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.