Rapid decline of macroalgae in Kaneohe Bay, Hawaii

The domination of reefs in Kaneohe Bay on the island of Oahu, Hawaii has long served as the most compelling example of the threat posed by nutrient pollution.  Reefs in the bay were dominated by the native green algae Dictyosphaeria cavernosa for nearly 40 years.  The virtual takeover by macroalgae and the subsequent decline of coral cover has long been attributed to local eutrophication (nutrient pollution) caused by sewage discharged into the bay until 1977.  Surprisingly, stopping the sewage discharge failed to substantially reduce the macroalgae, presumably due to a positive feedback that prevented the return to a coral-dominated state.

A new paper by Stimson and Conklin (2008) reports that the algae are finally gone.  After a 42-day period of rains and overcast skies, beginning in February 2006, the cover and biomass of Dictyosphaeria declined dramatically.  Two year later, there was still no recovery.

The cause of the Dictyosphaeria die off is unclear.  Stimson and Conklin attribute it to prolonged low light levels associated with the front.  A series of lab experiments indeed indicates Dictyosphaeria is quite sensitive to low light.  But I wonder if there was another cause; possibly a disease or an outbreak of micrograzers.  Regardless, this is really good news for reefs.  There are a growing number of reports of rapid reef recovery following what were considered to be more or less permanent phase shifts to macroalgal dominance.

In 2003 and 2004 I worked with a group that resurveyed Dairy Bull reef on the north coast of Jamaica (Idjadi 2006).  In the early 1990s when I worked in Jamaica as an undergrad and MS student, like most of the local reefs, Dairy Bull was pretty trashed.  14 years later, coral cover was increasing and macroalgae were on the run.  There still aren’t many fish and I heard that more recent bleaching has knocked the coral back somewhat.  But I am still pleased to see such recovery, especially in places like Jamaica, where there is virtually no local management and intense fishing pressure.  I also met some Palauan scientists at the ICRS meeting who reported that Palau’s reefs are recovering nicely (Golbuu et al. 2007) from the bleaching-induced coral loss we reported in 1998 (Bruno et al. 2001).

I am always on the lookout for this kind of good news.   If you hear about any other examples, please let me know.  We can’t ignore the threats and evidence of decline, but we also have to be clear that there is hope and that we haven’t given up.  (Also see Rich Aronson’s plenary talk at the ICRS link).

Reporting from Heron Island, GBR.   File under “cautious optimism”.

Bruno, J. F., C. E. Siddon, J. D. Witman, P. L. Colin, and M. A. Toscano. 2001. El Niño related coral bleaching in Palau, Western Caroline Islands. Coral Reefs 20:127-136.

Golbuu, Y., S. Victor, L. Penland, D. Idip, C. Emaurois, K. Okaji, H. Yukihira, A. Iwase, and R. van Woesik. 2007. Palau’s coral reefs show differential habitat recovery following the 1998-bleaching event. Coral Reefs 26:319-332.

Idjadi, J. A., S. C. Lee, J. F. Bruno, W. F. Precht, L. Allen-Requa, and P. J. Edmunds. 2006. Rapid phase-shift reversal on a Jamaican coral reef. Coral Reefs 25:209-211

Stimson, J., and E. Conklin. 2008. Potential reversal of a phase shift: the rapid decrease in the cover of the invasive green macroalga Dictyosphaeria cavernosa Forsskål on coral reefs in Kaneohe Bay, Oahu, Hawaii. Coral Reefs 27:717-726.

Leave a Reply

Your email address will not be published. Required fields are marked *