The influence of leaf-burying crabs on energy flow and export from mangrove forests at Hinchinbrook Island and Chunda Bay, north Queensland

Comparisons of leaf turnover by crabs (Sesarma messa) and microbes were carried out at Chunda Bay. Yellowing leaves of Rhizophora stylosa, which were about to fall, were collected. Ten leaves were retained, dried, weighed individually, ashed at 500°C for 24 hours and reweighed to calculate a conversion factor from fresh weight to ash-free dry weight (AFDW). Groups of four leaves were weighed and placed in nylon bags with a 2 mm² mesh size. Single leaves were weighed and 1m lengths of nylon twine were attached to the petiole.

Bags (restricting access to leaves by crabs) and individual leaves (allowing access by crabs) were tied to the forest floor within the low intertidal region of a Rhizophora forest at two sites, 100m apart. At intervals of 2 and 6 weeks, three replicates of each treatment were removed from each site. Leaves were cleaned of sediment, dried, weighed and ashed.

Measurements of removal of leaves by crabs and leaf litter fall were made on five occasions between April 1985 and January 1986 at Coral Creek in Missionary Bay, Hinchinbrook Island. On each occasion, leaf removal experiments were conducted during both day and night low tide periods.

Six 25 m² plots were marked out in the low to mid-intertidal zone of the mixed Rhizophora forest. Sixteen litter catchers were deployed randomly around the experimental plots on each sampling occasion. All litter was removed after 7 days, dried at 100°C for 3 days, sorted into components and weighed.

Yellowing leaves of Rhizophora spp. were collected and the area of each leaf measured with a Lambda Area Meter. A 1m length of nylon twine was tied to each petiole. At the beginning of each low tide period, all leaves on the surface of each plot were removed and then a number of the measured leaves were tethered within each plot. All tethered leaves were retrieved after 6 hours and a record was made of whether the leaf had been taken down a crab hole. Additional leaves falling into each plot were also collected.

The surface area of each tethered leaf was measured and the loss of leaf area converted to dry weight loss using a pre-established relationship between surface area and dry weight. The area of leaf lost to crabs was visually estimated for the additional leaves falling into each plot.

In January 1986, modified leaf removal experiments were carried out in other areas of mixed Rhizophora forest at an additional site at Coral Creek as well as Priest Creek and First Creek in Missionary Bay.