Seasonality and interannual variability of freshwater inflow to a large oligohaline estuary in the Northern Gulf of Mexico

Estuarine, Coastal and Shelf Science 68 (2006) 619e626 www.elsevier.com/locate/ecss

Seasonality and interannual variability of freshwater inflow to a large oligohaline estuary in the Northern Gulf of Mexico Y. Jun Xu a,*, Kangsheng Wu b a

School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA b South Dakota Water Resources Institute, Ag Engineering 211, Box 2120, Brookings, SD 57007, USA Received 28 February 2005; accepted 21 February 2006 Available online 19 May 2006

Abstract Quantity, timing, duration, and fluctuation of freshwater inflow are important factors affecting the development and health of aquatic and adjacent wetland ecosystems in coastal estuaries. This study assessed six decades of freshwater inflow from the Amite River, Tickfaw River, and Tangipahoa River watersheds to Lake Pontchartrain, a large oligohaline estuary in the Northern Gulf of Mexico, whose flood waters caused recent damage to the city of New Orleans in the aftermath of Hurricane Katrina. By utilizing the long-term (1940e2002) river discharge and climatic data from the three major tributary watersheds, monthly and annual freshwater inflows have been quantified and their spatial and temporal variations have been analyzed. On average, the three rivers discharged (
standard error) 0.27
0.04 km3 freshwater monthly and 3.29
0.15 km3 freshwater annually into the lake estuarine system, with the highest inflow from the Amite River (0.16
0.03 m3 mon1, and 1.91
0.09 km3 yr1) and the lowest inflow from the Tickfaw River (0.03
0.00 km3 mon1, and 0.34
0.02 km3 yr1). A distinct seasonality was evident with over 69% of the total annual inflow occurring during December and May (wet months) and with a low flow period from August to November (dry months). The monthly inflow during the wet months was positively correlated with the monthly precipitation (r2 ¼ 0.64), while the monthly inflow during the dry months was subject to evapotranspiration. Furthermore, the study found a 20-year low flow period from 1954e1973 (2.76
0.24 km3 yr1) and a 24-year high flow period from 1975e1998 (3.84
0.24 km3 yr1), coinciding with both the climate variation and population growth in the watersheds. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: freshwater; riverine; coastal watersheds; oligohaline estuary; Lake Pontchartrain; Gulf of Mexico; Louisiana

1. Introduction The influence of freshwater flowing into estuaries on biological processes and ecosystem development has long been recognized. The magnitude, timing, and duration of freshwater can affect fluctuations in estuarine chemical and physical properties including, among others, salinity, temperature, turbidity, and concentrations of nutrients, sediment, and dissolved oxygen. The fluctuations may have crucial impacts on estuarine species, habitats and productivity (e.g., Drinkwater and Frank, 1994; Loneragan and Bunn, 1999; Gillanders

* Corresponding author. E-mail address: [email protected] (Y.J. Xu). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.02.024

and Kingsford, 2002). To understand interactions between the environmental and biological variables of a specific estuary, long-term freshwater data are needed and the assessment on seasonal and interannual variations of long-term freshwater inflow to the estuary is especially valuable. Lake Pontchartrain, a 1619 square kilometer oligohaline estuary in southeastern Louisiana, has been subject to numerous anthropogenic impacts over the past half century including urban and agricultural runoff, shell dredging, saltwater intrusion, shoreline alteration, and industrial discharge (Howarth et al., 1991; Francis and Poirrier, 1999; Penland et al., 2002). However, the lake and the environmental issues were largely unknown until the recent Hurricane Katrina broke through levees and swamped New Orleans with the water of Lake Pontchartrain. The entire lake drainage basin is a 12,170

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square kilometer watershed encompassing 16 parishes in southeast Louisiana and four counties in Mississippi. Nearly 1.5 million people directly live around the lake that supports various species of fish, birds, mammals, and plants. As with many estuarine ecosystems in the world, water quality and freshwater inflow from the upstream watersheds have been among the most critical issues in sustainable freshwater resources and restoration efforts for the basin region (Sikora and Kjerfve, 1985; Calvert and Emmer, 1992; Mossa, 1995; Day et al., 2000; LDEQ, 2000; DeLaune et al., 2003). From a study on suspended solids in the upper Lake Pontchartrain Basin, Smith (1992) showed substantial variations in streamflow and water quality from several north shore streams and bayous. Low dissolved oxygen concentrations (<5 mg l1) were found during summer periods in the downstream stations of the Amite River (Ismail et al., 1998). With a five year water quality dataset (1986e1991), Smith (1994) reported a significant increase in instream concentrations of lead and total dissolved solids from the Amite River. Templeton (1998) made an effort to connect human activities, represented by population, total income, and per capita income, with streamflow in a subwatershed within the Amite River watershed. Due to multicollinearity of economic and demographic variables in his study, however, a statistically sound relationship between population variables and stream flow was not found. The Amite River watershed contributes the largest amount of freshwater to Lake Pontchartrain, and it is also the basin that experienced the greatest land use change in the region during the past four decades. Urbanization-induced land use changes and hydrologic alterations such as sand and gravel mining, urban runoff, sewage treatment plant discharges, and flood controls all have been attributed to water quality degradation in the region (Vernon et al., 1992; Mossa and McLean, 1997; LDEQ, 2000). In addition to water quality degradation and associated alterations in the terrestrial environment, freshwater inflow also has been linked to estuarine productivity in Lake Pontchartrain. In a recent study on relationships between fish habitats and environmental variables in Lake Pontchartrain, O’Connell et al. (2004) found that the fish assemblages collected by trawls in the lake have changed over the past 50 years. Changes in the fish assemblages from nearshore and pelagic habitats were closely related to environmental fluctuations. Annual yield of penaeid shrimp in the Gulf of Mexico was inversely related to the annual discharge of the Mississippi River, which may have been associated with the reduced salinity in the estuarine nursery areas with higher freshwater flow (Turner et al., 1992). Freshwater input has been considered to be the most important factor governing salinity distribution across Lake Pontchartrain (Sikora and Kjerfve, 1985), and it has, therefore, been attributed to controlling the biological processes and the health and stability of wetlands around this large oligohaline estuary (Turner et al., 1992; Thomson et al., 2001). Although a number of studies on stream discharge and freshwater inflow to Lake Pontchartrain have been published, there is no comprehensive investigation on the long-term trends of freshwater inflow from the major river watersheds

to the estuary. Researchers (e.g., Day et al., 2000; Turner et al., 2002; Shaffer et al., 2004) have recognized that restoring historic riverine freshwater inflows are among the few resources for restoring degraded wetlands in the Lake Pontchartrain drainage basin. However, there is generally a lack of knowledge as to how the freshwater inflows seasonally, interannually and interdecadally fluctuate, and how the variations are associated with hydrometeorological variables in the region. The recent flood from Lake Pontchartrain and environmental damage in the aftermath of Hurricane Katrina has especially reinforced the immediate need for a comprehensive analysis of freshwater inflow to the lake. Such hydrological information is critical for coastal resource planners and managers to conduct integrated assessment on the coastal watersheds and develop the best management practices (BMPs) for maintaining desirable environmental conditions within the basin. The aim of the present study was to provide a detailed examination of long-term freshwater inflow from three major tributary rivers to Lake Pontchartrain e the Amite, Tickfaw, and Tangipahoa Rivers. The specific objectives of the study were to: (1) characterize seasonal and spatial distribution of freshwater inflow from the upper Lake Pontchartrain Basin; and (2) determine six-decadal-long variations of the freshwater inflow. Such information can be crucial for the planning and management of freshwater resources in the cities and towns within the Lake Pontchartrain Basin. Also, the information on hydrologic regimes of these rivers can benefit many ongoing research and wetland restoration projects around the estuary. 2. Methods 2.1. Watershed characteristics The Amite, Tickfaw, and Tangipahoa Rivers are situated northwest of Lake Pontchartrain, which is an oligohaline estuary emptying into the Northern Gulf of Mexico (Fig. 1). The three rivers drain an area of 8728 km2, whereby the Amite River has the largest watershed (4822 km2) and the Tickfaw has the smallest watershed (1896 km2). Together they represent over 72% of the total drainage area of Lake Pontchartrain. Elevation in the basin area ranges from 0 m to 150 m above sea level. The long-term annual average air temperature is about 19  C, with the lowest monthly average of 12  C in January and the highest monthly average of 28  C in July. The long-term annual average precipitation is about 1600 mm, varying from 1108 mm to 2178 mm. The highest monthly average precipitation in the area occurs in July (159 mm), while the lowest is in October (86 mm). Although generally the monthly precipitation is evenly distributed, winter and spring are the seasons when regional flooding often occurs (Rohli and Grymes, 1995). Forest and agricultural lands are two dominant land use types in the region. About 58% of the total land area of the Amite River watershed is covered by forests, and the Tickfaw River and Tangipahoa River watersheds have a forest cover of

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621

Fig. 1. Geographical location of the Amite, Tickfaw, and Tangipahoa River watersheds (the numbers denote USGS stream gauge station identification).

66% and of 54%, respectively. Compared to the Tickfaw and Tangipahoa River watersheds, the Amite River watershed has the highest percentage of urban area. Dominant soil types in these watersheds are Tangi-Ruston-Smithdale, Toula-Tangi, and Maytt-Guyton (USDA ARS, 1991).

freshwater inflow and precipitation. Two-sample t-tests were used for the comparison of means of discharge between the watersheds. All statistical analyses were performed using the SAS software package (SAS Institute, 1996). 3. Results and discussion

2.2. Historical weather and discharge data sources and analyses

3.1. Seasonality of freshwater inflow

Data used in this study included daily precipitation and daily discharge data. Eleven weather stations in the study area were selected. Daily precipitation was obtained from the National Climatic Data Center (NCDC). Daily discharge data was collected from three U.S. Geological Survey (USGS) gauge stations near the mouth of each of the three rivers by way of the USGS National Water Information System website (NWISWeb). The discharge data covered the periods from 1938 to 2002 for the Amite River near Denham Spring (USGS gauge station 7378500), from 1940 to 2002 for the Tickfaw River at Holden (USGS gauge station 7376000), and from 1938 to 2002 for the Tangipahoa River at Robert (USGS gauge station 7375500) (Fig. 1). Long-term monthly and annual average precipitation was calculated from data collected at the eleven rain gauge stations from 1948 through 2000. Annual average precipitation in low flow period (1954e1973) and high flow period (1975e1998) were calculated individually in each of the three watersheds. The daily discharge was summed up to monthly and annual freshwater inflow from the three tributaries. Runoff in depth (mm) was calculated by dividing inflow by its drainage size for these three watersheds. A ratio of annual inflow from the three rivers to the precipitation amount within the watersheds was calculated to describe the watershed rainfall e runoff characteristics during the high and low flow period. Regression was employed to determine the relationship between

The Amite River showed a monthly average freshwater inflow (
standard error) of 0.16
0.03 km3, ranging from 0.07 to 0.27 km3; The Tickfaw River averaged a monthly inflow of 0.03
0.00 km3, ranging from 0.01 km3 to 0.05 km3; The Tangipahoa River had a monthly average of 0.09
0.01 km3, ranging from 0.04 km3 to 0.14 km3. On average, the monthly freshwater inflow from the three rivers to the lake totaled 0.27 km3. A significant seasonal variation in monthly freshwater inflow was found, with high flows in the winter and spring (wet months) and low flows in the late summer and fall (dry months). A total of monthly freshwater inflow in the six months from December through May accounted for 74%, 73%, and 69% of the total annuals in the Amite River, Tickfaw River, and Tangipahoa River (Table 1), respectively. There was spatial variation in high flow among the three watersheds. From the Amite River, the highest monthly freshwater inflow occurred in January. In the Tickfaw and Tangipahoa Rivers, however, the highest monthly freshwater inflow occurred in February and March, respectively. The seasonality of flow was mainly affected by the temporal distribution of precipitation and evapotranspiration (ET) within the river watersheds. In a spatial hydrologic modeling study of these watersheds, Wu and Xu, (in press) found a distinct seasonality in evapotranspiration with peak ET rates occurring in June and July (about 160 mm mon1) and lower ET

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622

Table 1 Monthly freshwater inflows and their standard errors from three coastal watersheds Freshwater inflow (km3)

January February March April May June July August September October November December

0.259 0.270 0.274 0.254 0.162 0.093 0.089 0.081 0.073 0.068 0.086 0.177

(
0.0262) (
0.0229) (
0.0230) (
0.0294) (
0.0217) (
0.0144) (
0.0082) (
0.0081) (
0.0087) (
0.0098) (
0.0095) (
0.0188)

Monthly average

0.157 (
0.0251)

Tickfaw

Tangipahoa

Total

0.046 0.050 0.050 0.045 0.029 0.017 0.015 0.016 0.015 0.012 0.017 0.032

0.125 0.138 0.140 0.130 0.089 0.059 0.058 0.054 0.052 0.044 0.056 0.097

0.429 0.458 0.464 0.429 0.281 0.169 0.162 0.149 0.140 0.124 0.158 0.306

(
0.0045) (
0.0045) (
0.0041) (
0.0051) (
0.0039) (
0.0021) (
0.0012) (
0.0015) (
0.0017) (
0.0014) (
0.0019) (
0.0034)

0.029 (
0.0044)

rates from December to February (40 mm mon1). During the wet months (from December through May), ET was low even with sufficient soil moisture, leading to higher monthly drainage. During dry months (from June through November), however, ET was higher due to higher air temperature, resulting in lower monthly drainage because much of the rainfall was evapotranspired. Therefore, the monthly freshwater inflow during the dry period did not correspond closely to the monthly precipitation, as compared to the relationship between the monthly flow and precipitation during the wet period (Figs. 2A,B). Our data showed that during the wet period, average monthly runoff could increase 90 mm (from 80 to 170 mm) with an increase of monthly precipitation from 200 mm to 300 mm, and that during the dry period, however, average monthly runoff could increase by only 30 mm (from 35 mm to 65 mm) for the same change of monthly precipitation. The results indicated a strong influence of evapotranspiration on freshwater supply during the summer months. Many studies have found that hydrologic regimes such as the magnitude, timing, duration, and variability of freshwater inflow impact the growth, survival, and population viability of estuarine fishes. A direct link, in particular, between summer flow and fisheries productivity has been observed by several researchers. For instance, in a study of the Logan River in southeast Queensland, Australia, Loneragan and Bunn (1999) found strong positive relationships between summer flow and commercial catches of prawns, crabs and flathead. Nislow et al. (2004) observed significant reductions in juvenile stream salmonid growth in association with low summer flow in the upper Connecticut River Basin. Though the mechanism of this linkage has not been well understood, researchers have pointed out the environmental changes associated with low summer flows including higher water temperature, lower dissolved oxygen content, and higher salinity level. In the Amite, Tickfaw, and Tangipahoa Rivers, lower dissolved oxygen concentrations were often found during the summer months (Ismail et al., 1998; Xu, 2003), which is apparently associated with the low flow and high temperature of that season. From these three rivers, the total flow during the dry months from

(
0.0106) (
0.0119) (
0.0106) (
0.0149) (
0.0091) (
0.0053) (
0.0042) (
0.0037) (
0.0046) (
0.0027) (
0.0056) (
0.0091)

0.087 (
0.0109)

(
0.0639) (
0.0642) (
0.0673) (
0.0604) (
0.0379) (
0.0243) (
0.0202) (
0.0186) (
0.0177) (
0.0165) (
0.0204) (
0.0410)

0.273 (
0.0404)

June to November between 1940 and 2002 ranged from 0.405 km3 in 1963 and 2.013 km3 in 1975, a nearly five-fold variation. It is not clear if and how this seasonal variation of flow may have contributed to the declined stream habitats, estuarine production, and degraded ecological functions in the Lake Pontchartrain drainage basin as reported by other 400

Freshwater inflow = 16.829e0.0079 P r2 = 0.64

A

350

Freshwater inflow (mm)

Amite

300 250 200 150 100 50 0

0

50

100

150

200

250

300

350

400

450

Monthly precipitation (mm) 400

B

350

Freshwater inflow (mm)

Month

Freshwater inflow = 10.681e0.0058 P r2 = 0.53

300 250 200 150 100 50 0

0

50

100

150

200

250

300

350

400

450

Monthly Precipitation (mm) Fig. 2. Relationships between monthly freshwater inflow and monthly precipitation during the wet months (DecembereMay, A) and the dry months (Junee November, B) in the studied watersheds.

Y.J. Xu, K. Wu / Estuarine, Coastal and Shelf Science 68 (2006) 619e626 0.6

researchers (Turner et al., 1992; Turner, 1997; DeLaune et al., 2003).

Amite Tickfaw Tangipahoa

Long-term annual freshwater inflows from the Amite, Tickfaw and Tangipahoa Rivers to Lake Pontchartrain averaged 1.91
0.09 km3 yr1, 0.34
0.02 km3 yr1, and 1.05
0.05 km3 yr1, respectively. In total, the three rivers discharged an average of 3.29
0.15 km3 freshwater annually to the lake estuary. A considerable large variation over the past 60 years was observed from all the watersheds: from 0.58 to 3.58 km3 yr1 for the Amite River, from 0.09 to 0.58 km3 yr1 for the Tickfaw River, and from 0.45 to 1.94 km3 yr1 for the Tangipahoa River. Comparably, the interannual variation was highest in the Tickfaw and Amite Rivers, a six-fold variation between the lowest and the highest flows, and was lowest in the Tangipahoa River, a four-fold variation between the lowest and the highest flows. The rivers’ annual freshwater inflows were positively correlated with the annual precipitation within their drainage areas. Fig. 3 shows a close linear relation between annual runoff and precipitation in all three watersheds. Over 80% of interannual variations in the runoff could be explained by the annual precipitation (r2 ¼ 0.85, 0.80, and 0.82 for the Amite, Tickfaw, and Tangipahoa Rivers, respectively). An average ratio of 0.34 for long-term annual average runoff to annual precipitation (runoff efficiency) was found for the three upper Lake Pontchartrain watersheds (Fig. 4). The runoff efficiency was high (0.50) in the high flow years and was low (0.15) in the low flow years, indicating a large variation in evapotranspiration from these three coastal watersheds. A spatiotemporal variation of extreme high and low flows among the three watersheds was found. Table 2 summarized three years with extreme high annual freshwater inflow and the three years with extreme low annual freshwater inflow from 1940 to 2002. The highest annual freshwater inflow from the Amite and Tickfaw Rivers occurred in1983, while the highest flow from the Tangipahoa River occurred in 1961. The second highest freshwater inflow occurred in 1400

Annual runoff (mm)

1200 1000 800

Amite River Tickfaw River Tangipahoa River

Tangipahoa Q = 0.709 P - 509 (r2 = 0.82)

Runoff efficiency

0.5

3.2. Interannual variations of freshwater inflow

0.4 high inflow year

0.3

average

0.2 0.1 low inflow years

0

0

250

500

750

1000 1250 1500 1750 2000 2250 2500

Annual precipitation (mm) Fig. 4. Long-term annual averages of runoff efficiency in the three years with extreme high freshwater inflow, the three years with the extreme low freshwater inflow and the overall average in the Amite, Tickfaw, and Tangipahoa Rivers.

1980 for the Amite River, in 1977 for the Tickfaw River, and in 1983 for the Tangipahoa River (Table 2). However, during the past 62 years, the three lowest annual flows from these three watersheds occurred in the same years: 1952, 1963, and 2000. Sikora and Kjerfve (1985) analyzed the salinity records from 1951 through 1981 at Pass Manchac within Lake Pontachartrain (Fig. 1) and found highest salinity levels in 1952 and 1963 (>2 ppt) over the 31-year period of record. Lowest salinity levels (mostly <0.5 ppt) were found in the years 1961, 1973, 1977, 1979 and 1980 (Sikora and Kjerfve, 1985). These five years were also found in this study to have the highest annual discharge from the three studied watersheds with values of 5.22, 5.10, 4.88, 4.81, and 5.39 km3, respectively. Because Lake Pontchartrain is connected with marine water in its fareast region, the results indicate that high salinity water can intrude further westward when freshwater inflow from the upper Lake Pontchartrain Basin is low. Annual freshwater inflow has been recognized as contributing to high productivity of estuaries (e.g., Loneragan and Bunn, 1999). However, low flow years have also been found to have both positive and negative effects on fish growth and survival (Deegan et al., 1999; Good et al., 2001; Arndt et al., 2002). The positive effect may be a lagged influence of the flow of a previous year, as lagged correlations between

Table 2 Three years each with the highest and the lowest annual freshwater inflows from three coastal watersheds

Amite Q = 0.704 P - 572 (r2 = 0.85)

600

Inflow and occurring year km3 (year)

400 Tickfaw Q = 0.634 P - 496 (r2 = 0.80)

200 0 900

623

1100

1300

1500

1700

1900

2100

Amite

Tickfaw

Tangipahoa

Highest

3.58 (1983) 3.27 (1980) 2.91 (1973)

0.59 (1983) 0.55 (1977) 0.55 (1995)

1.95 (1961) 1.94 (1983) 1.66 (1973)

Lowest

0.54 (2000) 0.58 (1963) 0.79 (1952)

0.09 (2000) 0.13 (1963) 0.16 (1952)

0.35 (2000) 0.45 (1963) 0.53 (1952)

2300

Annual precipitation (mm) Fig. 3. Relationships between annual runoff and annual precipitation in the Amite, Tickfaw, and Tangipahoa Rivers.

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624

freshwater flow and fisheries production typically used to support speculation that flows affect the survival of fish during their first year of life (Staunton-Smith et al., 2004). Using original field notes and museum records of Lake Pontchartrain, O’Connel et al. (2004) compared fish survey data from 1954 (assigned as dry period), 1978 (wet period), October 1996e September 1998 (wet period), and October 1998eOctober 2000 (dry period). The researchers found a change in fish assemblages in the estuary over the time, but concluded that the change was not in response to natural wet and dry environmental fluctuations. In this study, we found (Fig. 5) that the year of 1954 was a low flow year (0.81 km3) in the upper Lake Pontchartrain Basin, but the year before (1953) was a high flow year (2.76 km3). The year of 1978 was a year with a more or less average flow (1.71 km3), but the year before (1977) was a high flow year (2.90 km3). The annual flows

3.5

7500

Amite low flow

high flow

3.0

6500 5500

2.5 2.0

4500

1.5

3500

1.0

2500

0.5

1500

Annual precipitation (mm)

Freshwater Inflow (km3 yr-1)

4.0

0.0 500 1938 1946 1954 1962 1970 1978 1986 1994 2002

Year Tickfaw

7500 low flow

high flow

0.6

6500 5500

0.5

4500

0.4 0.3

3500

0.2

2500

0.1

1500

Annual precipitation (mm)

Freshwater Inflow (km3 yr-1)

0.8 0.7

500 0.0 1938 1946 1954 1962 1970 1978 1986 1994 2002

Year Tangipahoa

low flow

high flow

7500 6500

2.0

5500

1.5

4500 3500

1.0

2500

0.5

1500

Annual precipitation (mm)

Freshwater Inflow (km3 yr-1)

2.5

500 0.0 1938 1946 1954 1962 1970 1978 1986 1994 2002

in 1996, 1997, 1998, 1999 and 2000 were 1.21, 2.63, 2.15, 1.14, and 0.54 km3, respectively, showing that actually the only dry year was 2000. Based on the freshwater inflow results, we argue that a more careful study is needed to explain the impacts of dry and wet years on fish assemblages in Lake Pontchartrain.

3.3. Interdecadal variations of freshwater input Two distinctly different periods of freshwater inflow were observed over the past 62 years. One was a 20-year-long low flow period from 1954 to 1973, and another was a 24-yearlong high flow period from 1975 to 1998 (Fig. 5). During the low flow period the Amite, Tickfaw and Tangipahoa Rivers averaged an annual inflow of 1.47
0.14 km3 (
standard error), 0.28
0.03 km3 and 0.89
0.08 km3, respectively, while during the high flow period the three rivers discharged (in the same order) 2.28
0.14 km3, 0.38
0.03 km3, and 1.17
0.07 km3 (Table 3). In all three watersheds annual average freshwater inflow in the period from 1975 through 1998 was significantly higher than those from 1954 to 1973 (Table 3). A significant change in annual precipitation during the two periods also was found in the Amite River watershed (P ¼ 0.003) and Tickfaw River watershed (P ¼ 0.030), but not in the Tangipahoa River watershed (P ¼ 0.124) (Table 4). Interestingly, the increased rate of freshwater inflow was different among the three rivers: the Tickfaw and Tangipahoa Rivers showed a similar increase rate (36% and 31%, respectively), while the Amite River showed a much higher increase rate of flow (55%) (Fig. 6). Based on the U.S. census data (U.S. Census Bureau, http://www.census.gov/), we found an increase in the population density from 4.8 km2 in 1960 to 10.8 km2 in the Amite River watershed, but no significant change in population density in the Tickfaw (1.1 km2 in 1960 and 1.3 km2 in 2000) and Tangipahoa (1.1 km2 in 1960 and 1.3 km2 in 2000) River watersheds. Although we do not have information on the land use change over the 40-year period, the 125% increase of population density in the Amite River watershed may serve an indication on the fast urbanization in the watershed. The results suggest that the large increase of freshwater inflow from 1970s to 1990s is a result from both the climate variation and human impact in the coastal watersheds. Further research on land use change in

Table 3 Comparison of annual average freshwater inflow between the low flow period (1954e1973) and the high flow period (1975e1998) from three major tributaries to Lake Pontchartrain (means and standard errors are in km3 for the rivers) Period

N

Mean

Std error

t value

Pr > | t |

Amite

1954e1973 1975e1998

20 24

1.47 2.28

0.138 0.138

4.22

0.0001

Tickfaw

1954e1973 1975e1998

20 24

0.28 0.38

0.026 0.026

2.67

0.0107

Tangipahoa

1954e1973 1975e1998

20 24

0.89 1.17

0.083 0.066

2.83

0.0072

Year

Fig. 5. Time series plot of five-year moving average annual freshwater inflow from the Amite River, Tikfaw River and Tangipahoa River to Lake Pontchartrain; Circles indicate annual averages of freshwater inflow; Needles denotes annual average precipitation in the watershed.

Y.J. Xu, K. Wu / Estuarine, Coastal and Shelf Science 68 (2006) 619e626 Table 4 Comparison of annual average precipitation between the low flow period (1954e1973) and the high flow period (1975e1998) over the drainage areas of three major tributaries to Lake Pontchartrain (means and standard errors are in mm for the precipitation) Period

N

Mean

Std error

t value

Pr > | t |

Amite

1954e1973 1975e1998

20 24

1416 1665

60 53

3.13

0.0032

Tickfaw

1954e1973 1975e1998

20 24

1627 1837

75 54

2.31

0.0300

Tangipahoa

1954e1973 1975e1998

20 24

1547 1680

72 50

1.57

0.1240

this area is needed to quantify the magnitude of anthropogenic impacts on hydrologic regimes in the upper Lake Pontchartrain Basin. Researchers have postulated that the terrestrial material flushed into estuaries provides a significant input to estuarine food webs (e.g., Fry and Wainright, 1991; Mallin et al., 1992; Lee, 1995; Newell et al., 1995). The species composition and population size of aquatic organisms can be affected by the long-term variation of hydrologic regimes. Following a 2-year drought in Florida, primary production and the total numbers, biomass and species richness of fish decreased in the Apalochicola Bay system (Livingston et al., 1997). The trophic diversity and the biomass of herbivores in the fish community also decreased markedly after the drought. Some studies have shown that an increasing streamflow would increase nutrient loads and water column stratification in the northern Gulf of Mexico, which may exacerbate the already serious problems of eutrophication and hypoxia in the basin (Turner and Rabalais, 1994; Justic et al., 1996). In a recent retrospective study on Lake Pontchartrain, O’Connell et al. (2004) found that fish assemblages in the lake have changed over the past half century. Assemblage instability was most pronounced for fishes collected from demersal habitats (O’Connell et al., 2004). The researchers have attributed this

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long-term change to multiple anthropogenic stressors, although no specific stressor was mentioned. A river diversion project designed to control flood levels in the Comite River, a tributary of the Amite River, has been seeking funding for its startup since the 1990s. This Comite River Diversion Project with a proposed budget of over $160 million (USACE; Amite River Basin Commission, 2003) will construct a 19-km long channel to divert freshwater from the Comite River into the Mississippi River. We know that, after the diversion has been in place, a large amount of freshwater will not flow into the Lake Pontchartrain estuary. It is, however, unknown what short-term and long-term effects this reduction in freshwater input from the Amite River will exert on biological processes, estuarine habitats, fish communities, and aquatic and wetland ecosystems in and around this large oligaholine estuary. 4. Conclusions This study assessed the magnitude, timing, and variation of freshwater inflow from the Amite, Tickfaw and Tangipahoa Rivers into Lake Pontchartrain during the period from 1940 to 2002. It is the first comprehensive report on the long-term seasonality and interannual variability of freshwater discharge from the upper Lake Pontchartrain Basin. Based on our findings, we conclude that over the past six decades precipitation and runoff patterns in the Amite, Tickfaw, and Tangipahoa River watersheds have changed. The changes may have caused significant impacts on estuarine habitats and aquatic ecosystems in and around Lake Pontchartrain, though a thorough study on long-term relationships between hydrologic conditions in the Lake Ponchartrain Basin and biological and ecosystem changes in the lake is needed. We suggest that any hydrologic modifications in the Lake Pontchartrain Basin should not be undertaken without a full appreciation of their potential impacts. Acknowledgements

60

55

Change percentage (%)

50 40

36 31

30 20

References

15

10 0

Precipitation

The authors wish to thank the Louisiana District of the USGS Water Resources for making the long-term river discharge data available. Direct financial support for this research was provided by USDA McIntire-Stennis funds. This manuscript was greatly improved by the comments of two anonymous reviewers.

Amite

Tickfaw

Tangipahoa

Fig. 6. Changes in overall annual average precipitation and annual average freshwater inflow during the high flow period (1975e1998) as compared to the low flow period (1954e1973) in the Amite, Tickfaw and Tangipahoa Rivers.

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