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Effects of Channelization on Sabine Map Turtle Habitat in the Mermentau River Drainage, Louisiana: Use of Original vs. New Channels
Carissa A. Hartson, Emily L. Ilgen, Olivia S. Zaleski, and Peter V. Lindeman

Southeastern Naturalist, Volume 13, Issue 1 (2014): 119–127

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Southeastern Naturalist 119 C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 22001144 SOUTHEASTERN NATURALIST 1V3o(1l.) :1131,9 N–1o2. 71 Effects of Channelization on Sabine Map Turtle Habitat in the Mermentau River Drainage, Louisiana: Use of Original vs. New Channels Carissa A. Hartson1, Emily L. Ilgen1, Olivia S. Zaleski1, and Peter V. Lindeman1,* Abstract - Graptemys sabinensis (Sabine Map Turtle) occurs in the Sabine, Calcasieu, and Mermentau river drainages of Louisiana and Texas. The Mermentau River was altered by construction of seven cut-offs between the towns of Mermentau and Lake Arthur, LA, in the 1970s, and several cut-offs also have been constructed along the river’s largest tributary, Bayou Plaquemine Brule. We surveyed basking turtles on lower Plaquemine Brule and the Mermentau River to determine if the new channels are used to the same extent as the original channels. The original channels had nonsignificantly higher counts of basking Sabine Map Turtles and total turtles than the new channels. However, the shorter new channels had significantly higher densities of Sabine Map Turtles and total turtles than the original channels. The new channels create a bypass for excess water on the river following heavy rainfall, allowing higher flow on new channels, which may be preferred by Sabine Map Turtles. There was no significant difference in Sabine Map Turtle density between the inner and outer banks of the Mermentau’s original channels, presumably because dredging made the depths along the inner and outer banks similar; new channels also divert water away from the original channels during high-flow events, which may maintain the anthropogenically homogenized condition of the original channels. Introduction Although alteration of habitat plays a major role in the decline of river turtles around the world (Moll and Moll 2000, 2004), the impacts of river channelization on populations of freshwater turtle species have not been well studied. Channelization is a process that straightens rivers and speeds the flow of water through channelized sections of the system; typically it is undertaken for drainage (i.e., to facilitate agriculture or control against flooding) or for navigational purposes (Brookes 1988, Surian 2008). Channelization may impact turtle species through habitat destruction and food-source alteration, as the dredging associated with such projects eliminates sandbars, which are important for turtle basking and nesting, and may also alter habitats of mollusks and arthropods, which are key food sources for several species of turtles (Moll and Moll 2004). Reduction in food supply and alteration of habitat can be a threat to turtle populations and leave more vulnerable species at a greater risk of extinction. Three species of turtles declined or were extirpated in the Illinois River following river degradation that included channelization (Moll 1977, 1980), and channelization was also implicated as one of several causes of declines of riverine turtle species in Iowa (Vandewalle and Christiansen 1Department of Biology and Health Services, Edinboro University of Pennsylvania, 230 Scotland Road, Edinboro, PA 16444. *Corresponding author - Manuscript Editor: John Placyk Southeastern Naturalist C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 120 1996). In contrast, dredging of a channel on the lower Apalachicola River in Florida to facilitate navigation may have benefited resident turtle species in one sense, by increasing the number of nesting sites due to dredge spoil deposited on the floodplain (Ewert and Jackson 1994). Overall, however, the effects of channelization of rivers are believed to be negative for North American turtles, particularly for two highly river-dependent genera, Graptemys (map turtles and sawbacks) and Apalone (North American softshells) (Moll and Moll 2004). Between 1915 and 1935, the upper Mermentau River and its major tributaries in southwestern Louisiana were altered for navigation and drainage purposes (Gammill et al. 2002). The river was straightened, deepened, and cleared of vegetation. In the 1970s, the upper Mermentau was altered again, when seven cut-off channels were constructed to speed drainage to Lake Arthur and marshes further downstream after rainfall. Numerous smaller cut-offs have also been constructed on the river’s largest tributary, Bayou Plaquemine Brule (Fig. 1). In addition, very Figure 1. Map showing the location of the six cut-offs on the Mermentau River (A through G, excluding D) and eight cut-offs on lower Bayou Plaquemine Brule (upper inset map, H through O) that were surveyed for basking turtles. Lower inset map shows the regional location in southwestern Louisiana. Southeastern Naturalist 121 C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 few portions of the Mermentau drainage have not been dredged extensively (Vidrine 2010). Cagle (1953) described Graptemys sabinensis Cagle (Sabine Map Turtle; sensu Lindeman 2013), a narrow-headed species of Graptemys, based on an extensive collection of specimens from the Sabine River in Sabine Parish, LA. The species was first collected six decades earlier, however, from the Mermentau River (Mermentau, Acadia Parish, LA) and a site presumably on the lower Sabine River (Vinton, Calcasieu Parish, LA) (Cagle 1953; Ilgen et al., in press; Lindeman 2013). No further specimens of Sabine Map Turtles were collected from the Mermentau drainage between the 1890s and our studies, and several recent sources have erroneously omitted the Mermentau drainage as part of the species’ geographic range (Ilgen et al., in press; Lindeman 2013). We conducted a survey of basking turtles, in particular Sabine Map Turtles, on the Mermentau River and lower Bayou Plaquemine Brule to determine how construction of cut-off channels has affected turtle species. We surveyed Sabine Map Turtles and other basking turtles on 14 cut-offs, with pairwise comparison of basking abundance and density on the original and new channels. Study Area Taxodium distichum L. (Baldcypress) and Nyssa aquatica L. (Water Tupelo) are the dominant tree species along the banks of the Mermentau River. Houses, shipyards, and other industry occur on a few portions of the old and new channels, but most of the riparian forest is intact. The water is murky and sluggish, and sandbars are lacking; thus, the drainage is atypical habitat for Graptemys species in the southeastern United States (Lindeman 2013). Much of the character of our study area on the Mermentau River and Bayou Plaquemine Brule derives from the fact that the water level is less than 1 m above sea level throughout; thus, flow is often negligible and can even move slowly in an upstream direction during periods of heavy withdrawal for agriculture during droughts (Skrobialowski et al. 2004). At the town of Mermentau (between the cut-offs of lower Plaquemine Brule and the Mermentau), the river is ≈100 m wide and has a median discharge of 36 m3/s (mean = 42 m3/s). At the time of our study, a sustained drought during the spring and summer of 2011 caused flow to be nonexistent, with substantial saltwater encroachment affecting lower sections of the drainage. Along the riverbanks, there were fallen trees, often with emergent crowns, and other deadwood substrates which are used by turtles for basking. The Mermentau channel is 80–180 m in width and has been dredged to depths of 3–4 m, while lower Bayou Plaquemine Brule is 25–50 m wide and is maintained to a depth of 2 m (Gammill et al. 2002). The river engineering on the Mermentau has provided opportunities for habitat expansion by Sabine Map Turtles and other freshwater turtle species. Some of the original channels bypassed by cut-offs on upper sections of Bayou Plaquemine Brule have begun to become filled in and closed off (P.V. Lindeman, 2010–2011 pers. observ.), conditions that do not favor persistence of Sabine Map Turtles (Hively 2009). However, the original channels in our study area on the Mermentau Southeastern Naturalist C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 122 and lower Bayou Plaquemine Brule have remained wide and deep and may still provide suitable habitat for Sabine Map Turtles. Additional turtle species observed during the present study were Trachemys scripta (Schoepff) (Pond Slider), Pseudemys concinna (Le Conte) (River Cooter), Chrysemys picta (Schneider) (Painted Turtle), Sternotherus carinatus (Gray) (Razor-backed Musk Turtle), and Apalone spinifera (LeSueur) (Spiny Softshell Turtle). Graptemys pseudogeographica (Gray) (False Map Turtle), Sternotherus odoratus (Latreille) (Eastern Musk Turtle), and Chelydra serpentina (L.) (Snapping Turtle) also occur in the Mermentau drainage (Ilgen et al., in press) but were not observed during our surveys of cut-offs and original channels. Methods Turtle surveys We surveyed basking turtles on six of the seven cut-offs on the Mermentau (D was not surveyed due to logistical concerns) and the eight lowest cut-offs on Bayou Plaquemine Brule in May 2011 (Fig. 1), with one additional survey of cut-off A from May 2010 included in the data set. We conducted all surveys on sunny warm days between 0900 and 1700 hr. and used a jonboat with a 40-hp outboard motor to conduct basking counts. One person steered the boat within 12–18 m of the bank, while two spotters used 20x Bushnell binoculars and one used 18x Canon binoculars with an image stabilizer. We sighted turtles on deadwood, fallen or bent-over trees that retained foliage, stumps, riprap, and cypress knees, as well as occasionally swimming in the water. We used head and shell patterns and shape to identify turtles, and we kept the jonboat at a low speed to facilitate identification. We also recorded turtles that jumped before identification or were partially hidden from view, classifying them as unidentified. Bayou Plaquemine Brule is narrow enough that we could survey the banks on both sides with one pass down the center of the channel. For the wider Mermentau, we made two passes in each channel, along mainland and island banks. We surveyed most cut-offs more than once and averaged data from multiple surveys. On the Mermentau, we surveyed cut-offs A and G each four times, F three times, B twice, and C and E each once; we conducted no surveys on cut-off D. On Bayou Plaquemine Brule, we surveyed cut-offs H–J each three times and K–O each twice. In an attempt to remove possible bias resulting from turtles in one channel being startled into the water during our survey of the other channel, we systematically alternated whether we surveyed the new channel or original channel first for cutoffs surveyed multiple times as well as whether we surveyed the mainland or island banks first for Mermentau cut-offs. For the same reason, we systematically alternated surveying a series of original channels on Bayou Plaquemine Brule in one pass and then their corresponding new channels on a return pass and vice versa. Channel measurement and turtle densities We measured to the nearest 0.01 km the total length of each old and new channel along its midstream using Google Earth’s satellite map imagery (2011). We used the Southeastern Naturalist 123 C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 total distance surveyed to calculate turtles seen per kilometer. For the Mermentau, we measured each of six original channels along their outer (mainland) and inner (island) banks and calculated basking densities along each bank for comparison. Because some turtles jumped into the water prior to being identified or were too hidden from view to identify, we estimated the proportion of unidentified turtles that were Sabine Map Turtles using a broader data set on relative abundance (Ilgen et al., in press). We calculated total abundance of Sabine Map Turtles by first using the percentages of all identified turtles that were Sabine Map Turtles in the broader Mermentau and Plaquemine Brule surveys (90% and 86%, respectively; Ilgen et al., in press) to estimate the number of Sabine Map Turtles comprising the unidentified category, and then combining confirmed and assumed Sabine Map Turtles. Total turtles included all species observed, including unidentified turtles. We calculated basking densities of confirmed and total Sabine Map Turtles and of all turtles by dividing basking abundance numbers by distances surveyed. Statistical analyses We used S-PLUS to make statistical comparisons of both absolute numbers of turtles seen and turtle densities as paired by each set of an original and a new channel. Due to the paired nature of the data, any potential effects of weather condition, time of day, date, and other factors on turtle basking abundance were nullified. We used the Kolmogorov-Smirnov test to check data for normality (α = 0.05). If the raw data were not normally distributed, we used natural-log transformations of data to achieve normality. For normally distributed data, we used F-tests to confirm equality of variance and then used a paired t-test to test for significant differences in numbers or densities of turtles between original and new channels. For data that could not be transformed to normality, we substituted the Wilcoxon signed-rank test using raw data. We used the same protocol to test for significant differences in Sabine Map Turtle numbers between paired island banks (inner bends) and mainland banks (outer bends) of the original channels on the Mermentau River. Results For all surveys combined, we recorded 656 turtles on original channels (86% identified to species) and 251 turtles on new channels (87% identified to species). The number of confirmed Sabine Map Turtles averaged 12.9 on original and 7.3 on new channels (Table 1). Estimated total average numbers of basking Sabine Map Turtles were 15.0 on original and 8.1 on new channels. The total of all species averaged 17.0 turtles on original and 8.9 turtles on new channels. Despite the fact that in all cases means were greater on original than on new channels, there were no statistically significant differences in paired comparisons of the number of confirmed Sabine Map Turtles, total Sabine Map Turtles, or total turtles (Table 1). A contrasting result was found when numbers of turtles were converted to densities, due to the much greater lengths of the original channels (Fig. 1, Table 2). There were significantly greater densities of confirmed Sabine Map Turtles, total Sabine Map Turtles, and total turtles on new than on original channels (Table 1). Southeastern Naturalist C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 124 For the original channels of the Mermentau, there was no significant difference in the density of confirmed Sabine Map Turtles between island and mainland banks (t = 2.04, df = 5, P = 0.097; Fig. 2). Discussion While the higher numbers of basking Sabine Map Turtles and total turtles on the original river channels relative to new cut-off channels on the Mermentau and Bayou Table 1. Statistical comparisons of absolute numbers of turtles and densities. “Confirmed Sabine Map Turtles” are the total number positively identified to species, while “Total Sabine Map Turtles” combines confirmed Sabine Map Turtles with 90% of unidentified turtles seen in Mermentau surveys and 86% of unidentified turtles seen in Plaquemine Brule surveys (see text). Densities were found by dividing the raw data by the kilometers of river channel surveyed. Original channel New channel Test (mean ± SE) (mean ± SE) DataA StatisticB P Total confirmed Sabine Map Turtles 12.91 ± 4.18 7.32 ± 1.70 raw Z = 1.35 0.1800 Total Sabine Map Turtles 14.98 ± 4.94 8.12 ± 1.75 ln t = 1.68 0.1200 Total turtles (all species) 16.97 ± 5.42 8.86 ± 1.69 ln t = 1.85 0.0880 Confirmed Sabine Map Turtles/km 16.19 ± 2.04 33.84 ± 5.87 ln t = −3.21 0.0068* Total Sabine Map Turtles/km 18.74 ± 2.39 39.05 ± 6.34 ln t = −3.38 0.0050* Total turtles/km 21.45 ± 2.76 44.26 ± 6.53 raw Z = −2.73 0.0040* *Significant difference (α = 0.05). ARaw values or natural-log transformations used to achieve normality. BPaired t-test or Wilcoxon signed rank test (Z) used to test for significant dif ferences. Figure 2. Mean Sabine Map Turtle densities on original channels of cut-offs of the Mermentau River, separated according to bank on which they were seen. Tic marks show standard errors on the means of the six cut-offs. Southeastern Naturalist 125 C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 Plaquemine Brule did not achieve statistical significance, densities of basking turtles were significantly higher on the new channels. Thus, creation of the cut-off channels has created new and apparently more favorable habitat for the Sabine Map Turtle, even as the original channels maintain their status as suitable habitat for substantial populations, which are still high in number if not in density. A possible explanation for the higher densities on new channels is that they may be more lotic in character during high-flow events following rainfall (Brookes 1988, Surian 2008) and carry more nutrients, which may favor the prey base of the Sabine Map Turtle, including caddisfly larvae and other members of algal aufwuchs communities associated with deadwood (Hively 2009, Shively and Jackson 1985). While a period of high flow driven by groundwater might prompt higher basking frequency in cooler new channels than in warmer original channels, the severe drought that took place in 2011 makes this an unlikely explanation for our results. Table 2. Cut-off distances and widths for original and new channels on the Mermentau River (A through G) and Bayou Plaquemine Brule (H through O), the number of surveys conducted on each cut-off, and mean densities of Sabine Map Turtles (including 90% of unidentified turtles seen in Mermentau surveys and 86% of unidentified turtles seen in Plaquemine Brule surveys; see text) and total turtles. Mean Number Sabine Map Mean Cut-off Channel Distance (km) Width (km) of surveys Turtles/km turtles/km A Original 1.13 0.12 3 7.65 10.91 New 0.22 0.07 3 14.45 18.18 B Original 0.58 0.11 2 17.24 22.41 New 0.25 0.07 2 14.00 14.00 C Original 0.96 0.09 1 26.98 29.17 New 0.34 0.07 1 55.88 55.88 E Original 0.60 0.11 1 35.83 38.33 New 0.30 0.07 1 79.33 80.00 F Original 1.08 0.10 3 23.86 26.85 New 0.34 0.07 3 12.44 14.71 G Original 2.19 0.09 4 33.29 36.87 New 0.45 0.07 4 30.11 32.22 H Original 0.26 0.10 3 20.88 21.81 New 0.07 0.06 3 31.29 52.43 I Original 0.44 0.04 3 12.45 15.91 New 0.13 0.04 3 30.38 30.77 J Original 0.17 0.02 3 23.24 25.47 New 0.09 0.04 3 62.56 70.33 K Original 0.38 0.03 2 12.97 15.79 New 0.16 0.05 2 21.88 37.50 L Original 0.27 0.03 2 7.41 12.96 New 0.12 0.04 2 75.00 79.17 M Original 0.91 0.03 2 11.63 12.09 New 0.34 0.03 2 27.74 29.41 N Original 0.32 0.03 2 13.84 14.06 New 0.13 0.04 2 67.08 76.92 O Original 0.54 0.03 2 15.09 17.59 New 0.16 0.04 2 24.56 28.13 Southeastern Naturalist C.A. Hartson, E.L. Ilgen, O.S. Zaleski, and P.V. Lindeman 2014 Vol. 13, No. 1 126 On a sandbar-dominated stretch of river—regarded as typical Graptemys habitat in a Gulf Coastal drainage—one would expect outer bends of a river channel to be deeper with faster current and thus have more deadwood and higher basking densities (Lindeman 1999, 2013). We found no significant difference in Sabine Map Turtle density along the mainland versus the island banks of the six Mermentau cut-offs we surveyed; in fact, while the data approached significance despite low statistical power (only six cut-offs were used in the analysis), higher averages were on the inner, island banks, rather than on the outer, mainland banks. Dredging of the river channel likely made the depths of the channels relatively uniform; in addition, the low gradient of the Mermentau drainage causes flow to be virtually nonexistent except after heavy rains, thus causing little in the way of bank erosion and sediment deposition. Moll and Moll (2004) suggested that channelization and its associated dredging produce negative impacts for turtles by lowering the water table and speeding run-off of water, thus eliminating backwater habitats. In the Mermentau drainage, the primary floodplain backwater habitats would be ponds (platins) and marshes (marais) that have been virtually eliminated in the drainage by agricultural conversion to rice fields and crawfish ponds (Vidrine 2010, Vidrine et al. 2004). While the platins and marais would almost surely have provided additional habitat to several other turtle species (some of which utilize the many crawfish ponds that have replaced them; P.V. Lindeman, 2011 pers. observ.), whether or not these shallow wetlands were ever highly utilized by more river-restricted species like Sabine Map Turtles is uncertain. Our results show that new channels dug to straighten the Mermentau River and lower Bayou Plaquemine Brule, together with dredging that has maintained the connections to original channels, has increased riverine habitat for Sabine Map Turtles and other turtle species of the drainage and may help to maintain larger populations than would be present in the river without the cut-offs. Acknowledgments We thank W. Selman and the Rockefeller Wildlife Refuge of Louisiana for use of their jonboat. W. Selman and S. Shively read an earlier draft of the manuscript and made several helpful comments. A Pennsylvania State System of Higher Education Faculty Professional Development Grant funded our fieldwork in 2011. Literature Cited Brookes, A. 1988. Channelized Rivers: Perspectives for Environmental Management. John Wiley and Sons, Chichester, UK. 326 pp. Cagle, F.R. 1953. Two new subspecies of Graptemys pseudogeographica. 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