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Variability in algal growth potential and related parameters in four central Texas ponds.

ABSTRACT. -- The Algal Assay Procedure-Bottle Test (AAPBT), developed by the Environmental Protection Agency, has been used to determine the nutritional status (algal growth potential--AGP), limiting nutrients, and the presence of inhibitory substances in many freshwater environments. However, the AAPBT has been used only to a limited extent in small ponds. The AGP of these bodies of water can vary significantly between seasons, depending upon rainfall and the particular stage or compartment in which nutrients exist at the time of sampling. Nutrients incorporated in detritus and plankton, and also those in dissolved form, after autoclaving pretreatment have varying degrees of availability to the assay organism. Surface water samples from different locations within a small pond during periods of calm weather may be expected to exhibit minimal differences with respect to AGP. In contrast, subsurface samples taken during warmer months may vary greatly between locations, and may be extremely different from surface AGPs. Key words: algal assay; algal bioassay; algal growth potential; ponds.


Most limnologists would agree that although many sophisticated chemical analyses are available for assessing the productivity of freshwaters, biological tests are necessary for an effective evaluation. Algal assays are useful in that they reflect all the fertility factors present in the water, many of which may not be measured specifically by traditional chemical methods (Miller et al., 1974). According to Marvan (1979), the information obtained from algal assays "is important and surpasses the significance of the sum of all partial data about the relative abundance of particular water components." Algal assays have been used primarily to 1) assess the nutritional status of the water in question; 2) determine the limiting nutrient for phytoplankton growth; and 3) determine the presence of toxic substances (Claesson and Forsberg, 1978). Studies in which algal assays have been used to relate AGP (algal growth potential) to indigenous phytoplankton standing crop include those of Miller and Maloney (1971), Miller et al. (1974), Walmsley and Toerien (1975), Green et al. (1975, 1976), and Davis and DeCosta (1980). Payne (1976) performed AGP tests during the winter on several Indiana lakes and found that high AGP values obtained during that season were followed by high chlorophyll a, low Secchi depth, and high phytoplankton dry weight values in the field the next spring. Weiss (1976) examined 44 bodies of water in North Carolina and concluded that the maximum algal biomass produced in the assay could be important in describing their relative trophic states.

Although research has been done using algal assays to determine the AGP of aquatic environments and its relationship to various limnological parameters such as phytoplankton standing crop, nutrient concentrations, and others, few studies have been done to determine the variability in AGP for small ponds throughout the year. Possibly an appropriate sampling strategy would enable one to collect samples at any time of the year and still be able to make useful comparisons between these small aquatic ecosystems. Also, there has been little work done to compare AGP results with total ecosystem primary productivity. Most, if not all, studies designed to determine the "accuracy" of the algal assay have related AGP results to phytoplankton growth, measured most frequently with the light-dark bottle method (Gaarder and Gran, 1927) or chlorophyll a determinations. However, when these techniques are used, a pond that has a high primary productivity from vascular plants or periphyton (or both) will yield low productivity results if the phytoplankton population is small (Canfield et al., 1983). Nutrient determinations made by traditional methods during the winter, in order to predict the relative degree of spring productivity, face certain limitations, inasmuch as all nutritional factors or toxic substances may not be measured. Similarly, during the growing season, measurements of in situ primary productivity may yield highly variable results depending upon weather conditions, types of autotrophs present, and so forth. The objectives of this study were to 1) determine the variability in AGP at various locations and depths within a small pond, leading to the selection of an appropriate sampling procedure, and 2) determine the seasonal variability in AGP and selected physicochemical parameters (including gross community photosynthesis) in a series of ponds of different trophic states.


Algal growth potential (AGP) was determined using the Algal Assay Procedure-Bottle Test (AAPBT) developed by the Environmental Protection Agency (1971). Algal density in the samples was monitored daily by determining in vivo chlorophyll fluorescence, using a Turner 110 fluorometer, and fluorometric units were reporting as cells/mL or [micro]g chlorophyll a/L (r = 0.9981 and 0.9990, respectively). A small inoculum (<0.2 mL) containing Selenastrum capricornutum was used, which yielded approximately 10,000 cells/mL, or approximately 0.86 [micro]g chorophyll a/L, as the initial algal concentration. The algal growth potential was the maximum fluorescence attained. Afterwards, the limiting nutrient was determined by "spiking" the samples with [N.sub.3] or [P.sub.4]. Water samples were pretreated by autoclaving for 25 minutes at 121 C and 18 psi. Three replicate flasks were used for each sample and the maximum values were averaged. Total phosphate phosphorus (total P) was determined by the method outlined in Hach Chemical Co. (1979). Dissolved oxygen readings were made with a Yellow Springs (model 54) oxygen meter. The oxygen probe was extended out from the shoreline through a PVC tube 3.1 meters long and 3.8 centimeters in diameter. Dissolved oxygen was measured at three locations at depths of half-meter intervals in order to obtain an average value for the pond within the photic zone. The gross community photosynehesis rate (GCP), recorded as mg [0.sub.2]/[m.sup.2]/day, was determined using the three-point method outlined in Lind (1974). Indigenous chlorophyll, recorded in [micro]g/L, was determined by extraction with acetone (Jeffrey and Humphery, 1975).

The study ponds were located in Brazos County, Texas, approximately seven miles southwest of College Station (see Taylor, 1986). Physical characteristics of these ponds are listed in Table 1. For seasonal data, surface and integrated samples (using a weighted tygon hose to collect a water-column sample from the surface to approximately 0.25 meters off the bottom) were collected monthly from each pond for 15 consecutive months. The surface sample was collected from the center of the pond; the integrated sample consisted of three separate integrated samples taken from different locations and then combined. The AGP variability, which might be expected during a single sampling period at various surface locations within a single pond, was determined for Sebesta's Pond during late summer. Three surface samples were collected at each of three stations along a transect, one station at each end and one in the middle. All stations were located at least five meters from the nearest shore. Variability in AGP with respect to surface samples and depth was determined for Sebesta's pond and Robinson's no. 2 Pond. During mid-summer, surface and bottom samples were collected at each of three stations in both ponds. AGP and P were determined for all samples during the study, whereas indigenous chlorophyll was determined for the surface samples only. The trophic state for individual ponds was determined by the method outlined by Carlson (1977).


Spatial Varibility in Algal Growth Potential

The AGP, indigenous chlorophyll, and total P data for three surface samples collected at the shallow, middle, and deep stations of Sebesta's Pond are given in Figure 1. The 95 percent confidence intervals (CI) for the surface AGP at these stations were 18-22, 18-19, and 21-22 [micro]g chl/L, respectively. Although the confidence intervals for the middle station do not overlap those of the shallow and deep stations, indicating a significant difference, the closeness of the values suggests that collecting more samples might have yielded overlapping values, as evidenced in the shallow station's wider range, for example. The values are close, however, when compared to the difference in values obtained during different seasons. Like the AGP, values for surface chlorophyll and total P were only slightly variable among stations on this sampling date. The shallow, middle, and deep stations had intervals of 8-11, 4-15, and 10-15 [micro]g chl/L, respectively for surface chlorophyll, and 3-6 [micro]g/L for total P. Because the intervals overlapped, it was concluded that there was no significant difference among stations for the parameters. Similar AGP values would be expected at different surface locations within a small pond during the warmer months, as long as physical disturbances of organically rich sediment or an influx of nutrients from the watershed does not occur, and if the pond is not heterogenous with respect to the distribution of planktonic populations. Plankton can affect AGP results by incorporating nutrients into refractory compounds (Taylor et al., 1990; Reed, 1989). During algal blooms, this phenomenon may dramatically affect short-term AGP results. Attached or floating vegetation can affect AGP results by removing nutrients from the water before water samples are collected. Likewise, using only a filtration pretreatment can reduce significantly the nutrient supply in the sample by removing those incorporated in microorganisms.



Depth Variability in Algal Growth Potential

The AGP, total P, and surface chlorophyll values for Sebesta's Pond and Robinson's no. 1 Pond at deep, middle, and shallow stations during mid-summer are given in Figure 2. In Sebesta's Pond, the higher AGP values in samples taken near the bottom at the deep station could be attributed to a significant amount of decaying organic matter, with little or no mixing with the surface water. The more similar surface and bottom AGP at the middle and shallow stations suggests a more homogeneous condition. During this summer sampling date, Robinson's no. 2 Pond exhibited a sharp temperature decline from the surface down to 1.5 meters, whereas nearly isothermal conditions were recorded below 1.5 meters. The Secchi depth was quite shallow, the turbidity, due to suspended clay, restricting effective light penetration to the upper 25 centimeters. Decomposition of organic material was evidenced by the strong odor of hydrogen sulfide in bottom samples. Only small differences were observed in total P between surface and bottom samples, as opposed to large differences in AGP between the two samples. Results of nutient spiking revealed nitrogen to be the limiting nutrient; therefore, the dissimilar relationship between AGP and total P in the surface and bottom waters would be expected.

Seasonal Variation in Algal Growth Potential

The mean values and ranges for the parameters investigated during the 15-month study are shown in Table 2. The data demonstrate the great amount of variability that can be expected, not only between ponds, but also within a pond throughout the year. Monthly data for AGP, GCP, and indigenous chlorophyll are shown in Figure 3.

Sebesta's Pond. -- Compared to the other ponds, Sebesta's Pond showed the least variability in terms of surface water AGP throughout the year. The pond was relatively deep and had a stable watershed, densely covered with tall grasses, and a stable shoreline with a significant amount of vascular vegetation (Nitella sp.) in the littoral regions. The higher AGP values during the summer possibly were due to the release of nutrients incorporated in plankton to an available form during the autoclaving pretreatment. The AGP range of 49.7 [micro]g chl/L for the integrated samples compared to a range of 18.5 for the surface samples indicated that the bottom waters were having a substantial influence on integrated samples. Noticeable difference between surface and integrated samples were observed during the stratifying period, most of this being attributed to the much higher values at the deepest three meters station. The shallower stations had smaller differences in AGP between integrated and surface samples throughout the study period, indicating more homogeneous water conditions. Overall, the surface water AGPs were less variable throughout the year than integrated samples. The higher values in both sample types during the stratifying period might be attributed to a significant reduction in pond volume, 32 percent below the volume during the cooler months, resulting in the concentrating of limiting nutrients.

Sebesta's Pond exhibited phosphorus limitation during the cooler months and nitrogen limitation during the warmer months. This was the only pond in which phosphorus limitation was demonstrated at any time during the study. Nitrogen limitation was demonstrated in the other ponds year round. The water column was more homogeneous during the cooler months as indicated by relatively uniform oxygen-temperature profiles, and also the greater similarity between integrated and surface sample AGP and total P values. Carson's trophic state index using chlorophyll (TS[I.sub.chl]) revealed that this pond ranged from mesotrophic to eutrophic. The GCP was predominantly higher during the stratifying period, similar to indigenous chlorophyll values. However, a relatively low correlation (r = 0.6332, P = 0.0113) between these two variables indicates more of an influence of attached vegetation in the photosynthetic input of dissolved oxygen. There were instances when the dissolved oxygen near the thick algal mats was markedly higher than in surface waters. The lack of significant correlation between total P and the other variables indicates that sufficient quantities of phosphorus and nitrogen become available at different times during the year. The assay results may change significantly throughout the year with little change in total P. However, the proportion of available nitrogen to available phosphorus undoubtably changes.


The AGP for integrated samples was more highly correlated with indigenous chlorophyll than surface AGP (r = 0.8775, P = 0.0001 and 0.7863, P = 0.0001, respectively). The TSI using total phosphorus (TS[]) ranged from oligotrophic to eutrophic; therefore, it appears evident when comparing the TS[I.sub.chl] and the TS[] that the indigenous chlorophyll concentrations depend more on nitrogen than phosphorus, which is further supported by the AGP results.

Robinson's no. 1 Pond. -- This pond did not exhibit an easily discernible trend in AGP over the study period. It had the smallest surface area of the ponds studied, and was fairly well protected from wind by shrubs and trees. The monthly values were somewhat erratic, with a general trend of higher values during the warmer months. The available nitrogen was higher during the warmer months, in contrast to declining phosphorus concentrations. The bottom substrate was covered by thick growths of Cladophora sp. around the shallow periphery during most of the warmer months. The majority of the watershed was covered with dense grass. The landowner had fertilized the watershed with equine manure within a year prior to this study. A major rainfall (13 inches) in October had a stimulating effect on the integrated samples AGP, but not on the surface AGP. This was the wettest October ever recorded in this area, setting a record for the most consecutive days of rain with 10. Both integrated and surface total P rose abruptly during this month; however, no apparent increase in available nitrogen (determined by nitrogen spiking) occurred. An influx of nitrogenous material sinking to the bottom, or a mixing of the nitrogen-rich sediment into the bottom waters, may have been the cause of a higher integrated AGP.



This pond lost approximately 36 percent of its volume by summer, with a 16 percent decrease in surface area. The effects of a week of continuous rainfall before the October sampling date can be seen in an increase in total P and integrated AGP, a sharp decrease in surface chlorophyll and GCP, and a dramatic reduction in the Secchi disk transparency. The decrease in surface AGP might be attributed to a diluting effect of the rain water, much of which, being less dense, was sampled near the surface. Changes in weather may affect significantly the equilibrium of small ponds by causing great reductions or increases in volume, changes in the watershed, and so on. The highest indigenous chlorophyll values were noted during the cooler months, which might be attributed to nutrient-rich sediment being brought to the surface under the isothermal conditions. Small ponds generally have relatively greater basin surface areas for their size than large lakes or reservoirs; therefore, they can be expected to be influenced more by their bottom deposits, especially if the sediment is unprotected (devoid of vegetation). A cladoceran "bloom" in October accompanied a decrease in phytoplankton populations. During the early morning hours, many small bass (Micropterus sp.) were observed swimming onto the bank, being adversely affected by extremely low oxygen concentrations (<0.1 mg/L).


A sharp decrease in the GCP in October indicated a flushing of the "old" water, with its phytoplankton, by the heavy rainfall. This water replacement also could help explain the abrupt change in integrated AGP values. The TS[I.sub.chl] and TS[] indicated mesotrophic to eutrophic conditions. Indigenous chlorophyll and GCP were not significantly correlated, which indicated more of an influence of attached primary producers in the photosynthesis input in this pond.

Robinson's no. 2 Pond. -- This pond showed considerable variability in AGP. The integrated and surface sample AGP values, when compared during the warmer months, had a greater dissimilarity than when compared during the cooler months. The similar mean integrated and surface AGP during the cooler months indicated a relatively homogeneous condition. The sudden decrease in October was attributed to the major storm. This pond was apparently "flushed out" by the rain. The pond's basin, which had been excavated two years prior to the study, had no attached vegetation and, therefore, nothing to secure the sediment. Before October, the bottom sediment contributed a great amount of turbidity (Secchi depth = 10 centimeters). In October the visible characteristics of the water were much different than those in September; the apparent color changed from a turbid gray to a clearer, "tea" color (Secchi depth = 40 centimeters). The watershed was moderately sloped and allowed for a great amount of runoff to enter the pond. The dramatic reduction in AGP in October, with a gradual increase from that month to March, indicated a flushing of nutrient-rich water and gradual accumulation of nutrients in the "new" water.

During the warmer months before the October rain, the pond exhibited a high temperature gradient. The nutrient content near the bottom was noticeably high, as evidenced by the presence of hydrogen sulfide in the integrated samples. Surface waters, being much warmer and less dense, did not mix readily with subsurface water. The difference between the surface and integrated samples could be attributed to a higher total nutrient supply in subsurface waters. Nutrients incorporated in detritus in this sample, being in a more reduced form, may have been more labile under the heat and pressure of the autoclave; hence, they would be more available to the assay organisms. Both integrated and surface samples were nitrogen-limited throughout the study period.

A high correlation between surface and integrated AGP samples was noted during the mixing months (r = 0.9210, P = 0.0012). Surface chlorophyll showed a trend of increasing values from January to May, which was abruptly halted in June, but began again until the flushing event in October. An Anabaena sp. bloom was observed in April. During the warmer months, and abundant population of Euglena sp. formed a reddish powderlike "film" on the water's surface.

Oxygen concentrations dropped from 14 mg/L at the surface to 0.5 mg/L at depth of 0.5 meter in April. Likewise, temperatures dropped from 30[degrees]C at the surface to 21[degrees]C at a depth of 0.5 meter. The primary productivity was due entirely to planktonic or surface floating producers. The surface AGP was correlated well with surface chlorophyll for the 15-month study (r = 0.9231, P = 0.0001), whereas, surface chlorophyll and GCP had an "r" of 0.7461 (P = 0.0014). The total P values showed a sharp drop in October, coinciding with an increase in Secchi depth. The Secchi depth gradually decreased later, indicating a steady return to the original turbid condition. This pond had lost 42 percent of its volume and 41 percent of its surface area by late summer. As detritus accumulates, less mud will become suspended; thus, the turbidity may steadily decrease in the future. This was the pattern observed in Robinson's no. 1 Pond, by the landowner, a few years after it was constructed.

Parsons' Pond. -- High AGP values for both integrated and surface waters from December through March were apparently due to an increase in available nitrogen, either from the extremely soft, thick-layered sediment or from runoff. Samples during these months were collected within a few days of rainfall events. Significant runoff, coupled with cool water temperatures and windy conditions, effectively would mix this "unprotected" pond. The similarity of the high integrated and high surface AGP values during these months reinforce this hypothesis. Cattle grazed around the pond periodically, and an abundance of manure was present along its periphery. The surface AGP values were usually similar to those of integrated samples with the exception of two peaks, recorded in May, one day after a major hailstorm, and in August. During August, a Synura sp. (a colonial, motile golden alga) bloom was highly visible in the surface sample but only a few cells were observed in the integrated samples. These cells apparently released an additional nitrogen source when autoclaved. Samples from Parsons' Pond were nitrogen-limited throughout the study period.

This pond showed the most dramatic change in volume and surface area of all the ponds. By October, a 60 percent reduction in volume and a 44 percent reduction in surface area compared to the highest water levels was noted. Because AGP values were not higher during this period, the lack of a nutrient-concentrating phenomenon may be attributed to abundant duckweed (Lymnea sp.) and waterfern (Azolla sp.) on the pond's surface. During several summer months, these plants covered 50 to 70 percent of the pond, and by September, approximately 95 percent of the surface was covered. These plants may serve as important biofilters of nutrients. Azolla contains a symbiotic, cyanobacterium, Anabaena sp., which is capable of nitrogen fixation. Surface chlorophyll values were usually high year round compared to the other ponds. The GCP was typically higher during the stratifying period with a virtual cessation in September due to a complete coverage of the pond by Lymnea sp. and Azolla sp. Because this pond was always nitrogen-limited, the Azolla, containing the nitrogen-fixing, symbiotic Anabaena would be able to compete better than other autotrophs. This "mat" of vegetation effectively restricted light penetration. The mats were moved periodically by wind action, leaving some surface areas exposed. The rain storm in October completely removed all floating vegetation. A gradual increase in GCP from December through March indicated that the pond was returning to its original condition. The GCP and indigenous chlorophyll was moderately correlated (r = 0.7258, P = 0.0033). The total P was always extremely high due to a continuous inflow from the watershed and autochthonous nutrient loading from the soft, unsecured sediment, or both. The shallow nature of the pond provided high correlations between integrated and surface AGP (r = 0.9989 in the winter and 0.9628 in the summer).


Analysis of Variance (ANOVA) and Duncan's Multiple Range tests were used to determine significant differences among ponds for the various parameters (Figure 4). The pond grouping arrangement indicates that the AGP is in agreement with the other traditional methods of classifying aquatic ecosystems.


1. Algal growth potential (AGP) can be an extremely useful tool in distinguishing the potential conditions of a series of small ponds.

2. The algal assay procedure developed by the EPA can provide useful information with respect to the kind and quantity of algal-limiting nutrients. However, the assay results are susceptible to significant variation throughout the year.

3. Surface water samples from different locations within a small pond during periods of calm weather exhibit minimal differences with respect to AGP. In contrast, subsurface samples may vary greatly between different locations, and may be extremely different from the surface samples.

5. The AGP of a small pond can vary significantly from one season to the next, or within a single season, depending on rainfall, wind conditions, conditions of the watershed, and so forth.
TABLE 1. Ranges for selected physical parameters in the four central
Texas ponds during the 15-month study.

 Sebesta's Robinson's no. 1
 High Low High Low

Maximum length (m) 106 84 27 24
Maximum depth (m) 4.5 3.8 2.2 1.6
Mean depth (m) 1.8 1.6 1.3 1.0
Shoreline length (m) 325 200 79 70
Surface area ([m.sup.2]) 2529 1892 464 390
Volume ([m.sup.3]) 4514 3074 588 375

 Robinson's no. 2 Parson's
 High Low High Low

Maximum length (m) 39 28 73 50
Maximum depth (m) 2.5 1.8 1.7 1.2
Mean depth (m) 0.9 0.9 0.7 0.5
Shoreline length (m) 146 88 200 123
Surface area ([m.sup.2]) 627 370 1067 597
Volume ([m.sup.3]) 584 338 707 297

TABLE 2. Means and ranges for the parameters measured in the four
central Texas ponds during the 15-month study. AGP = algal growth
potential (ug chl/L; Total P = ug/L; GCP = gross community
photosynthesis (mg [0.sub.2]/[m.sup.2]/day); Secchi depth = meters.

 Warmer months Cooler months
 15-month study (April-Oct.) (Nov.-March)
 Mean (Range) Mean (range) Mean (range)

Integrated 21.4 (5.5-55.2) 29.9 (5.5-55.2) 14.0 (9.0-21.6)
Surface AGP 10.6 (3.7-21.9) 13.9 (3.7-21.9) 7.7 (3.2-13.4)
Chlorophyll 7.3 (1.3-17.9) 11.6 (4.9-17.9) 3.6 (1.3-5.5)
Core total 40.0 (0-150) 30.0 (10-60) 40.0 (0-150)
Surface 30.0 (0-150) 20.0 (10-20) 30.0 (0-150)
 total P
GCP 3.3 (0.8-7.6) 4.3 (2.1-7.6) 2.4 (0.8-4.5)
Secchi 1.8 (0.8-3.1) 1.5 (0.8-2.3) 2.1 (1.4-3.1)

Robinson's no. 1
Integrated 22.7 (7.0-40.8) 28.6 (17.3-40.8) 17.5 (7.0-37.6)
Surface AGP 20.1 (7.0-40.4) 23.3 (17.7-28.8) 17.2 (7.0-40.4)
Chlorphyll 62.0 (1.5-324.8) 42.2 (13.2131.0) 79.4 (1.5-324.8)
Core total 120.0 (0-280) 80.0 (30-180) 150.0 (60-280)
Surface 100.0 (40-220) 70.0 (40-130) 130.0 (70-220)
 total P
GCP 5.3 (0.3-9.7) 6.9 (0.3-9.7) 3.9 (0.6-6.4)
Secchi 0.8 (0.3-1.6) 1.1 (0.5-1.6) 0.6 (0.3-1.3)

Robinson's no. 2
Integrated 65.0 (8.0-232.7) 118.4 (13.1-232.7) 18.2 (8.0-32.3)
Surface 34.5 (8.4-131.9) 54.6 (10.7-131.9) 16.9 (8.4-33.0)
Chlorophyll 67.9 (12.0-215.5) 97.8 (12.0-215.5) 41.8 (25.8-76.7)
Core total 180.0 (70-260) 220.0 (100-260) 140.0 (70-240)
Surface 170.0 (70-270) 200.0 (70-270) 140.0 (70-240)
 total P
GCP 6.9 (2.0-15.6) 11.0 (2.2-15.6) 3.3 (2.0-4.6)
Secchi 0.2 (0.1-0.5) 0.2 (0.1-0.4) 0.3 (0.1-0.5)

Integrated 117.0 (17.9-431.8) 55.0 (30.2-74.6) 171.1 (17.9-431.8)
Surface AGP 122.4 (14.5-419.9) 81.2 (14.5-259.6) 158.5 (15.0-419.9)
Chlorophyll 95.8 (4.7-676.0) 168.1 (19.8-676.0) 32.6 (4.7-105.2)
Core total 680.0 (190-1470) 610.0 (190-1090) 750.0 (430-1470)
Surface 680.0 (130-1140) 610.0 (130-1070) 740.0 (410-1400)
 total P
GCP 10.9 (2.5-24.2) 16.7 (9.0-24.2) 6.5 (2.5-11.4)
Secchi 0.6 (0.2-1.0) 0.7 (0.5-1.0) 0.5 (0.2-0.8)


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Department of Biology, Baylor University, Waco, Texas 76798, and Department of Wildlife and Fisheries Sciences, Texas A & M University, College Station, Texas, 77843
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No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Taylor, Mark F., Jr.; Clark, William J.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:May 1, 1991
Previous Article:Historical biogeography of Sigmodon ochrognathus in Texas.
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