## Materials and Methods
### Experimental Set-Up and Cosm Construction  
The experiment was conducted outdoors under an open-sided transparent plastic shelter that allowed sunlight through while preventing rain or debris entering the cosms. The cosms were randomly distributed between 4 troughs. Two of these troughs had cool water flow-through pumped directly from the sea. The other two were maintained at c. 5 \({}^{\circ}\)C above ambient using aquaria water heaters (2 \(\times\) 300 W heaters per trough). Two water pumps ensured thorough mixing of the warmed water in the troughs. Photographs of the set-up can be found in Figure S1. Distilled water was added to the cosms five times during the experiment to compensate for evaporation and maintain constant salinity.  
Figure 1: Timeline of cosm construction and sampling. HBCDD—Hexabromocyclododecane; CNP—carbon, nitrogen, phosphorus.  
During the experiment, the temperature of the ambient treatments varied between c. 12-20 \({}^{\circ}\)C, depending on the time of day and the weather. The warm treatments varied between c. 17-26 \({}^{\circ}\)C. The maximum daily variation was 7.6 \({}^{\circ}\)C and 9.1 \({}^{\circ}\)C in the ambient and warm treatments, respectively. The salinity in the cosms was c. 7.5 psu and oxygen water concentrations were consistently \(>\)8 mg.L\({}^{-1}\).  
The cosms were prepared as follows. Water from the hatching tank was drained off into holding containers. The sediment was then thoroughly mixed and divided equally (610 mL wet sediment each) into 58 separate 5 L buckets (cosms). These were factory-new, made of polypropylene, and approved for food use. Two of these cosms were taken immediately for analysis of CNP and HBCDD in the water and CNP content of the sediment (see Section 2.5.4). Hatching tank water was then equally distributed (3.75 L each) between the remaining 56 buckets, which were then placed in the troughs and allowed to stand for 3 days. After this time the sediment had settled out to form a layer 2 cm thick. The number of buckets allowed for 4-6 replicates of each of the four treatments (ambient control (A\(-\)), warm control (W\(-\)), ambient with HBCDD (A\(+\)), and warm with HBCDD (W\(+\))), and for destructive sampling at three time points (1, 6, 13 days after HBCDD addition; t1, t6 and t13).  
The previously collected organisms were sieved out of their holding sediment and four _L. balthica_ (11.5 \(\pm\) 1.2 mm), two _C. glaucum_ (shell length c. 16 mm and c. 8.5 mm), and 10 Hydrobiidae snails were added to each of the treatments. This represented natural field densities of 187, 93, and 468 individuals per m\({}^{2}\), respectively. The _L. balthica_ burrowed quickly into the sediment.  
Finally, 4.5 mL HBCDD-spiked plankton suspension was added to half of the cosms (A\(+\) and W\(+\) treatments) to give a nominal amount in each cosm of 78.75 \(\upmu\) HBCDD. The same amount of unspiked suspension was added to the A\(-\) and W\(-\) cosms to ensure equal amounts of extra organic material were added to all cosms (63 mg dw or 21.4 mg TOC (total organic carbon) per cosm).  
The experiment was run for 13 days. During this time the weather was in general warm and sunny.