LAKE KIVU CHANGED ITS COLOUR AGAIN?

1.   Introduction

In the last few days, in late August 2016, the waters of the shore of Lake Kivu have changed its colour (Figure 1 ), but for reasons completely different from what happened in April 2016 (Figure 2).

Figure 1: Pictures published by Igihe.com on 31st August 2016, about Lake Kivu colour on 30th August 2016.

2.   Reminder about whitening of Lake Kivu

As reported previously by different local media houses, whitening occurs when a lake with hard water faces specific weather conditions. In April 2016, high air temperature, high humidity and high irradiance led to stratification of the oxygenated Lake water layer i.e the Mixolimnion which was really shallow (~28 m deep), with high water temperature (25.5O) and pH above 9.2. In those conditions, the carbonates, which are usually dissolved in the lake, were oversaturated and precipitated, calcium ions precipitated in the form of calcite particles. As these particles are white, it gave a milky or turquoise appearance to the lake’s water, when observed from near, observed from far, the lake seemed to be blue/green because of green algae (Cyanobacteria) that have multiplied in the lake. They are actually characteristic of the rainy season in many lakes.  

Figure 2:  whitening of Lake Kivu on 13th April 2016, photo by Lake Kivu Monitoring Programme.

3.   Colour change observed late August 2016

The change of the colour on the shore of Lake Kivu, in Rubavu, observed by the end of August 2016, is a normal phenomenon due to high wind speed. In fact, during dry season, the lake is beaten by strong winds blowing toward the north of the Lake (Gisenyi). Normally, the speed of wind is low in morning hours and you cannot see change of colour, then over the day, the wind increases with time (Figure 3) and becomes high in late hours of morning and in the afternoon.

These winds have seasonal and daily impacts on the lake:

Ø Seasonal impacts of winds include, but are not limited to mixing of the oxygenated waters (surface layer) and uptake of deep nutrients and other dissolved matters from deep waters into the surface layer.

Ø Daily impacts, which are of concern to explain the present shore colouring of the lake, include presence of waves on the surface water and maintain of the plume of inflowing rivers along the shore. The wind indeed prevents the plume to disperse into the lake and force it to spread along the shore.

The daily impact is actually happened recently, with the highly charged in suspended matters plume of the Sebeya River (as it often occurs). Normally, winds on Lake Kivu are predominantly blowing from the South (Figure 4). As South wind speed increases during the day, we thus observe, along the shore of Lake Kivu, a strip of clearly different colour of the water corresponding to the colour of the water of Sebeya River in the process of mixing with water of Lake Kivu. This strip is already slightly seen on the picture taken at 9:00 on 31st August 2016 (Figure 5c), but it was not there earlier in the morning (Figure 5a&b). Note that 

 

Figure 3: Maximum hourly wind speed (m/s) from  29th to 31st August 2016. Wind speed is the highest around 11:00 and is blowing from the south, toward the north (not shown on the figure).

 

Figure 4: Wind direction for August 2016, (Orientation: blowing from)

Figure 5: Pictures of the shore of Lake Kivu at7:00(a. &b.) and at 9:00 in the morning (c.), taken by LKMP on 31st August 2016.

On the other hand (Figure 6), data collected on the Lake on 31st August 2016 shows that stratification is deep in the biozone (~50 m depth), as expected during dry season; pH was 9.16. In addition, all other parameters measured indicated that the Lake was stable.

Figure 6: Full range profiles taken in the Northern basin of Lake Kivu with the CTM 725. – 31st August 2016. Note the stratification of the biozone at 50 m depth.

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