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1. Introduction
The marine ecosystem is the largest, most diverse, complex, and influential for all life on Earth, including humans. It plays a fundamental role in the functioning of the global ecosystem and services such as providing the balance of global water and geochemical cycles, the buffering of global climate change, and the fisheries production for human society [1,2]. However, the global ocean health has been under severe threat, particularly recently, predominantly due to anthropogenic pressure [3,4]. Various alarming signals such as decreasing fisheries production, oxygen depletion, and acidification have been identified in the global ocean [4]. The coastal ecosystem, in particular, might be the most critical component of ocean health since it is affected by multifaceted stressors including various anthropogenic activities (e.g., coastal developments) as well as natural variabilities. The ecosystem resilience following (human) disturbance is one of the essential components in securing a sustainable ecosystem and it has been evaluated by determining the function, structure, and biodiversity of the target ecosystem [5].
Marine microbial communities are complex and highly diverse; on average, ocean water contains one million microorganisms per milliliter, with thousands of unique microbial taxonomic groups [6]. They play a pivotal role in a number of ecological processes such as biogeochemical or nutrient cycling and energy flow, serving as regulators of oxygen, carbon, and nutrient dynamics (i.e., ecosystem functioning) [7]. In shallow coastal areas, microbial activities in bottom sediments and water column are particularly crucial for understanding coastal ecological processes, e.g., coastal benthic-pelagic coupling [8].
Evaluation of the structure and diversity of microorganisms or bacterial communities in coastal ecosystems has recently been suggested to be critical to gauge their environmental status, particularly for those that have been affected by human-driven stressors [9,10]. Microbial organisms can be a reliable indicator for detecting or diagnosing changes in seawater ecosystems because they are known to be sensitive to hydrologic and water quality changes exhibiting a rapid response to those changes (e.g., [11]). They can also rapidly respond to seasonal fluctuations, including phytoplankton abundance, grazing pressure, and nitrogen, phosphate and silica concentrations [12,13,14]. Moreover, environmental parameters such as temperature, which is dependent on seasonal changes, have an effect on the bacterial community [12,15]. Therefore, patterns of a detectable change in the structure and diversity of microbial...