Effect of diel and interday variations in light on the cell division pattern and in situ growth rates of the bloom-forming dinoflagellate Heterocapsa triquetra

Abstract

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.

Effect of Biotic and Abiotic Factors on In Vitro Proliferation, Encystment, and Excystment of Pfiesteria piscicida

“…Positive and negative controls are indicated. the large numbers of putative resting cysts observed at the end of the decline phase when the culture was virtually depleted of algal prey supported this view, which is consistent with the results of studies of other heterotrophic dinoflagellate species (5,18,19,51,52). In contrast, several culture medium supplements (ammonia, urea, and extracts of chicken manure, fish, and soil) that have been reported to modulate the proliferation of other dinoflagellate species (24,26,66,67) had no effect on P. piscicida.…”

Seasonal niche strategy of the bloom-forming dinoflagellate Heterocapsa triquetra

“…Low PAR in combination with an average 9 h day length versus a 16 h day length in midsummer produces the lowest incident light levels of the year. In situ growth rate studies conducted during the 1983 time-intensive study indicate that Heterocapsa triquetra blooms undergo transient day to day light limitation due to increased cloud cover (Litaker et al 2002). The transient light limitation, however, does not affect bloom formation, indicating that H. triquetra is well adapted to low light conditions.…”

“…The significantly lower growth rate below 15°C is also supported by field data. Minimal daily in situ growth rates for H. triquetra estimated during the 1983 study varied from 0.02 to 0.14 d -1 over a 7.6 to 12.0°C temperature range (Litaker et al 2002). Each of the observed H. triquetra blooms occurred when average daily water temperatures were generally <12°C, and often <10°C.…”

Top-down control by micro- and mesozooplankton on winter dinoflagellate blooms of Heterocapsa rotundata

“…Past research on winter dinoflagellate blooms on the east coast of the United States has focused on the physical changes to the environment that initiate blooms (Cohen 1985, Sellner et al 1991, Litaker et al 2002b. High rainfall and increased river flow have been shown to trigger such blooms by creating a stratified water column (Cohen 1985) and causing a large influx of nutrients (Litaker et al 2002b).…”