside the main, Darss Sill (ΣI=0.16 μM, 13 S) and that within the Sea, itself (ΣI=0.09 μM, 6 S). Meanwhile, iodate concentrations were high in the intruding water but very low behind the sills (∼0.04 μM), showing little change at salinities less than 12. Organic iodine was generally low (0.010 μM) but as high as 0.040 μM at 60 m at one station.

Dissolved iodate and total iodine along the British east coast

The distribution of iodine in the North Sea, along the east coast of the Britain in late winter (March 1999) was found to be largely consistent with that reported earlier for the British west coast during various seasons. This supports the contention of a steady state condition for iodine in British waters with mixing between three end-members, one immediately offshore, and others in the central North Sea and the ocean proper (the North Atlantic Current). In the English Channel the oceanic end-member seems to be a more southerly Atlantic water containing significantly lower iodate concentration than the North Atlantic Current. This observation shows that the steady-state condition is limited to high latitudes. In this survey the beginnings of an estuarine mixing system was encountered and, for the first time, this paper has been able to link estuarine iodine chemistry directly to that well offshore. It is deduced that the iodine chemistry of coastal water is established on a longer time scale than that needed for estuarine mixing, and that it is the iodine chemistry of coastal waters which determines the behaviour of iodine in the estuary.

Iodine distribution in the Southern Benguela system during an upwelling episode

Iodate and total iodine concentrations have been measured at eight stations along the coast of South Africa, between Cape Town and Cape Columbine, during a period of upwelling. At most, iodine uptake from the water amounted to 0.020 μM, and appeared to be entirely iodate; iodate reduction to iodide was negligible. Meanwhile the profiles of the major plant nutrients and chlorophyll-a showed that the water column was heavily influenced by processes of productivity. Overall then, the system reflected minimal iodate reduction accompanied by extensive uptake of nutrients. This challenges the conventional view, gained from the study of tropical and sub-tropical waters, that iodate reduction in the oceans is coupled closely to nutrient uptake and primary production. The implications of this are explored.

A re-assessment of Redfield correlations between dissolved iodine and nutrients in oceanic waters and a strategy for further investigations of iodine

Abstract The use of the Redfield nutrient model to explain the behaviour of iodine in the oceanic water column is re-assessed using a rigorous theoretical approach together with existing hydrographic information. It is shown that tests of the models applicability need to be based upon stations which display a marked turning point in their nutrient profile. Stations that do not, e.g. North and South Atlantic, should be avoided. The earlier use of mixtures of the two types is shown to have biased the conclusion in favour of a fit with Redfield. Reasons are given for believing that, in comparison with the plant nutrients, a greater proportion of iodine is re-cycled in the shallow domain of the profile. The possible correlations between inorganic iodine and the Redfield variables are considered from both theoretical and empirical standpoints.