What Supports NWHI Reef Productivity?
A Trophic Detective Story with Implications for Whole Ecosystem Management
Written By Dan Suthers based on a proposal by Carolyn Currin and conversations with Carolyn Currin and Randy Kosaki.
This series of feature articles began with three articles on how scientists conduct field research in the Northwestern Hawaiian Islands (NWHI), and two more articles on what they are discovering through this research. In those last two articles we learned that the NWHI fish populations are dominated by predators, and that the benthic (bottom) environment is dominated by algae even more than coral.
The present article explores the relationship between these two groups of dominant organisms: how does algae support the fish population of the NWHI? This article is based on a research study that has just gotten underway to explore this question. The conclusions of the study will not be known for a while, but we can learn a lot about the NWHI by understanding the questions being asked, and this study has implications for resource management that tie into the Hawaiian concept of an ahupua`a. Also, the design of the study is very cool!
Algae as Primary Producer of Energy for Reef Ecosystems
Any ecosystem must have a source of energy. For the majority of ecosystems on Earth, that source is the sun. (In this discussion, we'll ignore ecosystems driven by the energy of deep sea vents, as their contribution is small in proportion to solar energy.)
In order for solar energy to be useful to living organisms, it must be converted into food energy. Plants play this role: they convert solar energy to caloric energy (sugars) through photosynthesis. In addition to solar energy, plants also need nutrients to create food. These nutrients may come from the local reef environment itself, e.g., the substrate material and decaying matter; and they may come from elsewhere via ocean currents.
All other living things in a sun-driven ecosystem depend on the food energy produced by plants. This food energy is consumed by the herbivores (plant eaters), which are in turn consumed by smaller carnivores and apex-predators. Even coral, which is an animal, relies on symbiotic algae to live: see our feature article on coral bleaching.
Algae is the dominant plant in reef ecosystems. Therefore, algae produces the majority of energy driving a reef ecosystem. Another article discussed benthic algae: algae that lives on the bottom, below the reef water column (benthos = bottom of sea). Benthic algae are the plants you can see growing on the bottom in reef areas. Benthic algae derives most of their nutrients from the reef environment.
However, algae also occurs in planktonic form. Planktonic algae (planktos = wanderer) are microscopic algae that float in the water column. They are also called phytoplankton because, as plants, they convert light energy into sugars and oxygen through photosynthesis. There are estimates that the phytoplankton in the ocean contributes up to 50% of the oxygen in the atmosphere, so they are a more important source of the oxygen we breathe than even the rain forests. Planktonic algae derive their nutrients from the ocean water within which they float.
A previous article described reef biologists' finding that over half the fish biomass associated with NWHI reefs consists of apex predators (those at the top of the food chain). An apex predator must eat many times its own mass of other animals. Since there is a small standing stock of prey animals (less than 50% of the biomass), this stock must be replenished rapidly as it is consumed by predators. Therefore, the reef must be extremely productive at the level of herbivore fish: the fish that eat algae and convert it into food that predators can use (the flesh of the herbivore). Algae must be putting a lot of energy into the reef ecosystem through photosynthesis in order to support the replenishment of herbivores fast enough to support such a large population of predators.
But which algae are most responsible for this productivity? What proportion of the reef's productivity is based on benthic algae, and what proportion on planktonic algae? That is the question being answered by the study that I will describe below. The study is being conducted by Carolyn Currin of the NOAA Beaufort Laboratory in collaboration with Randy Kosaki, the NWHI Coral Reef Ecosystem Reserve Research Coordinator, and other NOAA researchers. This research complements an ongoing National Marine Fisheries Service (NMFS) study led by Frank Parrish to determine the food webs supporting monk seals in the NWHI, and NMFS researchers are providing samples of monk seal prey items (fish, shrimp and lobster) for analysis (you'll find out why below).
Tracing food energy through the ecosystem
Here's the problem that biologists face: We want to know which form of algae is primarily responsible for the large apex predator population in the NWHI, but those predators don't eat the algae directly. How do we trace food energy from the algae that created that energy to the predators when this path may involve several intermediate steps of smaller fish being eaten by bigger fish? It would be impractical if not impossible to do this by direct observation, watching lots of fish eat algae and then following those fish until they are eaten by other fish, and following those fish and so on until the diner is an apex predator. There are too many pathways in the food "chain" (which is really a web), and our presence would interfere with the ecosystem.
We need to find a "signature" in the apex predators that will tell us what algae originated the energy on which their meals were based. Fortunately, such a signature exists in the form of carbon isotope ratios.
All atoms of a given element (for example, carbon) have the same number of protons in their nuclei. However, atoms of an element can take on different forms by having different numbers of neutrons in their nuclei. For example, 99% of the carbon in the natural environment has an atomic weight of about 12, because it has 6 protons and 6 neutrons. It is called carbon-12 or 12C. However, a small percentage of carbon has an extra neutron. It is called carbon-13 or 13C. Unlike heavier isotopes of carbon (such as 14C), 12C and 13C are stable (they are not radioactive).
OK, but how can this help us track energy through a fish feeding frenzy? The simple answer is that the two groups of algaes, benthic and planktonic, incorporate 12C and 13C into their food energy at slightly different ratios. Food energy consists of sugars, and carbon is one of the major elements that make up sugar compounds, along with hydrogen and oxygen. This carbon works its way up the food chain and ends up in the flesh of predators. By sampling the flesh of predators and analyzing them to identify the ratios of 12C to 13C, we can get an idea of how much of the ecosystem is driven by each type of algae. Cool, huh?
You might wonder why different kinds of algae would use 12C and 13C differently. I did, so I asked Dr. Currin: here is my simplification of her explanation. Some biological processes, specifically including photosynthesis, are more likely to incorporate the lighter 12C than 13C. However, this "preference" is influenced by the availability of 12C: the organism takes what it can get. Benthic algae must incorporate more 13C than planktonic algae for several reasons. Planktonic algae are in the water column where the supply of 12C is constantly replenished. Benthic algae are at the bottom where a thin boundary layer of water is less mixed, so as the 12C gets used up by organisms in that layer, they are "forced" to use more 13C. Also, carbon is taken from dissolved bicarbonate (HCO3) as well as from dissolved carbon dioxide (CO2). Bicarbonate has a greater proportion of 13C than carbon dioxide. Due to factors influencing water chemistry, the layer of water at the bottom has more carbon available as bicarbonate than carbon dioxide. Therefore, many benthic algal species have adapted to use bicarbonate, so they take in more 13C.
In addition to identifying sources of primary production of food using carbon isotopes, this study is also analyzing nitrogen isotopes to determine the fish's "trophic position": how many steps above the food chain it is feeding. The isotope 15N is enriched as one goes up the food chain, so scientists can use it to determine (for example), whether a fish is eating fish that eat algae, or eating fish that eat fish that eat algae, etc.
For example, in this graphic provided by Dr. Currin, microscopic animals have a nitrogen level of +3. At the next level in the food chain, invertebates have a level of +6, and fish that eat those invertebrates have a level of +9. So, nitrogen tells us the "tropic position" of the animal. Also, planktonic algae has a carbon level of -22 while the level for benthic algae is -16. There's also seagrass, at -10. The levels of carbon are not shown for the predators because the study has not been completed yet! Once they are known we can infer which of the plants support more of the food chain.
Conducting the study
Presently this study has just begun. During the NWHI RAMP 2004 expedition, samples of smaller predator fish were collected, but scientific staff time was not allocated to collecting large fish or algae samples. This sampling has been proposed for future expeditions. Planktonic algae will be collected by collecting and filtering seawater samples (similar to the samples collected by this year's oceanography team). Benthic algae collection will include hand-collection of macroalgae (the visible plants) and filtering samples of sediment taken from the bottom.
That doesn't sound too hard, but how would you collect samples of large predators? The study does not require that scientists catch the whole fish or shark, just a sample of its flesh. The proposal is to use a coring device attached to a pole spear. Similar methods have been used to collect small tissue samples from bottlenose dolphins without injury to the animal. The study will focus on the ever-present ulua (Caranx ignobilis) as well as bluefin trevally (Caranx melampygus) as representatives of apex predators. It's easy to get close to ulua: I have on many occasions been able to touch them while snorkeling. I'd imagine that it is riskier to core sample a shark's flesh, so I can understand the researcher's decision not to do so! The study is also examining isotope ratios in the food of monk seals, using samples provided by the NMFS.
The work can also be extended beyond the reef to "subphotic" ecosystems. Opakapaka, onaga, and other fish live at 100 fathoms or more in depth. (A fathom is about 6 feet). The level of light at that level is insignificant. Presumably the ecosystem at that depth depends on photosynthesis taking place at the surface. But little is known about where deep water ecosystems get their energy. Does it come from phytoplankton, or from macroalgae growing on reefs? How is the food energy transferred to the depths? From fish that swim away from the reef and get eaten? Do they die and sink down, or do the deep water fish swim up to get their food energy? How important are reefs to pelagic (open ocean, away from land) ecosystems? There is so much left to be discovered about our marine ecosystems.
Whole Ecosystem Management
Why should we care? In addition to human curiosity, we should care because there are implications for reef ecosystem management. We need to know what primary energy source fuels this incredibly productive ecosystem in order to ensure that it continues to be so productive. In particular, it will matter whether the productivity of the reef is dependent on outside factors such as oceanic currents versus is more self-contained. Looking in the other direction, to manage pelagic fisheries we need to know whether fish in the open ocean depend on food energy that originated in the reef but transferred to the pelagic environment. Through the combination of carbon and nitrogen isotopes, scientists will obtain a "profile" of the food web that supports the high productivity of the NWHI, being able to tell for example the extent to which a given type of fish in a give area are dependent on the reef versus the surrounding ocean for their food.
There is currently a movement towards ecosystem management: the unit of management should be defined by the flow of energy and nutrients rather than artificial distinctions we impose. Managers have to be able to manage a whole system in order to be successful. For example, if deep water fish depend on energy produced in the reefs, the two should be managed together.
The idea of ecosystem management extends to land as well. Currently management of the NWHI is divided up between some government agencies for the terrestrial environment and other government agencies for the marine environment. However, there are strong connections between the two environments. For example, the birds that nest on the land take their food from the sea. Their guano washes back into the sea and affects algae growth in the area. Monk seals and sea turtles depend on both land and sea resources for their survival. Perhaps this entire system should be managed as a unit.
This concept of ecosystem management has an ancient predecessor: the Hawaiian concept of an ahupua`a. Before western contact resulted in changes to land management, Hawaiians divided up their land into pie-slice like units that corresponded to watersheds running from the top of the mountains to the reefs and beyond. Management units were defined by their self sufficiency and interdependency within the system rather than artificial distinctions such as "land" "reef" and "ocean." A similar approach to the Northwestern Hawaiian Islands and indeed to the Main Hawaiian Islands will enable more effective and sustainable ecosystem management.