Effect of Watershed Rainfall and Runoff on Estuarine Processes

    The Lower Cape Fear River Program has collected data for over three years. These include a wet summer in 1995, a hurricane-influenced summer in 1996, and a relatively dry summer in 1997. Thus, the relationship between meteorological factors and estuarine water quality can begin to be teased out from the influences of acute incidents and normal background effects. In this chapter we examine the relationships among a number of variables to explore this concept. A previous paper published through the North Carolina Water Resources Research Institute (Mallin et al. 1997b) has allowed us to draw conclusions about the relationships among light, nutrients, and phytoplankton parameters, some of which are reviewed in this chapter.

3.1 Statistical Methods

    Correlation analyses between rain, river flow, and estuarine physical, chemical and biological response variables were conducted using the entire data set from July 1994 through December 1997. Effect of river flow was assessed by taking the average daily flow (CFS) for the seven-day period preceding sample collection measured at Lock and Dam #1, about 65 km upstream of the estuary. Effect of rain was measured by using total rainfall at Greensboro (approximately 300 km upstream of the estuary) for the 28 day period preceding sample collection. All parameters were tested for normal distribution using the Shapiro-Wilk test. Non-normally distributed parameters were log-transformed prior to statistical analysis. Correlation procedures were run using the Statistical Analysis System (SAS).

3.2 Hydrological Control of Turbidity and Nutrient Delivery

    Data in Table 3.1 are from Station M42, a mid-estuary site chosen for presentation because this station is below the anthropogenic impact zone from seasonal dredging but upstream of major ocean dilution (Fig. 1.1). Rainfall in the upper watershed was strongly correlated with flow rates in the river proper (Table 3.1). Estuarine salinity was inversely correlated with average river flow at Lock and Dam #1 for the week previous to estuarine sample collection (Table 3.1), as well as with total rainfall at Greensboro for the preceding month (Table 3.1). River flow was also significantly correlated with turbidity in the estuary, indicating long-distance transport of material from upper Coastal Plain and Piedmont regions (Table 3.1). Flow was also strongly correlated with increases in light attenuation in the estuary (Table 3.1). Both turbidity and light attenuation were inversely related to salinity as well. It is thus evident that watershed rainfall and river flow can be potentially important physical forcing mechanisms in this estuary by increasing light attenuation through delivery of turbidity particles downstream. Increased flow in blackwater tributaries may also cause increased light attenuation through inputs of darkly-stained riparian swampwater.
    River flow was significantly correlated with orthophosphate and silicate concentrations at M42 (Table 3.1). Farther downstream at M35 and M23 flow was also significantly correlated with total nitrogen concentrations (r = 0.459; p = 0.016 and r = 0.569; p = 0.002, respectively). Nitrate, ammonium and silicate concentrations showed significant inverse correlations with water temperature (Table 3.1), possibly indicating increased delivery to the estuary during winter, and/or less utilization of these nutrients by phytoplankton and riparian wetland plants during winter.
    The seasonal pattern of increased summer estuarine chlorophyll a concentrations alluded to in Chapter 2 was corroborated by a significant positive relationship between chlorophyll a and water temperature (Table 3.1). Chlorophyll a was positively correlated with salinity but inversely related to river flow and rainfall in the Piedmont (Table 3.1). Strong currents likely inhibit chlorophyll a increases while calmer water enhances phytoplankton productivity.
    The Cape Fear is the largest river in North Carolina with an open connection to the sea, allowing for generally high flushing and flow velocities. Periods of low flow increase water clarity, reduce nutrient loading, and increase the likelihood of nutrient limitation rather than light limitation. As mentioned, watershed rainfall and increased river flow suppresses chlorophyll a accumulation in this system. This is in contrast to other large estuaries that are less heavily flushed. In the Neuse Estuary, watershed rainfall and river flow are significantly correlated with increases in nitrogen loading and phytoplankton productivity (Mallin et al. 1993). In tributaries of the Chesapeake Bay, river flow has been related to increases in nitrate delivery and chlorophyll a biomass (Malone et al. 1988; Jordan et al. 1991; Gallegos 1992). Additionally, in estuaries such as the Neuse, Pamlico, and Patuxent, winter dinoflagellate blooms occur as a response to winter nutrient pulses (Kuenzler et al. 1979; Sellner et al. 1991; Mallin et al. 1993; Mallin 1994), whereas winter blooms do not occufr in the Cape Fear Estuary.
    It is evident that the Cape Fear Estuary is strongly linked to the middle and upper watershed, including the distant Piedmont. Since this analysis has shown that rainfall and river flow are both significantly correlated with turbidity and nutrient concentrations in the estuary, non-point source runoff is thus an important source of turbidity, nutrients (and probably other pollutants) to the estuary.

3.3 Effect of Physical Factors on Estuarine Nutrient Limitation Patterns

    Turbidity is known to be an important factor influencing phytoplankton productivity in riverine estuaries (Cloern 1987). The Cape Fear originates well into the Piedmont, and carries a significant sediment/turbidity load downstream to estuarine waters (Chapter 2; Table 3.1). Additionally, winter dredging activities associated with the Port of Wilmington contribute variable amounts of turbidity directly to the estuary. High turbidity caused by estuarine dredging and upstream rainfall and runoff may reduce nutrient limitation of phytoplankton production in at least two ways. An obvious factor is the creation of physical conditions in which light, rather than nutrients, becomes limiting. Another likely turbidity effect is that dredging and runoff activities may increase delivery of nutrients to suspended phytoplankton cells. Clay particles adsorb phosphorus, organic nitrogen, ammonium and other materials, which are then carried into streams during runoff events (Froelich 1988; Balogh and Watson 1992; Burkholder 1992; NRCS 1995). Our data show significant correlations between turbidity and both total phosphorus and nitrogen (Table 3.1). Additionally, in the CFRE total phosphorus concentrations were highest at the turbidity maximum (M54), as has been found in other estuaries (Lebo and Sharp 1993). Turbidity-bound phosphorus may be desorbed in estuarine waters either because there is a concentration gradient in the less phosphate-rich mesohaline waters or because P is outcompeted for surface sites by other more abundant anions as salinity increases (Froelich 1988). This desorption would then help relieve P limitation in the water column. Ammonium is also bound to sediments and concentrations in the CFRE were likewise highest at the turbidity maximum (M54). Concentrations are also often significantly higher in CFRE bottom waters than surface waters (Mallin et al. 1996); thus, turbidity pulses and dredging activities may have the effect of delivering ammonium to the upper water column to relieve N limitation.
    Factors controlling phytoplankton production demonstrate two distinct patterns, spatial and temporal, in the Cape Fear Estuary (Mallin et al. 1997b; Table 3.2). Spatially, the estuary shows a strong longitudinal trend of increasing sensitivity to nutrient additions along with increasing salinity. Phytoplankton production at the oligohaline station NAV appears to be limited principally by light. There is no generally accepted bioassay procedure to confirm light limitation. Rather, light limitation is assumed from high ambient nutrient levels, lack of phytoplankton stimulation by nutrient addition bioassays, and/or by mathematical modeling procedures (Pennock and Sharpe 1994). Light is constrained at NAV due to the inputs from blackwater tributaries as well as particulate turbidity. The river at NAV is deep and well mixed, likely keeping phytoplankton cells under aphotic conditions for extended periods. Farther downstream, the mesohaline area near Channel Marker 54 is a transition zone between light and nutrient limitation. Meteorology plays an important role in the regulation of phytoplankton productivity in mid-estuary. Upstream rainfall and consequent watershed runoff increase loading of nutrients, turbidity, and water color, leading to stronger light attenuation and consequent light limitation. Dry, low flow conditions lead to decreased nutrients, turbidity and light attenuation, and increased phytoplankton biomass, with phytoplankton production becoming nutrient limited. The estuary near M23 was nutrient limited most of the year (Table 3.2). Light limitation was apparently not as important a factor at M23 compared with M54, although the degree of nitrogen stimulation in the bioassays was inversely related to light attenuation (i.e. clearer water meant stronger stimulation or limitation - Mallin et al. 1997b).
    In the Cape Fear Estuary there is a second (temporal) pattern concerning phytoplankton limiting factors, which is the seasonal switching of controls on phytoplankton production. Nitrogen limitation prevails in summer (Table 3.2), and in our bioassay experiments water temperature was significantly correlated with nitrogen stimulation (Mallin et al. 1997a). Coincidently, late summer is typically the period of highest phosphate concentrations in North Carolina estuaries (Rudek et al. 1991; Mallin 1994). Low DO conditions prevalent during that period can allow sediment-bound phosphorus to enter the water column, which drives the N/P ratio downward. Summer N/P ratios in the Cape Fear were near the Redfield ratio.
    In late fall and winter, particularly in mid estuary, experimental nutrient additions have little or no effect on phytoplankton growth and light apparently becomes the limiting factor (Table 3.2). In the upper to middle estuary, winter nutrient levels are high, light penetration is poor in the turbid, highly colored waters, and chlorophyll a levels are low. River flow, normally high in winter, is inversely correlated with chlorophyll a concentration in the estuary (Table 3.1). Dredging and periodic rainfall and runoff episodes add turbidity pulses to the estuary, driving the system to periodic light limitation. This is most prevalent in winter, with permitted channel dredging activity and increased flow to the lower estuary because of reduced upstream evapotranspiration in cold weather. Light attenuation by turbidity was determined to be the major limiting factor during winter in the Delaware estuary, subsequently switching to phosphorus limitation in late spring (Pennock and Sharp 1994). Light limitation is thus important on a temporal scale, particularly in the upper estuary and near the turbidity maximum.
    During spring in the Cape Fear there is significant phosphorus limitation, demonstrated by both bioassays and high inorganic N/P molar ratios (Mallin et al. 1997a; 1997b; Table 3.2). This may be a result of high winter-spring flow bringing inorganic nitrogen to the lower estuary (Table 3.1), and possibly the remineralization of organic N compounds formed during the previous year's biological activity. Agricultural runoff has been described as a cause of spring N+P colimitation in other North Carolina estuaries, while elevated summer primary productivity, increased P availability, and reduced runoff lead to strong summer N limitation (Rudek et al. 1991; Paerl et al. 1995).
    Research in other estuaries has also shown that nutrient loading, and nutrient ratios and limitation dynamics, may vary seasonally. In the Patuxent River, Maryland, nitrogen was the limiting nutrient during summer and phosphorus during spring (D'Elia et al. 1986). A similar pattern has been described for the Chesapeake Bay (Fisher et al. 1992; Malone et al. 1996), while in the Rhode River, a Chesapeake tributary, there was strong summer N limitation and weak spring P limitation (Gallegos and Jordan 1997). In that region the spring freshet, (with the high N/P ratio from agricultural runoff) contributes to spring P limitation, whereas minimum discharge, coupled with increased relative importance of wastewater effluent with its lower N/P ratio, contributes toward summer N limitation. Phosphorus availability varies seasonally in estuaries: In spring, phosphorus is rapidly taken up by estuarine biota (Lebo and Sharp 1993) but regenerated during summer months (Lebo and Sharp 1992).
    All North Carolina estuaries are N limited to various degrees (see Mallin 1994 and references within) but the Cape Fear is N limited mainly in summer and early fall. There appears to be a continuum in regards to limiting nutrients among the large riverine estuaries tested in North Carolina. Inorganic phosphorus concentrations in the Cape Fear Estuary are much lower than in either the Pamlico River Estuary or the Neuse River Estuary, while inorganic nitrogen concentrations are as high or higher (Stanley 1987; Christian et al. 1991; Rudek et al. 1991; Paerl et al. 1995; Mallin et al. 1996). The New River Estuary (with excess P from wastewater treatment plant inputs) and the Pamlico River Estuary (phosphate mining inputs) are mostly N limited (Keunzler et al. 1979; Mallin et al. 1997b). The Neuse River Estuary (wastewater treatment plant inputs and non-point agricultural runoff) is colimited by N+P in spring in the lower estuary but equally limited by either N or P in the oligohaline area near New Bern (Rudek et al. 1991; Paerl et al. 1995). The lower Cape Fear Estuary is seasonally divided by N and P limitation, with P limitation stronger and more persistent than in any of the North Carolina riverine estuaries previously studied. In this respect the Cape Fear is more similar to the Chesapeake Bay than its closer neighbors to the immediate north. The sensitivity to phosphorus in the Cape Fear system may prove to be especially important because of the vast amounts of phosphorus brought into the Cape Fear watershed in swine and poultry feed (Cahoon et al. 1998). Large amounts of this "new" phosphorus are then released in animal wastes into holding lagoons. This material may later enter neighboring water bodies through lagoon accidents or spray field runoff (Burkholder et al. 1997; Mallin et al. 1997c; 1998b).

Table 3.1. Significant (a = 0.05) correlation coefficients between physical / meteorological factors and estuarine response variables for Station M42 in the Cape Fear River Estuary.^a Pearson correlation coefficient (r) / probability (p).

^aTEMP = water temperature; SAL = salinity; FLOW = average river flow (CFS) at Lock and Dam #1 for 7-d period preceding estuarine sample collection; RAIN = total rainfall at Greensboro airport for 28-d period preceding sample collection; TURB = turbidity; Light = light attenuation coefficient; TN = total nitrogen; NIT = nitrate; AMM = ammonium; TP = total phosphorus; PHOS = orthophosphate; SIL = silicate; CHLA = chlorophyll a.

Table 3.2. Results of nutrient limitation bioassays showing nutrient treatments yielding chlorophyll a responses significantly (a = 0.05) greater than control. NAV experiments run July 1994 - June 1995; M23 experiments run July 1995 - June 1996 (from Mallin et al. 1997b).

^a experiment not run

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