Spatiotemporal variability of microcystin concentrations in water of 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 where they can remain in the soils for extended periods and be absorbed by root systems. While studies have reported spatial and temporal distributions for cyanotoxins in various freshwater sources, little has been reported for agricultural irrigation ponds. This research aimed to determine if persistent spatial and temporal patterns of the cyanotoxin microcystin occur in agricultural irrigation ponds. The study was performed at a working irrigation pond in Maryland, USA, during the 2022 summer sampling campaign consisting of 6 sampling dates over a fixed spatial 10-location grid. Concentrations of microcystin were determined using ELISA microcystin-ADDA kits. Ten water quality parameters were obtained using fluorometry and in-situ sensing. Relative differences (RDs) between a sampling location’s microcystin concentration and average concentrations across the pond were computed for each sampling date. Mean relative differences (MRDs) were computed for each sampling location for all sampling dates. 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 established. The pond’s flow conditions and bank proximity to sample locations were indicative of the MRD values signs and amplitude. The highest absolute values of the Spearman correlation coefficients were found between microcystin and pH (-0.777), and microcystin and phycocyanin (0.669). The lowest absolute values for correlation coefficients were found for colored dissolved organic matter (0.226) and chlorophyll-a (0.289). Correlations between microcystin relative differences and water quality relative differences were generally low and not statistically significant. Results of this work show that microcystin concentrations can exhibit stable spatial and temporal patterns in irrigation ponds, indicating that water quality sampling for cyanotoxins and placement of water intake should not be arbitrary. Research of the spatiotemporal organization 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.