Introduction

This analysis explores the acute toxicity of DDT metabolites on freshwater fish species, using data from the US-EPA ECOTOX knowledgebase. The DDT metabolites, DDE and DDD, were exposed to fish species native to South East Asia and North America like Channa spp., Heteropneustes fossilis, Lepomis macrochirus, Poecilia reticulata (native to northeast South America and the southern Lesser Antilles) at different life stages for different durations (days) and with different exposure types to determine the LC50 values - the concentration required to kill 50% of a population. The LC50 values for each species were compared to each other with a higher LC50 value signaling higher resilience and tolerance in the species compared to the others and a lower value signaling a lower tolerance to the DDT metabolite.

library(tidyverse)
library(readxl)

data <- read_excel("R- Ecotox/ECOTOX-Aquatic-Export_week 1.xlsx")

Data Exploration

The imported dataset from the USEPA data base was used to analyze the LC50 values across multiple fish species. The data was cleaned to extract the most useful sets for this analysis. The most important part of the data set which is the Conc 1 mean (standardized) was plotted against the species name. The interpretation of the data showed that Heteropneustes fossilis had the highest conc 1 mean (standardized) value, hence the highest LC50 value. This signals a higher resilience and tolerance to DDT metabolites compared to the other species. However, Lepomis macrochirus (bluegill) had the lowest LC50 value signaling a lower tolerance or resilience level to DDT metabolites.

Another observation from this dataset was that there were 5 different LC50 values for Channa striata as different experiments were carried out on it. The difference in these values could be attributed to differences in the duration of exposure (days), ecological, physiological, and environmental conditions. However, due to the presence of more than one LC50 value for this species, the data had to be summarized to create an average LC50 value for each species. After calculating and plotting the mean LC50 values, Heteropneustes still had the highest mean LC50 value, however, Poecilia (guppy) had a lower mean LC50 value than Lepomis (bluegill) (0.052 and 0.057 respectively). This means that guppies are slightly more sensitive to DDT metabolites on average than bluegill. The limited sample size affects the confidence in this result as there is only one representative data for both species.

A bar chart was plotted using the calculated mean LC50 values to give a visual for the summarized data. A dot plot was also plotted using the original values of the Conc 1 Mean (standardized) to give a spread of all the values of Channa striata across all different experiments. This is to give an accurate visual and spread to tell the whole story of the different experiments.

data_clean <- data %>%
  select(
    `CAS Number`,
    `Chemical Name`,
    `Species Scientific Name`,
    `Organism Lifestage`,
    `Exposure Type`,
    `Conc 1 Mean (Standardized)`,
    `Conc Min 1 (Standardized)`,
    `Conc 1 Max (Standardized)`,
    `Effect`,
    `Endpoint`,
    `Observed Duration Mean (Days)`
  )
data_summary <- data_clean %>% 
  group_by(`Species Scientific Name`) %>% 
  summarise(mean_LC50 = mean (`Conc 1 Mean (Standardized)`))
ggplot(data_clean, aes(x = `Species Scientific Name`, 
                       y = `Conc 1 Mean (Standardized)`)) +
  geom_bar(stat = "identity") +
  labs(title = "LC50 values of DDT metabolites across fish species",
       x = "Species",
       y = "LC50 (mg/L)") +
  theme_minimal() +
  theme(axis.text.x = element_text(angle = 45, hjust = 1))

ggplot(data_clean, aes(x =`Species Scientific Name`,
                       y = `Conc 1 Mean (Standardized)`)) + 
  geom_jitter(size = 3, width = 0.3, color = "steelblue") + 
  labs(title = "LC50 values of DDT metabolites across fish species",
       x = "Species",
       y = "LC50 (mg/L)") +
  theme_minimal() +
  theme(axis.text.x = element_text(angle = 45, hjust = 1))

ggplot(data_summary, aes(x = `Species Scientific Name`, 
                         y = mean_LC50,
                         fill = `Species Scientific Name`))+
geom_bar(stat = "identity") +
  labs(title = "Mean LC50 values of DDT metabolites across fish species",
       x = "Species",
       y = "Mean LC50 (mg/L)") +
  theme_minimal() +
  theme(axis.text.x = element_text(angle = 45, hjust = 1), 
        legend.position = "none")

Conclusion

The analysis established that amongst all the species analyzed in this study, Heteropneustes is the most resilient to the DDT metabolites while bluegill is the least resilient. However, one of the limitations of this analysis is the limited sample size. Asides from Channa striata, the data obtained for each species was from a single experiment and this could affect the accuracy of the analysis. Another limitation of this analysis is that the parent chemical, DDT wasn’t used in any of the experiments and only metabolites, DDD and DDE, were used. This analysis doesn’t give us the full picture on how these species would react to other metabolites or the parent chemical itself. Another observation is that a juvenile Lepomis was used in this analysis while the lifestage of other species is unspecified. We’re unable to confidently tell if the lifestage informed the LC50 value and to what extent. The sample collection process is also unspecified as differences in water quality parameters, feeding habits, anthropogenic activities could also affect the resilience and tolerance of the species. Future works could explore the bioaccumulation, biotransformation, and biomagnification of these chemicals across multiple species. Also, the presence of diseases in the species samples could affect their immunity to these chemicals, so future work could explore how diseases affect susceptibility to DDT and its metabolites.