No abstract

Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d periods to characterize phytoplankton biomass variability at 2 to 96 h periodicities via spectral analysis. In all, 28 parameters were monitored, encompassing meteorology, hydrology, water chemistry and phytoplankton production physiology. The spring (May 1982) and fall (November 1982) periods of rapidly changing water temperatures are compared. In spring, the estuary was in transition from riverine to lagoonal condition. Temperatures rose steadily (22 to 28"C), salinities (23 to 35%) and chl a biomass (22 pg I-') were high, and dissolved inorganic nitrogen (DIN) low (0.13 PM). Growth-limiting N was supplied as NH,' by biological remineralization, rising (0.2 to 3.0 PM) on 11 of 14 nights, then declining each following day to below detection. Higher ebb-tide chl a biomass produced a tidal signal. Die1 tidal height variations and daily wind stress changes accounted for about half the daily 2-fold oscillation in chl a, the remainder was driven by daytime phytoplankton growth followed by nighttime grazing losses. In fall, the estuary was more riverine (20 to 32%) and temperatures variable (12 to 20°C). Chl a biomass was low (4 pg 1-'1 and DIN high (2.0 PM). Chl a varied at the tidal and interday periods.Interday changes were correlated with water temperature changes, in turn coupled to the passage of atmospheric fronts at 3 to 4 d intervals. In both spring and fall periodic ecological processes operating on time scales equivalent to phytoplankton cell division times were found to be important In controlling chl a biomass changes.

A persistent diel cycle in chl a biomass was observed In the Newport Fbver estuary, North Carolina, USA d u n n g summer Chl a concentratlons were highest in the late afternoon (mean 22 pg I-') and least around dawn (mean 12 pg 1-l) The day to night change In chl a was paralleled by a commensurate decllne In volumetnc cell count Thls change In cell number was not correlated with wind resuspension, tldal dilution, or species composition differences, but rather is attributed to grazing A diel cycle in NH,' concentrat~on was also observed, due to temporal vanations in uptake relative to water column remlneralization Ammonium concentrations reached a maxlmum (> 2 u i v several hours after dawn, then decllned to a minimum ( < 0 5 p m by early afternoon as light-dependent uptake exceeded remlnerahzabon By late afternoon remineralization again exceeded uptake and NH,+ concentrahon increased continually throughout the rught In vltro ennchments demonstrated that the dayhme chl a increase was dependent on the early morrung uptake of nitrogen, whether In the form of N H 4 + , NO3-or urea Phaeopigment concentrahons also followed a diel pattern with an early afternoon minimum and a near-dawn maximum, mlrronng changes in chl a This pattern was due to photodestruction of phaeopigment dunng the day and accumulation at night Phaeopigment production was attributed to microzooplankton grazing When converted to chl a equivalents phaeopigment appearance accounted for 50 % of the chl a loss

Heterocapsa triquetra is an important bloom-forming dinoflagellate found in estuaries and nearshore regions worldwide. In an initial time-intensive study, the shallow, tidally mixed Newport River estuary, North Carolina, USA, was sampled from a fixed point located in the middle of the estuary every 2 h for 2 wk during the development of an H. triquetra bloom. The objective of this study was to investigate how short-term, high-frequency changes in temperature, light and salinity affected diel and interday cell division patterns and in situ growth rates of H. triquetra. During this study, phytoplankton samples were preserved in buffered formaldehyde and mitotic indices determined by acridine orange staining. The diel division pattern showed a nocturnal maximum between 23:00 and 05:00 h with reduced division during the day, a pattern characteristic of most dinoflagellates. The relative proportion of binucleate cells present during the day was influenced by interday variations in total irradiance, increasing during 2 of the 3 periods when there were 3 or more consecutive high light days (> 28 E m -2 d -1). Approximately 40% of the overall variation in interday division rates could be accounted for by differences in daily irradiance. The interday light differences were largely due to well-developed atmospheric frontal systems that brought increased cloud cover to the study area at regular 3 to 4 d intervals. The initial study, however, was of insufficient length to determine if the transient day-to-day light limitation could significantly affect seasonal bloom formation. A second longer-term, spatially intensive study was therefore undertaken to assess the relative importance of the incident light levels and nutrient inputs in controlling H. triquetra bloom initiation. During the second study, the estuary was monitored for photosynthetically active radiation (PAR), salinity, temperature, inorganic nutrients and cell densities of H. triquetra at 9 locations every wk from 23 December 1997 to 27 March 1998. Maximal H. triquetra bloom formation occurred during a 2 wk period when daily incident light levels were at or near the annual low. This suggested that H. triquetra is well adapted for utilizing low light levels and that variation in in situ growth rates in response to daily changes in PAR had little effect on bloom development. Instead, bloom initiation began with inputs of nitrogen-rich water following a runoff event, indicating that nutrient inputs are much more important in controlling bloom development than is light.