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Voltammetry in the study of unusual phenomena in the sea Part I: organic matter characterization during red tide and gelatinous zooplankton blooms in the northern Adriatic

Organic carbon (C) is a constituent of many processes in the sea and can be an important indicator of changes in marine system [1]. Due to climate changes, extreme natural weather events (droughts, floods, tropical cyclones, heat waves, intense precipitation i.e. heavy rains, abrupt changes in temperatures, salinity, circulation and mixing, strong winds, Saharan dust transport) are becoming more frequent. Such events lead to unusual phenomena such as mucilaginous algal blooms, red tides, invasive jellyfish blooms, which contribute to eutrophication characterized by anomalous accumulation of organic C in the sea [1-4]. In seawater, organic matter (OM) is a complex of organic compounds (proteins, carbohydrates, lipids, humic and fluvic acids) with different surface active properties [1, 2]. Surface active substances (SAS) can be autochthonously produced mainly through primary and secondary production, release or as secondary metabolites under feeding pressure or allochthonously introduced into the marine system from land and atmosphere. An important property of SAS is that it contains hydrophobic (e.g. fatty acid chains, aromatic rings or hydrocarbons) and hydrophilic functional groups (e.g. -NH2, - COOH, -OH or -SH) that allow the accumulation of SAS at different phase boundaries (e.g. marine surface- atmosphere, particle-aqueous phase, bottom layer- sediment, pycnocline in a stratified water column). Due to such properties, the presence of SAS in water samples can be monitored by adsorption processes at the hydrophobic surface of the mercury (Hg) as a working electrode, by a simple electrochemical method of phase sensitive alternating current (AC) voltammetry [1-3]. Changes of the Hg electrode double layer reflected on the capacitance current are approximately proportional to all (dissolved and particulate) SAS in the solution. The total SAS can be separated into particulate and dissolved fractions by filtering through a 0.45 μm or 0.7 μm filters. SAS content usually can be expressed in the equivalents of the selected model OM substance of Triton-X-100 (eq. T-X- 100) which is considered as a good representative of the most reactive part of the SAS in natural waters [1-3 and references therein]. SAS by normalization to dissolved organic carbon (DOC) content can be expressed as normalized surfactant activity, NSA=[SAS(eq. T-X- 100)/DOC] [2, 3], comparable to NSA of model substances, as representatives for SAS composition of different natural samples. This study will present properties of OM compounds, studied based on their reactivity (hydrophobic properties) during unusual phenomena in the northern Adriatic Sea: a) gelationous zooplankton blooms of the invasive ctenophore Mnemiopsis leidyi (Fig.1.A) [4] and b) red tide, mostly composed of Noctiluca scintillans bloom (Fig.1.B). These events were characterized by remarkable increase of organic C content in otherwise oligotrophic area. OM with surface active properties was recorded both in dissolved (SAS 0, 081-15, 379 mg/L ; DOC 0, 875- 318, 100 mg/L) and particulated fractions (SAS 0, 113-18, 122 mg/L ; POC 1, 666-18, 122 mg/L). The NSA values indicate the presence of more hydrophobic nature of DOC during the gelatinous zooplankton bloom. References [1] I. Ciglenečki, I. Vilibić, J. Dautović, V. Vojvodić, B. Ćosović, P. Zemunik, N. Dunić, H. Mihajlović, Sci. Total Environ. 730 (2020) 139104. [2] Ćosović, B., Vojvodić V., 1998. Electroanalysis, 10(6), 429- 434. [3] I. Ciglenečki, J. Dautović, A. Cvitešić, G. Pletikapić, Croat. Chem. Acta 91(4) (2018) 455- 461. [4] Ciglenečki, I., Paliaga, P., Budiša, A., Čanković, M., Dautović, J., Đakovac, T., Dutour- Sikirić, M., Kraus, R., Kužat, N., Lučić, D., Marić Pfannkuchen, D., Njire, J., Pasarić Z., Supić, N., Under review: Journal of Marine Systems.

voltammetry ; unusual phenomena ; organic matter ; northern Adriatic Sea ; red tide blooms ; gelationous zooplankton blooms

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