Hydrologic Variability and Its Control of Phytoplankton Community Structure and Function in Two Shallow, Coastal, Lagoonal Ecosystems: The Neuse and New River Estuaries, North Carolina, USA

  • Paerl H
  • Hall N
  • Peierls B
 et al. 
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Hydrologic conditions, especially changes in freshwater input, play an
important, and at times dominant, role in determining the structure and
function of phytoplankton communities and resultant water quality of
estuaries. This is particularly true for microtidal, shallow water,
lagoonal estuaries, where water flushing and residence times show large
variations in response to changes in freshwater inputs. In coastal North
Carolina, there has been an increase in frequency and intensity of
extreme climatic (hydrologic) events over the past 15 years, including
eight hurricanes, six tropical storms, and several record droughts;
these events are forecast to continue in the foreseeable future. Each of
the past storms exhibited unique hydrologic and nutrient loading
scenarios for two representative and proximate coastal plain lagoonal
estuaries, the Neuse and New River estuaries. In this synthesis, we used
a 13-year (1998-2011) data set from the Neuse River Estuary, and more
recent 4-year (2007-2011) data set from the nearby New River Estuary to
examine the effects of these hydrologic events on phytoplankton
community biomass and composition. We focused on the ability of specific
taxonomic groups to optimize growth under hydrologically variable
conditions, including seasonal wet/dry periods, episodic storms, and
droughts. Changes in phytoplankton community composition and biomass
were strongly modulated by the amounts, duration, and seasonality of
freshwater discharge. In both estuaries, phytoplankton total and
specific taxonomic group biomass exhibited a distinctive unimodal
response to varying flushing rates resulting from both event-scale
(i.e., major storms, hurricanes) and more chronic seasonal changes in
freshwater input. However, unlike the net negative growth seen at long
flushing times for nano-/microphytoplankton, the pigments specific to
picophytoplankton (zeaxanthin) still showed positive net growth due to
their competitive advantage under nutrient-limited conditions. Along
with considerations of seasonality (temperature regimes), these
relationships can be used to predict relative changes in phytoplankton
community composition in response to hydrologic events and changes
therein. Freshwater inputs and droughts, while not manageable in the
short term, must be incorporated in water quality management strategies
for these and other estuarine and coastal ecosystems faced with
increasing frequencies and intensities of tropical cyclones, flooding,
and droughts.

Author-supplied keywords

  • Coastal ecosystem
  • Hydrology
  • Neuse River Estuary
  • New River Estuary
  • Phytoplankton

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  • Hans W. Paerl

  • Nathan S. Hall

  • Benjamin L. Peierls

  • Karen L. Rossignol

  • Alan R. Joyner

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