As we have seen in previous blog posts, light pollution not only affects organisms on land, anthropogenic light sources also penetrate into deep waters and is a significant factor that has disrupted the livelihood of various aquatic creatures such as sea turtles, corals and fish. In this post, we will see the extent to which light is able to reach the waters of the northern Gulf of Aqaba.

The Gulf of Aqaba is part of the Red Sea, between two cities Eilat, Israel and Aqaba, Jordan (Tamir et al, 2017). Due to urbanisation at the shores of the Gulf of Aqaba, the marine environment there has been heavily exposed to the effects of anthropogenic light sources. These cities are known for its thriving tourism industry as well as recreational marine facilities that further aggravate the effects of light pollution. Additionally, the geographical conditions of Gulf of Aqaba such as its low water turbidity and low nutrient concentration allow increased penetration of natural and anthropogenic light into the waters to great depths with 1% of subsurface irradiance reaching depths exceeding 110m (Tamir et al, 2017). 

In this study, underwater downwelling irradiance (to evaluate the penetration depth of light) of the Gulf of Aqaba was measured on the night of September 9th 2014 and August 12th 2015. Measurements were taken at different sampling points (Figure 1) along the Israeli segment of the gulf.

Figure 1: Sampling points

Figure 2: GIS map of Downwelling irradiance 

Figure 2 shows the downwelling irradiance in the Gulf of Aqaba at 1m (a, d), 5m (b, e) and 10m (c, f) depth respectively n the wavelength channels of yellow light at 589nm (a,b,c) and blue light at 443nm (d,e,f). The black dots represent the sampling locations from i1 to i19. We see that artificial light from the city Eilat (see sampling location i1) was detected as one of the highest near the shores of the Gulf of Aqaba. In addition, sampling location i4, also shows relatively high irradiance. This location point coincides with Eilat’s main port and oil jetty terminal’s offshore pier that is located less than 300m from i4 which have also significantly contributed to increased artificial light in the gulf (Tamir et al, 2017).  

Figure 3: Vertical cross-section map of downwelling irradiance 

From figure 3, we see a clear depiction of downwelling irradiance of yellow (3a) and blue (3b) light with respect to depth versus horizontal distance from the city of Eilat. Tamir (2017) also noted that there is a distinct horizontal gradient of irradiance seen in figure 3 and 4. For example, at station i4, the irradiance in the yellow light wavelength channel was measured at 4.6 × 10−4 μW cm−2 nm−1, while at the station furthest away from the city of Eilat, i15 (6.3km from station i1), the measured wavelength was that of 1 × 10−6 μW cm−2 nm−1 at the same depth of 1m. 

Figure 4: Vertical profile of downwelling irradiance 

Apart from the horizontal gradient of irradiance, figure 4 also shows the level of irradiance at station i4 with respect to depth according to three wavelength channels: 443nm (blue light), 520 nm (green light) and 589nm (yellow light). We see that just beyond the first few metres of the water column from the city of Eilat, yellow and blue light of the emitted light spectrum are already able to penetrate into the waters at relatively high spectral irradiance. It is also interesting to note that down to the depths of 7m, yellow light irradiance was higher than blue light irradiance. However, of depths below 7m, blue light irradiance became higher than that of yellow light irradiance. This could be because at the surface of the water, where air and water interact, yellow light is refracted in directions further away from vertical when compared to that of blue light (Tamir et al, 2017). Hence, yellow light propagates along a longer path from measurement depth near the surface waters versus measurement depth under the water. 

Overall, such light irradiance information at different sites of the gulf is important to evaluate the extent and type of impact light pollution has on marine environments. The fact that irradiance of artificial light is not constant across locations, depths and distance across water bodies indicate that the impact of light pollution on aquatic creatures may not be uniform (Tamir et al, 2017). 

 

Stay tuned for more!

Jean

 

References 

• Tamir, R., Lerner, A., Haspel, C., Dubinsky, Z., & Iluz, D. (2017). The spectral and spatial distribution of light pollution in the waters of the northern gulf of aqaba (eilat). Scientific Reports, 7(1), 42329-42329.