Environmental Filtering Weakens with Trophic Level in Urban Coastal Ecosystems

Urban coastal ecosystems face increasing anthropogenic pressures
and environmental variability, yet the consequences for multitrophic
biodiversity and ecosystem networks remain poorly resolved. Here,
we combine environmental DNA metabarcoding, visual surveys, flow cytometry,
and environmental measurements to examine the spatiotemporal dynamics
of marine metazoans, protists, and prokaryotes across estuarine, transitional,
and oceanic habitats in Hong Kong’s urbanized coastal waters.
Using permutational multivariate analysis of variance (PERMANOVA),
we demonstrate that environmental control over community composition
weakens systematically at higher trophic levels. The variance explained
by seasonal and spatial interaction was highest for prokaryotes (R2 = 0.76) and protists (0.59), but notably lower
for benthic fauna (0.41) and bony fish (0.32). Co-occurrence network
analysis revealed that oceanic habitats, dominated by heterotrophic
prokaryotes, omnivorous fish, and hard corals, supported the most
complex and stable multitrophic networks, with an average complexity
of 0.54 compared to estuarine (0.23) and transitional habitats (0.29).
Structural equation modeling further revealed habitat-specific drivers:
temperature exerted the strongest direct effect in estuarine habitats
(>0.44), while biotic interactions involving primary producers played
a dominant role in oceanic habitats (direct effect >0.28). In contrast,
transitional habitats lacked significant environmental or biotic drivers,
indicating a system in flux where community dynamics are likely governed
by complex variables beyond standard environmental or biotic regulation.
These findings demonstrate the gradient-dependent interplay of environmental
filtering and biotic regulation in shaping coastal ecosystem stability.
Our results also highlight the value of an integrated eDNA-based framework
for monitoring biodiversity and ecosystem change, providing insights
for the management of urban marine environments under global change.