After the long dry spell through 2024 and early 2025, the lagoon looked spectacular – crystal water, corals thriving, fish everywhere. But nature is always balancing her books, and the wet winter that followed tipped things back the other way.
Rain is, of course, essential. Every household depends on it for drinking water, gardens, and life itself. The island’s tanks and aquifers are filled by these winter downpours. But when heavy rain hits bare soil, pasture or roads, the water doesn’t just recharge tanks – it runs off the land and seeps through the ground, carrying with it nutrients and fine sediments into the bays.
That nutrient pulse feeds more than just the corals’ microscopic algae. It also fuels Lyngbya – a brownish, stringy, filamentous growth often mistaken for seaweed. It starts to appear in late September and usually hangs around until March.
Figure 2: Only about 75 metres from the first site, but a world apart. Brown, filamentous Lyngbya mats have begun to creep across coral and rubble, fed by nutrient-rich runoff and warmer water – a seasonal reminder that what happens on land doesn’t stop at the shoreline, taken on 7 November 2025. See the map below for where this one was taken (2).
Figure 3: This aerial map shows where the two images above were taken, two days apart. Click to enlarge.
When Lyngbya blooms, it spreads across coral surfaces and rubble, forming drab, fuzzy mats that block light and take up space. Most grazing fish avoid it, so the usual reef cleaners leave it untouched. The mats trap fine sediment, alter oxygen levels, and make it difficult for coral larvae to settle or recover from disturbance. Over time, the reef’s living surface shrinks beneath a film of brownish scum.
Then, as summer peaks, a second wave arrives – the red matting cyanobacteria. Beginning in mid January, reddish or rust-coloured films spread across the reef, often growing over what’s left of the Lyngbya. For a month or two, the reef wears both – brown filaments tangled with red felt. By late autumn, as the water cools, the mats finally start to die back. The nutrients remain, but lower temperatures and shorter days curb their growth.
The contrast between these two photos, above – taken only about 75 metres apart – shows how quickly things can change. It’s a reminder that rainfall, temperature, and nutrient flow are all connected. What runs off the land doesn’t stop at the shoreline. When the lagoon turns slimy, it’s the reef telling us the balance has tipped too far – and that every drop of water, from our household tanks to tide, is part of the same story.
What Lyngbya really is
(for the curious)
Lyngbya looks like an alga, but it’s actually a cyanobacterium – a photosynthetic bacterium. Unlike true algae, which are eukaryotes with a nucleus and chloroplasts, cyanobacteria are prokaryotic. Their light-harvesting pigments are scattered throughout the cell, giving them that blue-green or brownish hue.
Some Lyngbya species produce toxins; others don’t. But all can form dense mats when the conditions line up – warm water, strong sunlight, calm seas, excess nutrients, and low grazing pressure. The precise temperature threshold isn’t well established, but blooms typically appear as the lagoon warms into the low 20s and nutrients rise with runoff.
Figure 4: Brown, hairy Lyngbya on the inside of the lagoonal reef, taken on 7 November 2025. Click to enlarge.
Figure 5: Red cyanobacterial matting, taken on 18 January 2025. Click to enlarge.