4 | Discussion
Understanding the relative role of the processes in the community assembly is a key task in community ecology (Velland, 2016). Our nMDS analyses revealed a clear trend of differential dispersal through size fraction — smaller/larger size-fractions communities had higher/lower zooplankton compositional homogeneity (Figure 2: smaller/larger area shared by smaller/larger size fractions). Furthermore, we observed an increase in the effective number of communities toward a larger size fraction at 0D and 1D beta diversities (Table 3). These observations demonstrate a shift in the role of dispersal through the size fractions: higher dispersal in the smaller- and lower dispersal in larger-zooplankton species. In contrast, the trend — decreased effective number of communities toward a smaller size fraction — is weaker in 2D beta diversity assessments (Table 3). This indicates that the pattern is caused by rare and common species, not abundant ones. In contrast to the higher homogeneity in the small-sized communities, daytime and nighttime community samples were clearly separated in the largest size communities (Figure 2).
Ecological communities are assembled by four fundamental processes: ecological drift, selection, dispersal, and speciation (Velland, 2016). Ecological drift is a random process; therefore, we expect that the process has no role in shaping the pattern (smaller/larger size-fractions communities had higher/lower zooplankton compositional homogeneity). Speciation is a process that creates new species in communities. In our study, the pattern was discovered on a very small spatial-scale, which indicates that the speciation process also has little role in our target community. Consequently, our results suggest that the balance of two processes shaped the pattern: higher dispersal and lower selection in smaller and lower dispersal and higher selection in larger zooplankton communities. Nocturnal increase of coral reef zooplankton abundance and biomass have been reported in a few studies (Nakajima et al., 2008; 2009). These observations are concordant with the present study, and larger size fractions contribute to the variation of daytime and nighttime. Avoidance of visual predation, as a process of natural selection, is the primary adaptive explanation for the evolution of zooplankton vertical migrations (De Robertis, 2002). The size of zooplankton might be one of the critical characters in achieving the balance between the processes described above.
Overall, we found that Time, Transect, and especially Size Fraction affected OTUs composition and diversity on all levels (rare 0D, common 1D, and dominant 2D). In general, Station affected neither diversity nor composition of the zooplankton, which suggests that distance to shore has little influence on the coastal zooplankton community. These results are supported by both diversity analysis and PERMANOVA (Tables 1-3, Figures 3 and 4).
Temporal and spatial effects on the diversity and composition of zooplankton have been studied previously (Alldredge and Hamner, 1980; Hayward and Mcgowan, 1979; Hwang et al., 2010; McManus and Woodson, 2012; Quetin et al., 1996). Their roles in shaping zooplankton community are more or less understood, especially for the temporal change, which influences plankton distribution by inducing diel vertical migrations (Lampert, 1989). In addition, differences between the North-West and South transects might be related to the Kuroshio current, which constantly passes through Green Island from south to north, and creates different oceanographic conditions (Coyle, 2005; Hernández-León, 1988; Leis, 1986; Liszka et al., 2022). Oceanic islands and mounts under the influence of strong currents, like the present studied Green Island, play a crucial role in providing nutrients to the surface water layers through upwellings, promoted by water mixing (Acabado et al., 2021; Chen et al., 2022). Therefore, higher zooplankton diversity was expected from the samples collected closer to the coast (Hamner & Hauri, 1981). However, no increment in diversity towards the coast was observed in the present study (Figure 3).
It is almost universally accepted that there is a gradient in zooplankton abundance and diversity from nearshore to offshore stations in continental platforms worldwide (Hernández-León, 1988; Queiroga et al., 2005; Baliarsingh et al., 2014). The major difference between these and our approach was the size of the zooplankton species analysed. Majorities of traditional morphological zooplankton studies identify only adult individuals with higher propensity on larger species. In contrast, we analyzed small-size zooplankton individuals, including both adult, juvenile and larval individuals. With some exceptions, morphological analyses of zooplankton communities are rarely conducted on smaller sample sizes (e.g. Böttoger-Schnack 1996; Nishibe et al., 2009). Importantly, such small-sized zooplankton are essential prey items for larval fishes, the survival of which impact e.g., recruitment patterns and success (Huang et al., 2021). We believe that the ability to analyze small size plankton is another advantage of diversity analyses using the molecular approaches.
In the present study, we collected zooplankton samples only from the surface. The sunlit ocean plays a vital role in geochemical cycles, functioning as a membrane between the atmosphere and the ocean interior (de Vargas et al., 2015). Therefore, it is important to understand the ecological processes of the surface community. Another practical reason that we focused only on the surface layer is the difficulty of performing multi-layer sampling in shallow oceans. However, whether or not the homogeneous species composition persists in the vertical profile is an intriguing question that should be addressed in the future research.