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Heterocapsa triquetra is one of the most common bloom-forming dinoflagellates found in estuaries and near shore regions around the world. This work examined the environmental factors associated with 3 separate wintertime H. triquetra blooms in the shallow tidally mixed Newport River estuary, North Carolina, USA. During 2 of the blooms in 1982 and 1983, the estuary was sampled from a fixed, single location every hour for 14 d. During the third study, the estuary was sampled at 9 fixed locations over its entire length each week from late December 1997 through March 1998. This time period included the formation and decline of the H. triquetra bloom. Barometric pressure, precipitation, photosynthetically active radiation, salinity, temperature, nutrient concentrations, and chl a were measured in each study. During the 1997–1998 study, pigments were analyzed using HPLC to characterize the phytoplankton assemblages and the dominant dinoflagellates in each sample were counted. The prevailing environmental conditions associated with the wintertime blooms were largely the result of atmospheric forcing. Low pressure systems moved through the study area at 3 to 4 d intervals and were accompanied by low ambient air temperatures and regular rainfall. Runoff following the rainfall events supplied inorganic nutrients critical for bloom initiation and development. It also created a mesohaline frontal zone in the middle portion of the estuary with salinity and hydrodynamic conditions favorable for H. triquetra growth. Here, the H. triquetra bloom reached its maximal development with chl a levels >100 μg 1-1 and cell densities between 1 and 6 x 106 1-1. As the H. triquetra bloom developed, nutrient inputs from the river became insufficient to meet growth demand and H. triquetra began feeding mixotrophically, supplementing its nutritional requirements and reducing competition from co-occurring dinoflagellates. Cloud cover associated with the low pressure systems transiently limited H. triquetra growth as did low temperatures. More importantly though, low temperatures limited micro- and macrozooplankton populations to such an extent that grazing losses were minimal. Hence, in order to bloom, H. triquetra optimizes a suite of factors including low grazing pressure, increased nutrient inputs, alternative nutrient sources, and favorable salinity and hydrodynamic conditions, as well as the negative factors of temperature-limited growth, short day lengths, and periods of transient light limitation.

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