Effects of Weir Management o n Marsh Loss, Marsh Island, Louisiana, USA JOHN A, NYMAN* ROBERT H. CHABRECK School of Forestry, Wildlife, and Fisheries Louisiana State University and Agricultural and Mechanical College Baton Rouge, Louisiana 70803, USA R. G. LINSCOMBE Louisiana Department of Wildlife and Fisheries New Iberia, Louisiana 70560, USA ABSTRACT / Weirs are low-level dams traditionally used in Louisiana's coastal marshes to improve habitat for ducks and furbearers. Currently, some workers hope that weirs may reduce marsh loss, whereas others fear that weirs may accelerate marsh loss. Parts of Marsh Island, Louisiana, have been weir-managed since 1958 to improve duck and furbearer

Weirs have been used in the coastal marshes of Louisiana for over 30 years for waterfowl and furbearer management (Louisiana Wild Life and Fisheries Commission 1964, 1965) and also have been used with hopes of reducing marsh loss (Berry and Voisin 1989). Weirs are constructed in the drainage systems of a marsh and function as low-level dams. T h e crest of a weir is usually set 15 cm below the elevation of the marsh (Chabreck and Hoffpauir 1962). This prevents complete drainage of tidally influenced waterbodies during lowest tide, but still allows the marsh to be flooded during high tide. This has several desirable effects. Ponds behind weirs contain water during winter months when north winds and low tides often drain unmanaged ponds (Chabreck and Hoffpauir 1962). When unmanaged ponds are drained, their use by wintering waterfowl and nongame birds is reduced, but the birds continue to use to weir-managed ponds (Spiller and Chabreck 1975). During winter, trappers and hunters are able travel in weir-controlled waKEY WORDS: Marshes;Weirs; Marsh loss; Marsh management; Louisiana

*Author to whom correspondence should be addressed. Current address: Laboratoryfor Wetland Soils and Sediments, Center for Wetland Resources. Louisiana State University, Baton Rouge, Louisiana 70803, USA

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habitat, Using aerial photographs, marsh loss that occurred between 1957 and 1983 in a 2922-ha weir-managed area was compared to that in a 2365-ha unmanaged area. Marsh loss was 0.38%/yr in the weir-managed area, and 0.35%/yr in the unmanaged area. Because marsh loss in the two areas differed less than 0.19%/yr, it was concluded that weirs did not affect marsh loss. The increase in open water between 1957 and 1983 did not result from the expansion of lakes or bayous. Rather, solid marsh converted to broken marsh, and the amount of vegetation within previously existing broken marsh decreased. Solid marsh farthest from large lakes and bayous, and adjacent to existing broken marsh, seemed more likely to break up. Marsh Island has few canals; therefore, marsh loss resulted primarily from natural processes. Weirs may have different effects under different hydrological conditions; additional studies are needed before generalizations regarding weirs and marsh loss can be made.

terways of a marsh during the lowest tides. Weir management also increases the amount of aquatic vegetation in marsh ponds (Chabreck and Hoffpauir 1962, Louisiana Wild Life and Fisheries Commission 1964, 1965, Chabreck 1968, Larrick and Chabreck 1976). Furthermore, because they maintain a body of water to dilute incoming water, changes in salinity proceed at a slower rate than in unmanaged ponds (Herke 1971, p. 34). Weirs in saline and brackish marshes are opposed by some because weirs interfere with the movement of marine organisms (Herke and others 1987). It is not clear when weirs were first used in coastal Louisiana. Although Viosca (1928) suggested the use of weirlike structures to counter the effects of canals in Louisiana coastal marshes, Arthur (1928) made no mention of weirs in his book on Louisiana fur animals. However, O'Neil (1949, p. 91) presented a copy of a 1942 marsh management plan that called for low dams (weirs) in access ditches that allowed the marsh to flood at high tide, but prevented the ditches from draining completely on low tide. T h e purpose of the dams was to prevent vegetative changes in the marsh resulting from excessive drainage caused by the ditches. Weir construction became a common marsh management practice in the late 1940s and early 1950s, and we believe the most active period of construction was 1955-1965. Weirs apparently were constructed to counter the effects of canals which, in the words of Penfound and Hathaway (1938, p. 46), "pro-

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mote r a p i d r u n o t t a n d also bring about a b n o r m a l fluctuations in water level and salinity d u e to storms and tidal action." At ~me time lhe growdl of Ruppi, mm{tima was great e n o u g h to choke marsh lagoons ( P e n f i m n d and lI:lltl:ma,, 1938, p. 26), and S~bfl,.~ of ~m'i, a plant vahtable [o f u r b e a r e r s a n d walert0wl, was d o m i n a n t in inanv brackish marshes ( ( : h a b r e t k a n d Narcisse 19Sl, ()'Nell 1949). Sc,~p,s olm'~v began declining in dm 1950s, and the decline ~as asnocialed with canals (Chabreck and Narcisse 1981). It was h o p e d ltiat by r e d u c i n g water exchange, weirs would c o u n t e r these perceived etlects o f canals. At tirst weirs were used without adjacent levees; however, weirs are now often f l a n k e d by levees and may be m o d i f i e d with variable crests or flap gates t m a d e changes to the Mississippi Rixcr, marsh loss in onc area was ahvavs m o r e than ol]7,et b~ m a , s h creation in anothei a l e a (Gagllano and others 1981 ). In lhis m a n n e r , tile m:ushes
Louisiana bv landowners who belie~ e that networks o f canals have increased the hydrological link between interior marshes a n d the G u l f o f Mexico. 'l'wo proposed nmchanisnis by which increased hydrological linkage increase marsh loss are increased sahwater intrusion and increased substrate loss by tidal flushing (Chabreck 1981, Mendelssohn and others 1983, Sasser and others 1986). By r e d u c i n g the rate ot water exchange between marshes a n d o p e n waterbodies, weirs mav reduce saltwater intrusion and substrate loss and restore m o r e natural hydrological conditions to marsh altered by canals. T b e use o f weirs is o p p o s e d by those who believe that the network of lexees associated with canals is increasing marsh loss by decreasing ttie hydrological link between interior marshes a n d the G u l f o f Mexico. T w o p r o p o s e d inechanisnis b~ which hydrological isolation increase m a r s h loss are sediment deprivation and plant slress resulting from soil waterlogging (Mendelssohn and others 1983, Cowan and others 1988). Weirs reduce water exchange (Chabreck and H o f t p a u i r 1962, H e r k e 1971, pp. 34, 35), which may r e d u c e s e d i m e n t availability, and p r e v e n t complete drainage, which may. increase soil waterh)gging su-ess. C o n s i d e r e d from this point o f view, weirs ,ha) f u r t h e , negativel} aher hydrological conditions in marshes aftected b~ canals. A l t h o u g h there have bccn no studies c o m p a r i n g m a r s h loss in x~eir-managed marsh a n d u m n a n a g e d marsh, inany researchers believe that weir inanagem e n t accelerates, r a t h e r than reduces, m a r s h loss. Day a n d others (1986) reviewed w e t l a n d - m a n a g e m e n t practices and noted that marsh was lost in i m p o u n d ments ot all types in Louisiana marshes. T h e y r a n k e d the effectiveness o f various types o f i m p o u n d m e n t s a n d c o n c l u d e d that weirs were second best to no mana g e m e n t at all a n d stated that weirs negatively affected p r o d u c t i o n o f organic matter, which is i m p o r t a n t in land-building processes. Cowan and others (1988) c o n c l u d e d fronl a review o f m a r s h - m a n a g e m e n t programs that n o n e o f the p r o g r a m s were m o r e than marginally o r partially successful in controlling saltwater intrusion o r marsh-loss rates in Louisiana. T h e y questioned w h e t h e r weirs have a positive or negative eflect on marsh-loss rates. Wicker and others (1983, pp. 4 - 1 7 ) noted that a weir-managed marsh at Rockefeller Wildlite Refuge in southwestern Louisiana had undergone a significant transition to open water and speculated that levee enclosure and weirs had contributed to m a r s h b r e a k u p bv maintaining h i g h e r than n o r m a l water levels, t t o w e v e r , no u n m a n a g e d marsh was available with which m a r s h loss could be compared. T h e p u r p o s e of this study was to d e t e r m i n e the

Effects of Weir Management on Marsh Loss

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effects of weir management on marsh loss at the Marsh Island Wildlife Refuge.

Study Area Marsh Island is a large island on the central coast of Louisiana (Figure 1). It is bordered on the south by the Gulf of Mexico and on the north by Vermilion Bay, West Cote Blanche Bay, and East Cote Blanche Bay. T h e Marsh Island Wildlife Refuge, operated by the Louisiana Department of Wildlife and Fisheries, occupies all of Marsh Island. T h e island consists of approximately 31,000 ha of brackish marsh. O'Neil (1949) considered Marsh Island to be the meeting place of the Chenier Plain and the Deltaic Plain, whereas Chabreck (1970) placed it entirely in the Inactive Delta Zone. For at least the last 40 years, the dominant vegetation has been Spartina patens and subdominant vegetation has been either Scirpus olneyi or Juncus roemerian~ (O'Neil 1949, Orton 1959, Chabreck and Hoffpauir 1962). T h e soil is highly organic, over 50% in some places (Chabreck 1970). The climate is humid subtropical with 152-163 cm (60-64 in.) of precipitation per year, a 300 to 310-day growing season, and an average annual temperature 20-21~ (69-70~ (Neuton 1972)9 Coastal Louisiana is subject to Ix~th winter storms and tropical hurricanes (Huh and others 1989)9 Lunar tides are weak and range 0.3 m, and winds often dominate water-level fluctuation. Persistent north winds associated with winter storms produce extremely low water levels and completely drain many marsh ponds (Chabreck and Hoffpauir 1962). T h e r e may be 2 0 - 3 0 storms each winter (Huh and others 1989). A program of weir construction on Marsh Island began in 1958 to increase the quality of duck and furbearer habitat (Louisiana Wild Life and Fisheries Commission 1959). T h e largest weir-managed area is drained primarily by seven large bayous, each equipped with a fixed-crest weir. It is approximately 5260 ha in size and is situated east of Bird Island Bayou (Figure 1). Two study areas were used and were separated by Bird Island Bayou (Figure 1). The weir-managed marsh was located east of Bird Island Bayou and contained 2922 ha. A 2365-ha section of a unmanaged marsh area west of Bird Island Bayou was used as a control. Both areas consisted of solid marsh and broken marsh interlaced with lagoons and numerous ponds of various sizes. Marsh Island has relatively few canals, thus any marsh loss that has occurred was primarily the result

~, k , .

Figure 1. Ix)cation of Marsh Island, Louisiana, and the weirmanaged and nonmanaged study areas compared; the dashed box is the area shown in figure 2. of natural processes. Bird Island Bayou was dredged shortly befi)re aerial photographs were made in 1957 and a short (<2000 m) access canal was dredged from Bird Island Bayou into each study area after 1957. A small portion of the unmanaged study area was under the influence of two weirs li)r several years in the mid-1960s, but these weirs were destroyed by a hurricane shortly after they were constructed. Weirs in the area east of" Bird Island Bayou were constructed between 1958 and 1960. Other than the weirs in the weir-managed marsh, no other man-made alterations to the natural hydrology of the island were present that would affect water- and salinity-level dynamics. T h e water- and salinity-level dynamics of the unmanaged marsh were dominated by north-draining bayous and thus were unaffected by either the spoil bank on Bird Island Bayou or an impoundment levee to the south.

Methods Black-and-white aerial photographs of Marsh Island made in 1957 were obtained from the US Department of Agriculture and used to estimate the amount of marsh in the study at that time. These photographs had a scale of approximately 1:21,000, were 23 x 23 cm (9 x 9 in.) in size, and were taken approximately one year before weirs were constructed on the island. T h e western half of the island was photographed 14 February 1957, and the eastern half was photographed 15 March 1957; there was no detectable difference in marsh photographed in both months. Because there was considerable overlap of the area included by individual photographs, a mosaic of the island was constructed from the photographs.

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Marsh d r a i n e d by bayous that later became weirm a n a g e d was e x a m i n e d using the 1957 aerial photographs. S i m i l a r - a p p e a r i n g marsh in d r a i n a g e systems o( bayous that are not now w e i r - m a n a g e d was selected as a control. h d r a r e d pht)tographs o t Marsh Island taken 19 Noventber 1983 were used to estimate the a m o u n t o f m a r s h in 1983. T h e s e p h o t o g r a p h s were large e n o u g h that only four were r e q u i r e d to cover both arcas; t h e r e f o r e no composite was made. T h e scale o f these p h o t o g r a p h s was a p p r o x i m a t e l ) 1: 16,000, and m o r e detail was visible than in the 1957 p h o t o g r a p h s . Because o f difficulties m d e t e r n t m i n g land water imerfaces, marsh was d e t i n e d as vegetated land, and marsh loss was d e t i n e d as a reduction in the a m o u n t o f area occupied by vegetated land. ( ) n Marsh Island, the d o m i n a n t vegetation is persistent y e a r - r o u n d and unvegetated m u d |]ats are licit tound. T h e change in percent marsh in the study areas t)etween 1957 and 1983 was c o m p a r e d to d e t e r m i n e if the a m o u n t of change d i f f e r e d between the w e i r - m a n a g e d area a n d the u n m a n a g e d area. A ditt'erence in marsh loss o f less than 59} over the 26 years (0.19C/c/year) was chosen a priori as attributable to natural ~ariation in m a r s h loss. A dot grid was used to estimate tim l)crccntagc ~1 ntarsh in each study area in 1957 and 1983 (copyright 1942, bv Milton M. Br~an). T h e gild is an acetate t r a n s p a r e n c y on which small dots are regularly arranged (9.9/cm 2, 6t/in. e) and was tlsed in the t b l h m m g m a n n e r . T h e grid was r a n d o m l y placed on the area ~ t interest, and with the aid o f a magnifying glass, the total n u i n b e r o f dots that lay over the area were counted. T h e n u n t b e r of dots over marsh were then c o u n t e d a n d the p e r c e n t a g e o f marsh in the area was c o m p u t e d . In both years, tire r e m o v e d vegetation fronl sonle areas, a n d these areas were considered marsh even t h o u g h they were devoid ~)t vegetation. T h e grid was not large e n o u g h to cover the study areas, so each area was divided into units small e n o u g h to be covered by a r a n d o n t throw o f the grid. In the 1983 p h o t o g r a p h s , areas that were canals o r spoil banks (<0.15} o f either area) were omitted. Each study area was sampled t h r e e times with tim dot grid to estimate the technique error. This p r o c e d u r e p r o v i d e d an estintate o f variation inllerent in the m e t h o d arid was used to calculate 95(/c confidence limits about the estimates o f percenl marsh in each area in each )'ear. Sample units did not overlap a n d covered the entire study areas. T h e r e t o r e , no sampling e r r o r was m a d e as the entire p o p u l a t i o n was sampled.

Results and Discussion In 1957, the interior o f Marsh Island had a high

Figure 2. A 1(,)57 aerial photograph showing solid (s) and broken (B) marsh, typical of the interior o[ Marsh Island. [ x)ttisiana. interspersion o f water. Many small p o n d s and potholes were spread tim mgh~mt, but their density was grealc>t in the central part of the island where the marsh was quite b r o k e n (Figure 2). In 1(,)57, 81.99~ +_ ().45~ o f the u n m a n a g e d area was marsh, a n d ~ 1.57c -+ ().35'~ of the area that was later wt'ir-managed w a s n l a l s h . T h e rem a i n d e r o f tim areas was o p e n water, p r e d o m i n a n t l y in the t o r m o f small ponds. In 1983, 72.8% +- 0.5c~:~ of the u n m a n a g e d area was inarsh a n d 71.5% +- 0.5% o f the w e i r - m a n a g e d area was marsh. Between 1983 a n d 1957, the w e i r - m a n a g e d area lost 10.0cJ (0.38~/yr), and the u n m a n a g e d area lost ,9.1f7c (().355'~/vr) o f the area to o p e n water: tim difference in marsh loss rates was (I.03%/vr. T h e combination o f b r o k e n marsh areas a n d o t h e r areas affected by recent fire m a d e the dot grid the best choice for this study. Methods that use planimetry entail decisions that are to some d e g r e e subjective, i.e., w h e r e to place the b o u n d a r y between solid and b r o k e n marsh, a n d what is the nfinimum sized feature to be noted. T h e s e decisions may vary a m o n g workers, or within a w o r k e r over time. For these reasons, the dot grid may be s u p e r i o r to m e t h o d s using planimetry for detecting small changes in b r o k e n marsh areas. Pixel analyses could have p r o v i d e d a m o r e precise measurem e n t o f percent marsh, a n d the data could have been stored on c o m p u t e r to allow additional analyses o f the data. However, the early p h o t o g r a p h s were black and white, a n d the later p h o t o g r a p h s contained areas recently affected by tire, a n d this p r e v e n t e d the use o f pixel analyses. F u t u r e workers should consider using a

Effects of Weir Management on Marsh Loss

dot grid to determine percent marsh if the study areas contain broken marsh and portions affected by recent ~re. Because marsh loss in the two areas differed less than 5% over the 26-year interval between photographs, i.e., <0.19%/yr, we concluded that weir-management did not affect marsh loss in this study area. Although the significance level was not exceeded, it appeared extremely conservative in relation to the range o f marsh loss rates f r o m brackish marshes in Barataria Basin, Louisiana. Adams and others (1976) f o u n d that marsh loss there ranged 1.26%/yr f r o m 0.63%/yr to 1.89%/yr. It therefore appears that natural variation in marsh loss rates may exceed the significance limits chosen for this study. Although this had no effect on the conclusions reached in this study, future workers should consider using a larger significance level to avoid incorrect conclusions. Appreciable marsh loss occurred, with roughly 10% o f both areas converting f r o m marsh to pond. T h e increase in water area between 1957 and 1983 was not a result o f the expansion o f lakes and bayous. Rather, the area o f broken marsh increased, and the a m o u n t of vegetation within previously existing broken marsh further declined. T h e conversion of solid marsh to broken marsh appeared to spread from areas o f broken marsh existing in 1957. T h e conversion o f solid to broken marsh was not random; instead, marsh farthest f r o m large lakes and bayous seemed more likely to convert to broken marsh. This pattern was also noted by Sasser and others (1986). Marsh is being lost despite a vertical accretion rate o f 0.70 crrdyr on Marsh Island (DeLaune and others 1987). This accretion rate may be insufficient to counteract the effects o f eustatic sea-level rise and subsidence. T h e a p p a r e n t result o f insufficient vertical accretion is p o n d formation as the area o f broken marsh increases at the expense o f solid marsh. It is possible that marsh loss on Marsh Island may reverse in the next century as the delta o f the Atchafalaya River expands. Marsh Island lost marsh less rapidly than Barataria Basin, Louisiana. As noted, Adams and others (1976) f o u n d that marsh loss in the brackish marshes o f Barataria Basin ranged from 0.63%/yr in some locations (8.91% lost between 1962 and 1974) to 1.89%/yr in other locations (34.17% lost between 1956 and 1974). T h e difference in marsh loss between Marsh Island and Barataria Basin may have been a result o f lower accretion rates in Barataria Basin, 0.59 cm/yr (Hatton and others 1983), than at Marsh Island, as well as o f alterations in the water- and salinity-level dynamics o f Barataria Basin. Differences in subsidence rates between Marsh Island and Barataria Basin also may have

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led to different rates o f marsh loss. Estimates o f subsidence are greater for Barataria Basin than for the central Louisiana coast, 0.24 cm/yr (Adams and others 1976) vs 0.07 cm/yr (Coleman 1966). At Marsh Island, neither removal o f the existing weirs nor construction o f additional weirs is likely to affect marsh loss, but the conclusion that weirs did not affect marsh loss should not be generalized to other marshes with different hydrological conditions. It is possible that weirs may increase marsh loss where spoil banks increase marsh flooding and decrease marsh loss where canals allow rapid water exchange. Before generalizations can be made about weirs and marsh loss, additional studies o f this nature are needed in areas where canals and spoil banks affect water- and salinity-level dynamics.

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DeLaune, R. D., C.J. Smith, W. H. Patrick, Jr., and H. H. Roberts. 1987. Rejuvenated marsh and bay-bottom accretion on the rapidly subsiding coastal plain of US Gulf Coast: a second-order effect of the emerging Atchafalaya Delta. Estuarine, Coastal, and Shelf Science 25:381-389. Frazier, D. E. 1967. Recent deltaic deposits of the Mississippi River: their development and chronology. Transactions of the Gulf Coast Association of Geological Soc#ties 17:287-311. Gagliano, S. M., K.J. Meyer-Arent, and K. M. Wicker. 1981. Land loss in the Mississippi deltaic plain. Transactions of the Gulf Coast Association of Geological Societie~ 31:295-300. Hatton, R. S., R. D. DeLaune, and W. H. Patrick, Jr. 1983. Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana. Limnology and Oceanography 28(3):494-502. Herke, W.H. 1971. Use of natural, and semi-impounded, Louisiana tidal marshes as nurseries for fishes and crustaceans. PhD dissertation. Louisiana State University, Baton Rouge, Louisiana. 264 pp. Herke, W.H., E.E. Knudsen, P.A. Knudsen, and B.D. Rogers. 1987. Effects of semi-impoundment on fish and crustacean nursery use: evaluation of a "solution." Coastal Zone '87 5:2562-2576. Huh, O. K., L.J. Rouse, H. H. Roberts, S. A. Hsu, and D. A. Rickman. 1989. Cold front passages and the response of coastal water and sediment: Mississippi Delta Plain Region, USA. Pages 10-11 in US Fish and Wildlife Service and the State of Louisiana. Abstracts from the 3rd annual workshop on remote sensing and geographic information systems for coastal management in Louisiana. US Fish Wildlife Service. National Wetlands Research Center open file report 89-01. Larrick, W. D., and R. H. Chabreck, 1976. Effects of weirs on aquatic vegetation along the Louisiana coast. Proceedings of

the Annual Conference of Southeastern Game and Fish Commissioners 13:100-115. Louisiana Wild Life and Fisheries Commission. 1959. Biannual progress report, 1958-1959, W-29-R-II. Louisiana

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Jour,~l of the Limnotogy Socie~ oJ South Africa 9(2):63-75. Neuton, M. B.,Jr. 1972. Adas of Louisiana--a guide for students. Ceoscience Publications. Louisiana State University, Baton Rouge, Louisiana. 196 pp. O'Neil, T. 1949. The muskrat in the Louisiana coastal marshes. Publication of the Federal Aid Section, Louisiana Wild Life and Fisheries Commission, New Orleans, Louisiana. 152 pp. Orton, E. W. 1959. A geological study of Marsh Island, Iberia Parish, Louisiana. Louisiana Wild Life and Fisheries Commission, New Orleans, Louisiana. 28 pp. Peltier, W. R., and A. M. Tushingham. 1989. Global sea level rise and the greenhouse effect: might they be related? Science 244:806-810. Penfound, W.J., and W. S. Hathaway. 1938. Plant communities in the marshlands of southeastern Louisiana. Ecolog@al Monographs 8( 1)'.1- 56. Ramsey, K. E., and S. Penland. 1990. Relative sea-level rise in Louisiana and the Gulf of Mexico: 1908-1988. Journal of Coaztal Research 6:323-342. Sasser, C. E., M. D. Dozier, J. G. Gosselink, and J. M. Hill. 1986. Spatial and temporal changes in Louisiana's Barataria Basin marshes, 1945-1980. Environmental Managemerit 10(5):671-680. Spiller, S. F.,and R. H. Chabreck. 1975. Wildlife populations in coastal marshes influenced by weirs. Proceedings of the

Annual Conference of the Southeastern Association of Game and Fish Commissioners 29:518-525. Turner, R. E., and D. R. Cahoon. 1987. Causes of wetland loss in the coastal central Gulf of Mexico. Volume II: technical narrative. Final report submitted to Minerals Management Service, New Orleans, Louisiana. Contract No. 14-12-0001-30252. OCS study/MMs 87-0120. 400 pp. Viosca, P. V. 1928. Louisiana wet lands and the value of their wild life and fisheries resources. Ecology 9(2):216-229. Wicker, K. M., D. Davis, and D. Roberts. 1983. Rockefeller state wildlife refuge and game preserve: evaluation of wetland management techniques. Louisiana Department of Natural Resources, Coastal Management Section 56 pp.