Soil conditions in damaged mangrove forests in north Queensland

In October and November 1991, soil was sampled and the abundance of crabs estimated in nine mangrove forests, located between Bailey's Creek, Cow Bay (north of Port Douglas) and Chunda Bay (south of Townsville).

Three forests were chosen in each of following categories: Severely Damaged (mangrove trees removed, soils subjected to earth-works and either erosion or the dumping of spoil) - Spoil Dump, Port Douglas; Marina, Port Douglas; Oyster Point, Cardwell Damaged (mangrove trees removed but soils relatively undisturbed) - Bailey's Creek, Cow Bay; Wharf area, Dungeness; Chunda Bay, Cape Ferguson Controls (mangroves without any obvious physical damage) - Barron Creek, Cairns; Meunga Creek, Cardwell; Coral Creek, Hinchinbrook Island

Each forest was located within 1km of the sea and was, or had been, low-shore, mixed Rhizophora apiculata, Rhizophora stylosa, Bruguiera gymnorhiza and Ceriops tagal forests. The damaged forests had been impacted within the previous 5 years. Within each forest, three randomly selected sites were chosen for sampling during low tide.

Relative soil compaction was measured by releasing a metal dropstick (329x6mm, weighing 78.5g) within a 2.0m pipe (16mm diameter) and recording the stick's penetration in the mud. Twenty measurements were made at each site.

The relative abundance of crabs was compared by counting the number of crab holes in 5 replicate 1m² quadrats placed randomly at each site.

Replicate surface and subsurface (10cm deep) cores were collected at each site and frozen for analysis in the laboratory. The pH of cores was measured using an Orion SA520 pH meter with combination electrode. Grain size analysis was carried out by using a combination of wet sieving and a column settling technique.

Soil samples for element analyses were homogenized using an agate Wiley Mill after being dried for 3 days at 80°C. The percentage of total carbon and nitrogen (by weight) was obtained for subsamples of homogenized sediment using a Leco Model 600 CHN Analyser. Another subsample was analysed for percent (by volume) of total phosphorous, potassium, sulfur and iron using a Varian Liberty 200 ICP-Emission Spectrometer (ICP-OES), following a perchloric-nitric acid digestion procedure.