High-replication assessment of the effects of poor water quality on South Florida coral health and susceptibility to Stony Coral Tissue Loss Disease

Poor water quality is a major stressor on coral reefs, especially in South Florida. Understanding the effects of specific nutrient sources (e.g., NO3, NH4, and PO4), nutrient combinations, and concentrations on corals has been challenging due to the complexity of required experimental designs. To address this, we modified and expanded the Sequential Treatment Application Robot (STAR) system to enable continuous and precise multi-nutrient dosing into individual coral vessels. Enhancements included modifying the LabVIEW program for precise control over multiple dose sources, improving the stir
plate system, constructing new aluminum tank stands for stability, and designing larger coral vessels to buffer nutrient concentration changes. The system was first tested for nutrient stability without corals (n=572 nutrient samples), leading to modifications such as using artificial seawater instead of RO water in the nutrient dosing solutions to improve nutrient mixing in the vessels. This troubleshooting phase successfully achieved target NH4 and PO4 concentrations, although maintaining NO3 levels was challenging due to NO3 reduction to NO2.

We then conducted an experiment to test the impacts of multiple nutrient combinations on the threatened staghorn coral, Acropora cervicornis. Twelve fragments from each of six A. cervicornis genotypes were allocated to 72 vessels across six nutrient treatments, including a control (C) with only artificial seawater and five elevated nutrient treatments: elevated NO3 only (NO3), elevated NH4 only (NH4), elevated PO4 only (PO4), elevated NO3 and PO4 (NO3 + PO4), and elevated NH4 and PO4 (NH4 + PO4). The elevated nutrient concentrations targeted a +10 μM increase in NO3 and NH4 and a +2 μM increase in PO4. A total of 563 nutrient samples were collected during this phase. Upon introducing corals, NH4 and NO3 concentrations declined across all nitrogen-involved treatments, attributed to nutrient uptake by corals and single-celled diatoms. Data on the photochemical efficiency (Fv /Fm) and buoyant weight of the corals were collected before and after nutrient exposure to assess treatment effects on coral performance. Future work will statistically analyze the impacts of these treatments on coral growth and Fv /Fm.