VARIATION IN THE VERTICAL-DISTRIBUTION OF ZOOPLANKTON AND FINE PARTICLES IN AN ESTUARINE INLET OF SOUTH-CAROLINA

Patterns in the vertical distributions of estuarine plankton within th e tidally dominated North Edisto Inlet, South Carolina, were described from acoustic backscattering samples at four high frequencies (265 kH z, 420 1.1 kHz, 1.1 MHz, 3.0 MHz). Samples were collected between the near-bottom (1 m) and near-surface (within 2 m) at 0.5-m intervals eve ry 2 min for 21 d in June and 24 d in September-October 1994. Spring a nd neap tidal conditions were illustrated during both seasons to repre sent the limits of patterns observed in the 45 d of acoustic data. Inv erse calculations interrelating the returns from the four frequencies allowed estimates of biovolume density (mm(3) m(-3)) by size class for backscatterers at selected equivalent spherical radii (ESR). Distribu tions of very small scatterers (0.13 mm ESR), probably representing a composite of inorganic and organic fine materials, detritus, benthic m eiofauna, phytoplankton, and very small zooplankton, indicated cyclic resuspension in proportion to tidal current velocities. Mid-sized scat terers (0.79 mm ESR), probably representing much of the zooplankton po pulation (i.e., shrimp postlarvae and crab megalopae), formed subsurfa ce layers during the daytime. These layers fluctuated in depth tempora lly and were directly above the areas occupied by the small scatterers , in an apparent avoidance of high concentrations of the fine material s. Zooplankton was widely distributed vertically during nighttime in J une. Large scatterers (5.0 mm ESR), probably fish larvae or small pela gic fish, occurred in vertical patterns similar to those of the zoopla nkton, but their distributions often were slightly higher in the water column than those of the zooplankton. The highest vertical distributi ons of each size class occurred in June, corresponding to seasonal dif ferences in current velocities. The simultaneous application of multi- frequency acoustical techniques with complementary hydrographic data a llows detailed descriptions of the distributional interrelationships b etween plankton scatterers of different sizes and some environmental p arameters. This information should contribute much to interpretations of catches of estuarine plankton from net samples collected at specifi c times, depths, and tidal stages.