Exploring the causal relationships between environmental factors and benthos distributions in an intertidal algal reef ecosystem via a structural equation model

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Algal reefs are unique and valuable yet poorly understood marine ecosystems. The causal relationships between environmental driving forces and biotic community distributions in an algal reef ecosystem were assessed to fill large scientific gaps. Using a structural equation model (SEM) coupled with factor analysis, we established a well-fitted algal reef SEM. This model revealed that there were two subsystems, one operating primarily above the reef laminated substratum and the other beneath the surface. The crustose coralline algae (CCA) and non-CCA cover areas were affected, with an inverse trend between the two, by river inputs, as indicated by NH3-N concentrations, and by the seasonal rhythm of riverine NO3-N concentrations. The trends revealed spatial competition between the two algal groups. The CCA coverage and epifaunal gastropod density responded positively to the presence of pebble habitat. Notably, sand coverage and its variation did not hinder the coverage of the two macrophytes. However, the epifaunal gastropod and arthropod densities were negatively regulated by the sand coverage variability. The infaunal arthropod density was positively influenced by the sediment heavy metal content. Among the biotic communities, the non-CCA cover area negatively affected the infaunal polychaeta density. Furthermore, the increasing infaunal polychaete density increased those of sipunculans, bivalves, and arthropods, implying co-occurrence. This SEM revealed that riverine nutrients, pebble habitat type and sand coverage variability were significant environmental drivers in organizing the biotic communities in an algal reef ecosystem.