Sea Urchins in the Caribbean

[dcdevil]

I am very interested in figuring out why Caribbean coral reefs have declined so much in the last 30 or 40 years. There are many reasons for the deterioration of the reef, but it is clear that the die-off of the long-spined Diadema sea urchins in 1983 was and is still a major contributor to the loss of coral. Sea Urchins are instrumental in keeping algae in check and allowing room for corals. This summary by Martin Moe provides an excellent overview and summary:

"The large, pre-plague populations of Diadema that inhabited the reefs were efficient bioerroders of the limestone reef structure as well the keystone herbivores on the reefs. They created much of the coral sand that surrounds the reef and kept the reef substrates clear of the algae and sediment that inhibits the settlement and growth of juvenile corals. Algae and sediment now cover much of the reef structure so even when the remaining few corals successfully spawn in late summer and release millions of planula larvae, if there are only a few areas where the larvae can settle and grow, recruitment of new coral formations are few and far between...

Loss of this keystone herbivore has shifted the ecology of coral reefs from stony corals to dominance of macro algae and algal turf. Natural recovery of small pockets of Diadema populations in St. Croix , Jamaica, Belize, Dry Tortugas and other areas,... has shown that when adequate populations of Diadema are restored to coral reef areas, this trend is reversed and coral settlement and growth once again become dominant on the reef.

And Then They Were Gone

I am currently in St. Croix and have seen pockets of healthy sea urchins in large numbers in Coakley Bay, Chenay Bay, and many juvenile sea urchins in the very shallow rocky areas near shore in many locations in St. Croix including Boiler Bay and Coakley Bay. Also, while much of the elkhorn and staghorn is dead and gone, there are seemingly more and healthier brain corals, porites, pillar corals, and montastrea, as well as less algae.

I am wondering if other divers have seen evidence of any comeback of the sea urchins in other areas of the Caribbean?

 

[drbill]

I was dismayed to see the amount of algae on former coral reefs when I've dived in the eastern Caribbean (not so much in the western Caribbean as far as I remember). At first I thought it was a result of the lionfish decimating the young of herbivorous fish, but then I learned about this mass die-off of the Diadema urchins. We've had urchin wasting events here in SoCal due to El Nino events, but ours seem to recover reasonably well within a few years.

 

[Wookie]

I operate in Dry Tortugas. I used to operate in Flower Gardens. We seem to have a healthy population of Diadema in both places, but the algal blooms in Dry Tortugas still occur with great frequency in the spring. Anecdotally, I blame the nutrients coming out of the Everglades with the spring rains for the algae, which is a long (6-12") green filamentous algae that covers all hard surfaces. It shows up in April or May and is gone by May or June. The rest of the year, and especially last year, the stony corals were healthier than I have seen since before the bleaching event of 2005. We have very large patches of Acropora (300' x 100') on some of the Dry Tortugas sites coming back, this is wild acropora, not lab raised as so much is in the upper keys.

 

[Tjack]

The catastrophic urchin die off is a major factor, but declines in reef fish populations also contribute to a disproportionate algae population.

 

[drbill]

Frank has certainly pointed to another significant issue with respective to algae on the reefs. The same is happening on the Great Barrier Reef.