Multiyear spatial and temporal variability of microcystin concentrations in an irrigation pond in Maryland, USA

Cyanotoxins in agricultural irrigation waters pose a potential human and animal health risk. Cyanotoxins can be transported to crops and soil during irrigation; they can remain in the soils for extended periods and be absorbed by root systems. Spatial and temporal variations of cyanotoxin concentrations have been reported for various freshwater sources. However, little has been reported for agricultural irrigation ponds. The objective of this research is to determine if persistent spatial and temporal patterns of the cyanotoxin microcystin occur in agricultural irrigation ponds over several years. The study was performed at a commercial irrigation pond in Maryland, USA, during the 2022-2023 summer sampling campaign over a fixed spatial 10-location grid on 16 sampling dates. Microcystin concentrations were determined using ELISA microcystin-ADDA kits. Ten water quality parameters were obtained using fluorometry and in-situ sensing. Temporal and spatial persistence was assessed using mean relative differences (MRDs) between measurements in each location and averaged measurements across the pond on each sampling date. Positive (negative) MRDs were found in locations where concentrations were predominantly larger (smaller) than the pond’s average. Persistent spatial patterns of microcystin concentrations were found. The pond’s flow conditions and bank proximity to sample locations were indicative of the MRD values signs and amplitudes. The highest absolute values of correlation coefficients were found between microcystin and pH, and microcystin and phycocyanin. The lowest absolute values for correlation coefficients were found for CDOM and chl-a. Results of this work show that microcystin concentrations can follow stable spatial and temporal patterns in irrigation ponds over multiple years, indicating that water quality sampling for cyanotoxins and placement of water intake should not be arbitrary. Research of the spatiotemporal variability of other cyanotoxin concentrations as well as understanding the degree of site-specificity of cyanotoxin concentration relationships with water quality parameters presents an interesting research avenue.