Abstract
Recycling irrigation reservoirs (RIRs) are emerging aquatic environments of global significance to crop production, water conservation and environmental sustainability. This study characterized the diversity and population structure of cyanobacteria and other detected microbes in water samples from eight RIRs and one adjacent runoff-free stream at three ornamental crop nurseries in eastern (VA1 and VA3) and central (VA2) Virginia after cloning and sequencing the 16S rRNA gene targeting cyanobacteria and chloroplast of eukaryotic phytoplankton. VA1 and VA2 utilize a multi-reservoir recycling irrigation system with runoff channeled to a sedimentation reservoir which then overflows into transition and retention reservoirs where water was pumped for irrigation. VA3 has a single sedimentation reservoir which was also used for irrigation. A total of 208 operational taxonomic units (OTU) were identified from clone libraries of the water samples. Among them, 53 OTUs (358 clones) were cyanobacteria comprising at least 12 genera dominated by Synechococcus species; 59 OTUs (387 clones) were eukaryotic phytoplankton including green algae and diatoms; and 96 were other bacteria (111 clones). Overall, cyanobacteria were dominant in sedimentation reservoirs, while eukaryotic phytoplankton and other bacteria were dominant in transition/retention reservoirs and the stream, respectively. These results are direct evidence demonstrating the negative impact of nutrient-rich horticultural runoff, if not contained, on natural water resources. They also help in understanding the dynamics of water quality in RIRs and have practical implications. Although both single- and multi-reservoir recycling irrigation systems reduce the environmental footprint of horticultural production, the former is expected to have more cyanobacterial blooming, and consequently water quality issues, than the latter. Thus, a multi-reservoir recycling irrigation system should be preferred where feasible.
Highlights
It must be noted that the PCR annealing temperature used in this study was 55 ̊C which is lower than the recommended 60 ̊C for cyanobacteria in the original protocol [29]. These results suggest that these cyanobacterium primers may detect some bacteria under less stringent PCR conditions
28.3% of operational taxonomic units (OTU) in the cyanobacteria group remain unclassified with only 81.6% to 93.3% sequence similarities to those in the Silva 16S rRNA database (S1 Table)
The percentage of unclassified OTUs was significantly higher. 99% of these taxa including Candidate (97.3% clones) were not classified (Table 2), some of them such as MLE1-12 or Melainabacteria have recently been assigned to cyanobacteria [40]
Summary
Agriculture uses 70% of global consumptive water [1], and generates huge quantities of runoff which is a major source of impairments to rivers, lakes and wetlands in the United States. Capture and reuse of irrigation and precipitation runoff is of global significance to water conservation and environmental sustainability by increasing the efficiency of agricultural water use and reducing nonpoint source pollution. These benefits, do not come without cost. The primary objective of this study was to determine the complexity of cyanobacteria communities in RIRs and its relation to recycling irrigation system setup and other environmental factors. Eight reservoirs and one adjacent stream at three ornamental crop nurseries were surveyed mid to late spring, the busiest growing season of the year for these production facilities
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