The Impact of MOSE (Experimental Electromechanical Module) Flood Barriers on Microphytobenthic Community of the Venice Lagoon

Abstract

MOSE is a system of mobile gates engineered to temporarily isolate the Venice Lagoon from the Adriatic Sea and to protect the city from flooding during extreme high tides. Within the framework of the Venezia2021 program, we conducted two enclosure experiments in July 2019 (over 48 h) and October 2020 (over 28 h) by means of 18 mesocosms, in order to simulate the structural alterations that microphytobenthos (MPB) assemblages might encounter when the MOSE system is operational. The reduced hydrodynamics inside the mesocosms favored the deposition of organic matter and the sinking of cells from the water column towards the sediment. Consequently, MPB abundances increased over the course of both experiments and significant changes in the taxonomic composition of the community were recorded. Species richness increased in summer while it slightly decreased in autumn, this latter due to the increase in relative abundances of taxa favored by high organic loads and fine grain size. By coupling classical taxonomy with 18S rRNA gene metabarcoding we were able to obtain a comprehensive view of the whole community potential, highlighting the complementarity of these two approaches in ecological studies. Changes in the structure of MPB could affect sediment biostabilization, water turbidity and lagoon primary production.

Keywords: Venice Lagoon; benthic diatoms; classic taxonomy; microphytobenthos; next generation sequencing.

Figure 1

(A) Study area; (B) Experimental design: 18 mesocosms were placed in 3 groups of 6 to assure sampling in 3 replicates per 5 experimental times (* T4 not present in October 2020, ** not the topic of this work, see the text, ADn: additional backup mesocosms); (C) Mesocosms placed in Palude di Cona; (D,E) Detail of the mesocosms; (F) MOSE gates in operation (source: www.mosevenezia.eu, accessed on 23 March 2023).

Figure 2

Microphytobenthic total abundance divided in the main microalgal groups for each sampling time (T) and enclosure replicate (R): (A) in July 2019 and (B) in October 2020.

Figure 3

K-dominance curves applied to the microphytobenthic community divided by experimental time (n = 3) (A) in July 2019 and (B) in October 2020.

Figure 4

Non-metric MDS ordination plots calculated on the overall MPB abundances (A) from T0 to T4 in July 2019 and (B) from T0 to T3 in October 2020. The most abundant species (RA ≥ 5%) and the environmental variables are overlaid as vectors; circles delimit clusters obtained applying the SIMPROF analysis (complete linkage, similarity threshold = 0.05, n. of permutations = 999). (C-LIP = lipids, C-PRT = proteins, C-H₂O = water soluble carbohydrates, C-EDTA = EDTA soluble carbohydrates).

Figure 5

LEfSe results: MPB species discriminative with respect to the experimental time (α-value for the Kruskal–Wallis and Wilcoxon test = 0.05; logarithmic LDA score threshold = 2). (A) July 2019, (B) October 2020.

Figure 6

Comparison between the taxonomic composition of the MPB community obtained through (A) classical microscopy and (B) 18S RNA gene sequencing. The species/ASVs representing relative number of cells/number of reads ≥ 0.5% are shown; averages of three enclosure replicates at each sampling time are represented.