DIfferential Coral Bleaching at Two-Step Dive Site: Warming Waters or Runoff?

We've always liked the coral at Two-Step snorkeling beach, the waters directly across from the City of Refuge in south Kona district. Sadly, this area was in the epicenter of the recent dengue outbreak (August 2015 - March 2016) here on the Big Island. Because of this, we avoided the place after August, just returning this past Memorial Day to dive. The weather was good, though the water was a bit cloudy from recent rains. We saw a large whitetip shark cruising below the wall, along with turtles and many fish, so the overall reef life seemed healthy. However, we saw no shrimp, and some (just finger type hard corals, it seemed) of the coral had bleached. I wonder why?

At Twostep, the small polyp finger corals (Porites) were bleached and in poor condition, completely changed from last August (2015) when we last were here. On the other hand, the plate corals (Porites rus) at the wall were healthy and seemed to be growing in the area of reef it occupied at the expense of the bleached finger coral. Closer to shore, the encrusting false brain corals were multicolored and mostly healthy, unchanged I think from last year.

So, what could be the causes of the bleaching we saw between October 2015 and May 2016 at Two-Step?

First of all, there was a significant bleaching event in corals worldwide this past October, attributed to a recent warming cycle or less likely to El Nino water temperature elevations. So perhaps the finger coral was less tolerant of the warming conditions.

Second, it's possible that the spraying of pesticide on the vegetation near the water, needed to eradicate the dengue-infected mosquitoes of this district, caused some insecticide leakage into the water. I do not know why that would only affect the finger coral, though.

A third possibility is some local infection of the coral, perhaps in conjuction with the other two environmental factors, caused the finger coral bleaching.

Will the finger coral return, or will the wall at 2-step become a wall of mostly plate corals? We will see if this becomes a succession event. Hopefully the other corals will not follow the bleaching example of the finger corals later this year.

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