The Influence of Freshwater Inflow on Macroinvertebrate Species Composition

In Lavaca-Matagorda Bay, Texas


Julie Kinsey, University of Texas, Marine Science Institute, 2004



Purpose of Project

For my project, I attempted to show spatial and temporal changes in dominant benthic species composition in Lavaca-Matagorda Bay. I focused my study on this area because the Lower Colorado River Authority (LCRA) and the San Antonio Water System (SAWS) have initiated a water diversion project that could potentially transfer approximately 150,000 acre-feet of water annually (49 billion gallons per year) from the lower Colorado River to the City of San Antonio. Water will be stored in “holding areas” at the mouth of the Lower Colorado River, and then pumped approximately 170 miles back up to San Antonio. The chief environmental concern is that freshwater inflows will be seriously diminished, threatening the health of this commercially and recreationally important estuary. Because benthic macroinvertebrates are excellent indicators of water quality in both fresh and saltwater systems, I wanted to visualize how species dominance changes over time with varying freshwater inflows.


Study Area


Map of the Lavaca-Matagorda Bay System, with barrier islands included. The shapefiles I used to create the map were obtained from the National Hydrography Dataset (NHD), These shapefiles could not be merged in such a manner that I could remove the lines in between them. Also, the barrier islands were classified differently than the mainland, thus I could not color them in with the same color as the mainland. I chose to use the ESRI Texas shapefile for the rest of my maps to present a “cleaner” look.





The Benthos & Inflow

Historical studies have stressed the importance of freshwater inflow to estuarine systems, and its status as a major factor in proper estuary functioning and health has long-been established in many coastal areas across the globe, including those along the Gulf of Mexico (Chapman 1966, Copland et al. 1972, Kalke 1981, Kalke and Montagna 1989). Inflows serve a variety of important functions in these habitats, including the creation and preservation of low-salinity nurseries, sediment and nutrient transport, allochthonous (outside) organic inputs, and movement and timing of critical estuarine species (Longley 1994). Benthic macrofauna (>0.5mm) are especially sensitive to changes in inflow, and can be useful in determining its effects on estuarine systems over time.

Benthic macroinvertebrates are established indicators of water quality in both freshwater and marine systems, and can highlight different aspects of the environment, including pollutant levels, hypoxia/anoxia, turbidity, salinity changes, and more (Oglesby 1967, Merritt and Cummins 1984). Relatively sessile and long-lived, benthic macroinvertebrates can reveal temporal changes in the environment that simple hydrographic measurements and chemical analysis can either not determine, or are impractical to utilize because of sizeable monetary and time constraints. Ubiquitous and relatively inexpensive to collect and analyze, these macrofauna (>0.5mm) are excellent tools in assessing both short- and long-term environmental conditions.

An integral part of the trophic structure in estuarine environments, soft-bottom dwelling macrofauna species possess a wide range of stress tolerances, including varying sensitivities to changes in salinity (Kalke and Montagna 1989, 1991, Longley 1994). Thus, they are not only important members of the estuarine community, but are also extremely useful in assessing the effects of freshwater inflow in estuarine systems, where salinity gradients can vary dramatically over time. These shifts can occur both rapidly and over long periods.

 While many early studies in regional Texas estuaries focused on oyster reefs, much recent benthic work has concentrated on the soft-bottom dwelling macrofauna and their relationship to several key environmental variables, including freshwater inflow, salinity, temperature, depth, and sediment type (Kalke and Montagna 1991, Engle and Summers 2000). While grain size and type, as well as temperature, play significant roles in benthic macroinvertebrate community structure, freshwater inflow and corresponding changes in salinity are primary factors controlling the distribution of marine and freshwater organisms within an estuary (Mannino and Montagna, 1994, Kalke and Montagna 1989, 1991, Attrill et al. 1996, Montagna et al. 2002). In fact, while sediment type and depth are important to benthic organisms, they are relatively minor factors controlling benthic community structure in most Gulf of Mexico estuaries when compared with salinity (Engle and Summers 2000). Depth in particular is rarely a key factor, because most estuaries in Texas are relatively shallow (Baird et al. 1996, Engle and Summers 2000).

            Because freshwater inflow and salinity play such a major roles in benthic community structure, it is important to understand the effects of freshwater inflow within and among different estuaries in order to properly manage these systems.


Species Dominance in Lavaca-Matagorda Bay


In the sites that I studied, two particular species generally dominated, Streblospio bendecti and Mediomastus ambiseta. The polychaete S. bendecti is a pioneering species that can respond quickly to disturbances in the environment. A suspension-feeder, it is generally found in the first 3 cm of the substrate. Mediomastus ambiseta, an equilibrium species, is a subsurface deposit-feeding polychaete that can be found in both the first 3 cm of the substrate, as well as below in the 3-10 cm range (Kalke and Montagna 1989, Martin 1994). Both species have wide tolerance ranges for changes in salinity. When other species are strained or killed by hypersaline conditions, both can survive and dominate. They can also dominate at higher inflows, as well.  Streblospio benedicti can proliferate rapidly after significant disturbances, while M. ambiseta is most often able dominate after conditions stabilize.

Chironomid larvae are freshwater species that are found in the bays after relatively high inflows have occurred. They indicate that a moderate to large inflow has occurred in the recent past, even if they are found during a sampling period when inflow was low.

Macroinvertebrate species data was obtained from my advisor, Dr. Paul Montagna (University of Texas, Marine Science Institute). Dr. Montagna has been collecting data from this system since 1988. Freshwater Inflow Data was obtained from the United States Geological Survey (USGS),


Original Intent for Project

 My original intent was to track changes in dominant species over time using the Tracking Analyst Program. Unfortunately, it posed a variety of problems that would not allow this, including:


  • On the map itself: Would not “play” all the dates in an animation loop. You had to “scroll” through them one-by-one to show them all.


  • If you created an animation file, it would also not include all the dates.


  • Also, the main problem was that it would not let me show more than one variable at a time (such as inflow, species dominance, etc.). Normally, you can highlight more than one variable at a time, but the temporal layer created by the program would not allow it. For my map animation to be interesting, I would need to pair freshwater inflow with species changes, as well as the presence or absence of freshwater species.


  • There IS a feature that allows you to symbolically highlight certain events (such as freshwater inflow above or below a certain level, and the presence of freshwater species), but once the feature appeared, it would not disappear from the map on dates when freshwater species were absent.


 A Still from my Animation:



The text in the animation would only feature the species information, and would not include date or inflow, which was necessary in order for the map to make sense and/or be useful. The presence of chironomid larvae (a freshwater species) was symbolized by the yellow circles, but they would not disappear from the map on subsequent dates when freshwater species were absent.






*To effectively show how species changed with inflow through time, I ended up creating a series of maps that highlighted the most interesting changes.


Map Series


*Note: In general, the flow rates are calculated from the month previous to the sampling date to allow for a lag in response time that is typical of most benthic species. However, some months in my study were sampled towards the end of the month, and benthic species have responded to inflows during that month. In this case, inflows from the sampling month are used, and are noted as such.



7.31.90: Flow Rate: .72cfs 6/90

26.3cfs on 7/90


M. ambiseta was dominant at all stations on the previous sampling dates in Jan and Apr, and low inflows in June most likely accounted for the fact that it was not displaced prior to the July sample. A low-to-moderate pulse of inflow occurred during the sampling month (26.3 cfs), displacing M. ambiseta. Streblospio benedicti dominated at sites A-C in July, but inflows were not large enough to displace M. ambiseta at station D (the station under the most tidal influence).



4.24.91: Flow Rate: 33.2cfs 3/91

1388cfs on 4/91



During April of 1991, an entire species shift occurred. The upper reaches that contain sites A & B most likely shifted due to moderate disturbance by the 1388cfs inflow during that month. However, the inflow had not yet reached station D, which shifted to a climax community because of stable, low-flow conditions. We can extrapolate this because the polychaete Polydora caulleryi is known as a late-successional (i.e. climax) species that prefers higher salinities. The previous month had moderately low inflow (as did January), and the dominant species was M. ambiseta at Stations B-D (S. benedicti was dominant at station A).



7.24.91: Flow Rate: 71.7cfs 6/91; 160 cfs 7/91




Inflow rates were relatively low up until this point, except for the moderate inflow pulse in April. Mediomastus ambiseta dominated, most likely because system had once again stabilized (P. caulleryi was probably displaced due to lowered salinities from April’s pulse). However, chironomid larvae were found, their presence presumably caused by the April inflow event. They probably did not appear until July’s sampling date because of travel time (i.e. the pulse did not immediately carry them as far as site A in April) and lack of  very high flushing rates in the spring.  This is a good example of how we can track changes in inflow using bioindicator species. Even though the flow rate was relatively low just before and during the sampling period, we can tell that there was a significant amount of freshwater inflow into the system in the recent past, as evidenced by the presence of freshwater species.



7/12/92: Flow Rate 1758cfs. & 10/6/92, Flow Rate 32.6 cfs.



The flow rate was high for first six months of 1992. It was probably too high to allow S. benedicti to dominate due to the fact that the upper layer where S. benedicti is found was constantly being turned over. Chironomid larvae were found at Stations A & B due to copious amounts of freshwater inflow. By the October sampling date, the flow rate had stabilized to 32.4 cfs in 9/92 (35.8 in 10/92), yet chironomid larvae is still found at Station A. Thus, the effects of freshwater inflow (and the fact that it occurred previously in the year) can still be witnessed, even though flow rates just before and during the sampling period were low.







·        To produce my initial map, I used National Hydrography Dataset (NHD) shapefiles and an ESRI shapefile of Texas from Exercise 1. Because I could not remove the lines between NHD shapefiles, I only used them for the initial map that showed the location of all of my sites. The ESRI shapefile of Texas was used exclusively for subsequent maps.


·        I made my own shapefile for sites A-D by creating a .dbf table with lat/long coordinates (converted into decimal degrees), importing it into an ArcMap document, and then setting the XY coordinates.


·        I made sure that all shapefiles were projected in WGS 1984 (my original coordinates from my GPS unit were in WGS 1984).


·        To highlight species changes, I labeled dominate species with their respective names, and then changed the symbology to reflect the different species (all this was done in the “Properties” section of the shapefile).


·        I used Tracking Analyst to create an animation (.avi) file. (See the “Original Intent…” section of this webpage for problems associated with using TA).


·        After I realized that the TA program would not work for my data, I created a series of maps highlighting the most interesting temporal data that I had obtained.









Species Key for Graph



Dominant Species



Species Number

Streblospio benedicti


Rhynchocoela (unidentified)


Polydora caulleryi


Oligochaetes (unidentified)


Mulinia lateralis


Minuspio cirrifera


Mediomastus ambiseta


Lepton sp.


Cossura delta


Apseudes sp. A


Ampelisca abdita




                        Somewhat surprisingly, no significant relationship was found between dominant species and inflow rates (Sigma Plot). This, however, does not mean that the data is not important. It should be noted that we can see a definite difference between the most dominant species, M. ambiseta and S. benedicti and the rest of the species. The most evident pattern is that M.  ambiseta and S. benedicti dominate during all levels of inflow. However, the dominant species that occur more rarely are only found when inflows range from approx. 0-1700 cfs, and disappear after moderate- to high-inflow events (with the exception of M. cirrifera and Apseudes sp. A, which were found only once each during high flow events). The rarely dominant species reappear after a disturbance, while the pioneering and tolerant species are always present during a disturbance. This indicates successional community dynamics, whereby different species respond to varying disturbance regimes. In other words, there is a temporal relationship between which species will colonize an area with respect to when a disturbance has occurred. Species such as S. benedicti can quickly colonize an area after a disturbance, and M. ambiseta can tolerate a large variety of disturbance regimes, but generally dominate after disturbances occur. The other species generally occur when a community has been stable for a longer period of time.



Other Factors

Inflow and Salinity are important, but so are:


l     Sediment type

l     Nutrient loading

l     Mechanical/chemical disturbances, etc.



These secondary factors not within the scope of this project



Importance of Study


Determination of minimum inflows is necessary to preserve the health of the estuarine and surrounding coastal ecosystems. Because freshwater inflow and salinity play such a major roles in benthic community structure, it is important to understand the effects of freshwater inflow within and among different estuaries in order to properly manage these systems.





I’d like to extend my special thanks to Dr. Paul Montagna, Marc Russell, Harris Muhlstein, Rick Kalke, and Dr. David Maidment for their help in creating this project.

Sources & Literature Cited


Atrill, M.J., S.D. Rundle, and R.M. Thomas. 1996. The influence of drought-induced low freshwater flow on an upper-estuarine macroinvertebrate community. Wat. Res. 30 (2); 261-268.


Baird, C., M. Jennings, D. Ockerman, and T. Dybala. 1996. Characterization of Nonpoint Sources and Loadings to Corpus Christi Bay National Estuary Program Study Area. EPA Report: CCBNEP-05. January 1996. 256pp.


Chapman, E.R. 1966. The Texas basins project. In: R.F. Smith, A.H. Swartz, and W.H. Massmann (eds.), A symposium on estuarine fisheries. Amer. Fish. Soc. 95(4):83-92. Special Publ. No. 3. 154 pp.


Engle, V.D. and J.K. Summers. 2000. Biogeography of benthic macroinvertebrates in estuaries along the Gulf of Mexico and western Atlantic coasts. Hydrobiologia.

436: 17-33.


Kalke, R.D. 1981. The effects of freshwater inflow on salinity and zooplankton populations at four stations in the Nueces-Corpus Christi and Copano-Aransas Bay systems, TX from October 1977-May 1975. In: R.D. Cross and D.L. Williams (eds.), Proceeding of the International Symposium on Freshwater Inflow to Estuaries. Washington, DC: U.S. Dept. Int. Fish & Wildlife, pp. 454-471.


Kalke, R.D. and Montagna, P.A. 1989. A Review: The Effect of Freshwater Inflow on the Benthos of Three Texas Estuaries, pp. 185-218. In: P.A. Montagna (principal investigator) Nitrogen Process Studies (NIPS): The Effect of Freshwater Inflow on Benthos Communities and Dynamics. University of Texas Technical Report

No. TR/89-011. 370 pp.


Kalke, R.D. and P.A. Montagna. 1991. The Effect of Freshwater Inflow on Macro-benthos in the Lavaca River Delta and Upper Lavaca Bay, Texas. Contributions in Marine Science. 32: 49-71.


Longley, W.L. (ed.). 1994. Freshwater inflows to Texas bays and estuaries: ecological relationships and methods for determination of needs Texas Water Development Board and Texas Parks and Wildlife Department, Austin, TX. 386 pp.


Mannino, B. A. and P. A. Montagna. 1994. Effects of Freshwater Inflow and Sediment Characteristics on Small Scale Spatial Variation of Macrobenthic Community Structure in Nueces Bay. Thesis Report. 157 pp.


Martin, C. 1994. Corpus Christi Bay and La Quinta Channel: A Comparison of Benthic Diversity. Thesis Report, Texas A&M Corpus Christi (TAMUCC). 81pp.


Montagna, P.A., R.D. Kalke, and C. Ritter. 2002. Effect of Restored Freshwater Inflow on Macrofauna and Meiofauna in Upper Rincon Bayou, Texas, USA. Estuaries. 25: 1436-1447.


National Hydrography Dataset (NHD),


Oglesby, R.T. 1967. Biological and physiological basis of indicator organisms and communities: Section I – Biological basis, p. 267-269. In: T.A. Olson and F.J. Burgess (eds.), Pollution and Marine Ecology. Interscience Publishers, New York.


United States Geological Survey (USGS),