A simple device for recording the maximum force exerted on intertidal organisms1

A new dynamometer records the magnitude and direction of the maximum force imposed on individual organisms by rapid water flow. These measurements allow "exposure" to be accurately quantified, and maximum water velocity to be estimated. The device is rugged, simple to build, inexpensive, and can be used in a variety of habitats. Organisms living in wave-swept environments are exposed to substantial hydrodynamic forces. These forces have long been thought to play an important role in the structure of wave-swept communities (e.g. Newell 1979; Lewis 1964; Jones and Demetropoulos 1968; Paine and Levin 1981; Tunnicliffe 1981), but the accurate measurement of these forces has proven difficult (Denny 1982; Vogel 1981). Various mechanical dynamometers constructed for this purpose suffer from various problems. The apparatus of Harger (1970) measures wave force by measuring the movement of a plate attached by a friction fitting to a nail. The device thus measures cumulative wave force and cannot distinguish between the effects of a series of small forces and that of a single large force. Further, this device measures force in only one direction (perpendicular to the substratum), and it is difficult o interpret the force imposed on the measuring plate in terms of the force that a particular organism would experience. The device of Jones and Demetropoulos (1968) measures the force exerted on a drogue plate attached by a rope to a spring scale. Due to its size the apparatus cannot be used in a confined space (such as inside a bed of macroalgae or mussels) and because the drogue is free to move about, the direction in which the force is applied is not recorded. As with the device of Harger, it I This study was funded by ONR contract N0001479-C-0611 and with logistic support from NSF grant OCE 77-26901 to R. T. Paine. is difficult to interpret the force measured by the apparatus in terms of the force experienced by a specific organism. Further problems with this device have been noted by Denny (1982) and Vogel (1981). In light of these problems the use of these devices has been restricted to measuring relative differences in exposure between sites. Koehl (1976) used a strain-gauge force plate to measure the force exerted on sea anemones by individual waves. The primary disadvantage of this apparatus is the requirement for a cable between the dynamometer and the recording apparatus; the experimenter and the expensive recording apparatus must be dangerously close to the surf or the cable must be inconveniently long. Denny (1982) described a telemetry system capable of accurately recording the forces directly exerted on individual organisms by individual waves. This apparatus overcomes many of the deficiencies of the earlier devices and has proven useful in examining wave forces on a variety of organisms (Denny in prep.). For many applications a continuous record of individual wave forces is unnecessary, and in these cases the time and money spent in constructing the telemetry system described by Denny (1982) are not warranted. Here I describe an alternative, a simple dynamometer that records the direction and maximum magnitude of forces exerted on individual organisms or small standard shapes. The dynamometer is inexpensive (<$10 each in 1983), easily built, rugged, and requires no external power source. Accurate measurements of maximum force and direction, as provided by this dynamometer, would be useful for several aspects of intertidal studies. For example, these measurements, when compared to the adhesive tenacity or structural strength