Carotenuto, Angelo (2025) Phytoplankton production dynamics with automatic in-situ probe type WIZ (Water In situ Analyzer). [Tesi di dottorato]

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Abstract

Dissolved inorganic nutrients play a pivotal role in the regulation of marine primary productivity, exerting a profound influence on phytoplankton community, which respond rapidly with changes in terms of biomass and functional traits. The present study aims to investigate the short-term variations in the concentrations of macronutrients -nitrates and orthophosphates- in the surface waters of the Fusaro Lagoon (Italy) and their impact on phytoplankton community structure. To achieve this, a novel in-situ analyser instrument has been used, namely W.I.Z. probe (Water In-Situ Analyzer), designed for high-frequency automatic samplings. The W.I.Z. probe has been monthly deployed from October 2023 to October 2024, and set-up with samples frequencies of 40-minute intervals. To shed light on the potential effects of nutrient variations on the phytoplankton community structure and validate data from the W.I.Z. probe, surface water samples were collected manually at three-hour intervals for the analyses of phytoplankton functional traits and nutrient analyses in laboratory by an easy-chem plus nutrient analyser. The results showed high concentrations of nitrogen and phosphorus in December, in a period characterized by light limitation, contrarily to what observed in summer when values were almost undetectable, highlighting the pivotal role of biological communities in regulating nutrient availability in the water column. A significant correlation was found between the size class nano-phytoplankton and the fucoxanthin/chlorophyll a ratio in October 2024, indicating a predominance of diatoms in this period. This contrasts with what observed in August 2024, when the chlorophyll-a values reached 12 μg/L and the biomass was mainly dominated by cyanophycean and dinoflagellates as highlighted by the relative higher concentration of their marker pigments, zeaxanthin and peridinin, respectively. The findings highlight the WIZ probe's reliability in quantifying nitrate and orthophosphate concentrations with a high frequency sampling over extended periods. The WIZ probe serves as a reliable investigative tool for detecting nutrient concentrations in coastal waters representing and a valuable instrument for ecological studies. The fluctuations observed in nutrient concentrations, N/P ratios, total biomass, size classes and pigmentary spectra represent a fundamental interpretative element for the correct analyses of the ecological processes in the Fusaro lagoon, not only for dynamics on a circadian scale, but also seasonal and annual scales.

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