Downstream Effects of Restored Freshwater Inflow to Rincon Bayou, Nueces Delta, Texas, USA

Transcription

1 Estuaries Vol. 25, No. 6B, p December 22 Downstream Effects of Restored Freshwater Inflow to Rincon Bayou, Nueces Delta, Texas, USA TERRY A. PALMER, PAUL A. MONTAGNA*, and RICHARD D. KALKE University of Texas Marine Science Institute, 5 Channel View Drive, Port Aransas, Texas 8 ABSTRACT: Construction of two dams in 1958 and 1982 decreased freshwater inflow to the Nueces River and Nueces Delta marsh, which has become a reverse estuary where salinity increases upstream rather than downstream as would occur in a normal estuary. In 1995, an overflow channel was dug to breach the banks of the Nueces River to restore inundation of the marsh via Rincon Bayou, which is the main stem channel of the delta. Previous studies demonstrated a restoration of a normal salinity gradient and positive affects on benthos in the upper reaches of Rincon Bayou. The present study was performed to determine how far downstream the overflow channel had beneficial effects. A transect of eight stations was established and sampled quarterly between October 1998 and October Benthic characteristics were measured to assess ecological change. There were 12 precipitation and freshwater inflow events between the month preceding and ending sampling. The largest were in fall and restored normal salinity patterns. The macrobenthic community was different in three zones. The upper four stations ( 6. km from the overflow channel) were highly variable in water inundation and salinity. The broad salinity range caused the lowest diversity and allowed short-lived pioneer species Streblospio benedicti, Laeonereis culveri, and Chirominid larvae to dominate. Biomass blooms occurred during fall inflow events in the upper reaches. The central stations (11. to 1. km from the overflow channel) were brackish, had more narrow salinity ranges, and were more diverse. Mulinia lateralis, Mediomastus ambiseta, Cerapus tubularis, and Ampelisca abdita were dominant species. The third zone, the lower portion of Nueces Bay (2 km from the overflow channel), was distinct from other stations in the transect, because it had the greatest marine influence. It is in a later successional stage, with a more diverse community of larger organisms, e.g., Polydora caulleryi, Tharyx setigera, and Mysella planulata, which were dominant species and contributed to the larger biomass there. Overall, the overflow channel restored normal salinity patterns in the upper reaches ( 1 km) of Rincon Bayou and freshwater pulses ( 1 6 m ) in fall increased benthic productivity indicating ecological functions were restored as well. This restoration however, only occurs intermittently for short time periods because the freshwater events are isolated and inflow volumes during the current study were too small to affect the lower Rincon Bayou or Nueces Bay. Introduction A growing demand for freshwater in the Nueces River watershed has caused a dramatic decrease in freshwater inflow into the Nueces Delta over the last 5 years. Average annual inflow to the delta has been reduced by 99% since 1958 when the Nueces River was first dammed (Irlbeck and Ward 2). Before damming, the Nueces Delta was inundated by freshwater inflow events that caused high water levels in the Nueces River to spill into Rincon Bayou, which is the main stem channel for the Delta. The upper part of the Delta (i.e., upper and central Rincon Bayou) were dead-end lakes without a direct connection to the Nueces River. Delta inundation occurred by over-banking of Nueces River flood waters and tidal inundation during isolated storms and tropical cyclones (Bureau of Reclamation 2). The increasing demand for freshwater has reduced volumes and elevation of flood events so that natural marsh inundation was reduced to an average of one event every three * Corresponding author; tele: 61/9-69; fax: 61/9-6; paul@utmsi.utexas.edu. years (Irlbeck and Ward 2). Decreasing freshwater inflow has caused the Nueces Delta marsh to become a reverse estuary. This occurs when salinity increases upstream rather than downstream as would occur in a normal estuary. When freshwater inundation decreased, low salinity water from the Nueces River entered the marsh only upon rising tides and diluted water in the hypersaline marsh areas. The hypersaline marsh was created by evaporation of unflushed pools. Hypersaline conditions have had adverse effects on the ecology of the Nueces Delta and on commercially and recreationally important shrimp and oyster populations (Montagna et al. 1998; Riera et al. 2). The Rincon Bayou Demonstration Project was undertaken to determine if it was possible to ameliorate environmental effects of decreasing freshwater inflow (Bureau of Reclamation 2). The project approach was to lower the minimum flooding threshold of the upper Nueces Delta from 1.6 m above mean sea level to about m to increase the opportunity for larger and more frequent quantities of freshwater and tidal water to be diverted into upper Rincon Bayou. The minimum 22 Estuarine Research Federation 18

2 Downstream Effects 19 Fig. 1. Nueces estuary study area with sample stations in Rincon Bayou and Nueces Bay. flooding threshold was lowered by digging the Nueces River and Rincon Bayou overflow channels, which allow freshwater to flow into the Rincon Bayou and Nueces Delta from the Nueces River. The project was operational from October 1995 through September 2. Previous studies in the upper Rincon Bayou demonstrated that inflow restored normal salinity gradients (Bureau of Reclamation 2) and enhanced benthic communities (Montagna et al. 22) and marsh vegetation (Alexander and Dunton 22). The purpose of the current study was to determine how far downstream ecological benefits accrued as a result of the demonstration project. The approach was to sample a transect from the Nueces River diversion point to upper Rincon Bayou to the lower end of Nueces Bay. Salinity and benthic characteristics were measured. Salinity indicates effects of inflow. Benthos are good indicators of ecological change because they are relatively immobile and live several months to a year, thus integrating responses over longer temporal and larger spatial scales than plankton or nekton. Materials and Methods STUDY AREA The Nueces Delta (Fig. 1) is located in a semiarid, subtropical climate that is characterized by hot humid summers and mild dry winters with occasional freezes (TDWR 1981; Tunnel and Dokken 1996). Although precipitation rate is on average cm yr 1, evaporation rate is 152 cm yr 1 (Henley and Rauschuber 1981). Without inflow the region is prone to hypersalinity. Precipitation events typically occur as spring thunderstorms or late-summer or early-fall tropical storms with very little rain in-between and variable in time (TDWR 1981, 1982). High tides also occur in spring and fall; inflow on top of higher water elevation increases the opportunity for natural flooding of the Delta. The seasonal character of these events means that freshwater inflow and runoff are primarily discrete pulses rather than a more constant flow (Orlando et al. 199). STUDY DESIGN The goal of the present study was to determine how far downstream from the diversion that biological effects could be identified. Eight stations were sampled quarterly for macrofauna and hydrologic variables. Stations were located along a 2-km transect starting in upper Rincon Bayou, km from the Nueces overflow channel, and extending into lower reach of Nueces Bay (Fig. 1). The distance from the Nueces overflow channel was calculated for each station using a geographic information system, and data is reported as distance, not station name. The distance stations were located from the head of Rincon Bayou were.29 (station ),.96 (station ), 5. (station 5), 6. (station 6), 11. (station 11), 1.29 (station 1), 21.2 (station 21), and 2. km (station 2). The two Nueces Bay stations, 21 and 2, have been sampled quarterly since June 198 and reported on as stations A and B, respectively, in three previous studies (Montagna and Kalke 1992; Mannino and Montagna 1996, 199) and as R1 and R2, respectively, in one study (Carr et al. 2). The four Rincon Bayou stations,, 5, and 6 were also sampled in previous studies and were referred to C, D, E, and F, respectively (Bureau of Reclamation 2; Montagna et al. 22). Nueces Bay is a secondary bay connected to Corpus Christi Bay, which is connected to the Gulf of Mexico km to the eastnortheast. There is little water exchange between the two bays, because of the small tidal range and presence of a constriction as the result of a causeway at the mouth of Nueces Bay. Five quarterly samples were taken over a 12-mo period: October 26, 1998, January 11, 1999, April 1, 1999, July 6, 1999, and October 29, HYDROGRAPHY Hydrographic characteristics of the water column were measured at each station during each

3 15 T. A. Palmer et al. TABLE 1. Summary of hydrographic events during study period showing in the Rincon Bayou (from Bureau of Reclamation 2). Event Event Date Net Inflow (1 m ) Rainfall (mm) Sept 1, 1998 Sept 29, 1998 Oct 16, 1998 Oct, 1998 Nov 12, 1998 Mar 5, 1999 Mar 2, 1999 May, 1999 Jun 19, 1999 Jun Aug 19, 1999 Sept, , , sampling period, at the surface and near the bottom, using a Hydrolab Surveyor II multiparameter instrument. Measurements were made of temperature (.15 C), ph (.1 units), dissolved oxygen (mg L 1.2), specific conductivity ( mmhos cm 1 depending on range), redox potential (.5 mv), depth ( 1 m), and salinity (automatically corrected to 25 C, ). BENTHOS Macrofauna were sampled with a 6.-cm diameter core (5. cm 2 area); cores were sectioned at depth intervals of and 1 cm. Triplicate macrofauna samples were taken at each station and were preserved with 5% buffered formalin. In the laboratory, macrofauna were sorted on.5-mm sieves and identified to the lowest taxonomic level possible. Organisms from each sample were pooled into higher taxonomic categories (Crustacea, Mollusca, Polychaeta, and others) and weighed to the nearest.1 mg. Samples were dried for 2 h at 55 C and weighed. Mollusks were placed in 1 N HCl for a few minutes to dissolve carbonate shells and washed before drying. Species diversity was calculated by pooling all three replicate cores for each site-sampling period combination. Pooling was performed to increase the area per sample and because there was little overlap in species among cores. Diversity is reported for the area of 16 cm 2. Diversity was calculated using Hill s number one (N1) diversity index (Hill 19), which indicates the number of abundant species in a sample and is a measure of the effective number of species (Ludwig and Reynolds 1988). It is the exponential form of the Shannon diversity index (H ): N1 H. N1 will tend toward 1 as diversity decreases. Hill s N1 was used because it has units of numbers of species, and is more interpretable than most other diversity indices (Ludwig and Reynolds 1988). Fig. 2. channel. Salinity change with distance from Nueces overflow All data (except diversity) were log transformed prior to analysis to transform the data into a normal distribution. Two-way ANOVA was used to test for differences in macrofauna abundance, biomass, and diversity among stations and sampling dates. Where treatment effects were significant, Tukey multiple comparison procedures were used with the experimentwise error rate maintained at.5. Statistical analyses were performed using SAS software (SAS Institute Inc. 1991). Community structure of macrofauna species was analyzed by Principal Components Analysis (PCA) and displayed in bivariate plots. Only species with an abundance over 1% were analyzed with PCA. The results are visualized in two ways: as factor patterns and as loading scores. The factor pattern plot helps to visualize variability and similarities among abundant species. The loading score plot allows comparison among stations. Results There were 12 precipitation and freshwater inflow events just prior to and within the 12-mo sampling period, which started in October 1998 (Table 1). Event number in mid-october was by far the largest event, and was the pinnacle of five closely spaced events over a 96-d period in the fall During this period, two tropical storms hit the region yielding over 56 cm of local rainfall and diverting 5,92 1 m of water from the Nueces River into the Rincon Bayou overflow channel (Bureau of Reclamation 2). This was the only time during the study period that low salinities persisted throughout the entire Nueces Delta. Salinity continuously increased from near the freshwater source to 2 near the marine source (Fig. 2). This was the only event in the study period that would have allowed overflow into the delta before

4 Downstream Effects 151 TABLE 2. Analysis of variance on log transformed biomass (g m 2 ), abundance (no. m 2 ), and diversity (N1.1 m 2 ); and Tukey multiple comparisons tests where underlined station means are not different at.5 level. DF degrees of freedom. Source DF Mean Square F Value Pr F Biomass Date Station Date Station Error Abundance Date Station Date Station Error Diversity Date Station Error Station (km) Mean Biomass Station (km) Mean Abundance 2,62 2,6 16,2 11,52,18 6,15 5,592,6 Station (km) Diversity the overflow channels were built. There were no events during the winter 1999 and only two isolated events in spring Consequently, salinities rose quickly during the early part of the study period. High tides in spring and early summer pushed marine water upstream, causing negative inflow events, in spite of precipitation. The second largest event (no. 12) in September 1999 also created a normal, increasing salinity gradient throughout the Delta, but salinities were higher than the previous fall. Salinity increased from Fig.. channel. Biomass change with distance from Nueces overflow near the freshwater source to near the marine source. The greatest mean biomass (12.6 g m 2 ) was found at station 2 in lower Nueces Bay with the strongest marine influence (Table 2, Fig. ). At the other stations, mean biomass ranged from.8 g m 2 at station 11 to 2. g m 2 at station 1. Station 1 had the second largest average biomass. Station 1 is located at the mouth of Rincon Bayou at the lowest edge of the delta. Biomass was highest in the spring and lowest in summer. The greatest mean abundance (2, ind m 2 ) was found at station 6 (Table 2, Fig. ). Maximum abundance occurred at station 5 in October 1998 and January Similar to biomass, station 11 had the lowest mean abundance (,6 ind m 2 ). There was a significant date station interaction, which was driven by decreased density in spring in the upper reaches, but increased density in the fall, winter, and summer. Diversity, measured as the number of dominant species (N1) did not change among sampling dates (Table 2, Fig. 5), but there were significant differences among stations. Unlike biomass and abundance, there was a pattern of increasing diversity with distance from the river source. There were low diversity values (1.5 to 2.2 species) in the upper reaches of Rincon Bayou (stations 6), a slight increase in diversity (2.5 to. species) in the lower reaches of Rincon Bayou (stations 11 and

5 152 T. A. Palmer et al. Fig.. Abundance change with distance from Nueces overflow channel. Fig. 5. Diversity, N1 (Hill 19) for each 16 cm 2, change with distance from Nueces overflow channel. 1) and upper reaches of Nueces Bay (station 21), and maximum diversity (15 species) in the most distant station (2) in Nueces Bay. A total of 12 species were found during the study, but most were found only at station 2 (Table ). Community structure changed along the transect, but was dominated by a few species. Streblospio benedicti was the most abundant species overall with a mean abundance per station of 11, individuals (6.6% of total abundance). Mediomastus ambiseta and Laeonereis culveri were the next most abundant with means of 1,95 (9.9%) and 915 (5.%) individuals per station, respectively. Principal components (PC) analysis was performed on the 1 most dominant species. The first axis (PC1) explained 6.1% of the variance and the second axis (PC2) explained 2.% of the variance (Fig. 6). The PC analysis indicated that stations were divided into three distinct groups, each with their own characteristic species. The upper four stations (,, 5, and 6) were characterized by hav- TABLE. Dominant macrofauna species found at stations listed in order of dominance. Mean abundance (no. m 2 )( standard deviation) for stations over all dates and replicates. Station (km) Species Streblospio bendicti Mediomastus Laeonereis culveri Polydora caulleryi Mulinia lateralis Cerapus tubularis Chironomid larvae Ampelisca abdita Tharyx setigera Mysella planulata Dominant species Dominant abundance Other species Other abundance Total species Total abundance ,61 (21,8) 1,15 (1,9) 6 (968) 15, ,26 (15,86) 1,9 (1,652) 1,2 (2,16) 1, ,99,2 (,8) 151 (1), (,8) 8 (16) 151 (69) 5, ,198 21,2 (15,8) 65 (952) 1,588) (2,86) 19 () 511 (1,) 5 2, ,11,9 (1,559) 1, (1,6) 5 (159) 8 (1) 19 () 8 (1) 6, ,29 2, (1,1) 5, (2,892) 8 (16) 19 (1,9),215 (6,89) 1,51 (2,5) 6 1, ,15 2 1, 62 (8),1 (2,965) 9 (9) 1 (1,) 86 (,5) 9 (9) 1 (69) 5,56 2 1,9 1 6,5 95 (281),9 (1,952),59 (5,81) 2,8 (,6) 1 (69) 16 (256) 2,2 (1,519) 1,1 (2,2) 8 15,16 9 1, ,55

6 Downstream Effects 15 ing S. benedicti, L. culevri, and Chirominid larvae. The central part of the transect (11, 1, and 21) was characterized by the bivalve Mulinia lateralis, the polychaete M. ambiseta, and the amphipod Ampelisca abdita. Another amphipod, Cerapus tubularis, was the second dominant species at station 1. Stations 11 and 21 of this group was slightly influenced by the dominant species S. benedicti. The third zone was station 2 in the lower portion of Nueces Bay. Station 2 was different from all other stations in the transect because of its greater marine influence. Station 2 had the most diverse community and larger organisms. Common polychaetes Polydora caulleryi, Tharyx setigera, and the bivalve Mysella planulata are the dominant species of this station and contribute to its greater associated biomass. Fig. 6. Plots of the first two principal components (PC) resulting from analysis of macrofauna data. A) PC loadings for 1 most dominant species (see Table ) and B) PC station scores. Sb Streblospio benedicti, Ma Mediomastus ambiseta, Lc Laeonereis culveri, Pc Polydora caulleryi, Ml Mulinia lateralis, Ct Cerapus tubularis, CL Chironomid larvae, Aa Ampelisca abdita, Ts Tharyx setigera, and Mp Mysella planulata. Discussion The objective of this study was to determine how far downstream benefits of a freshwater inflow event accrued as a result of construction of the Nueces River overflow channel. Benefits were defined as restoration of normal salinity gradients and increased benthic characteristics. In terms of salinity gradients, only one event had a strong effect and allowed the entire Rincon Bayou to temporarily present a normal estuarine salinity pattern (Fig. 2). This event, in fall 1998 (Table 1), allowed a large volume of freshwater to flow into the delta ( m ). During other periods, with much lower inflow volumes, positive salinity effects extended only 6 km downstream. Nearly normal salinity gradients occurred in July and October Salinity gradients change without additional inflow. Little inflow followed the large event in October 1998 and salinity increased to 6 km downstream by January and April Reverse estuary conditions were present from 6 to 11 km downstream. Only large events ( 1 6 m ) can result in beneficial salinity impacts throughout the entire Nueces Delta. Freshwater inflow can enrich nutrients and increase primary production (Livingston et al. 199; Brock 21). This food source can be consumed by epibenthic filter feeders (e.g., bivalves) or deposited onto sediment surfaces and consumed by interface feeders (e.g., polychaetes and crustaceans). Biological effects of inflow should be detectable in benthic communities. The October 1998 event had minimal effects on macrofauna further than 6 km downstream. Stations 5 and 6, displayed a peak in abundance after the October inflow event. This peak was almost entirely caused by a bloom of the polychaete S. benedicti (99.6% of total fauna at both stations). This species is known to inhabit extreme salinities and to increase rapidly following freshwater events (Kalke and Montagna 1989; Montagna and Kalke 1995; Mannino and Montagna 199). By January 1999, with little rain, salinity had risen and average macrofauna abundance had decreased but S. benedicti still remained the dominant species (95% and 88% at stations 5 and 6, respectively). This small change in dominance happened when two

7 15 T. A. Palmer et al. and three new species started to reappear at stations 6 and 5, respectively, which is common after the emergence of the pioneer species (Flint and Kalke 1986a). Because of continued high salinities at stations 5 and 6 abundance remained low in April Effects due to the fall 1998 flood were not observable by spring Biomass and abundance were generally lowest at the two most upstream stations ( and ). There are two likely explanations. Flow velocities at these two stations may have been too high during flood events to allow organic material to deposit in the sediments. This could explain why abundance was highest further downstream at stations 5 and 6. If flow downstream slowed then material could be deposited to sediments stimulating benthos. A second explanation is the lack of low salinity tolerance of estuarine species. Estuarine species could have been killed or transported downstream during floods. Loss of organisms due to low salinity intolerance plays a large role in estuarine systems, especially when freshwater events are isolated and salinity fluctuations are great as is common in Nueces Delta (Kalke and Montagna 1989; Montagna and Kalke 1992). Diversity was lowest at station 5 in central Rincon Bayou. The typical estuarine diversity pattern is for species number to decrease from fresh to brackish water, and increase again as salinity increases to marine conditions (Remane and Schlieper 191). This U-shaped pattern is only slightly apparent in Rincon Bayou because stations 5 and 6 had lower diversity values than stations and, and higher values occurred downstream (Fig. 5, Table 2). One small event proved to be positive for the upper reaches of Rincon Bayou ( km). In early April 1999, almost three weeks after a small inflow event ( m ), organism abundance increased at stations and. Again, S. benedicti increased in abundance as did another polychaete L. culveri. These two species almost entirely replaced the species that had lived there prior to the event. Many of the organisms present before the event were larger in size, so biomass actually decreased at station after the event. However, the recruitment of small-sized polychaetes at station was so great that biomass increased markedly (Fig. ). A similar freshwater event just prior to July 1999 sampling did not display a similar increase in abundance. The different response indicates that either the amount of rainfall or a lagged response could have played an important role in controlling abundance. The event prior to the April sampling period had 55 cm of rain compared to only 1 cm immediately prior to the July 1999 sampling. A larger runoff volume could have washed in extra organic material stimulating growth in early April. A longer lag in response time after the event could have also been a factor in determining this productivity difference because the July sampling effort was made only 9 d after the event while the April sampling effort was made 19 d after the most recent event. This added lag in April could have been enough to allow the establishment of the population of S. benedicti whereas 9 d may have been too short for the particular population present. Other factors (e.g., seasonal recruitment or temperature) could have played a role in explaining the difference between the two events. Previous studies in the Nueces estuary have shown that salinity is a major factor controlling species composition (Mannino 199; Mannino and Montagna 199). This is because most benthic organisms in Nueces estuary are either sedentary or sessile, and are intermittently exposed to large salinity fluctuations (Montagna and Kalke 1992). The current study has shown that in the upper Nueces estuary (i.e., Rincon Bayou), the situation is more complex because spatial heterogeneity is a large cause of variability. There appears to be three zones of community composition along the transect (Fig. 6). Higher salinities in the upper reaches of Rincon Bayou can stress benthic organisms. Previous studies have shown that species richness decreases as the environment varies from the mean and diversity is low where the salinity gradient is the most extreme (Gunter 1961; Kalke and Montagna 1989; Montagna and Kalke 1992, 1995; Mannino and Montagna 199). In these circumstances populations generally have a greater number of individuals of the species present, but these organisms tend to be smaller in size, which is a common response to disturbance (Rhoads et al. 198). In the current study, the polychaete S. benedicti was by far the most common species comprising slightly over 6% of the total abundance. The next two most dominant species, both polychaetes, M. ambiseta and L. culveri make up almost 15% of total abundance. This high proportion of pioneer polychaete species is characteristic of the highly variable environmental conditions (Flint 1985; Flint and Kalke 1986b; Montagna and Kalke 1992). Although dominant, the polychaetes were small resulting in low biomass. In Nueces Bay, polychaetes make up % of total abundance while bivalves make up 9% of biomass (Mannino and Montagna 199). Dominance of small pioneer species high in abundance is evidence that the upper reaches of Rincon Bayou is stressed by the reverse estuarine conditions (Table ). The pioneer polychaete S. benedicti and the euryhaline deposit feeder M. ambiseta dominated stations 11 and 1 km from the overflow channel.

8 Downstream Effects 155 Both species are thought to have low competitive fitness but thrive where adverse conditions exclude others (Mannino and Montagna 199). These sites differ from others in that they are in a part of Rincon Bayou that is reduced to a narrow channel with little freshwater input. These stations are downstream from a widening of Rincon Bayou that could affect benthic production. When flowing water meets a broad system, the energy that suspends matter is dissipated, causing the particles to be deposited. In this way, much of the organic matter and nutrients could be lost before they reach the narrow channel of lower Rincon Bayou. If this deposition was great enough it could inhibit macrofaunal production and this could explain the consistent decrease of biomass and abundance at station 11 (Figs. and ). Further downstream at station 1, biomass and abundance generally increase. Station 1 is at the edge where Rincon Bayou (i.e., Nueces Delta) meets Nueces Bay (Fig. 1). Waves propagated in Nueces Bay by the predominant southeast wind dissipate energy into the area. Community characteristics differ from other stations because of the presence of tube building amphipods C. tubularis and A. abdita. These two species on average made up over 2% of abundance at station 1, but are barely present at other stations. The average was mainly due to a large increase in abundance in July 1999 when the two amphipods made up almost 65% of total abundance. Apart from this time, these species were barely present. This is consistent with two summer cohorts of A. abdita found at three sites in Jamaica Bay, New York, which were related to seasonal nutrient loading (Franz and Tanacredi 1992). The bivalve M. lateralis was only present at station 1 when amphipod abundance was small. This is similar to south San Francisco Bay where the tubedwelling amphipod A. abdita is thought to control the abundance of the native bivalve Macoma balthica (Nichols and Thompson 1985). In April 1999, before the amphipod boom, M. lateralis made up 2% of the total abundance at station 1, and the biomass increased. Succession, or seasonal changes, between polychaetes, bivalves, and amphipods is most pronounced at the delta-bay edge. This is likely an edge effect rather than a freshwater inflow effect. The lower portion of Nueces Bay is very different from all other stations in the transect, because it has the greatest marine influence. Station 2 exhibited a narrow salinity range (22.2 to.1 ) and the highest biomass and diversity. This is consistent with many studies that show diversity increases moving away from brackish water into marine (Sanders 1968; Remane and Schlieper 191; Flint and Kalke 1985; Day et al. 1989; Levinton 1995). Consequently, the community at station 2 was very distinct (Fig. 6). The polychaetes P. caulleryi and T. setigera, and the bivalve Mysella planulata are dominant species of this station and contribute to the larger biomass found. The more diverse community of larger organisms indicate station 2 is in a later successional stage than other stations upstream. The upper Nueces estuary and Rincon Bayou is a region of low diversity dominated by pioneering species (Fig. 6). The region is characterized by extensive periods of reverse estuary conditions where small volumes of freshwater inflow and high evaporation rates cause salinity to increase in the upper reaches of the estuary. Macrofauna blooms are related to freshwater inflow events that temporarily produce normal estuarine gradients where salinity increases from river to sea. In upper reaches of Rincon Bayou, km from the freshwater source, benthic communities have low diversity and are dominated by small pioneer polychaetes. Further downstream in lower Rincon Bayou and upper Nueces Bay, 11 to 21 km from the diversion, diversity increases but abundance and biomass are low. Smaller nutrient loads, geomorphological differences, and a different energy regime may explain these differences. The effect of large inflow events ( 1 6 m ) can reach about mid-way ( 1 km) into Rincon Bayou. A stable freshwater community does not exist upstream because events are ephemeral and isolated. Diversity does not increase upstream from brackish waters to freshwaters as it does in some estuaries. Macrofauna blooms caused by freshwater inflow do not last for long periods, unless the inflow event is very large. The overflow channel restored normal salinity patterns in the upper reaches of Rincon Bayou and benthic productivity increased following periodic freshwater pulses. The response indicates ecological function was restored, but only for short time periods because freshwater volumes are low and events episodic. ACKNOWLEDGMENTS The authors thank the ranch workers who provided access to private property to perform the study. The senior author performed this work while on an undergraduate student internship from the University of Waikato, New Zealand. This work was performed with partial support from the Bureau of Reclamation, General Investigations and Wetland Development Fund, and the Texas Water Development Board, Water Research Planning Fund. LITERATURE CITED ALEXANDER, H. AND K. H. DUNTON. 22. Effects of increased freshwater inflow on the emergent vegetation of a south Texas hypersaline salt marsh. Estuaries 25: BROCK, D. A. 21. Nitrogen budget for low and high freshwater inflows, Nueces estuary, Texas. Estuaries 2:

9 156 T. A. Palmer et al. BUREAU OF RECLAMATION. 2. Concluding Report: Rincon Bayou Demonstration Project. Volume II: Findings. U.S. Department of the Interior, Bureau of Reclamation, Oklahoma- Texas Area Office, Austin, Texas. CARR, R. S., P. A. MONTAGNA, J. M. BIEDENBACH, R. KALKE, M. C. KENNICUTT, R.HOOTEN, AND G. CRIPE. 2. Impact of storm water outfalls on sediment quality in Corpus Christi Bay, Texas. Environmental Toxicology and Chemistry 19: DAY, JR., J. W., C. A. S. HALL, W.M.KEMP, AND A. YÁÑEZ-ARAN- CIBIA Estuarine Ecology. John Wiley and Sons Inc., New York. FLINT, R. W Long-term estuarine variability and associated biological response. Estuaries 8: FLINT, R.W.AND R. D. KALKE Benthos structure and function in a south Texas estuary. Contributions in Marine Science 28: 55. FLINT, R. W. AND R. D. KALKE. 1986a. Niche characterization of dominant estuarine benthic species. Estuarine, Coastal and Shelf Science 22:65 6. FLINT, R. W. AND R. D. KALKE. 1986b. Biological enhancement of estuarine benthic community structure. Marine Ecology Progressive Series 1:2. FRANZ, D. R. AND J. T. TANACREDI Secondary production of the amphipod Ampelisca abdita Mills and its importance in the diet of juvenile winter flounder Pleuronectes americanus in Jamaica Bay, New York. Estuaries 15:19 2. GUNTER, G Some relations of estuarine organisms to salinity. Limnology and Oceanography 6: HENLEY, D. E. AND D. G. RAUSCHUBER Freshwater needs of fish and wildlife resources in the Nueces-Corpus Christi Bay area, Texas: A literature synthesis. Technical report FWS/ OBS-8/1. U.S. Fish and Wildlife Service, Office of Biological Services. Washington, D.C. HILL, M. O. 19. Diversity and evenness: A unifying notation and its consequences. Ecology 5:2 2. IRLBECK, M. J. AND G. H. WARD. 2. Analysis of the historic flow regime of the Nueces River into the upper Nueces Delta, and of the potential restoration value of the Rincon Bayou Demonstration Project, Volume II, Appendix B. In Bureau of Reclamation, Concluding Report: Rincon Bayou Demonstration Project. U.S. Department of the Interior, Bureau of Reclamation, Austin, Texas. KALKE, R. D. AND P. A. MONTAGNA A review: The effect of freshwater inflow on the benthos of three Texas estuaries, p In P. A. Montagna (ed.), Nitrogen Process Studies (NIPS): The Effect of Freshwater Inflow on Benthos Communities and Dynamics. Technical report no. TR/ The University of Texas Marine Science Institute, Port Aransas, Texas. LEVINTON, J. S Marine Biology: Function, Biodiversity, Ecology. Oxford University Press, New York. LIVINGSTON, R. J., N. NIU, F.G.LEWIS, III, AND G. C. WOODSUM Freshwater input to a Gulf estuary: Long-term control of trophic organization. Ecological Applications : LUDWIG, J. A. AND J. F. REYNOLDS Statistical Ecology. John Wiley and Sons, New York. MANNINO, A Effects of freshwater inflow and sediment characteristics on small scale spatial variation of macrobenthic community structure in Nueces Bay. Masters Thesis, Department of Marine Science, University of Texas at Austin, Austin, Texas. MANNINO, A. AND P. A. MONTAGNA Fine-scale spatial variation of sediment composition and salinity in Nueces Bay. The Texas Journal of Science 8:1 1. MANNINO, A. AND P. A. MONTAGNA Small-scale spatial variation of macrobenthic community structure. Estuaries 2: MONTAGNA, P. A., S. HOLT, C. RITTER, K. BINNEY, S. HERZKA, AND K. DUNTON Characterization of anthropogenic and natural disturbances on vegetated and unvegetated bay bottom habitats in the Corpus Christi Bay National Estuary Program study area. Publication CCBNEP-25. Texas Natural Resource Conservation Commission, Austin, Texas. MONTAGNA, P. A. AND R. D. KALKE The effect of freshwater inflow on meiofaunal and macrofaunal populations in the Guadalupe and Nueces estuaries, Texas. Estuaries 15: 26. MONTAGNA, P. A. AND R. D. KALKE Ecology of infaunal Mollusca in south Texas estuaries. American Malacological Bulletin 11: MONTAGNA, P. A., R. D. KALKE, AND M. C. RITTER. 22. Effect of restored freshwater inflow on macrofauna and meiofauna in upper Rincon Bayou, Texas, USA. Estuaries 25:xxx xxx. NICHOLS, F. H. AND J. K. THOMPSON Persistence of an introduced mudflat community in south San Francisco Bay, California USA. Marine Ecology Progress Series 2:8 98. ORLANDO, JR., S. P., L. P. ROZAS, G.H.WARD, AND C. J. KLEIN Salinity Characteristics of Gulf of Mexico Estuaries. National Oceanic and Atmospheric Administration, Office of Ocean Resources Conservation and Assessment, Silver Spring, Maryland. REMANE, A. AND C. SCHLIEPER Biology of Brackish Water, 2nd edition. John Wiley and Sons, New York. RHOADS, D. C., P. L. MCCALL, AND J. Y. YINGST Disturbance and production on the estuarine seafloor. American Scientist 66: RIERA, P., P. A. MONTAGNA, R.D.KALKE, AND P. RICHARD. 2. Utilization of estuarine organic matter during growth and migration by juvenile brown shrimp Penaeus aztecus in a south Texas estuary. Marine Ecology Progress Series 199: SANDERS, H. L Marine benthic diversity: A comparative study. The American Naturalist 12: SAS INSTITUTE, INC SAS/STAT User s Guide, Version 6, Fourth Edition, Volume 2. SAS Institute Inc., Cary, North Carolina. TEXAS DEPARTMENT OF WATER RESOURCES (TDWR) Nueces and Mission-Aransas estuaries: A study of the influence of freshwater inflows. Technical report TDWR, LP-18. TDWR, Austin, Texas. TEXAS DEPARTMENT OF WATER RESOURCES (TDWR) The influence of freshwater inflows upon the major bays and estuaries of the Texas Gulf Coast: Executive summary, 2nd edition. Technical report TDWR, LP-115. TDWR, Austin, Texas. TUNNEL, JR., J. W. AND R. D. DOKKEN Current status and historical trends of the estuarine living resources within the Corpus Christi Bay National Estuary Program study area, Volume. Technical report CCBNEP-6C. Texas Natural Resource Conservation Commission, Austin, Texas. Received for consideration, January 2, 22 Accepted for publication, August 2, 22