Table 1.
Load cell performance characteristics.
Figure 1.
A picture and a diagram of current meter components.
Top A shows an instrument deployed on a sheltered reef location with clear depictions of the guide rod construction and drag-sphere placement. Bottom B illustrates the internal components of the current meter and wiring connections. Notice that the height above the substratum where current measures are conducted can be adjusted by increasing or decreasing the guide rod length.
Figure 2.
An illustration of measuring locations (A) and current meter placement (B) on each reef slope around Lizard Island, Northern Great Barrier Reef, Australia (14°40S, 145°28E). Current meters were placed at 3, 6, and 9 m depths at mid-tide in exposed, oblique and sheltered habitats relative to the prevailing south-easterly trade winds. These placements followed the approximate depths of the crest, mid and deep-slope of each habitat.
Table 2.
Statistical comparison of current measures from 27 different sites on a coral reef spanning 3 levels of wind exposure (exposed, oblique and sheltered) and 3 different depths (3, 6 and 9 m).
Figure 3.
Correlation between drag forces (%) on the current meter sensor and water velocity (cms−1).
Calibrations for low velocity (0–50 cms−1) were made by placing the current meter in a unidirectional current within a flume, whilst high velocity (>50–400 cms−1) was calibrated next to a Vernier LabQuest Flow rate sensor following the boat-tow methodology of Utter and Denny [28]. The log-linear correlation is highly significant (F = 12,090.7, p<0.0001).
Figure 4.
Current velocity profile of the coral reefs surrounding Lizard Island, Northern Great Barrier Reef, Australia.
Maximum water velocities (cms−1) in exposed, oblique and sheltered habitats at 3, 6 and 9 m depth relative to a wind exposure of 0–22.5 knots hr−1 are shown. Boundary lines indicate the 25th and 75th percentiles, lines within the boxes represent median velocity values and the error bars indicate the 90th and 10th percentiles. Stippled lines indicate the most commonly encountered current velocity in each habitat relative to average daily wind velocities.
Figure 5.
Wind induced wave frequency and amplitude in an exposed coral reef habitat (mean ±S.E.).
Wave frequency (black markers) was measured on the exposed slope in front of Lizard Island, and wave amplitude (white markers) was estimated from marine forecasts for this location (www.bom.gov.au).
Figure 6.
A raw data stream from a 7 day continuous measure of current velocity.
Water velocity is measured as drag on the current meter drag-sphere and is here reported in Newton (N). In this data stream, drag forces were continuously measured at 16 MHz and for every 10 sec interval the single highest and single lowest measure were recorded. Notice how variation in drag forces increases with increasing current velocity. Notice also the stable minimum measures over time highlighting that no ascendible zero-drift occurred. The high measure (∼1.4 N) at the beginning of the data stream is a test pull made by the diver immediately after securing the current meter to the substratum, and indicates the start of data recording in the field location.