temperature of limpets in the rocky intertidal zone: effects of caging and substratum

Temperature of Limpets in the Rocky Intertidal Zone: Effects of Caging and SubstratumAuthor(s): Anita M. Hayworth and James F. QuinnSource: Limnology and Oceanography, Vol. 35, No. 4 (Jun., 1990), pp. 967-970Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2837402 .

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Notes 967

gan, Huron and Superior, p. 277-298. In A. D. Hasler [ed.], Coupling of land and water systems. Springer.

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iments of Lake Erie's central basin. Diatom Res. 2: 113-134.

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Submitted: 6 June 1989 Accepted: 28 September 1989

Revised: 20 February 1990

Limnol. Oceanogr., 35(4), 1990, 967-970 ? 1990, by the American Society of Limnology and Oceanography, Inc.

Temperature of limpets in the rocky intertidal zone: Effects of caging and substratum

Abstract-Depending on weather, temperatures (Te) of intertidal limpets (Collisella scabra) were decreased as much as 4?C by the types of cages used in ecological studies in the rocky intertidal zone. The cages blocked 60-80% of incident solar radiation. Differences in substratum may have larger effects on T,, however, than those introduced by cages.

Interspecific interactions in the rocky intertidal zone are perhaps the best under- stood of any ecological community. This situation results largely from the tractability

Acknowledgments Mike Judge assisted in all phases of this study. Support was provided by the Bodega Marine Lab-

oratory and the Committee on Research at the Uni- versity of California at Davis.

of caging techniques, which have been used successfully in numerous studies (see Paine 1977). Cage experiments have generally controlled for cage effects by including "roofed" cages, open at the sides to allow access of crawling invertebrates, but pre- sumably producing shading and hydrody- namic effects similar to those of the fully enclosed cages. Effects of caging on the physiological environment of study organisms have rarely been investigated directly. Therefore it seemed worthwhile to examine the effects of cages on the thermal environment.

Operative temperature (Te) is a means of describing the microclimate of an area with a model of the organism as the temperature probe. Te incorporates all the avenues of



968 Notes

heat gain and loss and results in a single temperature value, which serves as a measure of sensible heat transfer. Exact dupli- cates of the organism are constructed out of thin-shelled metal casts containing a thermocouple inside for temperature measurements. Bakken et al. (1983) described in detail the methods of construction of the taxidermic models. For this study, individ- uals of the limpet Collisella scabra, a common high intertidal species, were used to construct the Te probes.

Limpets used to construct the model of the body were collected at Bodega Marine Laboratory (Sonoma Co., California, 38020'N, 12304'W). The study was carried out at the granitic rocky shore at Bodega Head, 1 km north of the laboratory. The tides at Bodega Head are such that the mid- to-high intertidal zone is out of water for up to 18 h during many spring tides.

Mean dimensions (? 1 SD) of the three body models used were: length, 2.6?0.1 cm; width, 1 .9?0.1 cm; height, 1. I +0.1 cm. To construct the models, we removed the soft tissue from the shell and poured molten paraffin wax into it. When the wax had hardened it was pulled from the shell, and a 32-gauge copper-constantan (Cu-cn) thermocouple inserted into the center of the wax. A thin layer of Cu was electroplated to the model according to the methods of Bakken and Gates (1975). To complete construction of the probe, we glued the model ofthe body to the shell with thermal adhesive. Limpet Te probes were calibrated and checked against each other in an environmentally controlled chamber.

Te measurements were made on cages used in ongoing studies at Bodega, and on the "classic" cage design of Connell (1970) and others. In the first design (M. L. Judge in prep.), studied in 1986, black slate settlement plates (30 x 30 cm) were installed into the7 rocks and cages were bolted over them. The cages were constructed of a frame of 0.25-inch (6.4-mm) steel rod, 25-cm square and 6 cm high, covered by a 6.4-mm Vexar (DuPont) nylon mesh. The classic design was made of 3.2-mm stainless steel mesh bent into a cage ofO x 10 x 4 cm attached to the substratum with a central screw.

Microclimate measurements were taken

for 1 min every 15 min and consisted of Te in three positions (under the center of a cage, on an exposed slate plate, and on the granite surface as a control), solar radiation at nor- mal incidence with a LiCor 200s pyranometer under the center of a cage and from the open granite surface, and air temperature 1 cm above the substratum with a 32-gauge Cu-cn thermocouple. Measurements were taken until the tide came in. One limpet Te probe was placed in each of the three positions, and measurements were taken si- multaneously. The probes were glued to the surface with thermal paste to ensure good conduction. Temperatures were measured with an Omega CJ-1 cold junction com- pensator and Data Precision model 255 voltmeter (precision of ? 0. 1C). The LiCor 200s has a good cosine response. Pyranome- ter measurements were taken with a LiCor Calconnector and the Data Precision voltmeter.

Measurements were taken during day- time exposure of the intertidal zone to attain the maximum difference that might occur between microclimates of the exposed and caged situations. The sites chosen for measurements were located in the same area so that slope and aspect were identical. The rock and slate surfaces were dry when measurements were taken.

Due to a nonlinear spectral response, the LiCor pyranometer (silicon-cell type) gives inaccurate measurements of radiative heat flux when shaded (Unwin 1980). In spite of this difficulty, the readings will give an or- dination for comparative purposes.

Measurements were taken on 18, 19, 31 August, 1 September, and 14 October 1986 and 6 March 1987. The comparison with the classic cages was done on 26 March 1987. As with the other studies with cages of this design, no settlement plates were used. The 26 March midday low tide was calm and sunny, under which the maximum physiological stresses beneath the cages might be expected.

Beneath the Vexar cages, Te was highest for the uncaged black slate surfaces and low- est on the slate under the cage. Much greater differences occurred on sunny days (Fig. 1). Maximal variations among treatments in Te were observed under sunny, calm condi-



Notes 969

30- 18 Aug 1986

/'

20

1 0 E 30 19 Aug 1986

20 -

1 0 700 900 1 1 00

Time (hours)

Fig. 1. Effect of substratum and Vexar cage on operative temperature (Te) on days that were very different in overall weather. With the limpet Collisella scabra as the model, Te was measured on white granite (solid line), black slate (dashed line), and under a Vexar cage (dotted line) used in intertidal ecology studies. Mea- surements were made every 15 min during exposure of the rock to air at low tide; 18 August 1986 was a sunny, warm day; 19 August 1986 was a typically foggy and overcast day.

tions (Table 1). The maximal difference in Te between the rock surface and the open plate was 5.8?C on 18 August- 9.8?C higher than on the caged plate. The Te of the open plate reached a maximum of 33.7?C on 18 August 1986. Although higher rock temperatures have been recorded (Johnson 1975), the timing of low tide prevented temperatures from going any higher. The maximal difference in Te for the classic caging technique was 3.0?C and gradually decreased as the sun angle decreased (Fig. 2, Table 1).

21

S /

D 18 ...... E~~~~~~~~~~~~~ , o 18

1H ..

15

1 200 1 300 1 400 Time (hours)

Fig. 2. Effect of substratum and classic cage on operative temperature (Te) measured on white granite (solid line) and under a classic cage (dashed line) used in intertidal studies, with the limpet Collisella scabra as the model. Dotted line is temperature of the white granite. Measurements were made every 15 min during exposure of the rock to air at low tide on 26 March 1987.

Obviously a large contributor to Te is direct solar radiation (Porter and Gates 1969), which is also the component most likely to be directly modified by cages. The maximal difference occurred on 18 August with a difference of 351 W m-2 for the Vexar cages. The maximal difference for the classic cages was 680 W m-2. The mean percent reduction in solar radiation due to Vexar cages for all days was 56?7%. Mean percent reduction in solar radiation for the classic cages was 81?6%. Incoming solar radiation was reduced by as much as 69% for the Vexar cages (Table 1).

Table 1. Effects of substrate material and caging on operative temperature (Te) and incident solar radiation. Maximal differences in Te and maximal percent decrease in solar radiation for different substrata and for two commonly used intertidal caging techniques. Te was measured with the high intertidal limpet Collisella scabra as the model. Measurements were made on 6 d with different overall weather conditions for the Vexar cages and on 1 d for the classic cage type. Comparisons were made to the exposed granite surface.

Max difference Reduction in in Te (Q solar

radiation Duration Exposed due to cage

Cage type Start timq (h) slate Under cage (max %) Weather

Vexar 18 Aug 86 0730 3.00 5.8 4.0 53 Sunny 19 Aug 86 0800 2.75 1.8 1.8 69 Overcast 31 Aug 86 0800 1.75 1.5 2.8 56 Partly cloudy 1 Sep 86 0730 2.00 4.5 1.6 48 Sunny

14Oct 86 1600 2.75 3.3 0.8 59 Sunny 6 Mar 87 1330 2.00 2.8 2.7 51 Overcast

Classic 26 Mar 87 1230 1.50 - 3.0 81 Sunny



970 Notes

Little is known about the direct physiological consequences of caging techniques (Underwood and Denley 1984). The intertidal zone is known for widely varying environmental conditions. Temperature gradients in excess of 100?C occur in tidal mudflats (Harrison and Phizacklea 1987). These gradients indicate that the fauna must be able to tolerate such environmental variations physiologically or behaviorally. Nevertheless, small differences in temperature can influence competitive outcomes (Wethey 1984).

We found that Te can vary by as much as 10?C among the microclimates found in intertidal caging experiments. Standard caging materials may block 60-80% of the incident solar radiation. The effects of this reduction are apparently offset by thermal conduction through the rock. The classic cages, which filter out 80% of the incident light decreased Te by only 3?C, or less.

The rock surface appears to have greater effects on the operative temperature of organisms than do cages. Black slate probably represents the most extreme substratum found in intertidal zones. Its color maxi- mizes solar radiation absorbed, and it dries rapidly, minimizing evaporative cooling later in the tide. The white granite at Bodega Head represents close to the opposite extreme in both reflectivity and roughness. The difference between the two substrata can be as much as 6?C, roughly twice the effect of standard intertidal cages.

In this study, the greatest effects of microclimate on operative temperature appear on sunny days and would be especially apparent if low tide occurred at midday. Oc- casional hot, clear days may influence the development of the intertidal community (Paine and Levin 1981). Lethal temperatures of intertidal organisms are frequently much higher than the maximum temperature seen on the slate plate on the hottest day, but it is plausible that cages could pro- mote survival during rare physiological bot- tlenecks. Thus results from such caging experiments may be influenced by the change in microclimate due to the cage. Under nor- mal conditions, cages clearly lower the Te of the caged organisms, perhaps equivalent to being somewhat lower in the intertidal

zone, but it is not clear whether the differences are physiologically significant. Larger differences are due to variation in the char- acter of the substratum.

Anita M. Hayworth' James F. Quinn

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Submitted: 6 June 1989 Accepted: 21 August 1989

Revised: 13 February 1990

1 Present address: 10542 Montego Drive, San Diego, California 92124.



temperature of limpets in the rocky intertidal zone: effects of caging and substratum

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