Purpose: To study the effects of sampling frequency, depth, and timing relative to disturbance events on the microbial structure and chemical composition of surface waters used for irrigation/frost protection of strawberries.
Methods: Six hundred 1G-water samples were collected during 1.5-years from a natural surface water reservoir used for irrigation/frost protection of strawberries. Thirty-six samples were collected every 15 days at 3, 6, 9 and 12 m from the inlet of the irrigation pump and at 3 depth ranges; 0.5-1, 1.75-2.25, and 3-3.5 m below the water surface. Samples were filtered (modified Moore swab) and the pellet was used to characterize the physicochemical and microbiological composition/structure (generic E. coli, Enterococci, STEC, Salmonella, Bacterial-16S and Fungal-ITS regions) of water.
Results: Rain events significantly increased detection of Salmonella, STEC, generic E. coli and Enterococci. Sampling depth, 0.5 vs 2.5 m, had a significant effect on pH (6.95 vs 6.2), TOC (7.3 vs 5.1 mg/L-C), turbidity (10.4 vs 11.5 FTU), and on the microbiological diversity/structure, whereas distance from shore had no significant effects (P<0.05). The dominant microbial families between 0.5-2 m depths were Pseudanabaenaceae, Nostocaceae and Synechococcaceae, while between 2.5-3.5 m the dominant families were Moraxellaceae, Aeromonadaceae, Methanosaetaceae and Pseudanabaenaceae. All samples met the GM and STV set for generic E. coli by 2.5-fold, even in those where Salmonella and/or STEC were detected at multiple depths and during/after rain events.
Significance: Depth, sampling date, and rain events are the most important parameters when designing surface water monitoring plans in order to establish a baseline of microbial water quality.