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 the tidally dominated North Edisto Inlet, South Carolina, were described from acoustic backscattering samples at four high frequencies (265 kHz, 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 every 2 min for 21 d in June and 24 d in September-October 1994. Spring and neap tidal conditions were illustrated during both seasons to represent the limits of patterns observed in the 45 d of acoustic data. Inverse 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). Distributions of very small scatterers (0.13 mm ESR), probably representing a composite of inorganic and organic fine materials, detritus, benthic meiofauna, phytoplankton, and very small zooplankton, indicated cyclic resuspension in proportion to tidal current velocities. Mid-sized scatterers (0.79 mm ESR), probably representing much of the zooplankton population (i.e., shrimp postlarvae and crab megalopae), formed subsurface layers during the daytime. These layers fluctuated in depth temporally and were directly above the areas occupied by the small scatterers, in an apparent avoidance of high concentrations of the fine materials. Zooplankton was widely distributed vertically during nighttime in June. Large scatterers (5.0 mm ESR), probably fish larvae or small pelagic fish, occurred in vertical patterns similar to those of the zooplankton, but their distributions often were slightly higher in the water column than those of the zooplankton. The highest vertical distributions of each size class occurred in June, corresponding to seasonal differences in current velocities. The simultaneous application of multi-frequency acoustical techniques with complementary hydrographic data allows detailed descriptions of the distributional interrelationships between plankton scatterers of different sizes and some environmental parameters. This information should contribute much to interpretations of catches of estuarine plankton from net samples collected at specific times, depths, and tidal stages.