Jen Smith, head researcher
In 2005, members of the Benthic Team participated in the first research expedition to the Northern Line Islands. One of most striking findings from this expedition was just how much the coral reef communities differed from atoll to atoll. Most strikingly, the atolls inhabited by people had fewer reef-building organisms (both corals and crustose coralline algae) and more fleshy turf and macroalgae (seaweeds). We hypothesize that this is a result of local human impacts (fishing and pollution) on the inhabited islands. Of particular concern is the likelihood that these changes are reducing the ability of the reefs to recover from stressful events such as the coral bleaching triggered by the warmer temperatures associated with climate change. Many questions still remain. Our focus for this 2010 expedition is to determine how these observed differences between atolls affect the health and productivity of these reef ecosystems.
We study mainly the benthic organisms that live on the sea floor or on the reef structure. Many of these are, at least in part, primary producers, that is they are able to carry out photosynthesis. In that process, they convert carbon dioxide (CO2) and sunlight into complex, energy-rich sugars, i.e. they are said to “fix” carbon by converting it into the form of usable food. These “seaweeds” and marine algae form the base of the coral reef food web. The sugars they produce are consumed by other organisms on the reef, the captured energy flowing up through the system to the top level consumers. Photosynthesis also produces oxygen (O2) as a by-product; the amount of O2 released indicates of the amount of carbon fixed. New carbon fixed becomes new biomass and is a measure for ecosystem production.
There are hundreds of species of marine algae at the atolls we will be visiting, each with their own unique physiological requirements and rate of photosynthesis. Reef building corals are also, at least in part, primary producers thanks to the microscopic algae that live as symbionts inside their cells. These algae, the zooxanthellae, photosynthesize actively and translocate up to 90% of their fixed carbon directly to their coral hosts. If the corals had to depend on hunting the relatively scarce plankton for all their food and energy, they would not be able to grow and build the reef structures fast enough to stay ahead of erosion. Thus the rate at which the corals deposit new skeleton (the calcification rate) is another measure of reef production. Other benthic species that are not primary producers rely entirely on carbon fixed by others. Rather than producing O2, they consume it and simultaneously release CO2. So, ultimately the amount of carbon fixed and consumed and the amount of O2produced and taken up in the reef benthic zone will depend on what organisms are present.
Our primary goals are to determine how primary production, carbon fixation and consumption, and calcification differ on reefs within the Northern Line Islands that are experiencing varying levels of disturbance by humans. A daunting task! To achieve this, our team will undertake a number of experiments.
(1) Measuring the community rates of respiration, photosynthesis, and calcification in situ using our custom-built reef tents known as cBATs. We worked with some brilliant engineers to develop these innovative devices. Each cBAT provides a transparent chamber that can be securely attached to the reef to seal off a 1 m x 1 m x 1 m area for monitoring for a period of time. Underwater sensors deployed inside the chamber will continuously monitor temperature, salinity, dissolved oxygen, and pH. We will also withdraw water samples at various time points though a sealed port in the side of the chamber. Afterwards we will determine the species composition of the plot and harvest all of the algae to determine the algal biomass present. From these data we will be able to compare the amount of photosynthesis and respiration taking place in equivalent plots studied at all of the atolls.
(2) Measuring the growth rates of common species of algae at atolls with varying levels of human impact. At each site we will conduct small experiments known as DAWGS (Deployed Algal Weight Gain Studies). For each, we collect small samples of algae of equivalent size and place them in individual cages that protect them from grazing. After 2-4 days the samples are collected and the algal growth rates determined. This compares production by the same types of algae under the differing conditions found at each atoll.
(3) Comparing the physiology of the same species of coral and algae collected from the various atolls. Here we will measure rates of photosynthesis and respiration under standardized conditions on board ship. Will we find higher rates of growth and photosynthesis for algae collected from sites with a higher input of nutrients? Will we see evidence that corals are feeding more by hunting at sites with a higher abundance of plankton prey? To answer these questions, we will obtain standard photosynthesis versus irradiance curves for coral and algal samples incubated in individual temperature-controlled chambers.