Dynamics of a macrophyte-driven coastal ecotone: Underlying factors and ecological implications

Abstract

Ecotones are ecological boundaries or areas of transition between ecosystems. They are sensitive indicators of global change, and studying their dynamics can be useful to predict habitat shifts and landscape transformation. The Ulva-Zostera ecotone in Elkhorn Slough (California, United States) is a good example of a transitional area between habitat-forming macrophytes in a highly eutrophic coastal system. This study focused on understanding its driving factors, spatiotemporal trends, and overall ecological implications. A combination of habitat monitoring, recruitment and transplant experiments, and sediment analysis were used to describe the ecological characteristics of the system. Ecotone dynamics were driven by seasonality of Ulva spp., its distribution across the ecotone delimited by processes affecting its early developmental stages. While water quality and light availability were not linked to abundance and distribution of Ulva spp., sediment composition likely affected macrophyte distribution and overall ecosystem structure. Remarkably, physico-chemical conditions that hinder growth and expansion of Ulva spp. occur where eelgrass (Zostera marina) is present. Understanding the processes involved in habitat boundary dynamics is important to identify stressors and anticipate change at larger scales.