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.