# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 3 Results and Discussion
### Post-Spill Impacts of Oil Spill Volume and Time Gap on Vegetation Health  
Figure 4 summarises the post-spill NDVI value extracted for a sample of 176 spill sites from the first available Landsat satellite observation acquired immediately after oil spills that occurred during 2006-2018. From the results, the dense vegetation has the highest maximum post-spill NDVI values (\(\sim\)0.6), followed by the sparse vegetation (\(\sim\)0.55) and mangrove (\(\sim\)0.4). Similarly, the dense vegetation has the highest median value (\(\sim\)0.3). In contrast, mangrove has the lowest median NDVI (0.2) among all vegetation types, with over 75% of NDVI values being \(<\)0.3, making it the most degraded vegetation. Typically, the NDVI ranges from 0.2 to 0.5 for sparse vegetation, such as shrubs and grasslands, and from 0.6 to 0.9 for dense vegetation, such as that found in temperate and tropical \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Points** & **Spill Date** & **Spill Volume (bbl)** & \(\Delta\)**NDVI Coverage** \\ \hline M1 & 14 August 2013 & 264 & 2013 to 2018 \\ M2 & 26 June 2010 & 800 & 2010 to 2018 \\ M3 & 1 February 2010 & 1020 & 2010 to 2018 \\ M4 & 5 August 2010 & 1510 & 2009 to 2018 \\ M5 & 15 May 2009 & 1554 & 2008 to 2018 \\ M6 & 15 June 2009 & 2500 & 2008 to 2018 \\ \hline \hline \end{tabular} D—Dense vegetation sample site; S—Sparse vegetation sample site; M—Mangrove sample site.  
\end{table} 
Table 3: _Cont._forests or crops at their peak growth stage [59]. However, the post-spill minimum NDVI values are lower than the minimum NDVI values for typical healthy dense and sparse vegetation, whilst the maximum NDVI for dense vegetation is approximately the same as the minimum NDVI value for a typical healthy dense vegetation, which indicates the impact of oil spill on the health of the vegetation. Regarding healthy mangroves, Asian mangroves generally have the highest NDVI values, especially in Southeast Asia (0.80), whilst the average NDVI of African mangroves is typically 0.67 [60]. However, following exposure to oil, the highest NDVI values in the Niger Delta mangroves are 0.4 (\(\sim\)100%) and 0.2 (\(\sim\)50%) lower than those associated with Asian mangroves and typical West African mangroves, respectively, which also indicate the impact of oil spills on the health of the mangrove vegetation. The degradation of mangroves due to the oil spill could affect their sustainability. It may ultimately lead to the disappearance of the mangroves, which will have serious consequences for both the people and the fauna in the Niger Delta who depend on it. The Niger Mangrove ecosystem faces serious threats from crude oil spills [42]. Specifically, the degradation of mangrove forests will impact biodiversity and jeopardise the livelihoods of coastal communities that depend on them for food and resources, leaving them vulnerable to tidal inundation and extreme weather due to the loss of natural defences [43]. The study by [PERSON] et al. [61] discovered that, whilst healthy mangroves reported a net loss, degraded mangroves consistently reported a net gain between 1988 and 2013. To ensure the sustainability of the mangroves in the Niger Delta, there is a need for a policy that would mitigate the impact of the oil spill. [PERSON] et al. [43] suggested that the enforcement of laws, public awareness campaigns, educational programs, and strategies for restoring and conserving the Niger Delta mangroves are crucial not only for safeguarding biodiversity but also for ensuring sustainable resource management practices and mitigating environmental threats. Figures 5-9 show the correlation between post-spill NDVI and oil spill volume for dense, sparse, and mangrove vegetation. The relationships are between all volumes of the oil spill (Figure 5), above 225 bbl (Figure 6), between 225 and 400 bbl (Figure 7), between 401 and 1000 bbl (Figure 8) and \(>\)1000 bbl (Figure 9). The oil spill volume ranges were determined based on the work by [PERSON] et al.