Short-term in situ warming influences early development of sessile assemblages

Increased temperature is arguably the most important facet of global climate change, as temperature influences processes across all biological scales. In terrestrial systems, the influence of warming on community dynamics has been investigated through field manipulations of temperature but, in contrast, there have been very few warming experiments conducted in the sea. Here, we used heated settlement panels to manipulate microhabitat temperature in situ for >3 wk, to examine how short-term warming affects community development. We conducted 2 independent experiments in contrasting subtidal habitats in the Perth (Australia) metropolitan area, to determine the usefulness of the field-based approach and to examine consistencies in community-level response to warming. In the first experiment (Swan River estuary), a similar to 2 degrees C warming treatment resulted in a lower space coverage of a tube-building amphipod and higher coverage of a solitary ascidian, Ciona intestinalis, which contributed to significant differences in community structure. In a second experiment (Hillarys Harbour), the bryozoan Watersipora subtorquata, spirorbid polychaete worms, and C. intestinalis covered less space on warmed surfaces than controls. This was associated with greater biomass of a colonial ascidian and widespread marine pest, Didemnum perlucidum, under warmer conditions, which overgrew and probably outcompeted other taxa. Our results show that community responses to short-term warming are variable and are influenced by individual responses of assemblage dominants. We discuss limitations of the study and highlight the importance of community-level, field-based manipulations of environmental change factors which examine interactions between all available members of the local species pool.