## 2 Materials and Methods
### Sample Collection  
On day 14 the water flow to the buckets was stopped. snails were initially removed from the mesh bags and counted. While minimal snail losses occurred, the losses were comparable among treatments. DO measurements (mg/L) (HQ40D multi-meter probe, Hach Lange GmbH, Dusseldorf Germany) were taken from each mesocosm and recorded. To obtain respiration and primary productivity data, oxygen measurements were necessary from the mesocosms in dark (night) and light (day) environments. To simulate a nighttime environment, dark plastic bags were placed over each mesocosm to prevent light from reaching any biofilms that had developed on the clay tiles. Bags were pressed against the water line to prevent excess oxygen from entering the water as CR commenced. Incubation occurred for 65 min, after which oxygen measurements were re-measured to determine DO loss over that period of time. Mesocosms were then covered with clear plastic to allow natural light penetration but no oxygen penetration. This allowed us to measure the productivity of the biofilms alone and not measure excess oxygen input from the outside. Incubations lasted for 65 min, after which the DO measurements were re-taken to obtain GPP.  
After obtaining oxygen measurements, tiles were carefully removed from their respective mesocosms. Razor blades were cleaned with 70% ethanol for bacteriostatic effect and used to scrape biofilm material into sterile, pre-weighed, 2 mL Eppendorf tubes. Obtained biofilm materials were immediately weighed and frozen at \(-20\)\({}^{\circ}\)C to ensure the contents were fully frozen and solidified. The materials were then lyophilized (\(-85\)\({}^{\circ}\)C (Alpha 2-4 LDplus, Martin Christ Freeze Dryers, Osterode am Harz, Germany)) and sent to the Agri-Food and Biosciences Institute (Belfast, Northern Ireland) for C (% C) and N (% N) content analyses.  
The C and N content of lyophilized biofilm samples were determined by combustion elemental analysis on a Thermo Flash 2000 elemental analyzer (Thermo Fisher Scientific GmbH, Bremen, Germany). Briefly, biofilm samples were weighed and encapsulated in pre-weighed tin cups for analysis. Each sample was then combusted at 950 \({}^{\circ}\)C within a 10 sec stream of ultra-pure oxygen (flow rate 240 mL/min). exothermic oxidation of the tin cup raised the combustion temperature to over 1000 \({}^{\circ}\)C. The gaseous carbon and nitrogen oxides from each sample were carried in a stream of helium carrier gas (140 mL/min) through a column of cobaltous oxide catalyst to remove sulfur and halogenated compounds. The purified gas then passed through a column of reduced copper, held at 680 \({}^{\circ}\)C, which converted the nitrous oxide gases present to elemental nitrogen and retained excess oxygen. Any water vapor present was removed from the gas mixture by passage through a magnesium perchlorate-packed adsorption filter, after which carbon and nitrogen were separated on a chromatographic column and passed to a thermal conductivity detector. The instrument was calibrated following the manufacturer's instructions, using the K-factor calibration methodology with acetanilide as an analytical standard. Aspartic acid (Thermo Fisher part number: 338 400 22) and a commercial soil reference material (Thermo Fisher part number 334 400 25) were used as quality control (QC) materials to check the instrument calibration, with two replicates of each QC material analyzed within each batch of 30 samples. The ratios of C to N were examined to identify changes in ecological stoichiometry [35]. Lyophilized weights were used for biomass analysis. The mass of carbon and nitrogen within a sample was then calculated as:  
\[\text{S}\left(\text{C or N}\right)=\text{S}\left(\text{dry weight}\right) \times\left(\text{P}\_\text{(C or N)}/100\right) \tag{1}\]  
where S (C or N) is the mass of carbon or nitrogen within the sample, S (dry weight) is the dry weight of the sample, and P (C or N) is the percentage carbon or percentage nitrogen in the sub-sample analyzed by the elemental analyzer.