Some weird brown patches appeared just offshore on the wildside a few weeks ago. Wasn't sure what they were as there had been talk of a toxic red tide in the Cape Town area.
Then yesterday I spotted a few in the bay out front and decided to find out a bit more. Turns out they're most likely to be a non-harmful algal bloom, not a poisonous red tide. Shew.
Algal blooms are usually harmless and are part of the natural ocean cycle. When conditions are just right, ocean phytoplankton reproduce like bunnies, creating a thick, visible layer near the surface. These algae blooms - or what we often term as "red tide" - might look really sketchy during the day, but if the critters are the bioluminescent (such as Noctiluca scintillans), then look out for the night show!
This particular variety of phytoplankton glows blue when agitated, transforming dark ocean into a giant lava lamp. If the bloom comes ashore, watch the waves light up as they crash onto the beach in a weird neon blue glow. Unfortunately our patches have stayed offshore so I haven't seen the disco-glow as yet. Maybe someone should go do doughnuts out there on their jetski at night?
So why do these blooms occur? It's usually from some form of nutrient input. On the West Coast it's upwelling bringing bottom nutrients to the surface due to a combination of wind and bathymetry, combining with sunlight, and then "boom"....you get an explosion of phytoplankton production.
These critters are always in our waters, they just need the right fuel to bloom. In the Eastern Cape there are upwelling cells (mostly off Port Alfred) which can periodically bring nutrients to the surface when the winds line up correctly. Another factor which might spark a bloom is when organic nutrients are leaked into the ocean from sewerage, fertiliser, or river run-off, where the addition of these nutrients is enough to start a phyto feeding frenzy.
The blooms in our bay could arise from either one, but most likely are natural events resulting from wind-induced nutrient input from cold bottom waters.
The most likely suspect around PE is the glow-in-the-dark-dude, Noctiluca scintillans. He's a big-ass dinoflagellate, well, big-ass in the dyno world - clocking in at about 2 mm. The blooms usually are thick and bright red/orange and cause outboard motors to choke up (OK, so there goes the idea of getting a jetski to buzz about in them to make glow trails at night)
Other than draining oxygen from surface waters when decomposing or possibly clogging motors/gills/intake pipes the blooms are not toxic. We've had no record of poisonous blooms in the East Cape area to date.
The toxic red tides which affect the West Coast happen when blooms of a specific phytoplankton species die after depleting all of the upper layer nutrients and oxygen and then become toxic. The blooms are eaten by filter-feeders - and then that's why it becomes an issue to eat shellfish from those areas. Cos you essentially eat what they've eaten.
Maybe if you eat our blooms you can glow in the dark when you jump?!
(Um no, that was a joke. Please don't eat the algae! It probably tastes kak, and you definitely won't glow)
Thanks to Tim Parker-Nance, Gavin Rishworth & Dr Gavin Snow for their input