Antibiotics are used in dairy production to treat and prevent livestock infections. Antibiotic residues are excreted in manure, often applied to fields as a soil amendment, which is present in agricultural runoff, and these residues can exert selective pressure on environmental bacteria, promoting the proliferation of antibiotic resistance genes (ARGs). Using pharmaceutical diffusing substrates (PhaDS), deployed seasonally, we are exploring how two antibiotics (sulfamethazine and tetracycline) interact with nutrients to influence ARGs in heterotrophic biofilms above and below a source of dairy effluent. After each seasonal deployment (14-19 days) at the upstream and downstream sites, we retrieved cellulose sponge substrata, immediately measured community respiration (CR) via dissolved oxygen change using dark incubations for 5-6 hours and then quantified biofilm ARGs (tetW and sul1 gene copies ml-1) using digital PCR. During fall, we found that sulfamethazine (with and without nutrients) increased sul1 concentrations at the upstream site (Generalized linear mixed model [GLMM], p<0.001), while only sulfamethazine alone increased sul1 concentrations downstream (GLMM, p<0.001). In contrast, tetracycline alone had no effect on tetW concentrations (GLMM, p>0.05), but tetracycline with nutrients decreased tetW concentrations downstream of the dairy (GLMM, p<0.05). Additionally, below the dairy, sulfamethazine with nutrients, and tetracycline treatments (with and without nutrients) had lower biofilm CR compared to the control substrata (ANOVA, p <0.05), while there were no other significant treatments effects at the upstream site. Additional seasonal deployments are planned, but thus far, we have found that antibiotics can select for ARGs in microbial biofilms while also altering functional metrics.