An interdisciplinary and multiagency study was conducted in southern Florida to examine harmful algal bloom (HAB) dynamics on the Caloosahatchee River, which drains from Lake Okeechobee to the west into the Gulf of Mexico. Using in-situ mesocosm experiments, ammonium, nitrate, phosphate, and urea were altered from ambient conditions to determine the effects on cyanotoxin biosynthesis gene presence and quantity as measured via quantitative polymerase chain reaction (qPCR). Eight experiments were conducted seasonally from May 2019 through July 2021 at the W.P. Franklin Lock and Dam. The experimental installations consisted of three floating metal cradles each containing four opaque fiberglass chambers filled with local river water. Each chamber was open at the top to allow free flow of air and light but closed off at every other part. This ensured the chambers were in the river but not getting cross-contaminated by exchange with the river water. The twelve chambers were each enriched using one of five treatments depending on the experiment: phosphate (P), nitrate (N), ammonium (A), urea (U) or control (C) – with only four of the five treatments selected, each with three replicates, for every experiment. The P, N, A, and U treatment chambers were enriched by applying a concentrated dosing solution of either dibasic dodecahydrate sodium phosphate, anhydrous sodium nitrate, liquid ammonium hydroxide, or urea, respectively to elevate concentrations above ambient levels. The control chambers were left unenriched. The mesocosms were treated and sampled in approximately 24-hour intervals over a 72-hour period (Time 0, Time 24, Time 48, Time 72). May and July 2021 mesocosms were additionally sampled one week from the initiation of the experiment (T240). Deoxyribonucleic acid (DNA) was extracted from these samples and tested via six qPCR assays to determine the presence and quantity of different cyanotoxin biosynthesis genes. Each qPCR assay tests for a different cyanotoxin biosynthesis gene with one assay testing for cyanobacterial 16S DNA as an internal control. The five toxin biosynthesis genes tested were microcystin (mcyE), nodularin (ndaF), saxitoxin (sxtA), anatoxin (anaC), and cylindrospermopsin (cyrA).