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PROJECT DESCRIPTION Coral reefs are in startling decline: this is the broad consensus among coral reef scientists, and is strongly supported by empirical data from around the world. One summary estimate places 10% of the world’s reefs as already irreparably damaged, 30% in this state in 10-20 years, and a further 30% in 20 – 40 years (Wilkinson 1993). |
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The Fishes of Biosphere2 | |
The Corals of Biosphere2 | |
The Algae of Biosphere2 | |
An Introduction to the Ethology of Fishes | |
Live Web-Cams of the Biosphere2 Oceanarium |
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Naturally, the economic impacts of reef decline will be
greatest for developing nations, but this does not mean that the US
will get by unscathed. Many of the reefs showing the most serious degradation
are in US waters. Coral reefs from Florida, Puerto Rico, and the US
Virgin Islands to Hawaii and former US protectorates in the Pacific
region have all shown sharp declines in live coral cover and fish abundance.
We know that in a general sense these declines are due to the combined
effects of overfishing, coastal pollution, emergent diseases, global
warming, and changes in atmospheric chemistry. What we do not know is
how to translate our available science into policy, and in many places
the scientific data are not yet convincing enough to justify the economic
implications of effective mitigation measures. Nowhere is this more
apparent than in the effects of elevated atmospheric carbon dioxide
(CO2) on coral reef health. |
However, the coral/climate question is still more complex
than this. Corals do not live in isolation, but rather are productive members
of one of our planets most complex and diverse ecosystems; the coral reef.
Corals compete for space and food, are eaten by fishes and invertebrates,
and are a host for countless species of small invertebrates and microscopic
organisms. Like all of our natural world, corals rest in some unknown balance
with other members of their community.
When herbivorous fishes and invertebrates are eliminated
from coral reefs – often from over fishing - algae can quickly outgrow
and shade out corals (Lewis 1985, Hughes et al. 1987, Steneck 1988,
McClanahan et al. 1994, Hixon and Brostoff 1996, Shulman and Robertson 1996,
McClanahan and Muthiga 1998, Schmitt 1998, McClanahan et al. 1999).
The presence of symbiotic crustaceans (Glynn
1983)
and algae (McConnaughey et al. 2000)
can affect coral calcification rates and their competition for space. On
a microscopic scale, corals are constantly battling in a fight against fungi
and bacteria that would hope to consume them (Le
Campion-Alsumard et al. 1995, Bentis et al. 2000).
This complex web of ecological relationships could either mediate or exacerbate
the affects of increased CO2.
In general, the complex nature of reef systems suggests
there are any number of positive and negative feedbacks on increased atmospheric
CO2. Will photosynthetic rates increase to offset decreased calcification
rates? Will new proteins evolve in corals quickly enough to compensate for
decrease bicarbonate concentrations? Will some competitors be differentially
affected by the changes? Like many complex questions, these answers will
only come when we “take it to the numbers” so to speak. Our
understanding of the precise affects of global climate change on coral reefs
has yet to incorporate this ecosystem approach.
Work by our collaborators in the Biosphere2 mesocosm (www.bio2.edu) has corroborated earlier experiments
in small-scale aquarium systems in demonstrating that elevated levels of
CO2 in seawater cause a profound reduction in the rates at which
corals are able to secrete their calcium carbonate skeletons (Langdon
et al. 2000).
Our current collaboration in Biosphere2 is focused on the investigation
of CO2 levels and complex interactions between herbivores, coral,
algae, and fungi.
CARBON DIOXIDE and the CARBONATE LIMITATION HYPOTHESIS
Corals are particularly fascinating in that their functional
self consists of part animal (the coral itself) and part algae in the form
of symbiotic dinoflagellates called Zooxanthellae. Corals collect organic
material from the water to secure nutrients and minerals, but a large portion
of the raw energy for life comes in the form of sugars created via photosynthesis
by the algae. Zooxanthellae reside in the tissue space of the corals where
nutrients vital to the photosynthetic pathway are readily available. The
process of photosynthesis produces oxygen and consumes CO2, and
the interstitial concentration of CO2 is driven down by the photosynthetic
process.
In water, carbon dioxide gas absorbed from the atmosphere
persists in very small quantities since the bulk of this material undergoes
a rapid reaction with water
CO2 + 2H2O
<-> H2CO3 <-> HCO3-
+ H+<-> CO23- + 2H+
(1)
Increased CO2 affects pH via the generation of the negative
carbonate ions on the right side of equation 1. Inside corals, photosynthesis
decreases CO2 concentrations, increases pH, and increases the
bicarbonate concentration. This vital function of Zooxanthellae, perhaps
more important then the sugars they produce, facilitates the calcification
process by which corals build their skeletal material (aragonite) via the
reaction
Ca2+ + 2HCO3- <-> CaCO3
+ CO2 + H2O
(2)
During coral bleaching Zooxanthellae are ejected from the
coral tissue due to some stressor, and the process of calcification becomes
energetically impossible. Under natural conditions a battle exists between
the decomposition of aragonite by environmental forces and its deposition
by the calcification process. When calcification is halted, natural decomposition
continues and so corals may actually shrink during bleaching events. Geography,
light availability, nutrient concentrations, pH, dissolved CO2,
O2, HCO3-, temperature, and salinity interact
to determine “natural” or “background” rates of
respiration, calcification, and degradation; the components of coral growth.
Under normal conditions, within the biogeographical ranges of hermatypic
corals, environmental conditions support rapid coral growth, allowing corals
to compete successfully for space and survive natural disturbance.
Increased CO2 concentrations push equation 1
towards the far right, forcing the dislocation of negative carbonate ions
from their positive counterparts (be they H+ or Ca2+),
and making the calcification process more energetically expensive (Marubini
and Atkinson 1999, Marubini and Thake 1999).
Decreased calcification rates upset the balance between growth and degradation,
with potential system wide consequences. Rates of physical and biological
erosion are large when compared to deposition, and a decrease in the rate
of calcification of as little as 5% will lead to a net loss of calcium carbonate
(Hoegh-Guldberg
1999),
but even small scale changes in calcification may effect ecological relationships.
METHODOLOGY
In the Biosphere2 system, dissolved CO2 concentrations
are altered on a (approximately) quarterly basis between present, past,
and predicted future levels. We use this manipulation to study the importance
of interactions between herbivores, corals, and CO2. In the oceanarium
proper we maintain four arrays of Montipora and Porites nubbins, half of which are caged to exclude herbivores. As a control
we maintain coral arrays in an adjacent flow-through facility at light levels
equal to and greater then those experienced in the oceanarium. Half of those
corals are “grazed” by hand to simulate the effects of fish
herbivory. This design offers 6 treatments;
Oceanarium grazed
Oceanarium ungrazed
Low light grazed
Low light ungrazed
High light grazed
High light ungrazed
At the end of every CO2 regime each nubbin is
stained using Alizarin Red and photographed from four orthogonal directions.
The growth rates of corals are estimated from the digital photographs using
NIH Image software, and, when nubbins are removed from the experiment, the
density of endolithic fungi is estimated using scanning electron microscopy
of thin sections.
We monitor fish herbivory in the oceanarium via four underwater
cameras that are trained on our coral arrays. Video from the cameras is
transmitted over the Internet using an Axis 2100 video server, and we record
fish behavior using a web-based ethology tool. In the coming months we would
like to bring additional researchers into the project to assist with our
observation of fish behavior, and we would like to collaborate with educators
to use this Internet-based experiment as a teaching tool in classrooms around
the world. Please take a moment to explore our ethology tools, get to know
the biota of the Biosphere2 oceanarium, and observe our live web-casts.
We look forward to comments and criticisms, and are excited to hear from
potential collaborators.
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