# 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
[32], who investigated the factors influencing the detectability of oil spills using spectral indices in an oil-polluted environment in mangrove vegetation in the Niger Delta. From the results, the dense and sparse vegetation has the lowest spill points at volume \(>\)1000 (Figure 9a,b) due to the lack of available volume oil spills in that range. It can be observed that, for all volumes of spills and up to volumes \(>\)225-400 bbl, there was a weak relationship between the oil spill volumes and NDVI values  
Figure 4: Box plot statistical summary of post-spill NDVI values for each vegetation type. Min and Max are the lowest and highest values of NDVI, respectively, excluding the outliers (diamond symbols). Q1 is the first quartile (25 percentile), and Q3 is the third quartile (75 th percentile). for all the vegetation types with \(R^{2}<0.03\). However, it can be observed that, though the relationship is weak, different vegetation types respond differently to the oil spill volume and the time interval between the oil spill dates. When the full range of oil spill volumes is considered, the relationship between the post-spill NDVI and the volume of oil spill is weak for all vegetation types, with the highest \(R^{2}=0.0107\) for mangroves (Figure 5). These results indicate a weak to no linear correlation between NDVI and oil spill volume, concurring with the lack of correlation (\(R^{2}=0.0001\)) found by [PERSON] et al. [32] when considering all spill volumes in the mangrove of the Niger Delta. However, when considering specific volume ranges, the NDVI for sparse vegetation is more negatively correlated (\(R^{2}=0.5018\)) to oil spill volumes in the range of 400-1000 bbl (Figure 8b). Similarly, the post-spill NDVI associated with dense vegetation is found to be more negatively linearly correlated with oil spill volumes >1000 bbl, with \(R^{2}=0.4356\) (Figure 9a). Interestingly, at the same oil spill volumes >1000 bbl, the sparse vegetation is positively correlated with \(R^{2}=0.9700\) (Figure 9b). Overall, the post-spill NDVI associated with mangrove vegetation is generally weakly correlated with spill volume, although a moderate positive correlation (\(R^{2}=0.4520\)) is observed for 401-1000 bbl (Figure 7c). However, the lack of a clear relationship between post-spill NDVI and oil spill volume for mangroves does not necessarily indicate that the oil spill does not impact the health of the mangroves. From Figure 4, it was observed that the mangrove is the most degraded type of vegetation. Much crude oil has been discharged into coastal environments, which makes mangroves extremely vulnerable to oil and industrial waste [62]. Therefore, the lack of correlation could arise because of difficulty quantifying the oil spill volume in the dynamic aquatic setting where mangroves are located, where waves and ocean currents can readily disperse the oil. Furthermore, the mangroves are often exposed to oil through runoff of spills inland, again making it difficult to reliably quantify the volume of oil originating at the mangrove sites. Similarly, the biota of mangroves exposed to oil pollution may experience increased mutation [63]. Figure 5: The relationship between post-spill NDVI and oil spill volume for (**a**) dense vegetation, (**b**) sparse vegetation, and (**c**) mangrove vegetation. Figure 6: The relationship between post-spill NDVI and oil spill volumes greater than 225 bbl for (**a**) dense vegetation, (**b**) sparse vegetation, and (**c**) mangrove vegetation. Similarly, Figure 10 shows the relationship between NDVI and time after the spill for the dense, sparse, and mangrove vegetation. In terms of the relationship between the post-spill NDVI and the time after the oil spill, the dense vegetation shows gradual signs of degradation through a weak decreasing trend in NDVI values following an oil spill (Figure 10a). This is because the full extent of the oil damage on dense vegetation may not be obvious until 6-12 months after a spill incident [64].