Water Flow in Seagrass Ecosystems - Seagrass Density Effects on Water Flow

Brief description

Field and laboratory experiments were conducted to study the effects of water velocity on bending of shoots and canopy compression at different densities of Amphibolis antarctica and Posidonia sinuosa. A hydrodynamic study associated with the project investigated the differences in water flow caused by manipulating the density of an Amphibolis griffithii meadow. The laboratory experiments investigated the hydrodynamic differences between Amphibolis antarctica and Posidonia sinuosa, in terms of the bending behaviour of the plants under varying water flow regimes, and the degree of canopy compression as a result of bending.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Density manipulation experiment - The experiment was conducted in an extensive, uniformly dense Amphibolis griffithii meadow located at a depth of 3-3.5 m in Warnbro Sound, sound of the Perth metropolitan area (see thumbnail). Experimental units comprised 3 m diameter circular plots of various densities. Treatments consisted of five densities: 0, 25, 50, 75, 100 % of natural meadow density; 100% was unmodified meadow (approximately 500 shoots per metre2) and 0 % was completely cleared of seagrass plants. Six replicates of each treatment were established, making a total of 30 experimental plots. Plots densities were randomly assigned, and density was manipulated by random removal of whole shoots from the plot. Plot densities were maintained throughout the life of the project by periodic pruning of shoot regrowth. A metal stake with the plot's number was left at the centre of each plot as a permanent marker. *Water flow measurements* Measurements of the hydrodynamic environment within the experimental plots were carried out using the electronic current meters developed as part of this project (see Chapter 2 of thesis). To ensure compatibility of measurements over potentially different weather conditions, plots were sampled in groups of the five treatments. This ensured a minimum influence of weather differences between sampling the different treatments in any one replicate group. To ensure a minimum of time between sampling a complete group of manipulated densities, the plots were required to be close to each other within the experimental site. This meant that the support vessel could be anchored in the middle of a set of plots, and all those plots could be sampled from the one position. Each set of five treatments was sampled over a maximum period of 90 mins. After an initial 5 min calibration period each plot was sampled for a period of 5 min before the array was moved to another plot. All data was logged using a Datataker DT100F data logger (Data Electronics).

Statement: - Bending of seagrasses under different flow regimes - Fresh plants of Posidonia sinosa and Amhibolis antarctica were collected from Shoalwater Bay at a depth of less than 2 m. Only the above-ground shoots were collected, to minimise damage to the seagrass meadow. Specimens were collected on SCUBA, and stems cut with scissors at the sediment level for A. antarctica and just below the leaf bundle sheath for P. sinuosa, to maintain shoot integrity. Specimens were stored in aerated aquaria at constant temperature (16 degree Celsius and 14/10 hour light/dark regime) until use. All specimens were used within 48 hours of use. *Flume design* Bending experiments were undertaken in a perspex recirculating laboratory flume. The flume was 150 cm long, 25 cm wide and 20 cm deep. Water was recirculated via a PVC pipe (diameter 10 cm) by means of a plastic propeller driven by a variable speed mixer motor (Anderman multispeed stirrer). A flow-smoothing honeycomb filter was used to eliminate effects of curvature of the recirculating pipe, and to avoid the creation of a central jet of incoming water. For further method see section 4.3.2.3 of the thesis.

Notes

Purpose
To investigate some density related ecological parameters of seagrasses.