Seagrass dynamics and resilience

Abstract

The vulnerability of seagrass ecosystems, and the services they provide, to damage and loss from anthropogenic stressors has led to a surge of interest in understanding their resilience. This chapter examines patterns of change in tropical and temperate Australian seagrasses to identify underlying causes of the observed patterns. It then relates seagrass dynamics to ecosystem resilience, and examines how resilience can be measured, managed and enhanced. Seagrasses in tropical waters show strong seasonal patterns in many places, with seagrass extent and cover increasing during the winter dry season and decreasing during the summer wet season. This seasonality is overlaid by a striking longer term trend of increase during El Niño periods and subsequent loss during wetter, stormier La Niña periods. Seasonality is less evident in temperate waters, where mapping of dynamics has generally been used to show longer term patterns, especially large-scale loss after decades of stability, sometimes with partial recovery. Changes in some places have been linear and in others strongly non-linear, possibly indicative of systems breaching a threshold or tipping point in levels of stressors such as pollutants. Resilience theory has become a powerful tool for understanding the dynamics of seagrass change. Seagrass resilience requires several key traits: genetic and species diversity, good water quality, connected ecosystems and continuous habitats, and balanced trophic interactions. These traits are integrated through ecological feedbacks. In Zostera muelleri meadows, for example, the capacity for seagrass to resist decline during pulses of poor water quality depends on its ability to: (1) efficiently remove excessive nutrients from the water, thereby limiting phytoplankton growth and improving water clarity, (2) suppress resuspension of sediment for improved water clarity, and (3) provide habitat for grazing animals that remove epiphytic algae. The increased understanding of resilience is shifting the focus of seagrass ecosystem management towards the management of stressors to optimise key feedbacks, and thus ultimately to enhance resilience. The chapter culminates in descriptions of practical management actions demonstrated to effectively enhance key traits and overall seagrass resilience.