Ecological Archives M074-011-A4

Mark W. Denny, Brian Helmunth, George H. Leonard, Christopher D. G. Harley, Luke J. H. Hunt, and Elizabeth K. Nelson. 2004. Quantifying scale in ecology: lessons from a wave-swept shore. Ecological Monographs 74:513–532.

Appendix D. Details of the materials and methods.

1. Recording Dynamometers.

At each experimental location on the transects, a 1.6-cm diameter, 20-cm deep hole was drilled into the granite substratum.  The opening of this hole was countersunk to a depth of 2.5 cm and a diameter of 3.2 cm, and a PVC threaded female pipe fitting (1/2" NPT) was installed using epoxy putty.  This fitting served to mount either a recording dynamometer (for measurements of maximum wave-induced hydrodynamic force), or a recording thermometer (for measurements of maximum temperature). 

The recording dynamometers were similar in construction to those described by Bell and Denny (1994) and Denny and Wethey (2000).  A drag element (a wiffle golf ball) was connected by a short length of fishing line to a spring housed in a short length of plastic pipe.  Any force imposed on the ball stretched the spring, and the maximum was recorded by a rubber slider on the fishing line.  The housing fit snugly into a hole in the substratum.  Details of the construction of the device can be found in Bell and Denny (1994) or at www.stanford.edu/group/Denny

To make a measurement, the slider was reset using forceps and the device was screwed into its emplacement.  After an appropriate period (typically 4–10 days), the device was recovered and the position of the slider was measured using Vernier calipers.  The larger the force imposed on the drag element, the larger the displacement of the slider.  The proportionality between applied force and displacement was determined in the laboratory for each device by holding the device ball-down and hanging known weights from the ball.  There are two salient differences between the device described by Bell and Denny and that used here.  First, the housing of the current device is mounted rigidly in the substratum.  Because the whole device does not need to reorient with the flow during the passage of a wave, the response time of the current model is substantially less than that of the Bell/Denny device.  Second, the shorter length of fishing line used in the current device (in the absence of drag, the ball is held less than 1 cm from the housing), allows the device to be installed in close proximity to organisms on the substratum.  The "sweep" of the drag element when subjected to a large force is approximately 10 cm, suggesting that the spatial extent of the measurements made by an individual dynamometer is a circle with a diameter of approximately 20 cm.

2. Wave Gauge.

The calculation of significant wave height involves a correction for the attenuation of the wave-induced pressure signal as a function of depth. To avoid the possibility of artifacts due to this correction, the calculation of significant wave height was truncated at a wave frequency equal to approximately three times the peak frequency of the waves.

3. Recording Thermometers.  

The thermometers were housed in 1.3-cm diameter CPVC pipes of the same length used for the dynamometers, and were similarly held in place by threaded male pipe fittings.  The bulb of the thermometer extended out of the housing into a hole drilled into a 2.5-cm diameter brass ball.  The ball (which was rigidly attached to the male pipe fitting by a threaded brass coupling) served both to protect the bulb of the thermometer and to ensure that the area of the device exposed to solar irradiance was independent of the angle at which the device was installed in the substratum.  The brass ball was coated with a thin layer of matte-black rubber.  To make a recording, the thermometer was briskly shaken to reset the column of mercury, and the housing was screwed into the emplacement in the substratum.  The device was retrieved at a later time (typically 1–7 days), and the maximum temperature was recorded. 

4. Topography.

The topography of the substratum was measured along all transects. The objective of these measurements was to create an index of the topographic potential for a given location to be exposed to wave-induced hydrodynamic forces.  Experience led us to believe that this index should include (1) the horizontal angle (azimuth) of the local shore relative to the direction of wave approach, (2) the slope of the substratum at the location, and (3) the presence or absence of offshore obstacles in the path of wave approach.  A location with a vertical slope facing directly into oncoming waves with no offshore obstacles has the greatest potential to encounter large forces, whereas locations with lesser slopes facing obliquely to the waves and sheltered by obstructions have less potential.  We propose that the following topographic index appropriately quantifies these ideas:

(D.1)

Here w is the compass direction from which waves approach the shore, s is the compass angle of the horizontal component of the location normal, and s is the slope of the shore at the location fb is the blocking angle, the slope of a line drawn from the location to the top of the nearest “up-wave” obstacle . If the location is on a vertical surface facing into the oncoming waves, this line would be drawn directly down from the site to the water below . If the location is on a vertical surface facing away from the waves, the line is drawn vertically up W varies between 0 and 1.  As proposed, the index is highest for vertical locations (s = 90o) that face the oncoming waves (w = s) without obstacles (b = -90o) and is lowest for vertical locations on the down-wave sides of obstacles.  The value of c (which weights the contribution of offshore obstacles to the index) was chosen to provide the greatest correlation between W and maximum wave force.  A value of c = 0.6 was used here. See Helmuth and Denny (2003) for details of this index and its measurement.

5. Microalgal Primary Productivity .

Microalgal primary productivity was estimated as the rate of algal film accumulation in the absence of herbivores.  A 7.5 cm square settlement plate was installed at each location to measure the monthly accumulation of algal film.  Each plate was made of 0.5 cm thick polycarbonate plastic (Lexan) covered with gray, rugose, safety-walk sheet (3M Corporation, St. Paul, Minnesota, USA).  These plates are a standard apparatus for measuring the recruitment of barnacles (Roughgarden et al. 1988).  Each settlement plate was attached to the rock by a stainless steel threaded rod, which had been glued into a hole drilled into the substratum.  A 10 cm square sheet of copper foil was sandwiched between the settlement plate and the substratum.  The resulting 1.25 cm border of copper served as an effective barrier to molluscan grazers. 

Upon retrieval, the microalgal film was removed by scrubbing the plates with a small brush and the algal tissue, mixed with seawater, was concentrated by spinning at 2500 rpm in a refrigerated centrifuge.  After pouring off the supernatant, the algal tissue was extracted in spectrophotometric grade acetone (90%). To ensure complete extraction, the volume of acetone ranged from 5–50 mL, depending on the volume of the algal sample.  Extractions were done at -20 oC in complete darkness for 24 hours.  The concentration of chlorophyll a in the absence of grazing, assumed to be a rough proxy for microalgal primary productivity, was determined spectrophotometrically using standard techniques (Hill and Hawkins 1990). 

6. Temporal Variation in Physical Processes

Temperatures representative of the body temperature of intertidal organisms were measured in February, 2002. A brass ball (2.5 cm diameter) was painted flat black and tapped with a half inch NPT pipe thread. A small temperature logger (iButton, Dallas Semiconductor, Dallas, Texas, USA) was inserted in this hole and the ball was then mounted (using a threaded PVC coupling) on the intertidal rock at one of the locations on our transects. This "body" temperature was recorded to the nearest 0.5oC every 10 minutes for two weeks in February 2002.

Gaps in the wave record from the Farallon Islands (due to equipment failure) were filled by linear interpolation, and 11.7% of the time series was affected by gaps.  The largest single gap was 3.3% of the overall time series.

 

LITERATURE CITED

Bell, E. C., and M. W. Denny 1994. Quantifying "wave exposure": a simple device for recording maximum velocity and results of its use at several field sites. Journal of Experimental Marine Biology and Ecology 181:9–29.

Denny, M. and D. Wethey. 2000. Physical processes that generate patterns in marine communities. Pages 3–37 in M. Bertness, S. Gaines, and M. Hay, editors. Marine Community Ecology. Sinauer Asssociates, Sunderland, Massachusetts, USA.

Helmuth, B., and M. W. Denny. 2003. Predicting wave exposure in the rocky intertidal zone: do bigger waves always lead to larger forces? Limnology and Oceanography, in press.

Hill, A. S., and S. J. Hawkins. 1990. An investigation of methods for sampling microbial films on rocky shores. Journal of the Marine Biological Association of the United Kingdom 70:77–88.

Roughgarden, J., S. Gaines, and H. Possingham. 1988. Recruitment dynamics in complex life cycles. Science 241:1460–1466.



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