Figure 7. Full map of coastal North Carolina depicting the
τFC values sized by the magnitude of their
βFC alongside HUC 12 land areas and their respective
percent change in developed land use. SGAs B (a), E (b), and G (c) are
used as focal areas to understand how management planning, increase in
developed land, and estuary type affect the variation in FC trends. This
map was created in R version 4.1.0 with ‘sf’ version 0.9 package.
Focal Area B
SGA B (Figure 7c) is associated with waters at the mouth of the major
river cutting through this region, the Cape Fear River, and lagoonal
estuaries. SGA B is a class 6 estuary (Engle et al. 2007) characterized
by large area, moderate volume, high freshwater flow, and moderate depth
and salinity. This area contains higher rainfall thresholds ranging from
2 to 4 inches indicating that coastal FC concentrations within
conditionally approved or approved portions of these growing areas do
not respond as intensely to rainfall as compared to other areas of the
coast with lower rainfall thresholds. Prior studies have reported poor
water quality in SGA B (Alford et al., 2016; NCDEQ, 2022), likely
correlated with a high increase in the developed land up-river. Based on
the changes in land use observed over the study period, watersheds
adjacent to SGA B were associated with increased urbanization along the
NC coast (Figure 4). SGA B is also characterized by consistently
negative βSal values indicating decreasing salinity
values for the samples that have been taken over the past 20 years
(Figure 6c).
FC trends support our prior understanding regarding declining water
quality in this region (Figure 7c), as demonstrated by high
τFC and βFC values across most of the
study sites. However, some spatial variation in FC trends were observed.
The more southern areas within this SGA, associated with the mouth of
the Cape Fear River, have higher τFC values than the
lagoonal areas located in the northern portion of the region, which are
not generally directly affected by the Cape Fear River. The southern
Cape Fear River can experience diverted flow through a man-made waterway
under some high tide conditions, resulting in river flows discharging
directly into the lagoonal estuaries. This tidal overflow effect might
explain the similar trends in βFC from the southern Cape
Fear River area to the southern lagoonal estuary area despite being very
separate geographies. While there are negative τFCvalues in the lagoonal areas, the βFC is higher in the
areas where τFC is positive, meaning that the FC
concentrations towards the mouth of the Cape Fear River are increasing
at a faster rate than the decreasing, lagoonal FC concentrations.
Focal Area ESGA E (Figure 7b) represents a trunk and tributary estuarine system
surrounded by moderate urban development in the past 20 years. SGA E
contains systems classified as class 2 estuaries (Engle et al., 2007)
indicating moderate area with low volume, moderate freshwater flow,
and high salinity. SGA E supports a large number of up-estuary
shellfish leases within the tributary systems. This area responds
strongly to rainfall events in terms of FC load, as demonstrated by
the low rainfall thresholds (i.e., 1 inch to 4 inches), with the
lowest rainfall thresholds located up-estuary. This watershed, similar
to SGA B, experienced a moderate increase in developed land use within
the past 20 years (Figure 4). However, the τFC values
in this region were generally negative with a few very positive
τFC values within the river systems, suggesting an
improvement in baseflow water quality. This area has also shown more
positive βSal values (Figure 6c), indicating an
increase in salinity measured for the samples taken over the past 20
years.
Focal Area G
SGA G (Figure 7c) contains the Pamlico and Pungo Rivers. SGA G is a
class 4 estuary (Engle et al., 2007), which is characterized by moderate
area, moderate depth, low volume, and high freshwater flow. This area is
associated with the back-barrier Albemarle-Pamlico Estuary to the east,
which is a class 9 estuary (Engle et al., 2007) indicating very large
area, deep bathymetry, high volume, and high salinity environment. This
area is also characterized by low increase in surrounding development
and relatively fewer shellfish leases, demonstrating that this system is
relatively unimpacted by human activity as compared to focal areas B and
E. Accordingly, rainfall thresholds within SGA G are all at the highest
limit of 4 inches, indicating FC concentrations in these waters are not
highly sensitive to precipitation and stormwater runoff. There was a
variety of negative and positive τFC values in this area
with a majority of the stations exhibiting negative τFCvalues. The βSal values in this area are also variable,
exhibiting a range of both positive and negative trends.