Figure 2. The relationship between distance from the hot plate surface and magnitude of warming under different flow conditions (A). Mean temperature differentials (6SEM) between a control and a hot plate were calculated from 15 readings taken during one minute, following sufficient time for thermal gradients to stabilise. See methods for details of flow conditions. Mean (6SEM) temperature difference between control and hot plate at different positions across the plate surface, under different flow conditions, is shown in panel B. doi:10.1371/journal.pone.0016050.g002 variability and complex interactions that often cannot be simulated under controlled conditions [12]. Manipulations can also be conducted over various timescales, to simulate both gradual warming and short-term extreme events (i.e. heat waves), which are projected to increase in frequency with climate change [1]. Moreover, the system, which can be deployed in any aquatic habitat, can be manipulated like ‘standard’ settlement plates and will thus facilitate complex field experiments that combine multiple stressors (i.e. temperature with disturbance frequency or nutrient enrichment) and examine species interactions (i.e. competition vs. facilitation) within functional communities. Finally, unlike thermal pollution studies, hot plate experiments conducted at different times and places would be comparable, which would permit testing of important climate-related ecological theories, such as the relative effects of warming on organisms inhabiting polar versus tropical systems and thermally stable versus thermally variable environments. With regards to the biological data from the field deployment, the two key ecological responses to warming we recorded were an increased coverage of the ascidian Didemnum perlucidum and a greater total biomass of the sessile assemblage. The genus Didemnum includes community-altering invasive species, such as D. vexillum and D. perlucidum, which have been translocated from their native waters to new habitats on vessel hulls and in ballast Controlled Warming on Marine Community Structure waters, sometimes with community-wide consequences [13]. There is some evidence to suggest that the growth and invasive success of these species is correlated with increasing temperatures and reduced water quality [14,15]. However, very little is known about Didemnum spp. in Western Australia and further research is urgently needed. The greater total biomass values we recorded on hot plates compared with controls was most likely due to either increased recruitment or increased growth rates, or a combination of both. Again, more work is needed to investigate the relative importance of these key biological processes in driving this pattern. The potential for oceanic warming to cause an increase in biofouling rates on artificial surfaces has been suggested previous- ly, as microbial biofilm development may accelerate and aggressive fouling seasons may lengthen, particularly in temperate regions [16,17]. Here, we observed greater fouling on warmer surfaces; an observation that could have major implications for the multi-billion dollar anti-fouling industry. Most predictions on the consequences of global warming on marine biodiversity have been based either on climate envelope modeling or on inferences from highly artificial laboratory experiments, rather than from observations or manipulations conducted in real habitats. Although these approaches have provided valuable information, they are limited in that they generally do not consider direct or indirect species interactions under the influence of multiple stressors acting simultaneously [18], which even the most sophisticated mesocosms or computer models cannot simulate. Furthermore, most research has targeted individuals or populations, while communities and ecosystems have received far less attention [19]. These pressing knowledge gaps currently restrict our ability to predict, and plan for, the effects of oceanic warming on marine biodiversity and resources [19,20]. ‘Scaling-up’ results from small-scale experiments to make predictions about large-scale processes such as oceanic warming is challenging [21], and such approaches have been criticised [22]. However, it has been empirically demonstrated that small-scale interactions can generate large-scale patterns [23], while small- scale field experiments have considerably improved understanding of the fundamental principles of ecology [24]. As such, information on the influence of temperature on community structure, species interactions and invasibility gained from field manipulations will enhance our ability to predict the consequences of global environmental change on marine biodiversity and resources. As assigning cause and effect is best achieved through experimental manipulation under realistic conditions [25], the hot-plate system will be a useful addition to the research ‘toolbox’ available to climate scientists in that it will complement modeling, experimen- tal and observational approaches conducted at larger spatial scales. Materials and Methods Hot plates were similar in design to traditional and widely used settlement panels (20620 cm) except that the surface of the panel and the surrounding boundary layer of water were electrically heated to levels above ambient temperature. The mean magnitude of warming achieved in the Swan River (,0.7uC) reflected present-day increases above mean temperatures observed during heat waves [26], and the projected increase in mean sea temperature by 2030 [1,27]. Warming was achieved by mounting an electrical heat trace, sealed in silicon, beneath an anodised aluminium plate. Power was supplied to the heat trace via a 50 m cable that connected to a shore-based control unit and mains power supply. The magnitude of heat transfer through the aluminium plate was a simple function of the applied voltage, which was calibrated prior to deployment. Each panel drew ,2A PLoS ONE | www.plosone.org 3 January 2011 | Volume 6 | Issue 1 | e16050