3.1 Biomass
An interaction between species and season significantly influenced both aboveground (p < 0.001, c2 = 40.042, df = 3, Table 1a, 1b) and belowground (p < 0.001, c2 = 30.606, df = 3, Table 2a, 2b) biomass. Halodule wrightii aboveground biomass exceeded that of Z. marina in the fall by a significant margin (p < 0.001, t-ratio = 6.297, df = 71, Supplemental A), with no significant differences between species within the other seasons (Figure 1, Supplemental A). The same was true for H. wrightii belowground biomass, which exceeded that of Z. marina in the fall (p < 0.001, t-ratio = 7.248, df = 72, Supplemental A) with no significant differences between species in the other seasons (Figure 1).
3.2 Epiphyte biomass standardized to leaf area
An interaction between species and season significantly influenced epiphyte biomass when standardized to leaf area (p < 0.001, c2 = 55.088, df = 3, Table 3a, 3b). H. wrightii epiphyte biomass was significantly greater than that of Z. marina in spring (p = 0.0001, t-ratio = 4.961, df = 71), fall (p < 0.0001, t-ratio = 6.349, df = 71), and winter (p < 0.0001, t-ratio = 10.363, df = 71, Supplemental A). There was no difference in epiphyte biomass between the species in summer (Figure 2).
3.3 Aboveground to belowground ratio
H. wrightii aboveground to belowground ratio (AG:BG) was significantly influenced by an interaction between species and season (p < 0.001, c2 = 22.006, df = 3, Table 4a, 4b). AG:BG of H. wrightii differed significantly from Z. marina, with H. wrightii showing a lower ratio in winter (p = 0.0003, t-ratio = -4.705, df = 64) and spring (p = 0.0002, t-ratio = -4.915, df = 64), a greater ratio in summer (p = 0.0397, t-ratio = 3.208, df = 64), with no difference between the species in fall (Supplemental A, Figure 3).
3.4 Epiphyte Community
We identified thirty-one distinct epiphyte taxa spanning various phyla and life history stages. Analysis of similarities (ANOSIM) revealed seasonal variation in epiphyte communities when comparing the two seagrass species (R = 0.428, p = 0.001), however, there was no significant annual variation in epiphyte communities when comparing the two (R = 0.026, p = 0.008). Permutational Multivariate Analysis of Variance (PERMANOVA) highlighted a significant interaction between seagrass species and season (p = 0.001, R2 = 0.0395, df = 3, Table 5). Further pairwise comparisons showed significant differences between the epiphyte communities of H. wrightii and Z. marina in winter, summer, and fall (p = 0.001 for all), whereas there was no difference in the spring (p = 0.214, Table 6). Non-metric Multidimensional Scaling (NMDS) ordination (Figure 4a) revealed that fall and winter communities were related, summer formed a distinct group, and spring spanned across all seasons in multivariate space. Examining the data within each season individually (Figure 4b), there was little difference between seagrass species in the spring epiphyte community, however, fall, summer, and winter exhibited distinct clustering for Z. marina and H. wrightii.
SIMPER analysis revealed Acanthosiphonia echinata as the dominant winter epiphyte, found on 96% of H. wrightii shoots but only 10% of Z. marina shoots. This species alone accounted for 15% of the dissimilarity between the two seagrass epiphyte communities (Figure 5, Supplemental B). Winter epiphytes were primarily Acanthosiphonia echinata, Hummia onusta, and filamentous brown algae in the genera Ectocarpus, Hinksia and Acinetospora (Figure 6). Ectocarpus spp. were present on 70% of H. wrightii and 15% of Z. marina shoots during fall, accounting for 12.5% of the dissimilarity (Supplemental B). The fall Z. marina community shares similarity with the winter H. wrightii community (Figure 5); both are relatively abundant in Hummia onusta and Acanthosiphonia echinata, however filamentous brown species were not as prevalent on Z. marina in the fall. During summer, the cyanobacteria Lyngbya was present on 73% of Z. marina shoots but only 6% of H. wrightii shoots accounting for 11% of the dissimilarity (Supplemental B). An increase in the relative abundance of crustose coralline algae, non-macroalgal epiphytes such as Lyngbya, and hydroids was seen in the summer (Figure 6). Spring and summer were the most diverse communities (Figure 6). Species in the Acrochaetiaceae were found throughout the year on both seagrasses.
To highlight the primary contributors of the observed variations in the epiphyte communities across seasons, we focused on the top 18 species that accounted for 80% of the dissimilarity between seagrass epiphyte communities within summer, fall, and winter (Figures 5 and 7). Overall, the epiphyte composition was generally shared between the two seagrass hosts, apart from 5 rare species: Anotrichium sp., Hooperia divaricata, and Phaeostroma pusillum were associated with Z. marina, while Botrytella micromora and Hypnea cryptica were unique to H. wrightii, none of which were included in the SIMPER results due to their infrequent occurrence.