The Influence of El Niño Induced Drought on The Limnology and Dynamics of Cyanotoxins In A Shallow Tropical Reservoirkoka: Implications for Possible Public Health Threat and Restoration Efforts
No Thumbnail Available
Date
2020-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Studies predict that future climate-change scenarios such as global warming promote the proliferation and dominance of potentially toxic cyanobacteria due to their influence on the limnology of aquatic environments. Lentic systems in the tropics such as Koka Reservoir are largely of riverine origin. They are, therefore, more susceptible to changes in hydrological cycles or rainfall pattern induced by climate change anomalies such as El Niño induced drought and La Niña induced flooding. This study was conducted on Koka Reservoir, which is one of the most severely impacted water bodies by the recurrence of potentially toxic cyanobacteria. Our study period (May 2015-April 2016) coincided with the recent incidence of El Niño induced drought that was reported to be the worst in decades. Physico-chemical and biological parameters were determined using the standard methods. The drought caused a reduction in the flow of Awash River and amount of sediment transported and drying of Modjo River. This expectedly caused a concomitant reduction in the annual nutrient budget of the reservoir received from riverine sources. The consequent low levels of nutrients in the reservoir, which was most pronounced for nitrogen sources, led to the occurrence of Microcystis spp. in unusually low abundance and the emergence and dominance of diazotrophic cyanobacteria, Cylindrospermopsis spp.. The low nitrogen levels (μg L-1) observed in the open water (16.5-135) and near-shore (9.69 -250) sites also suggest that riverine input is probably the major source of nutrients to the reservoir. Total phosphorus (μg L-1), however, exhibited better availability, with its levels varying from 532 (Jul) to 668 (Mar) and 500 (Jul) to 698 (Mar) in the open water and near-shore sites, respectively. The large discrepancy between the model predicted phosphorus concentration (Pλ, 156.6 mg m-3) calculated from the total phosphorus input from allochtonous sources (riverine and atmospheric) and the observed in-lake phosphorus concentration (609 mg m-3), with the former constituting only 25.7 % of the latter, also suggest the supply of phosphorus from autochthonous sources. Frequent horizontal and vertical mixing of the reservoir may have enhanced phosphorus recycling. Horizontal mixing of the reservoir was reflected by the absence of significant differences in limnological parameters. Frequent vertical mixing manifested by turbidity (NTU, 48.39 -2820 and 48.29l-2970), TSS (g L-1), 0.015l-1.672 and 0.029-1.5), Secchi disk transparency (cm,3-17 and 3.05-17.01) between the open water and near-shore sites, respectively, may have promoted the presumed internal phaphorus loading. Despite the low abundance of Microystis spp, the detected intra-cellular concentrations (μg L-1) of the microcystins variants MC-LR (815), MC-YR (466.6) and MC-RR (265.68) by far exceeded the public health safety limit of 1 μg L-1. However, Dolichospermum, the co-occurring diazotrophic cyanobacterium, may have also been responsible for the observed levels of MCs although its confirmation requires further study. The extra-cellular toxins were relatively less concentrated and less frequently detected. However, concentrations (μg L-1) of up to 20 of MC-LR, 6.13 of MC-YR and 1.27 of MC-RR, which also exceeded the permissible limit (μg L-1) set by WHO, were measured. The most potent MC-variant, MC-LR, constituted the greatest proportion (52.6-77.6 %) of total microcystins concentrations in the majority of the samples suggesting the extremely high potential public health risk. The strong positive correlation among the extracellular MCs variants (Spearman Rank Order Correlation, P< 0.05) suggests the possible involvement of a non-specific release mechanism, namely cell lysis, which may have occurred following the collapse of the bloom in May. This study also revealed that all limnological conditions including water residence time (239.5-439.9 days), temperature (22.4 -31.3 oC), and salinity except the limiting level of nitrogen, were within the ranges reported to be optimal for the proliferation of Microcystis and production of microcystins. The strong, positive and significant correlation between Microcystis spp. abundance and nitrogen concentration (Spearman Rank Order Correlation (r= 0.7, P<0.05), and the lack of association with other limnological features suggest that nitrogen was the key environmental factor in the dynamics of Microcystis spp. and microcystins. Absence of correlation between the abundance of diazotrophic cyanobacteria and concentrations of nitrogen (r=-0.173, P>0.05) but the strong and positive correlation with the concentrations of total phosphorus (r=0.718, P<0.05) seem to suggest that phosphorus was the key nutrient influencing this functional group in the reservoir as nitrogen can never be limiting. Considering the long history of sediment deposition and the most likely concurrent nutrient loading since 1960s and the high retention efficiency associated with the prevailing long water residence time, the reservoir may have already accumulated a huge phosphorus reserve in the sediment. Emergence of nitrogen-fixing genera that are capable of satisfying their nitrogen requirement from the inexhaustible atmospheric sources and the presumably huge phosphorus reserve already accumulated in the reservoir implies that reduction in the external input of both nutrients can hardly help in reversing the current situation in Koka Reservoir at least in the near future.
Description
Keywords
Atmospheric deposition, Climate change, Cyanobacteria, cyanotoxins, diazotrophic, Koka Reservoir, Microcystis, Phosphorus recycling, nutrient-limitation, trophic state