October 12, 2011

Why don’t we count Halimeda? and related questions

Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs. If you have a question you would like answered, please email rcinfo@reefcheck.org.

Halimeda– record in ‘Other’ category
Coralline algae- record the substrate beneath

Reef Check Tropical – Why don’t we count Halimeda? and related questions

One reason that Reef Check data are considered reliable enough to be used by scientists in major scientific investigations of reef health is because the protocols were designed specifically to be used reliably by volunteer citizen scientists. There are limitations to what we can expect citizen-scientist volunteers to be able to learn in a relatively short training period. By restricting the level of identifications to a small subset of representative indicator organisms, we know our Reef Check volunteers can be trained to e.g. differentiate between a butterflyfish and a grouper, but we would not expect them to learn all species of grouper.

In a traditional ecological monitoring program designed to be carried out by trained scientists, it might be decided to differentiate various coralline and calcareous algae. But this is beyond the scope of Reef Check monitoring. This is not to diminish their ecological role on the reef.

What are coralline and calcareous algae and how does Reef Check handle them?

In recent years, biologists have moved algae out of the Plant Kingdom and into the Protist Kingdom and divided them into many sub-groups going well beyond the traditional red, green and brown algae divisions. When you consider many aspects of the life history of algae – such as a zooxanthella growing a tail and swimming away from a host coral, (“zoox” are the microscopic algae that live in corals), this change is not surprising. The Protist Kingdom is the catch-all group for organisms that don’t fit into the other Kingdoms.

Many species of calcareous algae take up calcium from seawater and create a calcium carbonate skeleton that can be either a tightly encrusting red or pink layer on rocks such as Lithophyllum, loosely encrusting or can form various branching structures such as the ubiquitous green Halimeda. There are over a hundred species of Lithophyllum and almost 40 species of Halimeda found in all tropical waters. There is only one brown genus, Padina. Halimeda “branches” serve as food for herbivores and after death, their broken segments make up as much as 90% of sand on some beaches and they create the bulk “in fill” of many fossil reefs. Coralline red algae also provide much of the “cement” that holds a reef together even after they have died.

Coralline algae are also very important as a stimulus for coral and other invertebrate larvae to settle on the reef. However, only a small amount of coralline algae is needed and the larvae do not necessarily settle directly on the algae but often on rocks nearby so there is not much added value in recording the amount of coralline algae. Therefore, the 2004 peer review of the Reef Check protocol by a dozen Reef Check scientists concurred with the original 1996 design and peer review that it was not necessary to measure the amount of calcareous algae including encrusting coralline algae. The reviewers suggested that these algae be allocated to the following categories:

1. Calcareous (except Halimeda) or encrusting coralline (record substrate under these e.g. Sand or Rock)
2. Halimeda sp. (put in “Other,” OT)
3. Turf algae (put in “Rock,” RC)

Reef Check is focused on human impacts, and Turf algae are considered the “normal” healthy reef condition due to appropriate numbers of herbivores and normal nutrient levels. So it makes sense to effectively “ignore” turf and only record these same species when their growth has exceeded 2.5 cm and they are considered Nutrient Indicator Algae under the Reef Check protocol. Since encrusting coralline algae are primarily found on rock, it also makes sense to record the underlying rock – their ecological function is simply as potential space for settlement just like rock. Halimeda can occupy large areas of sand in between reefs and even on reefs. Because of its importance, the review panel felt it was best to record Halimeda as “Other.”

Humans and especially scientists spend a great deal of time and effort trying to divide up the natural world in a sensible, digestible way. As we have seen with the recent transfer of all algae from the Plant Kingdom into the Protist Kingdom – this process can be refined as we learn more. But changing a protocol after several years of use entails costs because it is no longer possible to make direct comparisons of data. Reef Check divides up the reef world into categories that have proven useful for the goals of Reef Check – but it is not a perfect solution especially for those with a special interest in Halimeda. For such groups, every Reef Check team is welcome to subdivide categories during their local surveys and separate out e.g. Halimeda from the Other category for local analysis, and then just recombine the data when submitting to Reef Check HQ.