The fleshy macroalgal index (FMI) is a measure of the amount of large, soft, fleshy algae (sometimes called “seaweed”) on a reef. Common MAR varieties include Lobophora (a type of brown algae) and Halimeda (green algae). The FMI is defined as the product of fleshy macroalgal areal coverage and canopy height.16
When overly abundant, fleshy macroalgae can compete with corals for reef space, interfere with coral recruitment (F1), and reduce coral survival. In low abundances, these macroalgae are part of a healthy reef community, providing food for a variety of herbivores (F11, F12).
The abundance of fleshy macroalgae, which is relatively easy to measure, can serve as a clue regarding two important reef processes that are more difficult to measure:
- Herbivory intensity – Increasing FMI can be a sign of decreased grazing pressure (seen, for example, in the wake of the early 1980s Diadema die-off, F12).
- Nutrient availability – Increasing FMI can be a sign of increasing nutrients (seen, for example, in 2007 in a Costa Rican bay receiving inadequately treated sewage and golf course runoff)17.
We recommend that fleshy macroalgal abundance be routinely monitored in terms of the fleshy macroalgal index. Algal height and extent can be measured relatively inexpensively on the same transects used to assess coral cover (S4) and other parameters. As more data become available, habitat-specific FMI goals will be refined to take into consideration factors such as reef type and location.
A promising sign of Benchmark reef recovery would be a MAR-wide average FMI of 30.
- The 1980s urchin die-off (F12), overfishing of herbivorous fish (F11), and increasing nutrient loads (S8) have contributed to dramatic increases in macroalgae— so much so that algae have taken over many reefs formerly covered by corals. (In 2000, every reef surveyed in Mexico had Red Flag levels of fleshy macroalgae.18)
- Preliminary 2006 data show some declines in FMI.19