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Recent increases of rainfall and flooding from tropical
cyclones (TCs) in North Carolina (USA): implications
for organic matter and nutrient cycling in coastal
watersheds
Hans W. Paerl .Nathan S. Hall .Alexandria G. Hounshell .Karen L. Rossignol .
Malcolm A. Barnard .Richard A. Luettich Jr. .Jacob C. Rudolph .
Christopher L. Osburn .Jerad Bales .Lawrence W. Harding Jr.
Received: 4 April 2020 / Accepted: 28 July 2020 / Published online: 11 August 2020
ÓSpringer Nature Switzerland AG 2020
Abstract Coastal North Carolina experienced 36
tropical cyclones (TCs), including three floods of
historical significance in the past two decades (Hur-
ricanes Floyd-1999, Matthew-2016 and Florence-
2018). These events caused catastrophic flooding
and major alterations of water quality, fisheries habitat
and ecological conditions of the Albemarle-Pamlico
Sound (APS), the second largest estuarine complex in
the United States. Continuous rainfall records for
coastal NC since 1898 reveal a period of unprece-
dented high precipitation storm events since the late-
1990s. Six of seven of the ‘‘wettest’’ storm events in
this [120-year record occurred in the past two
decades, identifying a period of elevated precipitation
and flooding associated with recent TCs. We exam-
ined storm-related freshwater discharge, carbon
(C) and nutrient, i.e., nitrogen (N) and phosphorus
(P) loadings, and evaluated contributions to total
annual inputs in the Neuse River Estuary (NRE), a
major sub-estuary of the APS. These contributions
were highly significant, accounting for [50% of
annual loads depending on antecedent conditions and
storm-related flooding. Depending on the magnitude
of freshwater discharge, the NRE either acted as a
‘‘processor’’ to partially assimilate and metabolize the
loads or acted as a ‘‘pipeline’’ to transport the loads to
the APS and coastal Atlantic Ocean. Under base-flow,
terrestrial sources dominate riverine carbon. During
storm events these carbon sources are enhanced
through the inundation and release of carbon from
wetlands. These findings show that event-scale dis-
charge plays an important and, at times, predominant
Responsible Editor: James Sickman.
H. W. Paerl (&)N. S. Hall K. L. Rossignol
M. A. Barnard R. A. Luettich Jr.
Institute of Marine Sciences, University of North Carolina
at Chapel Hill, Morehead City, NC 28557, USA
e-mail: hpaerl@email.unc.edu
A. G. Hounshell
Department of Biological Sciences, Virginia Tech,
Blacksburg, VA 24061, USA
J. C. Rudolph C. L. Osburn
Department of Marine, Earth and Atmospheric Sciences,
North Carolina State University, Raleigh,
NC 27607, USA
J. Bales
Consortium of Universities for the Advancement of
Hydrologic Science, Cambridge, MA 02140, USA
L. W. Harding Jr.
Department of Atmospheric and Oceanic Sciences,
University of California, Los Angeles, Los Angeles,
CA 90095, USA
123
Biogeochemistry (2020) 150:197–216
https://doi.org/10.1007/s10533-020-00693-4(0123456789().,-volV)(0123456789().,-volV)
role in C, N and P loadings. We appear to have entered
a new climatic regime characterized by more frequent
extreme precipitation events, with major ramifications
for hydrology, cycling of C, N and P, water quality and
habitat conditions in estuarine and coastal waters.
Keywords Tropical cyclones Flooding Organic
carbon Nutrient cycling Phytoplankton Estuarine
Coastal North carolina
Introduction
Since the mid-1990s, coastal North Carolina, USA has
been struck by 36 tropical cyclones (TCs), indicative
of a recent increase in such events (Paerl et al.
2018,2019) (Fig. 1and Table 1). This increase
appears to reflect regional and global patterns, as the
frequency and magnitude of events have increased
over several decades (Webster et al. 2005; Holland
and Webster 2007; Seneviratne et al. 2012; Wuebbles
et al. 2014; NOAA National Hurricane Center-Fig. 2).
North Carolina’s low-lying, readily flooded coastal
region is on the ‘‘front doorstep’’ of these increases
with concomitant biogeochemical and trophic impacts
(Frankson et al. 2019). The number of TCs making
landfall in North Carolina is highly variable from year
to year, while TC intensity and rainfall rates are
projected to increase in a warming climate (Konrad
and Perry 2010). This low-lying region is also
impacted by sea level rise (Frankson et al. 2019),
which combines with frequent TCs to make coastal
North Carolina and the neighboring mid-Atlantic
region highly vulnerable to flooding.
In the last two decades (1999–2019), coastal North
Carolina experienced three major floods from TCs that
were considered 50-year events (i.e., an annual
probability of 2%): Floyd (1999), Matthew (2016),
and Florence (2018) (Paerl et al. 2019). An examina-
tion of major storm events impacting the North
Carolina coastline since 1898 indicated that 6 of 7 of
the highest rainfall events occurred during the last two
decades (Paerl et al. 2019), a trend in keeping with
increased precipitation associated with TCs globally
(Allan and Soden 2008; Asadieh and Krakauer 2015;
Lehmann et al. 2015). These storms led to unprece-
dented flood damage, accompanied by large inorganic
and organic C, N and P pulses in freshwater discharge
impacting estuarine and coastal waters (Bianchi et al.
Fig. 1 Tropical cyclone dates and tracks as they impacted coastal North Carolina, USA, since the recent period of elevated storm
activity which commenced in the mid-1990s. Updated from Paerl et al. (2018)
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198 Biogeochemistry (2020) 150:197–216
Table 1 Tropical cyclones impacting North Carolina from 1996–2019
Tropical cyclone Date Storm characteristics
(Paerl et al. 2018)
Precipitation
amount (mm)
Impact on
North Carolina
Tropical Storm Arthur 20 Jun 1996 Dry and windy 100 Moderate
Hurricane Bertha 12 Jul 1996 Dry and windy 80 Moderate
Hurricane Fran 6 Sep 1996 Wet and windy 406 Major
Tropical Storm Josephine 8 Oct 1996 Wet and windy 100 Moderate
Hurricane Danny 24 Jul 1997 Dry and calm 300 Minimal
Hurricane Bonnie 27 Aug 1998 Dry and windy 371 Major
Hurricane Earl 4 Sep 1998 Dry and windy 102 Moderate
Hurricane Dennis 4 Sep 1999 Wet and windy 506 Major
Hurricane Floyd 16 Sep 1999 Wet and windy 484 Major
Hurricane Irene 18 Oct 1999 Wet and windy 255 Major
Hurricane Gordon 19 Sep 2000 Wet and windy 145 Major
Tropical Storm Helene 23 Sep 2000 Dry and windy 211 Moderate
Tropical Storm Allison 13 Jun 2001 Dry and calm 406 Moderate
Hurricane Gustav 10 Sep 2002 Dry and windy 125 Minimal
Hurricane Isabel 18 Sep 2003 Dry and windy 115 Moderate
Hurricane Alex 3 Aug 2004 Dry and calm 127 Minimal
Tropical Storm Bonnie 13 Aug 2004 Dry and calm 77 Minimal
Hurricane Charley 14 Aug 2004 Wet and calm 150 Minimal
Hurricane Gaston 30 Aug 2004 Dry and calm 155 Minimal
Hurricane Ophelia 14 Sep 2005 Dry and windy 254 Major
Hurricane Ernesto 1 Aug 2006 Wet and windy 371 Major
Tropical Storm Barry 3 Jun 2007 Dry and windy 95 Minimal
Tropical Storm Gabrielle 9 Sep 2007 Dry and calm 200 Minimal
Tropical Storm Cristobal 20 Jul 2008 Dry and calm 87 Minimal
Hurricane Hanna 29 Sep 2008 Dry and calm 97 Minimal
Hurricane Earl 3 Sep 2010 Dry and windy 115 Moderate
Hurricane Nicole 29 Sep 2010 Wet and calm 573 Major
Hurricane Irene 27 Aug 2011 Wet and windy 360 Major
Tropical Storm Beryl 30 May 2012 Dry and windy 179 Minimal
Tropical Storm Andrea 7 Jun 2013 Dry and windy 188 Minimal
Hurricane Arthur 4 Jul 2014 Dry and windy 107 Moderate
Tropical Storm Ana 7 May 2015 Wet and calm 175 Minimal
Hurricane Joaquin 3 Oct 2015 Wet and windy 200 Major
Hurricane Hermine 2 Sep 2016 Dry and windy 200 Minimal
Hurricane Matthew 8 Oct 2016 Wet and windy 610 Major
Hurricane Florence 14 Sep 2018 Wet and Windy 913 Major
Hurricane Michael 11 Oct 2018 Wet and calm 330 Moderate
Hurricane Dorian 5 Sep 2019 Wet and Calm 250 Moderate
The tropical cyclones are listed with the date of impact in North Carolina, the characteristics of the storm, rainfall amounts in North
Carolina, and the impact on North Carolina
123
Biogeochemistry (2020) 150:197–216 199
2013; Paerl et al. 2006; Osburn et al. 2019a,b)
(Fig. 3). Increasingly frequent extreme events in
recent years appear to be altering hydrologic and
biogeochemical dynamics of estuarine and coastal
waters, leading us to pose the question: Are we
witnessing increasing occurrences of ‘‘hot moments’’
that ‘‘occur when episodic hydrological flow-paths
reactivate and/or mobilize accumulated reactants?’’
(McClain et al. 2003).
In this contribution, we draw on a long-term water
quality monitoring program (since 1994) to charac-
terize and quantify C, N and P inputs to the nutrient-
sensitive receiving waters of the Neuse River Estuary
(NRE), a major tributary of the 2nd largest estuarine
complex in the United States, and the Albemarle-
Pamlico Sound (APS), the US’s largest lagoonal
ecosystem and a major fisheries nursery. We also
examined hydrologic and nutrient impacts on the base
of the food web in the NRE where phytoplankton
account for [80% of primary production (Paerl et al.
1998). These events also promote hypoxia and anoxia,
which are commonly observed in periodically verti-
cally-stratified sub-estuaries of the APS (Mallin and
Corbett 2006). These impacts have been detailed for
the NRE in previous publications (Paerl et al.
2006,2018).
The objectives of this study were to quantify event-
scale responses to TCs compared to seasonal- to
interannual patterns of C, N and P loadings as baseline
conditions. We addressed two questions: (1) How does
the recent increase of precipitation and freshwater
discharge associated with TCs drive fundamental
changes of C, N and P cycling in the NRE-APS
continuum and adjacent coastal waters, and (2) What
are the overall biogeochemical and ecological impacts
of increased loadings?
Methods and materials
Study site: the Neuse River Estuary-Pamlico
Sound estuarine continuum
North Carolina’s Neuse River Estuary (NRE) is a
lagoonal, micro-tidal ecosystem, representative of *
20 global estuarine and coastal waters (Kennish and
Paerl 2010). It is the largest sub-estuary of the
Albemarle-Pamlico Sound (APS) system. The APS
drains approximately 50% of North Carolina’s and
20% of Virginia’s coastal plain regions via 5 major
rivers (Bales et al. 2000; Bales 2003) (Fig. 4). The
NRE receives inputs from a 14,600 km
2
watershed that
has experienced urbanization of its headwaters and has
intensive, rapidly expanding row-crop agriculture and
swine/poultry operations toward the coast (Stow et al.
2001; Paerl et al. 2019). As for many estuarine and
coastal waters, primary production and algal bloom
formation in the NRE are largely N-limited for much
of the year (Paerl et al. 1995,2007; Rudek et al. 1991).
High water column primary production ([300 g C
m
-2
y
-1
) results from excessive N loading and
efficient recycling, especially in the summer (Chris-
tian et al. 1991; Boyer et al. 1993,1994). Vertical
density stratification in the main channel also pro-
motes extensive bottom-water hypoxia (\2.0 mg O
2
L
-1
) in summer-fall (Buzzelli et al. 2002).
The full sequence of biotic responses to nutrient and
hydrologic perturbations in estuarine and coastal
waters dominated by freshwater discharge may not
fully develop or can be masked by rapid flushing
(Hopkinson and Vallino 1995). However, the APS is
poorly flushed due its large estuarine volume relative
to river inflows, the microtidal regime of the NC
coastline, and weak tidal exchange through three
shallow inlets along the Outer Banks. Residence times
generally range from one to three months for the NRE
and up to a year for Pamlico Sound (Pietrafesa et al.,
1996; Paerl et al. 2009; Peierls et al. 2012) and are
strongly impacted by changes in freshwater input.
Changes in residence time due to floods or droughts
are accompanied by changes in salinity and nutrient
concentrations that drive significant changes in the
Fig. 2 History of the tropical storms, hurricanes, and major
hurricanes (Category 3?) in the North Atlantic Ocean, derived
from the analysis of the National Hurricane Center, Miami, FL,
USA
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200 Biogeochemistry (2020) 150:197–216
phytoplankton community (Hall et al. 2008; Paerl
et al. 2018). The NRE has been the site of numerous
harmful algal bloom events, ranging from toxic
cyanobacterial blooms upstream (Paerl 1983; Fulton
and Paerl 1988; Lung and Paerl 1988) to dinoflagellate
and raphidophyte blooms downstream (Pinckney et al.
1997,1998; Valdes-Weaver et al. 2006; Paerl et al.
2007,2010,2013; Hall et al. 2008).
The Neuse River Estuary Modeling and Monitoring
Program (ModMon) (https://paerllab.web.unc.edu/
projects/modmon/) has produced a database of
observational, experimental research, and modeling
results on C dynamics, nutrient-productivity relation-
ships, and algal blooms (Luettich et al. 2000; Bowen
and Hieronymus 2000; Paerl et al. 2009; Peierls et al.
2012) for the Neuse River Estuary since 1994 and for
western Pamlico Sound (PS) since 2000. Other
observations are provided by the North Carolina
Department of Environmental Quality (NC-DEQ),
which conducts monthly water-quality monitoring
(including phytoplankton taxonomic composition) in
the NRE. Together, these activities comprise more
than 25 years of data with high spatio-temporal reso-
lution (c.f. Paerl et al. 2018).
Fig. 3 US Geological Survey’s Landsat 8 satellite showing
colored dissolved organic matter (CDOM) in floodwater
discharged to North Carolina estuarine and coastal ecosystems
following Hurricane Florence in September, 2018. Upper frame
shows a true color image, while lower frame has been processed
to emphasize CDOM. Photo courtesy of Landsat Data Webpage
(https://www.usgs.gov/land-resources/nli/landsat)
123
Biogeochemistry (2020) 150:197–216 201
ModMon sampling consisted of bi-weekly visits to
11 stations along the main axis of the NRE (Fig. 3),
including vertical profiles with collection of near-
surface and near-bottom water (Paerl 2005; Paerl et al.
2006). In western PS, samples were collected monthly
at nine stations (Fig. 4). At each station, profiles of
temperature, salinity, and dissolved oxygen were
made at 0.5 m depth intervals using a YSI 6600
multi-parameter water quality sonde (Yellow Springs
Inc, Yellow Springs, OH). Dissolved oxygen was
calibrated with water-saturated air according to the
YSI User’s Manual with accuracy of 2% or 0.2 mg
L
-1
. Discrete samples for nutrient concentrations,
phytoplankton biomass and community composition,
and inorganic and organic C species were collected
from near-surface (0.2 m depth) and bottom waters
(0.5 m above bottom) using a non-destructive dia-
phragm pump, dispensed into 4 L polyethylene
bottles. Samples were maintained in the dark at
in situ temperature in a large insulated container and
returned to the laboratory at the UNC-CH Institute of
Marine Sciences, Morehead City (IMS) within 4 h of
collection for processing.
Freshwater discharge, flushing time, C and nutrient
loading
Average, daily discharge from the Neuse River was
measured by the US Geological Survey at Fort
Barnwell (USGS 02091814) (Fig. 4). Daily material
loadings of C, N and P were calculated using
Weighted Regressions on Time Discharge and Season
(WRTDS) (Hirsch and DeCicco 2015; Hirsch et al.
2010) using daily average river flow and material
concentrations at the head of the estuary by ModMon
(or NC DEQ for total N and total P) (Fig. 4). Half
window widths of the tricube weight function were set
to default values: 6 months for seasonality, 7 years for
time, and 2 natural log units for discharge (Hirsch and
DeCicco 2015). Changes in flushing time of the Neuse
River Estuary that resulted from changing river flow
conditions were calculated based on freshwater dis-
charge at Fort Barnwell and salinity depth profiles
measured at ModMon stations 0 to 180 using the date-
specific freshwater replacement method (Alber and
Sheldon 1999). See Peierls et al. (2012) for details on
how the method was implemented.
Fig. 4 Maps showing (left) the location of the Neuse River
estuary, its watershed and the downstream Pamlico Sound. The
right hand side shows the Neuse River and its Estuary, including
the upstream Fort Barnwell USGS gaging station and the water
quality sampling stations of the Neuse River Modeling and
Monitoring Program, ModMon. The locations of the EPA Clean
Air Status and Trends Network (CASTNET) air monitoring site
BFT142 and National Data Buoy Center station CLKN7 at Cape
Lookout are also shown
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202 Biogeochemistry (2020) 150:197–216
Carbon and nutrient analyses
C and N measured at ModMon stations included: total
dissolved nitrogen (TDN), nitrate plus nitrite (NO
3
--
?NO
2
-
), ammonium (NH
4
?
), soluble reactive
phosphate (SRP), dissolved organic carbon (DOC),
dissolved inorganic carbon (DIC), particulate organic
carbon (POC), and particulate nitrogen (PN). Dis-
solved inorganic nitrogen (DIN) was NO
3
-
?NO
2
--
?NH
4
?
. Dissolved organic nitrogen (DON) was
computed by difference as TDN—DIN. Nutrient
analyses used 100 mL aliquots filtered through pre-
combusted (4 h at 525 °C) 25 mm diameter Whatman
GF/F filters into acid-washed and sample-rinsed
150 mL polyethylene bottles that were subsequently
frozen at -20 °C. Filtrates were analyzed for dis-
solved N forms and SRP with a Lachat/Zellweger
Analytics QuickChem 8000 flow injection autoana-
lyzer using standard protocols (Lachat method num-
bers 31-107-04-1-C, 31-107-06-1-B, and 31-115-01-
3-C, respectively) (Peierls et al. 2012). Particulate
organic carbon and nitrogen were measured on seston
collected on pre-combusted GF/F filters, analyzed by
high-temperature combustion using a Costech ECS
4010 analyzer (Peierls and Paerl 2010). DIC and DOC
were measured on a Shimadzu Total Organic Carbon
Analyzer (TOC-5000A) (Crosswell et al. 2014).
Phytoplankton biomass
Chlorophyll a(Chl-a), an indicator of phytoplankton
biomass, was measured on near-surface and near-
bottom samples by filtering 50 mL of NRE water onto
GF/F filters. Filters were frozen at -20 °C and
subsequently extracted using a tissue grinder in 90%
acetone (EPA method 445.0, Arar et al. 1997). Chl-
ain extracts were measured using the non-acidifica-
tion method of Welschmeyer (1994) on a Turner
Designs Trilogy fluorometer calibrated with pure Chl-
a(Turner Designs, Sunnyvale, CA).
Results and discussion
Loads of dissolved organic carbon (DOC), total N
(TN) and total P (TP) were driven by freshwater
discharge at the sampling station 0 furthest upstream
(Figs. 5,6,7,8). Seasonal patterns were evident as
periods of elevated discharge and nutrient loads: (1) a
winter-spring period with elevated rainfall and runoff;
(2) a summer-fall period with maxima in years
experiencing TCs, contrasted with years without
summer-fall TCs (e.g., 1997) when elevated nutrient
and DOC loadings were not observed.
Some TCs grazed the coastline and had little effect
on loads (e.g., Gordon in 2000, Charley in 2004, Ana
in 2015), while storms that made landfall significantly
affected loads, e.g. Fran (1996), Floyd (1999),
Matthew (2016), Florence (2018). These differences
can be traced to three factors: (1) the storm track,
especially after landfall; (2) forward speed of the
storm; (3) amount of rainfall associated with the
storm; and (4) storm surges. Storms that remained
offshore or grazed the coastline (e.g. Gordon, Charley)
generally delivered little rainfall, with highest winds
and rainfall typically occurring to the right of the
storm and remaining offshore. Forward speed of TCs
strongly affected rainfall, especially once landfall
occurred. This was the case with Fran, Floyd,
Matthew, and Florence (Figs. 5–8); slow-moving,
high-rainfall events that appear to be increasing in
frequency (Paerl et al. 2019).
Freshwater discharge associated with TCs strongly
affected hydrologic characteristics of the NRE,
including water residence times. On average, the
flushing time of the NRE is about 2 months (Fig. 9),
and commonly exceeds 4 months during protracted
dry periods (e.g. 2007). Increased river discharge from
TCs with high precipitation, however, rapidly
increases the flushing rate of the estuary. Following
extreme floods associated with Matthew and Florence,
residence time of the NRE was only about 10 days,
and for Floyd less than 5 days.
Impacts of discharge from a high-volume flushing
event associated with Florence (Sept. 2018) are shown
as disruptions of vertical profiles of physical–chemical
properties (salinity, temperature, dissolved oxygen)
that also flushed phytoplankton biomass (as Chl-a)
from the system (Fig. 10). In contrast, events with
lower discharge (e.g. TC Ernesto in 2006) led to
elevated levels of DOC, TN and TP, with strong
vertical stratification and significant horizontal gradi-
ents of physical–chemical properties (Fig. 11). These
conditions of increased nutrient delivery favored algal
growth and formation of blooms as freshwater
discharge was not high enough to flush phytoplankton
from the estuary. Furthermore, freshwater discharge
was not sufficiently high to disrupt vertical density
123
Biogeochemistry (2020) 150:197–216 203
stratification, and these conditions supported a bloom
of the toxic dinoflagellate, Karlodinium veneficum
(Hall et al. 2008) (Fig. 11).
In the case of a tropical depression in July 2002 that
delivered substantial rainfall to coastal North Car-
olina, high discharge to the NRE and the nearby
Pamlico River flushed phytoplankton from the system
into the PS (Paerl et al. 2007). Nutrient-rich waters
then supported algal growth and blooms in the PS, a
much larger system than adjacent sub-estuaries with a
substantially longer residence time of up to one year
(Pietrafesa et al. 1996).
Significant differences in annual loadings of DOC
and nutrients occurred in years with and without TCs.
In years without TCs (1999, 2004, 2010), typical
seasonal patterns of DOC, TN and TP loadings were
observed (Figs. 5,6,7,8). In years with TCs
accompanied by moderate rainfall, up to half the
annual loadings of DOC and nutrients were associated
with storm events. In years with TCs accompanied by
very high rainfall, more than half the annual loadings
were associated with storm events. Given the sampling
protocol, we were not able to quantify additional
contributions from storm surges.
Summarizing, TCs with high rainfall have the
potential to double annual loadings of DOC and
nutrients to the NRE and PS. For TC Floyd (1999),
approximately 80% of annual N and 60% of the annual
P loads were attributed to surface runoff and subsur-
face groundwater (Paerl et al. 2001,2006). With
regard to organic matter, approximately 80% of annual
allochthonous POC and an equivalent percentage of
annual DOC inputs were attributable to floodwaters
from Floyd (Paerl et al., 2001,2006). For the Pamlico
Sound alone, this single event provided 60% of the
annual load (Bales 2003; Paerl et al. 2006).
Biogeochemical and ecological impacts
Hurricane Floyd struck eastern North Carolina in Sept.
1999, overwhelming the NRE with extremely high
freshwater discharge, and essentially turning it into a
‘‘pipeline’’ that emptied into the APS (Paerl et al.
2001,2006). This event led to very large discharge of
Fig. 5 Daily Neuse River freshwater discharge at the Fort Barnwell (USGS 02091814) gauging station upstream from the head of the
Neuse River Estuary, as shown in Fig. 4, for selected years
123
204 Biogeochemistry (2020) 150:197–216
freshwater overlying denser saline water, resulting in
strong, persistent vertical stratification, and extensive
hypoxia in bottom waters of the APS, with massive
fish and shellfish kills and an abrupt increase in fish
disease (Eby and Crowder 2002; Adams et al. 2003;
Paerl et al. 2006).
Prevailing hydrologic conditions in a given year
strongly affect overall impacts of TCs on estuarine and
coastal waters. For example, Hurricane Matthew in
2016 occurred during a ‘‘wet’’ year with above-
average discharge from winter through spring
(Fig. 12). Thus, impacts of Hurricane Matthew on
annual C, N and P loadings were masked by ‘‘high-
flow’’ conditions compared to TCs passing through the
NRE and its watershed during ‘‘dry’’ years (Paerl et al.
2018). Moreover, precipitation from Hurricane Mat-
thew was largely focused in the upper regions of the
watersheds, and the pulse of waters associated with
Matthew took 2–4 weeks to reach the NRE and
Pamlico Sound proper. Nutrient loadings following
Hurricane Matthew accounted for *50% of the
annual SRP loading and *20% of the annual DIN
load to the NRE and Pamlico Sound (Paerl et al. 2018).
In the first two weeks following Matthew, 25% of
annual C loading for 2016 was delivered to the estuary
(Osburn et al. 2019a,b). The majority (67%) of the C
load was DOC (Osburn et al. 2019a,b), similar to
observations for other TCs such as Floyd (Paerl et al.
2018).
Hurricane Florence stalled off the North Carolina
coast for several days from late September to early
October, 2018, and upon landfall delivered [70 cm
of rainfall to eastern NC and the NRE watershed (Paerl
et al. 2019). Similar to Floyd and Matthew, floodwa-
ters from Florence led to extensive freshening of the
NRE (Fig. 9). The sheer volume of freshwater
discharge and high flushing rates prevented salinity
stratification. Initial estimates of DOC loading suggest
that the concentration doubled near peak river dis-
charge, with Florence accounting for a 75% increase
of the annual DOC load to the NRE (cf. Fig. 8).
The combined effects of Hurricanes Floyd, Mat-
thew, and Florence on hydrologic, C, N and P loadings
to the NRE and APS systems in the past 20 years were
Fig. 6 Daily total nitrogen (TN) load at the head of the Neuse River Estuary
123
Biogeochemistry (2020) 150:197–216 205
unmatched in recorded history of hurricanes and TCs
for coastal North Carolina (Paerl et al. 2019). While
long-term C, N and P cycling dynamics of the legacy
loads delivered by these storms are still evolving,
short-term biogeochemical and trophic impacts are
evident. A notable example is expansion of the
hypoxic zone in the estuarine continuum following
Floyd that compressed finfish and shellfish habitats
(Eby and Crowder 2002; Eggleston et al. 2010).
Enhanced nutrient loads associated with TCs
(Figs. 5–8) stimulated phytoplankton production, but
effects varied in time and space, depending on
estuarine flushing rates and the magnitude of high-
nutrient freshwater discharge. For example, in condi-
tions of moderate discharge and nutrient enrichment
but freshwater discharge insufficiently high to exceed
phytoplankton growth rates and flush algae out of the
system, stimulation of phytoplankton biomass as Chl
awas evident in the NRE shortly after storm passage.
Examples of this response included TCs Isabel (2003),
Charley (2004), Ernesto (2006) and Joaquin (2015)
(Fig. 13). Conversely, a delayed phytoplankton
response to TCs with high rainfall and discharge,
was exemplified by Floyd (1999), Matthew (2016) and
Florence (2016) that essentially turned the NRE into a
‘‘pipeline’’ where phytoplankton growth was not
stimulated until months after the event (Fig. 13).
Under these conditions, a large proportion of the short-
term (days to weeks) growth response of phytoplank-
ton occurred in the larger, longer residence time
downstream Pamlico Sound (Paerl et al. 2007). The
NRE response can lag by a matter of weeks, depending
on how quickly high river flow recedes. Examples of
this scenario include Floyd (1999–2000) and Florence
(2018) (Fig. 13).
Some TCs such as Ernesto (2006) generated large
pulse loadings of nutrients in freshwater discharge,
producing optimal conditions for development of
Fig. 7 Daily total phosphorus (TP) load at the head of the Neuse River Estuary
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206 Biogeochemistry (2020) 150:197–216
harmful algal blooms (HABs) of the toxic dinoflagel-
late, Karlodinium veneficum. These HABs occurred in
the mid-estuarine region of the microtidal NRE where
nutrient-rich freshwater discharge and moderate
flushing provided long enough water residence time
for blooms to proliferate (Hall et al. 2008) (Fig. 11).
Large phytoplankton blooms also occurred in the PS
with its longer residence time in response to nutrient
loadings in freshwater discharge following passage of
Hurricane Floyd (1999–2000) (Tester et al. 2003).
Freshening of coastal ecosystems in North Carolina
associated with an increased frequency of TCs and
associated precipitation affects salinity gradients in
the NRE-PS continuum, with periods of depressed
salinity extending well downstream (Paerl et al. 2006;
2018). These conditions favor low-salinity phyto-
plankton groups, such as chlorophytes and cyanobac-
teria (Pinckney et al. 1998; Hall et al. 2013; Paerl et al.
2013,2018). Other estuarine and coastal waters
experiencing increased discharge and flooding asso-
ciated with a higher frequency of major storm and
rainfall events show evidence of a similar
Fig. 8 Daily dissolved organic carbon (DOC) load based on concentrations measured at the head of the Neuse River Estuary
Fig. 9 Time series of flushing time of the Neuse River Estuary
in relation to the timing of ‘‘wet’’ tropical cyclones that resulted
in significant increases in river discharge. See Paerl et al. (2018)
for details on the objective definition of ‘‘wet’’ storms
123
Biogeochemistry (2020) 150:197–216 207
phytoplankton bloom response. For example in the
Mississippi Delta, northern Gulf of Mexico, increased
rainfall and flood events in the Mississippi watershed
produced large plumes of nutrient-rich freshwater that
extended into coastal bayous and bays (Bargu et al.
2019). These conditions promoted the downstream
expansion of harmful (toxic) cyanobacterial genera,
including Dolichospermum and Microcystis (Rieken-
berg et al. 2015; Bargu et al. 2019). In the APS, a
parallel scenario is observed in the brackish Albemarle
Sound that drains several rivers in eastern North
Carolina and southeastern Virginia, including the
Chowan and Roanoke. The Chowan River has a
history of harmful cyanobacterial blooms, or Cyano-
HABs (Dolichospermum and Microcystis) and is
currently experiencing a bloom resurgence (https://
www.albemarlercd.org/fighting-algal-blooms.html).
These systems have also experienced increased storm-
related freshwater and C, N and P loadings that create
periods of fresher and more nutrient rich conditions
that favors CyanoHAB expansion into oligohaline
Albemarle Sound where Dolichospermum,Microcys-
tis and Cylindrospermopsis populations have become
widespread and common (Calandrino and Paerl 2011;
NC DEQ 2019).
Increased DOC and DON loadings coincide with
increases of freshwater discharge and nutrient load-
ings associated with TCs. Bioassays conducted in-situ
on the New River Estuary, North Carolina showed that
organic matter in freshwater discharge stimulated
phytoplankton biomass and altered taxonomic com-
position, stimulating several dinoflagellate and
cyanobacterial taxa (Altman and Paerl 2012). These
findings suggest specific components of the DOM (i.e.,
DON) pool play a role in phytoplankton dynamics.
Bacterial production may also be stimulated by DOM
in freshwater discharge (allochthonous) or DOM
produced by phytoplankton (autochthonous) (Peierls
and Paerl 2010; Peierls and Paerl 2011).
Fig. 10 Physical–chemical impacts of freshwater discharge
resulting from Hurricane Florence (September, 2018) on the
Neuse River Estuary, NC. Shown are water column profiles
(dashed vertical lines show location of ModMon station profiles)
of salinity, temperature, dissolved oxygen, pH, chlorophyll a(as
an indicator of algal biomass) and turbidity, before (left hand
side) and after (right hand side) the storm made landfall
123
208 Biogeochemistry (2020) 150:197–216
Implications for carbon cycling
As observed for nutrient loadings, the increasing
intensity and frequency of TCs also affect C loading
and cycling in estuarine and coastal waters. Freshwa-
ter loads of nutrients and organic matter provide the
fuel for microbial consumption of oxygen, often
leading to extensive hypoxia and anoxia throughout
the APS (Buzzelli et al 2002; Paerl et al.
1998,2001,2006,2018) (Fig. 14). Furthermore,
breakdown and cycling of allochthonous organic
matter mediated by heterotrophic microbes (Peierls
and Paerl 2010) modulates nutrient availability, pri-
mary production of organic matter, and ultimately,
trophic state. The importance of ecosystem-scale DOC
loadings attributable to TCs are exemplified by
Hurricane Matthew (2016) which accounted for up
to 25% of the annual DOC load to the NRE and
Pamlico Sound (Osburn et al. 2019a,b), representing a
disproportionate impact on the annual DOC loading to
the estuarine system in 2016.
The source of this additional DOC loading was
largely a result of the flooding of freshwater wetlands
at the head of the estuary, shifting the quality of DOM
imported into the estuary from upstream, terrestrial-
sources to wetlands-derived sources following the
storm event (Rudolph et al. 2020; Osburn et al.
2019a,b; Table 2). Not only did this result in enhanced
DOC loading from the Neuse River to the NRE, but it
also enhanced DOC loading from the NRE to the
downstream Pamlico Sound, as the ‘‘pulse’’ of
wetland-derived DOC was ‘‘shunted’’ to the sound
(Rudolph et al. 2020; Osburn et al. 2019a,b; Houn-
shell et al. 2019; Table 2). In the weeks to months
following Hurricane Matthew, the wetland-derived
organic matter flushed into Pamlico Sound and was
oxidized to CO
2
thereby switching the system from a
CO
2
sink to a CO
2
source and changing the role of the
estuary in the processing of organic matter. (Osburn
et al. 2019a,b).
Changing quality and quantity of DOC flushed into
estuarine and coastal waters could have important
implications for the role of ecosystems, such as APS,
Fig. 11 A toxic dinoflagellate (Karlodinium veneficum) bloom associated with nutrient-enriched runoff from tropical storm Ernesto,
October, 2006, which set up strong vertical and horizontal salinity gradients in the NRE
123
Biogeochemistry (2020) 150:197–216 209
in the global C cycle (Bauer et al. 2013). With an
increase of extreme discharge events associated with
TCs, some estuaries, such as the Neuse River, may
switch from ‘processors’ of DOC to ‘pipelines’ with
direct export of terrestrial DOC to downstream sounds
and coastal waters, such as Pamlico Sound, which
themselves become important processors (Osburn
et al. 2019a,b; Hounshell et al. 2019). This could
result in estuaries and coastal waters experiencing
extended hypoxia and acidification (Wallace et al.
2014) as well as reorganizing coastal C cycles (Najjar
et al. 2018; Yan et al. 2020). Additional studies are
needed to assess the direct link between wetlands-
derived DOC flushed into estuarine and coastal waters
following TCs with observed CO
2
fluxes, but prelim-
inary results suggest these coastal ecosystems become
important locations for the transformation of this
wetland-derived DOC in the weeks and months
following storm events associated with extreme pre-
cipitation (Osburn et al. 2019a,b).
Fig. 12 River discharge, salinity and DOC for seven discrete discharge events in the Neuse River Estuary, starting with offshore storm
Joaquin September, 2015 and ending with Hurricane Matthew in September, 2016. From Hounshell et al. (2019)
123
210 Biogeochemistry (2020) 150:197–216
Additionally, following TCs with comparatively
less extreme precipitation, such as Hurricane Irene in
2014, Crosswell et al. (2014) observed a significant
release of CO
2
from the NRE and Pamlico Sound
following TC passage. Specifically, the authors
reported extensive DOM enrichment associated with
Hurricane Irene (2014) in the NRE and Pamlico
Sound, leading to a massive pulse of CO
2
-release to
the atmosphere, roughly equivalent to the annual
amount of CO
2
fixed by phytoplankton in these
systems (Crosswell et al. 2014). Results following
both Hurricane Matthew, a disproportionally high
precipitation event, as well as following Hurricane
Irene, demonstrate the importance of TCs on control-
ling riverine DOM loading and processing under a
range of TC characteristics.
DOM quality of exported DOC storm loads is also
likely an important factor in the response of estuaries
to TCs. For example, Other studies have reported
similar changes of DOM quantity and quality follow-
ing hurricanes. Letourneau and Medeiros (2019)
observed increases of DOC concentrations of terres-
trial quality following passage of Hurricane Matthew,
despite little change in freshwater discharge to Geor-
gia estuaries. The increasing terrestrial quality of
DOM associated with freshwater discharge was linked
to increased microbial degradation (Letourneau and
Medeiros 2019), pointing to potential linkages of TCs
to DOM quality, microbial degradation, and CO
2
production. Lu and Liu (2019) observed significant
enrichment of DOM in four Texas rivers draining into
the northern Gulf of Mexico following major storms.
The authors noted that DOM quality shifted from
protein-like and lipid-like compounds at base flow
conditions, to lignin, tannin and condensed aromatic
structure during high freshwater discharge. Similarly,
in the Newport River Estuary, near the APS, increases
in aromatic DOC were correlated to higher microbial
mineralization rates of aromatic C (Osburn et al.
2019b). Thus, terrestrial DOM flushed into estuarine
systems during high-flow conditions has been traced to
mobilization of organic matter from the watersheds
and wetlands due to flooding, consistent with findings
in the NRE and Pamlico Sound (Rudolph et al. 2020).
Results of previous studies and evidence presented
here show that increased TCs exert a disproportionate
impact on C, N and P loadings to estuarine and coastal
waters, with qualitative and quantitative effects on
primary producers and associated microheterotrophs
(Paerl et al. 2006,2018; Peierls et al. 2003,2011).
Fig. 13 Discharge (m
3
s
-1
) in relation to phytoplankton production (as Chl a) downstream in the Neuse River Estuary-Pamlico Sound
estuarine continuum
123
Biogeochemistry (2020) 150:197–216 211
Short- and long-term biogeochemical impacts, includ-
ing coastal C, N and P cycling, oxygen dynamics,
habitat alteration, and trophodynamics deserve careful
scrutiny as we have entered a ‘‘new normal’’ with
increased TCs and associated extremes of precipita-
tion and freshwater discharge (Seneviratne et al. 2012;
Wetz and Yoskowitz 2013; Asadieh and Krakauer
2015; Paerl et al. 2019).
Fig. 14 Conceptual figure, showing the interactive effects of
TC-related freshwater discharge, DOC and nutrient loading and
wind mixing on physical structure of the water column,
phytoplankton and associated microbial activities and biogeo-
chemical responses, including hypoxia and air–water CO
2
exchange in a lagoonal estuary like the Neuse River Estuary.
Illustrated are the before, during and after TC scenarios. During
moderate storms, the DOC and DON from the storms lead to a
down estuary bloom. During severe storms, the DOC and DON
from the storms is flushed out of the estuary into Pamlico Sound
123
212 Biogeochemistry (2020) 150:197–216
Acknowledgements We appreciate the assistance of J.
Braddy, A. Joyner, H. Walker, B. Abare, R. Sloup and all
students and technicians that participated in the field and
laboratory work supporting this publication. This research was
funded by NSF Projects DEB 1119704, DEB 1240851, OCE
0825466, OCE 0812913, OCE 1705972, OCE 1706009, and
CBET 0932632, North Carolina Department of Environmental
Quality (ModMon Program), Lower Neuse Basin Association,
North Carolina Sea Grant Program, and the University of North
Carolina Water Resources Research Institute.
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