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Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d in summer of 1982 and again in winter of 1983. This sampling regime was of statistically appropriate duration to allow characterization of the variability between periods of 2 to 96 h by spectral analysis of the time-series. The project (PULSE) took place in the Newport River Estuary, located behind the Outer Banks of North Carolina, USA. In all, twentyeight parameters were monitored, encompassing the meteorology, hydrology, water chemistry and phytoplankton-production physiology. Although the annual cycle was monitored, only the winter and summer seasons are compared here, i.e., the lowest water temperatures with the highest water temperatures. The physics, chemistry and biology of the estuary at the hourly scale were highly variable and non-random. The estuary is riverine in winter; growth-limiting nitrogen is supplied as nitrate (NO) and ammonium (NH) by runoff from the drainage basin. In summer, the estuary is lagoonal; nitrogen is supplied as NHby biological regeneration. Chlorophyll a biomass varies primarily at the ∼4 d period in winter and at the diel period in summer. Although finely tuned to environmental variability, phytoplankton abundances were at equilibrium insofar as daily chlorophyll production was balanced by losses, i.e., grazing, export and deposition. Most important, high-frequency processes, here periods at the scale of cell-division times, can be very important in phytoplankton ecology. [ABSTRACT FROM AUTHOR]

Ovigerous females of the crab Rhithropanopeus harrisii (Gould) were collected from an estuary having irregular tides and exposed to only a semi-diurnal tidal cycle in salinity change in the laboratory. A circatidal rhythm developed, in which larval release occurred around the predicted time of highest salinity (high tide). Thus, salinity cycles can entrain a tidal rhythm. After exposure to only a diel light/dark (LD) cycle, releases occurred mainly in the first 2 h of the dark phase. When exposed to both a tidal salinity cycle and an LD cycle, larval release depended upon the time of nocturnal high tide. Crabs only released at night. If high tide was near the beginning or within about 3 h of the end of the dark phase, releases occurred at the beginning of the dark phase. As high tide moved toward the middle of the night, an increasing proportion of crabs released around the predicted time of high tide. These data support the suggestions that (1) the LD and tidal salinity cycles control the specific time of hatching and (2) there is a hierarchy in the functional advantages for the timing of larval release, in which reduced vulnerability to visual predators is more important than avoidance of stressful salinities or intial seaward transport. [ABSTRACT FROM AUTHOR]