Brief description
An experiment was conducted over 13 months to test whether sea urchins and herbivorous fishes exert additive effects on the macroalgal assemblage. Specific predictions were that there would be effects on the biomass and species composition of the algal assemblages growing on the reef.Lineage
Maintenance and Update Frequency: notPlanned
Statement: The experiment consisted of four experimental treatments and a fifth, unmanipulated, control treatment.
The four experimental treatments were:
- exclusion of both fish and sea urchins (-F-U)
- exclusion of fish, allowing access by sea urchins (-F+U)
- exclusion of sea urchins, allowing access by fish (+F-U)
- a partial cage to allow access by both fish and sea urchins, while mimicking the effects of the structures used in the previous treatments (+F+U)
For the -F-U treatment, plots were caged, with the top and sides enclosed down to the reef surface. For the -F+U treatment, a roof was included to restrict grazing by fish, but sides were not enclosed so that sea urchins (and other invertebrates) could move in and out. For the +F-U treatment a 50cm tall fence structure was used to enclose each plot, thus preventing access by sea urchins while allowing access by fish. The +F+U treatment had three sides enclosed and a partial roof covering half the plot: this design is similar to cage controls used in some other studies.
There were four replicate plots for each treatment. Each plot measured 1.5m x 1.5m (2.25m2). The experiment began in April 2000, and the structures were regularly replaced to prevent excessive fouling. Sea urchins were removed from the plots allocated to the -F-U and +F-U treatments when the cage and fence structures were erected. Any sea urchins found in these plots during the course of the experiment were removed.
Statement: -Adult Algae-
On three occasions during the experiment (April, July and November 2000), the species of attached attached macroalgae present were identified, and their percent cover estimated, from a 0.25 m2 quadrat placed in the centre of each plot. At the end of the experiment (May 2001), attached algae were harvested from a 0.25m2 quadrat placed in the centre of each plot. The algae were placed in calico bags, transported to the laboratory and frozen. Algae were subsequently sorted to species level, dried in a oven at 80 degree Celsius for 48 hours, and weighed.
Statement: -Recruitment plates-
To test whether herbivores exerted detectable influences on recruitment of macroalgae, rectangular limestone plates measuring 300 x 160 x 30mm were used. They were first conditioned by leaving them in sea water in the laboratory for several days. They were then placed in situ adjacent to the study area for a period of several weeks - during this time they were not placed into the experimental plots to allow settlement of algal propagules (this was done because cage structures would potentially hinder settlement of algal propagules, and then interpretation about the effects of herbivores on recruitment could be confounded by variations in settlement). After this period, one plate was placed flat on the substratum in each plot and secured to a metal rod.
The total biomass of algae that had recruited to the plates was measured as ash-free dry weight (AFDW). To do this, all algae within a 140 x 100mm area in the centre of the plate were scraped off with a razor blade. The material was placed into a pre-weighed crucible and dried in an oven at 80 degrees Celsius for 48 hours (to yield dry weights). The crucibles were then weighed, put in an oven at 550 degrees Celsius for 2 hours and weighed again (to yield ash dry weights). Subtraction of ash dry weights from dry weights yielded AFDW.
Statement: -Caging artefacts-
To test whether the cage structures used were modifying local environmental conditions, the effects of cages on the rate of sediment deposition, and the intensity of light reaching the algal assemblages, were evaluated.
The rate of sediment deposition was estimated using sediment traps in January 2001. Traps were made from PVC pipe (16mm internal diameter, 150mm long) with one end closed. One trap was placed in each plot, secured to a steel rod, and left for ten days. When retrieved, traps were closed with a cap, then transported to the laboratory and refridgerated. Material from each trap was passed through a 125um sieve; the particles retained were put into a pre-weighed crucible, dried in an oven at 60 degree Celsius for 24 hours and then weighed. The rinse water that passed through the sieve was drawn into syringes and passed through pre-combusted Whatman GF/F glass microfiber filters; the filters were then put into pre-weighed crucible, dried in an oven at 60 degrees Celsius for 24 hours and then weighed.
Light intensity (uE m-2 sec-1) in cages was measured over 20 days in April-May 2001. Light loggers (Model 392, Dataflow Systems Pty Ltd) were deployed in two plots each of the -F-U, -F+U and unmanipulated control treatments. However, only two of the these were retrieved (one each from the -F-U and -F+U treatments): the others were lost during rough conditions, when they became detached and floated away.
Notes
CreditGary Kendrick - Supervisor
Created: 30 07 2007
Data time period: 2000-04 to 2001-05
text: westlimit=115.65; southlimit=34.09; eastlimit=115.66; northlimit=32.08
text: uplimit=8; downlimit=7
Subjects
25 202012 |
25 211002 |
25 247001 |
ANIMALS/INVERTEBRATES |
ANIMALS/VERTEBRATES |
BIOLOGICAL CLASSIFICATION |
BIOSPHERE |
Biosphere | Vegetation | Macroalgae |
COMMUNITY DYNAMICS |
Centrostephanus tenuispinus |
EARTH SCIENCE |
Echinoderms |
ECOLOGICAL DYNAMICS |
Fish |
Heliocidaris erythrogramma |
Oceans | Marine Biology | Marine Invertebrates |
Oceans | Marine Biology | Marine Plants |
Phyllacanthus irregularis |
Species Recruitment |
oceans |
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Other Information
(PhD thesis)
uri :
http://theses.library.uwa.edu.au/adt-WU2004.0086/
global : 8a9db950-3033-11dc-95ce-00188b4c0af8
Identifiers
- global : 48e59850-3e59-11dc-b523-00188b4c0af8
