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Aquatic Microbial Ecology

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AME 63:133-143 (2011)  -  DOI:

Environmental controls upon picophytoplankton growth and biomass in a eutrophic estuary

Michael S. Wetz1,*, Hans W. Paerl2, J. Christopher Taylor3, Jeremy A. Leonard2

1Texas A&M University—Corpus Christi, Department of Life Sciences, Corpus Christi, Texas 78412, USA
2Institute of Marine Sciences, The University of North Carolina—Chapel Hill, Morehead City, North Carolina 28557, USA
3NOAA Ocean Service, Center for Coastal Fisheries and Habitat Research, Beaufort, North Carolina 28516, USA

ABSTRACT: We assessed the role of top-down versus bottom-up factors in regulating picophytoplankton (PicoP) growth and overall phytoplankton size structure in a eutrophic estuary. PicoP biomass reached an annual maximum in summer/fall and was positively correlated with temperature. Ephemeral blooms (chlorophyll a > 20 µg l–1) of PicoP were observed in the upper and middle regions of the estuary despite inorganic nitrogen concentrations <1 µmol l–1. Nutrient-amended PicoP growth rates were similar to in situ growth rates in the upper estuary, and PicoP biomass was negatively correlated with river-derived inorganic nitrogen concentrations, indicating that regenerated nutrients are a major source of nitrogen supporting PicoP growth. Microzooplankton grazing rates routinely exceeded PicoP growth rates during summer; therefore, grazing must have become uncoupled from PicoP growth on timescales shorter than the interval between grazing experiments (i.e. 2 to 4 wk) for PicoP to have bloomed. Field data point to the possibility of trophic cascades involving copepods, protistan grazers, and phytoplankton as a mechanism for this growth–grazing uncoupling. These and other recent findings indicate that bottom-up factors alone cannot explain the PicoP blooms observed in some estuarine systems and emphasize the need for grazing control studies to better understand the regulation of primary production.

KEY WORDS: Picophytoplankton · Estuary · Microzooplankton · Mesozooplankton · Trophic ­cascade · Nutrients

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Cite this article as: Wetz MS, Paerl HW, Taylor JC, Leonard JA (2011) Environmental controls upon picophytoplankton growth and biomass in a eutrophic estuary. Aquat Microb Ecol 63:133-143.

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