Benthic macrofauna under extremes: Unraveling the response strategies from individual behavior to community structure in tidal flats

Abstract

Extreme weather events are increasing in intensity, duration, and frequency, and threatening tidal flat ecosystems due to global climate change. Benthic macrofauna plays essential roles in biogeomorphic feedback as ecosystem engineers, yet they are prone to extreme environmental fluctuations in storms and drying tidal regimes. Moreover, modern globalization has accelerated the invasion of introduced species, which compete with native species for natural resources. Therefore, studying how benthic macrofauna responds to climate-change-induced extreme weather events may convey an essential message on short-term equilibrium and the long-term development of tidal flat ecosystems. A mesocosm system was designed and customized in the lab to mimic the heatwaves with contrasting magnitudes and durations on tidal flats. Simulated heatwaves were imposed on the model bioturbator species Cerastoderma edule living in different micro-topographies and sediment types, to study the response of bioturbation activities under thermal stress (Chapters 2 & 3). Besides individual-level response, an in-situ experiment was applied to study the effects of repeated storm events on benthic community structure, using a raking treatment to mimic storm-induced sediment disturbance (Chapter 4). Moreover, the effects of compound extreme weather events on species shift were studied by imposing different salinity settings and heatwave profiles on the native C. edule and an introduced bivalve Ruditapes philippinarum (Chapters 5). Macrobenthos' response on the individual level is categorized into the “fight, flight, or freeze” framework. Bioturbators increase metabolic rates to “fight” against the thermal stress and maintain optimal physiological conditions (Chapters 2); they burrowed deeper during low tide to escape from (“flight”) the thermal stress, thereby causing more sediment mixings (Chapters 2 & 3); they reduce activity and metabolism (“freeze”) to maintain basic physiological functions under acute extreme stress or long-term intermediate stress (Chapter 5). On the community level, internal environmental stress can select species with specific biological traits, while species with unfitting traits decrease in abundance (Chapter 4). Moreover, compound extreme weather events may create “disturbance-driven establishing windows” that benefit the introduced species to overtake native species (Chapter 5).