Effect of Heliocidaris erythrogramma on benthic macroalgae and drift retention

Brief description

Two experiments were performed. The first was designed to test for effects of Heliocidaris at different densities, using fenced plots in which the densities of Heliocidaris were controlled. The second (which was repeated twice) was designed to test for effects of natural densities of sea urchins on benthos, using unfenced plots from which all sea urchins were removed.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: -Manipulating Heliocidaris density-
This experiment tested the null hypothesis that Heliocidaris has no influence on the biomass or species composition of attached macroalgae or drift macrophytes at any of the naturally occurring densities at Mewstone. The experiment consisted of six treatments: four densities of Heliocidaris (0, 6, 12 and 30 per plot) and two types of controls ('Half fence' and 'No fence') in which densities were not manipulated. The control plots were used to test for caging artefacts. In the 'Half fence' control, three sides of each plot were fenced, while the fourth was left open. In the second type of control, plots were simply marked at the corners with steel pickets. If there were no artefacts associated with the fence structure, there should be no consistent differences between these two treatments.

There were 4 replicate plots for each treatment. Plots were 1.5m x 1.5m (2.25m2), and were marked at the corners with 60cm steel pickets hammered into the rock to a depth of about 20cm. Plots in which densities were manipulated were fenced around the entire perimeter with 60 ply black polyethylene mesh (2 inch stretched), attached top and bottom to a border of double-braided polyester rope (10mm diameter). The mesh was tied to the steel pickets with elastic cord, and attached to the rock with galvanised steel pegs. The mesh was not attached to the rock in the 'Half fence' control, to enable urchins to move in and out.

The experiment was started on the 10-12 November 1999. Plots were allocated to treatments according to a table of random numbers. Urchins were removed from all plots except those designated as 'No fence' controls. Urchins were then added to treatments as appropriate. Urchins were returned to the 'Half fence' controls in their original densities - this was to control for possible effects of removing and repositioning urchins. In the final week of April 2000, a severe storm destroyed two high density plots, with the damaged mesh scouring away much of the substratum. The experiment was ended on May 2000.

Statement: -Effects of removing Heliocidaris-
This experiment tested the null hypothesis that removal of Heliocidaris would have no influence on the biomass or species composition of attached macroalgae or drift macrophytes. The experiment consisted of two treatments: sea urchins removed (-U) and sea urchins left (+U). The experiment was done twice, once over an 11-month period (June 2000 to May 2001), and once over a 5-month period (December 2000 to May 2001). At the beginning of each run of the experiment, four plots were marked by hammering steel rods into the rock: each plot was 4m x 4m (16m2). Two plots were randomly assigned to the +U treatment, the other two were assigned to the -U treatment: sea urchins were removed from the -U plots and released > 500m away. Tests of urchins accidentally killed were also removed. Plots were checked throughout the experiment, and any sea urchins found inside the -U plots were removed.

At the beginning of the experiments, sea urchins were counted in four 1 m2 quadrats per plot. Attached macroalgae were harvested from the plots that were established in June 2000 on the first day of the experiment, and on two occasions approximately 6 months and 11 months later. Attached macroalgae were harvested from the plots that were established in December 2000 on the first day of the experiment, then approximately 5 months after. Macroalgae were only harvested from the central 3m x 3m area of each plot to allow a 'buffer zone': this central area was divided into an imaginary grid of 36 x 0.25m2 squares and 5 randomly-chosen squares were harvested at each time to avoid harvesting the same area twice.

At the end of the experiment, drift algae and seagrass were collected by hand from three 1m2 quadrats haphazardly placed within each plot.

Harvested algae and drift were put into calico bags and frozen. Later, the contents were thawed and sorted into species. Algae were then dried at 80oC for at least 48 hours and weighed. Because of the potential for variations in biomass of small species (i.e. that were difficult to collect by hand) due to variations in harvesting effort, only values of > 0.5 grams dry weight were included in analyses.

Notes

Credit
Gary Kendrick - Supervisor

Credit
CSIRO