Tag Archives: Macroalgae

Rapid decline of macroalgae in Kaneohe Bay, Hawaii

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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”.

References
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.

Grazer composition an important factor in controlling macroalgae

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The redband parrotfish was one of the species studied as part of research into the importance of fish diversity for the health of coral reefs.

The redband parrotfish was one of the species studied as part of research into the importance of fish diversity for the health of coral reefs.

We know the biomass of macroalgae on coral reefs is largely controlled by herbivory and that one of the most important groups of grazers are parrotfish.  A new study published in PNAS (Berkepile and Hay 2008) indicates that the richness and composition of grazer species is also important.  In a nutshell, different fish consume different seaweeds because of their differing chemical defenses.  Similar work in other benthic marine systems has found that consumer species richness can be an important determinant of ecosystem functioning, yet this is the first such study on a coral reef.

Our study shows that in addition to having enough herbivores, coral ecosystems also need the right mix of species to overcome the different defensive tactics of the seaweeds.  explained Mark Hay, the Harry and Linda Teasley Professor of Biology at the Georgia Institute of Technology.

Despite different species of parrotfish in the Caribbean having different feeding behaviors, bioerosion rates, and preferred diets, parrotfishes are often considered as a unified functional group when inferring their effects on community structure. However, we found that redband and princess parrotfish had considerably different effects on communities, suggesting that grouping all parrotfishes may blur important distinctions among species.

Despite their different feeding morphologies, ocean surgeonfish and princess parrotfish generated similar macroalgal communities dominated by upright brown macroalgae (e.g., L. variegata and Sargassum spp.). In contrast, despite their more similar jaw morphology, the communities generated by redband and princess parrotfish differed considerably in the abundance of upright macroalgae. Similar to the work of Bellwood et al., these results show that fishes with different feeding morphologies can have similar effects on community structure, suggesting that relying primarily on jaw functional morphology to construct functional groups or infer a species’ impact may be unreliable.

Working out of the underwater Aquarius laboratory off Key Largo Florida, Hay and co-author Deron Burkpile – who is now at Florida International University in North Miami – constructed 32 cages on the reef. Each cage was about two meters square and one meter tall and was sealed so that larger fish could neither enter nor leave.

The number and type of fish placed into each four-square-meter cage varied. Some cages had two fish that were able to eat hard, calcified plants; some had two fish able to eat soft, but chemically-defended plants; some had one of both types, and some had no fish at all

For the cages in which we mixed the two species of herbivores, the fish were able to remove much more of the upright seaweeds, and the corals in those areas increased in cover by more than 20 percent during ten months, Hay said.

The data we are seeing in Fiji [from similar experiments] suggests that diversity may be even more important there than it was in the Caribbean.  There are a lot of different species doing a lot of very different things. These consumers are very important, and in areas where they are over-fished, the reefs are crashing.

Reference

Berkepile, D.E. and M.E. Hay. 2008. Herbivore species richness and feeding complementarity affect community structure and function on a coral reef. PNAS 105: 16201–16206

Diver Todd Barsby secures a cage to the coral reef during a study of the role of diversity among herbivorous fishes.

Diver Todd Barsby secures a cage to the coral reef during a study of the role of diversity among herbivorous fishes.