Dynamics of environmental conditions during a decline of a Cymodocea nodosa meadow

. The dynamics of the physicochemical and biological parameters were followed 16 during the decline of a Cymodocea nodosa meadow in the northern Adriatic Sea from July 17 2017 to October 2018. During the regular growth of C. nodosa from July 2017 to March 18 2018, C. nodosa successfully adapted to the changes of environmental conditions and 19 prevented H 2 S accumulation by its re-oxidation, supplying the sediment with O 2 from the 20 water column and/or leaf photosynthesis. The C. nodosa decline was most likely triggered in 21 April 2018 when light availability to the plant was drastically reduced due to increased 22 seawater turbidity that resulted from increased terrigenous input combined with resuspension 23 of sediment and elevated autotrophic biomass. Light reduction impaired photosynthesis of C. 24 nodosa and the oxidation capability of below-ground tissue. Simultaneously, a depletion of 25 oxygen due to intense oxidation of H 2 S occurred in the sediment, thus creating anoxic 26 conditions in most of the rooted areas. These linked negative effects on the plant performance 27 caused an accumulation of H 2 S in the sediments of the C. nodosa meadow. During the decay 28 of above- and below-ground tissues, culminating in August 2018, high concentrations of H 2 S 29 were reached and accumulated in the sediment as well as in bottom waters. The influx of 30 oxygenated waters in September 2018 led to the re-establishment of H 2 S oxidation in the 31 sediment and remaining of the below-ground tissue. Our results indicate that if disturbance of 32 environmental conditions, particularly those compromising the light availability, takes place 33 during the recruitment phase of plant growth when metabolic needs are at maximum and 34 stored reserves minimal, a sudden and drastic decline of the seagrass meadow occurs. 215°C, 225°C 2.17 peak identified family chain (ECL) GC

and non-vegetated sediment by divers using plastic core samplers (15 cm, 15.9 cm 2 ). For 129 granulometric composition, organic matter, prokaryotic abundance, total lipids and fatty acid 130 analyses, the cores were cut into 1 cm sections to a depth of 8 cm and lyophilized, except of 131 6 sections for prokaryotic abundance analysis, that were weighted (approx. 2 g) and fixed with 132 formaldehyde (final conc. 4% v/v) immediately after slicing the sediment core. For determining the prokaryotic abundance in seawater, 2 ml of formaldehyde (final conc. 4% 149 v/v) fixed samples were stained with 4,6-diamidino-2-phenylindol (DAPI, 1 μg mL −1 final 150 conc.) for 10 min (Porter and Feig, 1980). In sediment samples, prokaryotes were detached 151 from the sediment particles by addition of Tween 80 (0.05 mL) and ultrasonicated for 15 min 152 (Epstein and Rossel, 1995 The material from each quadrat was washed under running seawater to remove sediment.

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From each quadrat algae, leaves and rhizomes with roots were separated. The length of the 160 longest leaf on each shoot was measured and the shoots were counted. Species of macroalgae 161 were determined, and their coverage was estimated according to the Braun-Blanquet scale. fraction that passed through the 0.063-mm sieve was collected and analyzed following the 169 standard sedigraph procedure (Micromeritics, 2002). The material that was retained on the 170 sieves was dried and weighted. The data obtained by both techniques were merged to obtain a 171 continuous grain size range and analyzed with the statistic package Gradistat v 6.0. Sediments 172 were classified according to Folk (1954). The sediment permeability was calculated based on 173 median grain size (d g ) following the empirical relation by Gangi (1985). The organic matter     (Pirini et al., 2007). To evaluate the input of terrestrial organic matter relative to that of 220 marine origin in particulate matter, the terrestrial to aquatic acid ratio (TAR= C24+C26+C28 / 221 C12+C14+C16) was used (Cranwell et al., 1987;Bourbonniere and Meyers, 1996).

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In FAME chromatograms elemental sulfur (S 0 ), eluted as S 8 (m/z 256), was identified by     (Table S1). The abundance of 259 prokaryotes (2.6-11.3 x 10 5 cell mL -1 ) varied seasonally and significantly correlated to showed a similar trend as observed for UND (Fig. S2, Table S2). 2 ). In contrast to leaves and shoots, the belowground biomass was stable until March 2018 282 when a decline was observed that continued until October 2018 (30.5 ± 6.8 g m -2 ) (Fig. 2a). TL in the C. nodosa above-ground tissue (6.7 ˗ 25.3 ± 2.4 mg g -1 DW) increased until 286 February 2018, when maximum TL concentrations were measured (Fig. 2b). Thereafter, TL 287 concentrations decreased until August 2018. During this period, the belowground TL 288 11 concentration (6.3 ± 1.9 -15.9 ± 1.1 mg g -1 DW) was generally lower than the above-ground 289 TL concentrations and the trend was similar to that of leaves. The minimum concentrations of 290 TL were observed in September 2018, while in October 2018, concentrations similar to that 291 measured in October 2017 were observed (Fig. 2b).

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In general, O 2 penetration depth in the vegetated and non-vegetated sediment co-varied 359 with the O 2 concentration in the bottom layer, penetrating deeper when its concentration in the 360 bottom water was higher (Fig. 6b). In the vegetated sediment, O 2 was mainly depleted down 361 to 1 cm of depth. In the non-vegetated sediment, the oxygen penetration depth was up to 4 362 times higher than in vegetated sediments, except for the period from August 2018 to October 363 2018 when the penetration depths were similar (Fig. 6b). were comparable but the accumulation zones deeper (Fig. 7).

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S 0 mainly occurred in oxic (Eh > 150 mV) and suboxic (150 mV > Eh > 0 mV) layers of 389 both, vegetated and non-vegetated sediments (Fig. 7). Generally, the ranges of approximated  changes throughout the investigated period with significantly higher concentrations in upper 416 than in lower sediment layers (p < 0.05) (Fig. 9). 417 In the vegetated sediment, TL showed significant monthly changes in the upper (F = 418 11.418, p < 0.05) and lower sediment layers (F = 3.186, p < 0.05), in contrast to both layers of 419 non-vegetated sediment (p > 0.05). From July to October 2017, in the upper layer of vegetated 420 sediments, TL was significantly higher than in non-vegetated sediments (Fig. 9). From UNDs were similar, except for July and August 2018 when a considerable decrease of UND 442 was observed in vegetated sediments (Fig. 9). Chlorophyta was observed (Fig. S4, Table S2). From July to October 2017, April to May 447 16 2018 and September to October 2018, a contribution of 20PUFA attributed to phytoplankton 448 and Rhodophyta was also detected. 16PUFA and 22PUFA accounted for the smallest 449 contribution to the PUFA pool and were found in seston and macroalgae (Fig. S4, Table S2).

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The similarities between the sediments were also observed in the contribution of the main 451 SAT components to the SAT pool from July 2017 to March 2018 and from September to 452 October 2018 (Fig. S4, Table S2). From April to August 2018, an increase of the long-chain 453 (C≥ 24) and common (C16:0 + C18:0) fatty acids followed by the decrease of bacterial fatty 454 acids (BACT) contribution to the SAT pool was observed in both layers of the vegetated 455 sediment. In contrast, the contribution of these components to the SAT pool was fairly 456 invariable in non-vegetated sediments during the same period (Fig. S4, Table S2).   The belowground tissue followed a similar trend, but with less expressed changes. Still, their 499 recognizable remnants were found after the loss of the above-ground tissues.  FeS, FeS 2 ) or with organic matter after sulfurization (Jørgensen, 1977;1982   Beyond seagrass itself, this loss had extensive consequences as it has endangered many 632 species that depend on seagrass for food, shelter and nursery. Given the lack of data on the 633 ecological and conservation status of the still numerous seagrass meadows along the northern 634 Adriatic coast, the identification and monitoring of the main pressures acting on them are 635 needed to protect such valuable habitats from degradation and extinction.

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Summary statistics is given in Table S3.