Ring of green water

By Sue Matthews, Overstrand Estuary Management Coordinator

The bright green strip that has appeared along much of the Bot River estuary’s shoreline is a bloom of the filamentous algae Cladophora. This is  a nuisance algae in many aquatic systems throughout the country, and in other parts of the world. It is indicative of nutrient-rich waters, but blooms are typically triggered by environmental conditions. High levels  of nitrogen and phosphorus can be attributed to anthropogenic (human-induced) inputs, such as fertilizers, livestock manure and sewage effluent, but natural recycling of nutrients from the organic-rich bottom waters also plays a role. Strong winds help turn over the water column and bring these nutrients to the surface, where they can be taken up by plants. 

Cladophora occurs as both attached and free-floating forms, and it thrives in shallow, clear waters where there is abundant light for photosynthesis. Since water levels in the estuary have remained relatively low this year, the algae has been able to colonise large areas. In water supply and irrigation systems, Cladophora is a nuisance as it causes taste and odour problems in drinking water, and clogs pumps and pipes. On the Bot it is only when the algae dies and decays that it really becomes problematic. Dense mats stranded on the shore rot in the sun, resulting in a brown sludge that looks and smells like sewage. Bacterial decomposition processes deplete the oxygen in the underlying sediment, turning it into a slimy, sulphurous black mud. In a worst case scenario, overlying waters may also be affected, causing animals to move out of the area, or die if they cannot escape to well-oxygenated water. The bacteria associated with rotting Cladophora can also pose a risk to human health. 

Cladophora has become a major problem in North America’s Great Lakes over the past decade, creating a headache for holiday homeowners along their shores. Here the Cladophera species grows on submerged rocks, logs and other hard surfaces, with abundance peaking in spring and autumn. Die-offs occur in mid-summer, possibly due to higher water temperatures, and the filaments then break free and are swept ashore by wind and wave action. The pong from the rotting mats forces homeowners to keep their windows closed, and clears the beaches of bathers. 

Hopefully, the current bloom in the Bot is just a sign that spring has sprung, and with it warmer temperatures, longer days, more sunshine and, of course, strong south-easters. Or it could be the canary in the coal mine, warning us of a future crisis if we don’t take a hard look at possible pollution sources. 

Sue is contracted to the Overstrand Municipality in a position funded by The Table Mountain Fund, an associated trust of WWF-SA.