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Detailed information about the location and extent of zooplankton patches is fundamental to understand how abiotic and biotic forces interact to structure the spatial distribution of zooplankton. We mapped zooplankton patchiness in a Minnesota lake during spring, summer, and autumn with high-frequency (192-kHz) single-beam sonar. Conventional plankton samples of aggregations detected acoustically revealed that Daphnia pulicaria (mean body length 1.6 mm, mean target strength Ϫ120 dB) scattered most (ϳ63%) of the sound. Other taxa were smaller (Ͻ½ the length of D. pulicaria) and were usually less abundant and therefore scattered much less sound than D. pulicaria. Our acoustic estimates of Daphnia concentrations illustrate extreme patchiness, with concentrations varying by as much as four orders of magnitude over vertical distances of less than 1 m. Seasonal patterns of patchiness were related to predation by rainbow trout and to abiotic factors associated with stratification. Daphnia concentrations were highest from June to October in a deep-water ''refuge zone'' where oxygen concentrations were between 3 and 5 mg L Ϫ1 . These oxygen levels are suitable for Daphnia but are lower than those required by rainbow trout. Heterogeneity in Daphnia concentration along the lake's long axis was highest in May and June, when the population resided primarily in the oxic hypolimnion during the daytime. From July to October, as oxygen concentrations declined in the hypolimnion, the population became more metalimnetic and more uniformly distributed in the horizontal dimension. A diel study of the population in October indicated that the patchiness of population also changed dramatically between day and night. During the day the population aggregated densely in a thin layer (ϳ2 m thick) in the thermocline. After sunset the population dispersed into the epilimnion, where concentrations were ϳ100,000 m Ϫ3 less than they were during the day in the thermocline.Patchy spatial distributions and the low resolution of conventional sampling methods have impeded analyses of zooplankton populations. Zooplankton concentrations have been shown to vary by a factor of 1,000 within distances of meters horizontally or vertically, and the sampling resolution of conventional plankton nets and hoses is usually too coarse to identify the spatial limits of aggregations precisely (Coyle 2000). Variation in population density estimates due to sampling often cannot be distinguished from real changes of population density, and the effects of biological processes are difficult to distinguish from those of advective transport (Megard et al. 1997). Acoustic and optical plankton samplers developed during recent decades are major advances. They have very high sampling rates and spatial resolution, comparable to modern instruments (e.g., CTD profilers) used to measure environmental variables. Large numbers of plank-

The temporal and spatial variations of chlorophyll concentrations (n) and integral photosynthesis (Zp) are described for a large, morphometrically complex lake. The &ily x-ate of l Contribution No. 90, Limnological Research Center, University of Minnesota. Financial support for this investigation came from the U.

The mean transparency (37.5 m) measured by Secchi disk at "blue-water" pelagic sampling stations in the southeastern Mediterranean Sea during 3 yr was greater than in most other seawater. Our measurements in the Mediterranean and those by others in the English Channel, Atlantic Ocean, and Pacific Ocean obey the Lambert-Bouguer law if it is specified that transparency depends on the vertical attenuation coefficient for downward irradiance in the waveband of underwater sunlight that has lowest attenuance. Mean downward irradiance (+95% C.L.) at the depth where a Secchi disk disappears, calculated from 187 paired estimates of transparency and attenuation coefficients from these oceanic regions, is 22-t 1% of surface irradiance in this waveband. This value is significantly larger than the value (18%) that has been used widely since an early study by Poole and Atkins, who calculated attenuation coefficients incorrectly.As predicted by the Lambert-Bouguer law, reciprocal transparency was proportional to the attenuation coefficient for the most-penetrating waveband. Attenuance of this waveband and reciprocal transparency both increase linearly with increasing concentration of phytoplankton chlorophyll in pelagic water. Chlorophyll concentrations in pelagic Mediterranean water were very low (0.02-O. 14 mg m-3), but attenuance by units of phytoplankton chlorophyll was about 10 times larger than in more productive seawater, probably because blue light was predominant and many of the phytoplankton were very small coccolithophorids (cell diameters < 10 pm) with optically dispersive cell surfaces.Measurements of transparency by Secchi disk are as accurate and precise as estimates of attenuation coefficients calculated from irradiances measured at sea with photoelectric sensors. The Poole-Atkins equation for transparency in terms of the waveband with lowest attenuance describes the variation of transparency as well as the Duntley-Preiscndorfer equation does. Much of the information needed for interpreting satellite ocean-color imagery could be obtained very efficiently with closely spaced measurements of transparency from surface vessels during synoptic cruises.Planktonic algae can cause large variations in the transparency of seawater, especially in the open ocean. It is becoming increasingly important to understand how algae can cause such variability, because there is a global pattern of marine transparency. The transparency map compiled recently by Lewis et al. ( 1988) shows a pattern that is very similar to those of phytoplankton chlorophyll concentrations in- AcknowledgmentsContribution 308 from the University of Minnesota Limnological Research Center and also a contribution from Israel Oceanographic and Limnological Research, Ltd. Financial support provided by the U.S. Aid Program and the University of Minnesota Graduate School.We thank Y. Azov and A. Schneller for their assistance and J. T. 0. Kirk and A. Morel for critical comments and valuable suggestions on the manuscript.