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Small sample of soil and moss taken from edge of dry rockpool on top of dry sandstone hilltop and kept at room temperature for several weeks. Water added and kept at room temperature for several days. Samples taken by dropper over next few weeks and examined microscopically.

Small sample taken of soil and moss from edge of dry rockpool on exposed sandstone hilltop and kept at room temperature for several weeks. Water added and kept at room temperature for several days. Samples taken by dropper over next few weeks and examined microscopically.

Found smothering large parts of the seagrass beds below low tide level between the wharf and Hobbs Bay on Tiritiri Matangi Island in the Hauraki Gulf in December 2023.
Seems to be similar to the outbreak of Okeania off the shores of Waiheke Island recently. It is a filamentous marine cyanobacterium forming a dense mats covering the seagrass blades with mucus like layer, rising up to form black slimy club-shaped aggregations. These thick mat structures seem to obscure the light falling on the seagrass.
The photos show how it can form a dense mat of trichomes blocking out light and smothering anything growing beneath it.
No heterocysts and no visible slime tubes when viewed with the microscope.
Many Okeania species are tropical. The Hauraki Gulf is currently 2.2 degrees C warmer especially during the winter than records show 60 years ago.
It appeared shortly after about a million tonnes of raw sewage escaped into the Gulf when the Parnell wastewater pipe collapsed in 2023, raising the nutrient levels in the Gulf.
The black blobs are full of O2 bubbles which cause them to float free and bob to the surface where swimmers might ingest them. They release a toxin causing skin irritation and possibly blistering.

A) Aspect général d’un thalle.
B, C, D) Stade “Chantransia” (microscope).
E, F, G, H) Forme développée (microscope).
I, J, K) Habitat & site de collection (= source du lavoir municipal de la commune d'Orly).

Could it be Batrachospermum gelatinosum?