hydroids, bryozoans and barnacles, settled on the plates (Fig. 1D). Didemnum perlucidum was the major space occupier; it covered more than twice the space on the hot plates than on the unheated controls (Fig. 1C; t-test on arc-sine transformed data, df = 14, t = 1.56, P = 0.15). This observation suggested that initial growth and/or development and/or recruitment of Didemnum perlucidum may be enhanced by warmer substratum and seawater conditions. Total dry biomass of the fouling assemblage was greater on hot plates than control panels (t-test on Ln transformed data, df = 14, t = 1.91, P = 0.07), again a likely consequence of accelerated recruitment and/or growth rates of early life stages under warmer conditions. Further information on the behaviour of the thermal gradient over the hot-plate surface was obtained from an experimental flume system. The relationship between distance from the plate surface and the magnitude of warming was examined under three different flow regimes characteristic of the Swan River. Under low flow conditions, the heated boundary layer of water surrounding the hot plate was in excess of 8 mm in depth, while a maximum temperature differential of 3uC was achieved on the plate’s surface (Fig. 2A). However, the thickness of the warmed boundary layer and the magnitude of the warming treatment were strongly influenced by increased flow rates (Fig. 2A). Even so, mean warming of at least 0.2uC was recorded up to 4 mm from the plate’s surface at both medium and high current speeds. Crucially, the magnitude of warming across the hot plate surface was consistent under all flow conditions (Fig. 2B). Discussion We have presented a useful technique to modify seawater temperature in situ, which is analogous to previous manipulations Controlled Warming on Marine Community Structure of environmental factors influencing sessile assemblages on settlement panels. As with other manipulations conducted at small scales, the hot plate system only modifies the immediate environment (i.e. a thin layer of water surrounding the plate surface) and does not elevate the temperature of the water column or the substratum at large spatial scales. As such, the approach has some limitations in that it does not affect larval survivorship, fecundity of the local population, adult physiology, or the pool of species available for colonisation; all of which will be strongly influenced by oceanic warming. The technique is therefore most suited to experimentation on species that have short-lived (or no) life cycle stages in the water column and are primarily influenced by conditions near the substratum, such as microbial biofilms and many species of ascidians and macroalgae. Another limitation relates to the physical extent of the warming treatment, which is (at most) 10 mm in depth and is strongly influenced by water movement across the plate. Consequently, the technique is most appropriate for examining small organisms or life stages that exist within the warmed boundary layer of water, and for experimen- tation in habitats that experience low to moderate water flow, such as many embayments, lagoons and lakes. Modifications to the panel structure and design to maximise the thickness of the warmed boundary layer are ongoing. Despite these limitations, the hot plate approach described above has clear benefits. The major advantage of the hot plate system is that it facilitates investigation on the effects of temperature on larval choice, recruitment dynamics, growth and calcification rates, community development and species interac- tions in real aquatic habitats. This is crucial, as much of the artificiality associated with laboratory, and even mesocosm, approaches is removed by conducting manipulations in real habitats, which are characterised by natural environmental Figure 1. The ‘hot plate’ system was deployed in field (A) and seawater temperature 3 mm over the experimental settling surfaces on both a hot plate and a control plate was recorded for the duration of the deployment (B). Panel C shows mean (6 S.E.) percent cover of Didemnum perlucidum and total biomass of the sessile assemblage on hot plates and controls (n = 8). Note change of scale on biomass plot, for clarity. A representative sessile assemblage covering a control (left) and hot (right) plate after 36 days is shown in panel D. doi:10.1371/journal.pone.0016050.g001 PLoS ONE | www.plosone.org 2 January 2011 | Volume 6 | Issue 1 | e16050