Cows in our oceans?

By Cristi Richards

We have finished our second day of diving at French Frigate Shoals and work has been going well. The skies have been sunny and despite a small cold front that came through a few days ago, the seas aren't too rough. I'm glad for calm seas, especially because my role on this cruise is part of the installation team for Calcification Acidification Units or CAUs, which we affectionately pronounce 'cows'. These are small plastic plates sandwiched on a bolt and then threaded onto a stainless steel pin. We install a set of these CAUs at 5 survey sites per island and when the weather is calm, installation is much easier. These plates will act as settlement structures for calcifying organisms such as coral and algae. In two years, when we return to these sites, we will collect the CAUs to find out what organisms are present and how much they have grown. When I say 'calcifying organisms' I am referring to plants and animals that grow by producing their own calcium carbonate skeletons. Calcium carbonate is a hard, rock-like substance that corals and algae create from nutrients that are available in the sea water through a chemical process known as accretion or calcification. It is also the main component in pearls, eggshells and the shells of marine animals.

A calcification acidification unit, or CAU (pronounced cow), acts as a settlement structure for calcifying organisms such as corals and algae. By monitoring these organisms, scientists hope to better understand how they are affected by the changing chemistry of our oceans.

Why is this important to study? As the world's atmosphere changes due to the burning of fossil fuels and other natural and human influences, increasing amounts of carbon dioxide are affecting the world's oceans. Specifically, carbon dioxide that is being added to the atmosphere is being absorbed into the oceans where it forms carbonic acid – changing the chemistry of the whole ocean. Over the last 20 years, scientists have seen a rise in acidity levels of oceans around the world, which is impressive given their immense volume. Actually, the rate of this increase in acidity has changed from approximately 20% in the 1990s to 30% from 2000 to 2010. This is a significant increase in the rate that ocean acidity is changing and is the fastest increase documented since the industrial revolution. This increased acidity may have significant implications for the coral and algal communities, which would in turn affect numerous fish species, many of which are important food resources.

The amount of calcium carbonate that calcifying coral and algae are able to create is directly affected by the acidity of the water around them. Scientists are concerned that if pH levels drop too low, corals and algae won't be able to produce as much calcium carbonate. This has been shown to occur in laboratory experiments and in some areas of the Great Barrier Reef. A question that needs to be answered is, if the pH gets too low, will they be able to produce their skeletons at all? This is important because without a skeleton to provide protection from predators, the corals and algae won't survive.

It may not appear important to protect such small, and seemingly insignificant members of the ocean community, however the affect of these calcifying coral and algal species is far reaching. Beaches are dear to many people's hearts and to the economics of many coastal cities. Much of the sand that makes up our beaches is produced by Halimeda – one type of calcifying algae that could be in danger. This marine plant produces calcified, rock-like segments that break off and contribute to the sand on many of the Pacific beaches and it is estimated that Halimeda accounts for 30% of the sand production in some areas of the world. Calcifying algal species also provide a base for corals to settle and grow on, and provide food for many of the fish living on the reef. Without a place to settle, the number of corals and the structure they provide could decrease. Many of the fishes that hide among coral branches as juveniles would then be easier targets for predators. These same small fish are important to humans for food and are the base of the food chain for many of the larger fish that humans also eat. You may be getting the idea that the relationships between all of these organisms are very complicated, and you would be right. Everything is connected; starting with the unassuming alga and leading to the fish served on your dinner plate while overlooking a sandy beach while you are on vacation. The atmosphere affects the water, affecting the coral and algae that affect the fish and then our economy and a portion of our food supply.

To try to simplify and figure out some of these relationships, we are trying to break them down, studying one aspect at a time. Our hope is that by monitoring what species are building calcium carbonate structures and how much they are building, we will have an idea of how or if corals and algae are being able to adapt to the changing chemistry of the oceans. Will there be a change in how the skeletons are built or in their density? Will corals have sufficient protection with a skeleton that is less dense? Will the skeletons break down more easily during storms? If the algal skeleton that contributes to beach formation is not as dense, will it break down more easily leading to faster beach erosion? Installing CAUs is just one step in answering these questions. It is important for scientists to answer these questions so that we can determine how healthy our reefs are and can better predict their course for the future.