[Coral-List] Black reefs

Bruno, John F jbruno at unc.edu
Thu Sep 8 12:26:48 EDT 2011

Hi David,

It seems the more we learn about Kingman and other reefs in the Line Islands, the more abnormal their abiotic setting seems to be.  Kingman is highly oligotrophic and iron-limited, it has the luck of not having experienced significant warming (high temp anomalies) in the last few decades or as far as we know, any major storms, COTS or disease outbreaks, etc.  Couldn't these factors explain the high coral and low macroalgal cover recently observed there (particularly the iron/nutrient limitation)?  Or do you still argue the most parsimonious explanation is the presence of sharks and other top predators?  And do you still think this reef should serve as a global model for what a pristine reef is supposed to look like?  The more it is studied, the more anomalous is appears to be.

Sincerely, John

Message: 3
Date: Wed, 7 Sep 2011 16:44:43 +0300
From: David Obura <dobura at cordioea.org<mailto:dobura at cordioea.org>>
Subject: Re: [Coral-List] Black reefs
To: coral-list at coral.aoml.noaa.gov<mailto:coral-list at coral.aoml.noaa.gov>
Message-ID: <19877635-7A1E-4927-BB10-009B8F4B461A at cordioea.org<mailto:19877635-7A1E-4927-BB10-009B8F4B461A at cordioea.org>>
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HI all,

I think the key phrase in Forest's post was "emergent reefs".

I have similarly used steel mesh, re-bar and other relatively raw-iron parts in the water in Kenya and not had any toxicity problems for corals, apart from the physical problems of the rust sloughing and thick bands of powdery oxide building up in some places. And I hadn't noticed any problems with shipwrecks ... until I dived in the iron-depleted central Pacific. In the Phoenix and Line islands, where there is low iron in the oceanic waters and no iron-bearing rock (as in e.g. the main Hawaiian islands, i.e. no 'emergent' rock other than carbonates), any iron artefacts or shipwrecks on the reefs clearly result in the 'black reef' phenomenon.

The interesting thing about this case study to me is the illustration of what happens when you 'geo-engineer' and ecosystem - adding iron to oceanic waters to enrich planktonic production and sequester carbon may do that to some extent, but as in black reefs, it would likely completely change the nature of the ecosystem, very likely with many unintended consequences (such as, perhaps, a shut-down in the pathway that would leave to carbon sequestration).


David Obura

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