Notes 669. rameters of Daphnia. Limnol. Oceanogr. 28: PETIPA, T. S On the energy balance of Calanus

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1 Notes 669 Max Planck Institute for Limnology ORCUTT, J. D., JR., AND K. G. PORTER Diel Department of Physiological Ecology vertical migration by zooplankton: Constant and fluctuating temperature effects on life history pa- Postfach 165 rameters of Daphnia. Limnol. Oceanogr. 28: 72- W-232 Plijn, Germany 73. PETIPA, T. S On the energy balance of Calanus References helgolandicus (Claus) in the Black Sea, p Zn ALCAW, M:, AND J. R. STRICKLER Loco- Physiology of marine animals [in Russian]. motion in copepods: Pattern of movements and Akad. Nauk SSSR. energetics of Cyclops. Hydrobiologia 167/168: 49- STICH, H. B., AND W. LAMPERT Predator eva sion as an explanation of diurnal vertical migra- DODSON, S Predicting diel vertical migration tion of zooplankton. Nature 293: , AND -. of zooplankton. Limnol. Oceanogr. 35: 1195-l Growth and reproduction EDMONDSON, W. T., AND A. H. LITT of migrating and non-migrating Daphnia species Daphnia in Lake Washington. Limnol. Oceanogr, 27: 272- under simulated food and temperature conditions 293. of diurnal vertical migration. Oecologia 61: 192- HUTCHINSON, G. E A treatise on limnology V. 2. Wiley. STRICKLER, J. R Swimming of planktonic Cy- LAMPERT, W The measurement of respiration, clops species (Copepoda, Crustacea): Pattern, p Zn J. A. Downing and F. H. Rigler movements and their control, p Zn T. [eds.], A manual on methods for the assessment Y. T. Wu et al. [eds.], Swimming and flying in of secondary productivity in fresh waters, 2nd ed. nature. Plenum. IBP Handbook 17. Blackwell. VEIGA, J. M., AND J. CASTEL CoQt Cnergetique ~ The adaptive significance of diel verde la locomotion chez le Copepode Eurytemora tical migration of zooplankton. Funct. Ecol. 3: 2 l- hirundoides (Nordquist, 1888). C.R. Acad. Sci. 27. Paris Ser. 3 33: MORRIS, M. J., G. GUST, AND J. J. TORRES Propulsion efficiency and cost of transport for co- Submitted: 14 May 1991 pepods: A hydromechanical model of crustacean Accepted: 7 October 1991 swimming. Mar. Biol. 86: Revised: 22 October 1991 Limnol. Oceanogr., 37(3), 1992, , by the American Society of Limnology and Oceanography, Inc A new plankton trap for use in the collection of rocky intertidal zooplankton Abstract-Intertidal energy levels historically have precluded the use of conventional plankton sampling methods in rocky inshore habitats. A new sampling device (intertidal plankton trap) that exploits wave surge was used to determine the composition, distribution, and abundance patterns of zooplankton from the surf zone of a rocky shore habitat. On three separate dates, replicate traps were used to sample consecutive tides, concurrently at two locations, to assess diurnal differences in inshore zooplankton during day- Acknowledgments I thank M. Drawbridge, P. Dunn, and S. Nixon for assistance during sampling, C. Rawlinson and F. Steinert for engineering contributions, and A. Fukuyama and two anonymous reviewers for critical review. Special thanks to D. Behrens for input, support, and enthusiasm. This study was partially funded by a grant-in-aid of research from Sigma Xi. light and nocturnal tidal cycles. Diurnal differences in taxon abundances suggest diurnal vertical migration. As a consequence of sampling difficulty, information regarding the distribution of planktonic organisms in the surf zone of exposed rocky shore habitats is scarce. It has not been possible to sample such shores with traditional, open-water sampling methods, such as towed plankton nets and beach seines, primarily due to the physical constraints. Irregular bottom contours and depth profiles, wave energy, and tidal surge make sampling in this particular environment almost impossible. Thus, fundamental questions regarding the transport, dis-

2 67 Notes Fig. 1. Intertidal plankton trap with cut-away showing PVC pipe and rubber flapper valve. A-Removable plastic lid with Nitex mesh netting; B-opposing 5.-g magnets; C-brass spring-clips; D-stainless steel eye-bolts set in intertidal substrate. persal, and recruitment of larval organisms to and from these habitats are left largely unanswered, limiting understanding of the population dynamics in the intertidal community. A variety of plankton traps has been developed and successfully used in recent years (sue Riitzler et al. 198; Doherty 1987). Most have been fashioned after hght traps or emergence traps, designed to sample planktonic invertebrates and larval fishes from more placid environs, such as bays and inlets, tropical coral reefs and lagoons, and freshwater lakes. The designs of most other traps either limit sampling to areas where the flow of water is steady and unidirectional, or only allow selective capture of certain organisms in the water column, as is the case with light and emergence traps. Until recently, the physical transport processes influencing recruitment and settlement patterns of larval organisms in the intertidal have not been seriously addressed. Recent work by Denny (1987, 1988; Denny and Gaines 199) elucidates the formidable nature of water motion in wave-swept environments and ways in which intertidal plants and animals cope with the hydrodynamic stresses of such environments. Although Denny (1987) and Gaines and Roughgarden (1985) highlight the major role ofplanktonic larvae in the community ecology of intertidal habitats, Denny (1987) acknowledges that the mechanisms responsible for larval dispersal and transport largely remain unknown. This note introduces a new sampling device specifically designed to collect planktonic organisms in the surf zone of rocky shore habitats. My objectives were to investigate the composition, distribution, and abundance patterns of certain zooplankton occurring in the surf zone and to assess temporal differences in these patterns with respect to photoperiod. To my knowledge, this device is the first of its kind in both design and application. The study area was the extreme nearshore (surf zone) of a rocky shore habitat at Diablo Canyon, San Luis Obispo County, California ( M, W). The Diablo Canyon area is along a remote portion of the central California coast between Morro Bay and Pismo Beach and is typical of rocky intertidal habitats along this irregular coastline, characterized by sheer waveeroded cliffs and jutting headlands. The area is generally exposed to ocean swells, and large storms during winter generate nearshore waves that reach heights of up to 11 m (D. Kelly pers. comm.). The main body of the intertidal plankton trap consists of an 18-liter rigid, plastic food container with a removable lid (Fig. 1). A filtering mechanism is created by cutting a 27-cm-diameter section from the center of the removable lid (3-cm diam) and replacing it with 333-pm Nitex mesh netting secured with nontoxic silicone caulking. Seawater and associated organisms wash into the trap through a 17-cm long, 15-cmdiameter PVC (polyvinyl chloride) pipe positioned through the center of the container s base, also secured with silicone caulking. A rubber flapper valve (of the type used on exhaust ports of motorboats) is mounted on the inside end of this pipe which enables the trap to sample and retain plankton from intertidal waters. The hinge of the flapper valve is positioned at the vertical apex of the pipe (Fig. I), opening during an incoming surge and closing during the subsequent outgoing surge. Magnets are fastened to the

3 Notes 671 inside opening of the valve to hold the flap closed, ensuring that the sample is retained during periods of slack water; mesh netting (2 cm) is secured to the outside end of the PVC pipe to prevent large pieces of drift algae from fouling the flapper valve (not shown in Fig. 1). Underwater observations (with SCUBA) verified the trap s intended operation. For sampling, the trap is deployed and retrieved from a fixed intertidal location when the area is exposed at low tide. The trap is secured by four brass spring-clips, fastened to the underside of the trap with U-bolts and mounting brackets, in turn fastened to stainless steel eye-bolts drilled into intertidal bedrock. This arrangement allows rapid placement and retrieval. The distance between the eye-bolts is maximized so the trap straddles firmly against the intertidal bedrock, thus allowing the least movement during wave surge. At the time of placement, the trap is positioned so the mouth end (pipe opening) faces the incoming wave surge. The constant backwash of seawater prevents long-term clogging of the mesh netting (filter). Sections of 6- x 6- x.5 cm-thick Neoprene are fixed between the mounting brackets and the body, serving as shock absorbers against incoming waves and tidal surge. Sampling design required the construction of six traps. Three sets of mounting points were drilled into intertidal substrate at each of two sampling locations (designated PTl and PT2). Each set of mounting points was drilled into substrate at + 15 cm above Mean er Water (MLLW). The two sampling locations, separated by -.5 km, were similar in exposure to inshore waves. The three replicate traps at each location were positioned within 1.5 m of each other. Replicate traps were deployed and then removed from each location during consecutive low tides, thus sampling intertidal waters for the intervening period of the incoming, high, and outgoing tides; both locations were sampled concurrently. Periods of tidal inundation typically lasted between 11.5 and 12.5 h due to the local, mixed-semidiurnal schedule. Once retrieved, traps were taken to the laboratory where samples were rinsed with filtered sea- water into separate containers. Each replicate sample was preserved in a 7% solution of isopropyl alcohol and later sorted into major taxonomic groups under a dissecting microscope. Consecutive day and night low tides were sampled for three separate 24-h periods, in April and May 1987, to investigate differences in the occurrence and abundance of inshore zooplankton between tides varying in phase relationships with daylight. Tide and wave-height data for each sampling interval were collected (Table 1). Organism abundances from a total of 36 samples were determined based on sorting the entire sample without subsampling. These data were used to calculate mean abundances for dominant taxonomic groups, in turn used to compare locations and sampling intervals. Of the animals collected by the plankton traps, six taxa (Calanoid and harpacticoid copepods, gammarid and caprellid amphipods, mysid shrimp, and decapod megalopae) occurred most frequently and therefore were considered for comparisons. All six taxa were represented in each plankton sample for each sampling interval at both locations. Copepods were most abundant, followed by amphipods and mysid shrimp (primarily Acanthomysis spp.). Other animals that frequently occurred in the traps included isopods (Zdotea resecata and Cirolana spp.), pycnogonids (primarily Pycnogonum stearnsi), in an array of prosobranch gastropods, and immature stages of tunicates, molluscs, barnacles (cyprids), echinoids, ophiuroids, bivalves, and polychaetes. The identification of larval ani- mals, save fishes, was not performed due to my limited taxonomic skills. In most instances, for all three sampling dates, samples collected during nighttime high tides showed greater numerical abundances for each major taxon compared to daylight samples (Table 2). The traps collected 4 fish larvae, all but one of them during nighttime high tides. The number of individuals within each taxon varied greatly among replicate traps at both locations for each sampling interval (Table 2). Results of Mann-Whitney U-tests indicated that abundances of each major

4 672 Notes Table 1. Tidal and wave-height data, and estimated submergence time for plankton trap sampling of consecutive low tides (within the same 24-h period) at Diablo Canyon. - Sampling period Apr 13 Apr 26 Apr 1 May 11 May Trap Tide ht (cm) submergence Tide Time (MLLW) Wave ht+ range (m) time (h) L.ow L OW $ I l.2.9-I * Data from continuous wave height and period monitoring with telemetered wave gauge: Diablo Canyon Offshore Wave Monitoring Program, Pacific Gas SC Electric Company unpubl. rep. taxon (for a given tidal interval) did not PT 1) there were no significant differences (P differ significantly (P >.5) between the >.5) in the abundance of any taxon withtwo trap locations for each sampling date. in a tidal interval among dates. These re- Comparison of the three sampling dates at sults reflect the variability in individual each location by Kruskal-Wallis tests indi- abundances among replicate traps. cated that with one exception (megalops at Because the two high tides within each Table 2. Mean number of individuals (+ 1 SE) collected by replicate plankton traps (n = 3) for both daytime (D) and nocturnal (N) high tides. - Station PTI Station PT D N D N Caprellid amphipods 12 Apr go-t t t t May 78f c44 55t f39 Gammarid amphipods 12 Apr f47 279t May 19+-g 288-t69 97f14 211f69 Harpacticoid copepods 12 Apr * t f62 1 May * f t- 17 Calanoid copepods 12 Apr f c & f c89 1 May _46 155t c45 Mysid shrimp 12 Apr f13 47f c2 1 May 49k k-11 Decapod megalops 12 Apr t11 33* k t7 49t May k3 13-t7 14-t3 Larval fish* 12 Apr 4, 1.3kO.6 12, 4.-t May 1 7, 2.3t-.3 3, l.ok1. 1, 3.3-to.9 3, l.o-to.6 -- * Total and mean number (t. 1 SE) of fish larvae collected by plankton traps; larvae are represented by the intertidal fish genera Cebidichthys, Xiphister. Gobiesox, Pholis, and Oligocottus.

5 Notes Station IT1 Station FT2 Station PII M a l2apr SApr lomay l2apr 25Apr 1 May Sampling dates/am tides 12 Ap 1 May 12Apr 1 May Sampling dates/pm hdcs Fig. 2. Number of Calanoid copepods and gammarid amphipods per hour of sampling for replicate traps at stations PT 1 and PT2. Estimated trap submergence times for both stations are 11.7 h (AM high tide) and 11.4 h (PM high tide) for April, 11.7 h (AM high tide) and 11.3 h (PM high tide) for April, and 11.4 h (AM high tide) and 11.8 h (PM high tide) for 1-l 1 May. 24-h sampling period differed in maximal tidal height, day and night samples differed in their submergence time even though traps were positioned at the same tidal height (Table 1). Although the difference in submergence time for any sample comparison is < 1 h, such differences must be considered when comparing results. During sampling, the three traps at each location were positioned similarly with regard to orientation, tidal height (+ 15 cm MLLW), and tidal exposure. All traps also sampled for the same duration. For a given interval, therefore, the six traps at both locations should have sampled nearly equal amounts of water, if not among locations then at least within locations. This suggestion cannot be verified because water flow through each trap was not measured. It is not clear whether the variability in sampling rates (organisms per hour) among replicate traps reflects natural variability or whether replicate traps sampled unequally (Fig. 2). For example, the number of gammarid amphipods and Calanoid copepods sampled per hour of submergence varied considerably between replicate traps for both locations (Fig. 2). If individual traps sampled similarly, as suggested above, then the variability exhibited likely reflects the inherent patchiness typically associated with plankton distributions (Wiebe 197; Willason et al. 1986). If they did not, differences may be due to topographically induced variation in wave surge. The addition of a flow-measuring device would certainly improve this trap s quantitative capabilities. Although accurate

6 674 Notes measurement of flows and forces in waveswept environments is difficult, several techniques do exist (see Denny 1988). Some techniques are limited by design characteristics, others by expense. Water movement through the trap is unidirectional, so a propeller or rotor-blade-type flowmeter attached to the inside of the trap s intake (PVC pipe) would likely work best. Diurnal vertical migration (Friedrich 1969), in which the vertical distribution of planktonic animals is associated with photoperiod, could explain the apparent diurnal differences observed in organism al abundances during this study. One theory, as explained by Lerman (1986), speculates that midwater and epibenthic animals feed on abundant phytoplankton in the upper water at night. During daylight, the same animals seek refuge from predators in deeper, darker waters, or in the case.f nearshore shallow habitats, under and within numerous types of benthic substrates. Provided that vertical migration increases plankton abundance in the nearshore waters at night, greater numbers of organisms would be transported to and from the surf zone during this period. Organisms collected by plankton traps were probably transported to the surf zone by shoreward-moving water masses. Once transported there, plankton mobility is likely ineffective against inshore surge. IJnder such energetic conditions, sampling biases based on active and passive behaviors of plankters may be small. Such sampling biases, however, should be both modeled and measured. The new device introduced here provides nearshore ecologists with a viable tool to sample the logistically difficult rocky-shore surf zone. It is simple, durable, inexpensive, and easy to deploy. The filter mesh is interchangeable allowing selective sampling based on organism size. The design allows fabrication of larger or smaller versions and collection of live organisms. This trap can also be set and collected by SCUBA divers from sites underwater, a cap fits over the mouth-end (PVC pipe) excluding water during placement and retrieval. Spatial, temporal, and logistic limitations posed by other plankton sampling methods (beach seines and boat tows) are addressed by the trap s replicability. Although the trap proved durable over the course of the study, it was not subjected to inshore waves > 1.2 m in height (Table 1). Whether the device will hold up under considerably rougher conditions is not known. U.S. EPA, W-7-)1 75 Hawthorne Street San Francisco, California References Aaron Carr Setran DENNY, M. W Life in the maelstrom: The biomechanics of wave-swept rocky shores. Trends Ecol. Evol. 2: p Biology and the mechanics of the waveswept environment. Princeton. -- AND S. D. GAINES On the prediction of maximal intertidal wave forces. Limnol. Oceanogr. 35: DOHERTY, P. J Light-traps: Selective but useful devices for quantifying the distributions and abundances of larval fishes. Bull. Mar. Sci. 41: FRIEDRICH, H Marine biology. Sidgwick and Jackson. GAINES,~. D., AND J. ROUGHGARDEN Larval settlement rate: A leading determinant of structure in an ecological community of the marine intertidal zone. Proc. Natl. Acad. Sci. 82: LERMAN, M Marine biology: Environment, diversity, and ecology. Benjamin Cummings. R~~TZLER, K.,J.D. FERRARIS,AND R.J. LARSON A new plankton sampler for coral reefs. Mar. Ecol. 1: WIEBE, P. H Small-scale spatial distribution in oceanic zooplankton. Limnol. Oceanogr. 15: WILLASON, S. W., 9. FAVU~~I, AND J. L. Cox Patchiness and nutritional condition of zooplankton in the California Current. Fish. Bull. 84: Submitted: 16 December 199 Accepted: 24 September 1991 Revised: 5 December 1991