Applicability of a biotest battery developed for temperate regions to tropical environments: implications for sustainable wetland management – a Case study of Kilombero Ramsar Site
Loading...
Date
2015-04
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Leuphana Universität Lüneburg
Abstract
The increasing rate at which chemicals are released into the environment in Africa over the past
two to three decades has raised a concern about the immediate ecological impacts and associated
long-term environmental risks. Toxicity tests, which measure responses of organisms to various
contaminants, have the capacity to indicate ecological impacts of chemicals that are released into
the environment, and are therefore used in risk assessment studies. Intensive eco-toxicological
studies in the developed world have led to establishment of standardized biotest procedures in
order to assure reproducible and reliable results for risk assessment and for environmental
regulations. Although there are a lot of data and publications on water and sediment quality for
temperate countries using bioassays, tropical ecological risk assessment data using bioassays are
scarce.
The focus of this study was to apply biotests developed in temperate countries on tropical samples
and evaluate their suitability for assessing pesticide contamination in soil, water, and sediment
from Kilombero Valley Ramsar Site, in Tanzania. A total number of 143 dry and rainy season
samples (58 water, 68 sediments and 17 soils) was collected in eight plantations (sugarcane, rice
and teak). Toxicities were assessed by a battery of standardized tests comprising the algae growth
inhibition test with the freshwater green algae, Pseudokirchneriella subcapitata, the luminescence
bacteria test with Vibrio fischeri, and the bacterial sediment contact test with Arthrobacter
globiformis. Responses of test organisms to sample matrices were categorized and classified into
toxicity classes according to a fuzzy rule based-expert system. Suitability of using the P. subcapitata
bioassay for assessing pesticide contamination in tropical agronomic systems based on the results
of this study was evaluated. Chemical, geochemical and eco-toxicological analyses were combined
in a weight of evidence approach to provide environmental risk information for this Ramsar
wetland (wetland of national and international importance according to the Ramsar Convention of
1971). Interviews with plantation managers were conducted in order to get an overview over the
application of pesticides in the area. Together with the distribution of measured pesticides, an
evaluation of potential human risk has been carried out.
Toxicity responses of the three bioassays differed to a considerable extent depending on the
sample matrices and season. Dry season samples (soil and sediments) resulted in higher mean
inhibition of A. globiformis (25%-31%) than rainy season sediment samples (5%). Neither P.
xxSummary
subcapitata nor V. fischeri showed a significant difference between rainy and dry season samples
(water and sediments) even though there was a trend towards higher inhibition during dry season.
Growth of P. subcapitata was stimulated (to an average of 69%) by all dry season soil samples while
V. fischeri and A. globiformis were inhibited by the same samples at mean values of 34% and 25%,
respectively. The observation that toxicities tended to be higher in dry than in rainy season
sediment samples in the three bioassays indicate the presence of bioavailable inhibiting
contaminants, which are deposited in the banks of streams or rivers when the flow velocity is slow.
Low toxicities of rain season samples probably indicate a lower concentration of
pesticides/contaminants due to flooding and increased surface runoff. A few dry season sediment
samples collected from the outlets streams that receive drains from the rice paddies, as well as
those collected adjacent to pesticide mixing /sprayer filling point, showed elevated toxic responses
in three bioassays. However, there were no a direct relationship between such samples and the
detected pesticides residuals, suggesting that the toxicity was caused by other contaminants. Fuzzy
rule based classification of toxicity responses of the three bioassays showed that 73% of samples
posed little or no toxic potential risk, while 25% were identified to pose critical risk and only 1%
posed elevated critical risk.
The algae growth inhibition test showed stimulation in 131 samples out of 143 rainy and dry season
samples (sediments, soil and water). Laboratory experiments to evaluate the source of stimulation
in the algae test revealed that nutrients such as organic carbon and Vitamin B in the test
medium/matrices could stimulate P. subcapitata growth rate. Kilombero samples, however, had
relatively low C/N ratios (according to FAO-classification), varying from 10 to 20 for poorly drained
soils, suggesting that there is high decomposition of organic matter (enhanced by tropical climatic
conditions) and organic humus is transported from the highlands to these lowland flood plain
wetlands, thus enriching its organic nutrients.
Experiments were carried out to assess whether the blockage of electron transport system by
photosystem II inhibitors (specific herbicides) could have caused the stimulation in the P.
subcapitata growth inhibition test. It was revealed that: as opposed to chemicals with different
mode of action, the presence of PS-II inhibitors in the test medium stimulated the prompt
fluorescence (PF) and inhibited the delayed fluorescence (DF), a pattern that is caused by back
reactions when the electron transport system is blocked. Although pesticide residuals such as
diuron, glyphosate, AMPA, propoxur, atrazine, 2-hydroxy-atrazin, metribuzin, chlorpyrifos,
xxiSummary
desmethyldiuron, monuron, hexazinone and ametryin were detected in water, sediment and soil
samples, the DF and PF patterns were not detected in the environmental samples as they were with
single substance tests. Despite the potential to induce fluorescence, the presence of PSII inhibitors
in the samples could thus not be responsible for stimulation in the 72-hour growth inhibition test.
In conclusion, V. fischeri and A. globiformis bioassays can be used in tropical climates to identify
contaminated hotspots. However, the chronic P. subcapitata bioassay could not provide enough
information on the toxicity of sample matrices because of the observed stimulations. Thus, there
might be a potential to underestimate the toxicity of samples collected from agricultural fields,
especially for sediment and soil samples, which are rich in nutrients and are contaminated with low
concentration of pesticides. Supplementing the 72-hour algae test with a short bioassay, detecting
photosystem II inhibitors, could provide additional, useful information on the toxicity of herbicides
in environmental samples.
Another supplementary test with the yeast Saccharomyces cerevisiae showed toxic responses in the
same magnitude as in the bacterial bioassays when applied to the Kilombero samples. While
promising to become a meaningful addition to the biotest battery to indicate fungicides, this test
needs optimization due to unexplained reactions when exposed to organic pesticides.
The risks of pesticides in the ecosystem depend on the characteristics of a chemical and on the level
and pattern of exposure. Unsafe handling of pesticides in the Kilombero valley, which were
revealed during field surveys are: pesticide spillage at retail marketing places, spillage during filling
up the sprayers, cleaning of the spraying equipment in open waters, on-farm disposal of the
pesticide containers and mixing pesticides close to streams, wetlands and ditches. These water
bodies surrounding the surveyed plantations have been used by the adjacent households as a
source of water for domestic use. Therefore, continued use and unsafe handling of pesticides in
these wetlands pose a great risk to human health, wildlife and aquatic organisms. Ecotoxicology using simple, fast and cost effective bioassays, which are novel to Tanzanian setting, might help to
provide strategies for reducing the risk from agrochemicals in the Tanzanian wetland ecosystems.
Description
PhD Dissertation
Keywords
Eco-toxicology, Pesticide management, Tropical Environments, Sustainable Wetland Management, Kilombero Ramsar Site