As we were out diving Engine Reef today on the edge of the channel between Nusa Lembongan and Cenningan islands, there was a strong warm down current across the reef from the outgoing tide. The warm water was thick with silt and debris and visibility nearing zero. We dropped in for our dive on the eddy line at the leading edge of the current and swam through the warm murky water over the reef edge to 16m depth. Here a cold upwelling of crystal clear water was mixing with the warm current and creating a feeding frenzie for huge schools of fish. We hovered for several minutes and watched as tuna and travelly cut through the schools of smaller fish. After a few minutes hundreds of schooling bannerfish swam off the reef towards a large grey shadow which was hanging a few metres out from the reef edge. As the shadow came into view we could see the distinctive shape of a mola-mola, an oceanic sunfish, up from the deep following this upwelling to get cleaned by the bannerfish.
The warm down current began increasing in strength so we began our ascent up the reef slope. As we swam over the reef the water was full of floating Ulva sp. and several other species of Chlorophyta which was blanketing the reef. The entire reef was becoming green with an algal matting within a matter of minutes!
It would be interesting to know where the green algae bloom was coming from and the causes of if. Several large schools of herbavors (black surgeon fish) were feasting on the algae. So we will be returning to the site in the next couple days to see how the reef community has reacted and adapted to the invasion.