ARSENIC SPECIATION AND SEASONAL-CHANGES IN NUTRIENT AVAILABILITY AND MICRO-PLANKTON ABUNDANCE IN SOUTHAMPTON WATER, UK

The links between dissolved arsenic speciation, biological activity an d the availabilities of the nitrogen and phosphorus plant nutrients ha ve been investigated in a seasonal survey of Southampton Water (U.K.). Southampton Water (Hampshire, southern England) is an approximately 1 0 km long, and 2 km wide north-westerly extension of the Solent, recei ving water hom the rivers Test and Itchen. It is a partially mixed est uary bordered by broad intertidal mudflats with shingle and sand on th e eastern side, and a salt marsh to the west. Two sites were chosen: N W Netley Buoy is in a sheltered high-salinity estuarine environment wh ilst Calshot Buoy lies just outside Southampton Water and in a more ex posed location of less-variable salinity. The first evidence of arseni c(III) production at both sites occurred in the second half of April, during the decay of a major Skeletonema costatum diatom bloom. Arsenic (III) levels rose as Skeletonema was replaced by a numerically smaller but more chlorophyll-rich bloom of another diatom, Rhizosolenia delic atula. Rhizosolenia is therefore implicated as a possible source of ar senic(III). Methylated arsenic was absent whilst the mater temperature was low and during the initial Skeletonema bloom, but a week later, d uring the growth phase of the succeeding bloom of the diatom R. delica tula, they became detectable. Methylated arsenic levels gradually incr eased through the spring to a broad maximum covering the mid-summer, w hen Mesodinium rubrum, Scrippsiella trochoidea and associated microfla gellates also peaked. No subsequent single organism could be linked to the release of methylated arsenic into Southampton Water; organoarsen icals having been observed in the presence of flagellates, diatoms and ciliates. A large bacterial maximum was observed following blooms of S. trochoidea and M. rubrum but laboratory culture experiments of natu ral bacteria from Netley failed to produce significant changes in the concentration of any arsenic species. Phosphate depletion did not appe ar to be a prerequisite for arsenate assimilation. From the summer pea k methylated arsenic levels then gradually diminished to undetectable levels in the winter months. Monomethylarsenic, present at concentrati ons approximately 50% those of dimethylarsenic, persisted longer throu gh the summer. The arsenic species which can be measured using the hyd ride procedure may therefore represent intermediates in the decomposit ion of the bioarsenicals, such as arsenosugars, which are released, ei ther actively as excretion/secretion products or passively as part of the decay process. Currently unidentified precursors of hydride-reduci ble arsenic species ('hidden' arsenic) may explain the poor link betwe en planktonic activity and the levels of measurable arsenic species in the water column.