Rhenium distribution and behaviour in the salinity gradient of a highly stratified estuary and pristine riverine waters

The abundance and distribution of dissolved Re (DRe) has been determined in the freshwater part of the Krka River (Croatia), draining the karstic carbonate landscape, as well as in the salinity gradient of its highly stratified estuary. Due to low DRe concentration, the batch procedure consisting of pre-concentration step using an anion exchange resin (Dovex) and analysis of DRe in 8 M HNO 3 eluate using a high resolution inductively coupled plasma mass spectrometry (HR ICP-MS) was adopted. Due to the potential inconsistent recoveries, ranging from 60% to 87%, the quantification was performed by using the isotope dilution (ID) technique. The DRe concentration in the Krka River basically increased in the downstream direction, with the 6.2 pM at the spring site, reaching 11.9 pM before the estuarine region. The weathering of the surrounding carbonate lithology is assumed as the source of natural Re. The two specific anomalies were registered: a strong increase in DRe concentration due to the anthropogenic input near the town of Knin (27.5 pM) and a decrease at downstream site caused by the subterranean input of freshwater of the Zrmanja River leading to relatively low DRe concentration (8.5 pM). In the estuarine segment, a near-conservative behavior of DRe was found in the salinity gradient of the upper surface layer, with the DRe concentration ranging from 18 to 38 pM. An anthropogenic supply was suggested within the estuarine segment close to the urban area causing a small positive deviation from the conservative line. In the bottom seawater layer, a minor decrease of DRe concentration in the most upstream estuarine regions was evidenced, implying a potential weak scavenging of Re.

Control of salinity in an estuary by a transition

A theoretical reason is developed to explain why the mouth of a vertically stratified estuary should act as a constraint on the amount of salt water available for mixing in the estuary. Flume experiments designed to test this idea are described. The effect of tides on the control action and application to various estuaries is discussed.

Rhenium distribution and behaviour in the salinity gradient of a highly stratified estuary

<p>In oxic waters, Re<sup>VII </sup>is the stable oxidation state which undergo hydrolysis to the relatively unreactive perrhenate ion, Re<sup>VII</sup>O<sub>4</sub><sup>- [1]</sup>. The oceanic dissolved Re exhibits quite conservative behaviour with the concentration of about 40 pM <sup>[2]</sup>. Despite the frequent utilization of Re for the atmosphere and the ocean past redox state reconstructions, the geochemical behaviour of Re in the modern surface environments such as rivers, estuaries as well as in seawater is not well studied. Understudy is partially arising from the fact that Re has low seawater and riverine concentration of 4 pM and 16.5 pM, respectively<sup></sup><sup>[1, 3]</sup>. In the Amazon and the Hudson estuaries, in crease of Re concentration at low and middle salinity regions is observed<sup> [4]</sup>. On the other hand, Re exhibits complete conservative behaviour in Indian river estuaries, i.e. Narmada, Tapi and the Mandovi estuaries in the Arabian Sea and the Hooghly estuary in the Bay of Bengal <sup>[5]</sup>. Deviation from conservative behaviour in Re can be explained as the interplay of variety of factors including the nature and composition of the particles, Eh-pH conditions, biological productivity and fate of the organic matter. <sup>[5]</sup>.</p><p>Here we present the Re concentration profiles in the freshwater part of the karstic Krka river (Croatia) and its 23 km long estuarine segment, covering a full salinity range (0.1 to 38.6). Analysis of Re was performed by its preconcentration and separation using an anion exchange resin (Dowex 1X8) followed with the ICP-MS quantification using isotope dilution (ID) method. The Krka River spring is characterised by the low Re concentration (~6 pM). A noticeable anthropogenic influence at the point of the wastewater discharge of the Knin town was observed (27 pM). This input probably caused a progressive downstream increase of Re concentration to 12 pM at the freshwater end-member in the winter period (with a high Krka River discharge) and 17 pM in the summer period (low Krka River discharge). In the estuarine segment, a near-conservative behaviour of Re was found, with the "oceanic" concentration of 38 pM in the seawater end-member.</p><p> </p><p>References:</p><p>[1] Hasse AA et al., (2019) Coordination chemistry reviews 394: 135-161.</p><p>[2] Anbar AD et al., (1992) Geochimica et Cosmochimica Acta<strong> </strong>56:4099-4103.</p><p>[3] Miller CA et al., (2011) Geochimica et Cosmochimica Acta 75:7146-7179.</p><p>[4] Colodner D et al., (1993) Earth and Planetary Science Letters 117:205-221.</p><p>[5] Rahaman W and Singh SK (2010) Marine Chemistry 118: 1-10.</p>

Phytoplankton Dynamics in the Middle Adriatic Estuary, with a Focus on the Potentially Toxic Genus Pseudo-nitzschia

The Krka River estuary is a karstic, permanently stratified estuary due to the strong freshwater inflow. It is a special environment in which to study the phytoplankton community, especially because this area is an important aquaculture site. Among other potentially toxic phytoplankton species, the diatom genus Pseudo-nitzschia occurs frequently and is a potential source of domoic acid (DA), causing shellfish toxicity and human intoxication. The main objective was to examine the dynamics of the phytoplankton community and, in particular, the genus Pseudo-nitzschia in the upper part of the Krka estuary, through monthly sampling over two years. The phytoplankton community was analysed using light microscopy and scanning electron microscopy to determine the diversity of Pseudo-nitzschia species and characterise the environmental parameters associated with a high abundance of Pseudo-nitzschia species. Seven Pseudo-nitzschia species were identified in the investigation, with higher frequencies and abundances in the less variable layer, at a 7 m depth. Blooms of Pseudo-nitzschia were noted in the late summer/early autumn, dominated by P. delicatissima/arenysensis. Winter assemblages were characterised by P. pseudodelicatissima/cuspidata, P. calliantha, and P. subfraudulenta, and were associated with domoic acid occurrence in shellfish.