Posidonia oceanica

Mecca, Silvia, Casoli, Edoardo, Ardizzone, Giandomenico & Gambi, Maria Cristina, 2020, Effects of ocean acidification on phenology and epiphytes of the seagrass Posidonia oceanica at two CO vent systems of Ischia (Italy) Abstract, Mediterranean Marine Science 21 (1), pp. 70-83 : 73-75

Phenology of Posidonia oceanica

The Posidonia oceanica shoot density showed higher values for both the Vullatura (mean ± SD: Vu3 = 866 ± 148 shoots/m 2; Vu6 = 619 ± 106 shoots/m 2) and Castello Aragonese (N3 = 667 ± 155 shoots/m 2) acidified stations ( Fig. 2a ) than that at the control site ( NC = 391 ± 95 shoots/m 2). These differences were significant by ANOVA (F = 64.70, p -value <0.001); Tukey’s HSD test reported significantly different p -values between the control and acidified stations ( NC –N3: p adj <0.001, NC– Vu3: p adj <0.001, and NC–Vu6: p adj <0.001), and between N3–Vu6 (p adj = 0.03). Comparisons between N3–Vu3 and Vu3–Vu6 did not display significant differences. The number of leaves (adult, intermediate, and juveniles pooled together) significantly varied among months, with the same pattern in both the acidified and control stations ( Fig 2b ). Maximum values were reported in October and December, whereas the minimum values were found during the summer months (June and August). The mean number of leaves per shoot did not differ among pH conditions ( Table 1 ). Conversely, the mean leaf length was higher in June, decreasing up to the minimum reached in October ( Fig. 2c ). The mean leaf width ( Fig. 2d ) showed minimum values in June for NC, Vu3, and Vu6, and in August for N3. The maximum mean width was reported in August, October, and December for all the studied stations.

Significant differences in the mean leaf length according to the pH condition, month, and the interaction between these two factors were observed ( Table 1 , PERMANOVA test). The mean leaf width also differed among pH condition and month ( Table 1 ). In the acidified stations we recorded lower leaf length and width in all the considered months, respect to the control station.

The leaf area index ( LAI = m 2 of leaf surface/m 2 of sea bottom) ranged between 0.6 to 2.3 and was lower at all stations in October and higher in July (Supplementary Materials, Fig. S2 ). The LAI did not vary among stations/pH, since in the acidified stations, the lower leaf lengths, and therefore lower leaf surface areas, are compensated by higher shoot densities compared to those at the control station.

The leaf apex erosion (coefficient A) showed greater percentages of eroded leaves occurring at all the stations during the summer months ( Fig. 3 ). Grazing (biological erosion) was the main source of leaf damage, especially in the acidified stations N3, Vu3, and Vu6, whereas mechanical erosion was less represented. Differences in the type of apex erosion were detected among pH conditions, months, and the interaction between these two factors (PERMANOVA, see Table 2 ). Leaves grazed by the herbivore fish S. salpa were the most common and dominant in the acidified stations, with sea urchin and crustacean (mainly isopods) bites never exceeding 35% of the analyzed leaves ( Fig. 4 ). On the contrary, grazing by crustaceans and sea urchins was higher in the control station, where S. salpa bites were less frequent.

Figures

Fig. 2: Temporal variation in Posidonia oceanica morphological features at the studied stations: mean shoot density (a), mean number of leaves per shoot (b), and mean leaf length (c) and width (d). Bars represent the standard deviation. Gray colors indicate low pH conditions: N3 (pH 7.21 ± 0.34), Vu3, and Vu6 (pH 7.26 ± 0.48); and white indicates the control station (NC; pH 8.00 ± 0.08). Asterisks highlight features that show significant differences according to pH conditions.

Fig. 3: Percentage of leaves showing different types of leaf apex erosion (intact, mechanical, and biological) over time at the studied stations. At least 30 of the oldest leaves were examined at each month and station.

Fig. 4: Percentage of different types of biological apex erosion (Sarpa salpa, sea urchins, and crustaceans) during time at the studied stations and considering only the leaves showing biological erosion.