TRAPPING OF LAND-DERIVED SEDIMENT BY MACROALGAE ON THE SOUTH MOLOKAI REEF FLAT, HAWAII

Macroalgae is a natural and integral component of coral reef communities around the world. However, in recent years, marine scientists have documented "phase shifts" on reefs, during which large, fleshy macroalgae has outgrown coral and calcified algae to become the dominant substrate organism. Vast areas of the 1 km wide reef flat off of central south Molokai show degraded coral coverage, with high amounts of macroalgae and land-derived sediment. The source of this sediment is erosion of Molokai’s volcanic edifice, a process that has been accelerated since the late 19th century by human activities such ranching, farming, and the introduction of feral grazers. Our studies demonstrate that macroalgae traps a significant amount of fine-grained, terrigenous sediment, suggesting that algae may play an important role in sediment dynamics of nearshore tropical environments. The efforts of this study were focused on: 1) characterizing the composition, amount, and grain size distribution of sediment trapped by different macroalgae species; 2) determining whether sediment trapped by macroalgae is distinct from surrounding surface sediment; and 3) documenting shore-normal and cross-reef patterns of algae growth and sediment storage in relation to reef health.

Samples of sediment-laden macroalgae were collected along GPS-controlled, shore-normal transects in both February and July of 2003. At each sampling location, surface sediments (top 1-2 cm) were collected and a 0.25 m² quadrat was utilized to assess benthic algal cover. Grain size distributions were determined via settling tube analyses and laser diffraction; carbonate content in each size fraction was measured with a coulometer. The results of this study indicate that macroalgae preferentially trap terrigenous silt and clay (<0.063 mm), while nearby surface sediment is dominantly sand-sized (0.063 – 2.0 mm), carbonate material. Regions of the reef flat with less coral and more intense upland development tend to have the highest algal cover and the most sediment trapping. Preliminary findings indicate that this sediment may be contributing to algal growth, via nutrient transport or creation of optimal substrate and conditions, setting up a positive feedback that could be progressively degrading Molokai’s fringing reef.