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The impact of drainage maintenance strategies on the flora of a low gradient, drained Irish salmonid river

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In 1990, the Central Fisheries Board initiated research on how drainage maintenance practices and strategies might be modified to enhance the salmonid carrying capacity of the affected water while maintaining an acceptable degree of conveyance. Much
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  Hydrobiologia 34 : 197-203, 1996 197 J. M. Caffrey, P R. E Barrett, K. J. Murphy R M. Wade eds), Management and Ecology of Freshwater Plants. Q 1996 Kluwer Academic Publishers. Printed n Belgium. The impact of drainage maintenance strategies on the flora of a low gradient drained Irish salmonid river James J. King Central Fisheries Board, Mobhi Boreen, Glasnevin, Dublin 9, Republic of Ireland Key words: Channel maintenance, cross-section modification, floral succession, Sparganium erectum, low gradient  bstract In 1990, the Central Fisheries Board initiated research on how drainage maintenance practices and strategies might be modified to enhance the salmonid carrying capacity of the affected water while maintaining an acceptabledegree of conveyance. Much of the maintenance requirement was caused by dense in-channel weed beds impedingdischarge and facilitating siltation. The impact of various maintenance regimes on the aquatic flora was examined in the course of pilot studies on channels of base width 3-9 m. The findings from one of these, the R. TullamoreSilver which chokes annually with Sparganium erectum L., are presented. Overdigging the centre of the channeland placement of spoil at the margins confined S. erectum to a narrow marginal zone and facilitated development of a submerged, open-water flora. Introduction In Ireland, under the 1945 Arterial Drainage Act, the Office of Public Works (OPW) was given responsibility for the design and implementation of drainage schemeson a catchment basis. The OPW was also required to 'permanently maintain arterial drainage works to an adequate standard' (Howard, 1980). Maintenance is usually required in those channels with a low longitu-dinal gradient and/or those which were over-widened at the works stage and thus rendered incapable of self- cleansing. Siltation in marginal or open water areas in such channel sections facilitates the development of various macrophyte species leading to further siltation. This process of sediment accretion and macrophyte expansion can lead to impedance of water, causingback-up in side channels carrying run off from landdrains, reducing the conveyance in the channel and leading to demands from riparian owners for mainte- nance. While the impacts of channelization works onlotic ecosystems have been extensively documented(Swales, 1982), little study has been done on the eco-logical impacts of post-works maintenance. In 1990 the OPW requested the Central Fisheries Board to under- take research on alternative strategies which might be incorporated during routine mechanical maintenanceprogrammes involving desilting and weed removal.It was envisaged that such strategies should have an enhancing effect on the channel's salmonid carrying capacity while maintaining an acceptable degree of conveyance. While particular attention was given to thestatus of salmonid fish stocks, the impact of the exper-imental works programme on aquatic plants was mon-itored where appropriate. The results below describe the vegetation changes observed in one such channel,the R. Tullamore Silver, in the four year period (1991to 1994) since maintenance. Study site The R. Tullamore Silver, a tributary of the R. Clodiagh in the Brosna catchment, is an alkaline, moderatelyenriched channel of high conductivity. The study sites in this channel all lay in a 6 km section of uniform- ly low gradient (0.09 ). Channel base width rangedfrom 5.5-6.5 m with lateral sediment deposition form-ing low shelf or berm areas, also known as secondarybanks, close to the water surface. Tree/shrub coveroccurred only on the bankfull line and accounted for  198 less than 5 of channel length on any bank in the areas examined. While many bankside areas were un- fenced, a combination of relatively high banks andsteep bank slope restricted animal trampling and per- mitted a vigorous riparian cover of terrestrial grasses. The hydraulic regime was one of a continuous glide and the floral regime was characterised by two main types, an Apium sp.-dominated mixed flora in shallow (0.5- 0.75 m) reaches on a hard stony-clay bed and a Sparga-nium erectum L.-dominated flora in deep (0.75-1.5 m)glides on silt overlying firm sandy clay. The channel held a good stock of brown trout Salmo trutta L. with angling until mid-May each year. Subsequently,the growth of S. erectum excessively impeded angling. High water levels and frost in late autumn general- ly cleared the channel of S. erectum and conveyance was unimpeded by macrophytes in the winter-springperiod.The R. Tullamore Silver was arterially drained in the early 1950's. From 1972 to 1994 it has been main-tained on five occasions, on a c. 5-yearly basis. Thiswork is currently executed by hydraulic excavators. Materials and methods Hydraulic machine works programme The experimental maintenance programme stipulatedthat only material available on-site could be used in works and that all manipulation be done by hydraulicdigging machines. This precluded strategies involving chemical control or mechanical cutting of nuisanceweed.Standard maintenance aims to restore the channel'sdesign conveyance by re-profiling the cross-section,removing silt banks, instream macrophytes and oth-er obstructions. Maintenance was carried out in an upstream direction, on a 9 km section of the R. Tul- lamore Silver between October 1990 and February 1991, beginning at the channel's confluence with the R. Clodiagh. Five sites, lying between 3.5 and 6 km of the channel's downstream end, were selected for detailed examination and specific maintenance treat- ment applied. One of the sites was an Apium-dominated zone and four were areas of heavy, full channel-widthcover of Sparganium erectum. Qualitative assessment in autumn 1990, prior to maintenance, indicated an in- channel Sparganium erectum cover in excess of 75 at all four Sparganium sites chosen. The Apium site(site 1) was given standard maintenance as were two of the Sparganium sites (sites 2 and 3). A third Spar- ganium site had some degree of secondary bank area, colonized by Glyceria maxima (Hartman) Holmberg,along both banks. This site was left as an unmain-tained control (site 4) to see if the vegetated marginaldeposits might serve as a nucleus for further deposi- tion with consequent channel narrowing and increased velocities leading to self-cleansing. The fourth Sparga- nium zone was maintained in an experimental manner (site 5). The centre of the channel was overdeepened,removing deposited silt, bed clay and nuisance weedmaterial. The spoil produced was towed to the mar- gins to form a secondary bank of clay material, givingthe channel a V-shape in cross-section rather than the traditional trapezoidal cross-section. Sites 1, 2 and 4 were c. 100 m in length. Sites 3 and 5 were contiguous, the former being 25 m long and lying upstream of the latter, which was 75 m in length. Monitoring of macrophytes Monitoring of macrophytes took place annually from 1991 to 1994 in the September - early October periodbefore any plant die-back had occurred. In the case of sites 1, 2, 3 and 5 this took the form of a scaled mapping (1:100) of the macrophyte cover within a representative 25 m sub-section of the test site. Marker pegs wereset at 5 m-intervals along the bank in each mapping site. An engineering tape was stretched horizontally across the channel at each peg in turn and the width,from left-hand side, of each major vegetational cover form and of open-water recorded. These width valueswere transcribed onto metric graph paper for each 5-m transect and the points linked to show the extent of open water and major vegetative elements. Smaller stands of less prominent species or isolated stands of majorelements were then drawn in on the mapping. The percentage vegetation cover and contribution to cover of individual species was compiled. The same 25 m sub-sections were mapped annually. Caffrey (1990)considered that a 25 m channel length was adequate to reflect the instream status of macrophytes presentwithin longer, similar channel sections. In the first series of monitorings after maintenance, in September 1991, it was impossible to carry out any instream monitoring in site 4 due to the dense nature of the marginal and instream cover. Instead, an annualphotographic record was taken when other sites were being mapped. The natural development of a narrow open-water passage through this site in 1994 enabled  L.H.S 6 10 R.H.S. 12 Rodipp S.emcUm. | Open water Site 3: prganium tandard maintenance parganium experimental Figure 1. Characteristic channel cross-sections form sites 1 to 5 on R. Tullamore Silver, September 1993. Scales in metres. Left hand side(L.H.S.) and Right hand side (R.H.S.) with respect to observer facing downstream. 199 2   200 % omposition Apium standard maintenance Fringing herbs Mixed rm n r Phalais G.maxima S.erectum P.natans Other I 1 Site : Sparganium standard maintenance . I A J; Fringing herbs Mixed Phalaris G.maxima S.erectum P.natans Other Site 5: Sparganium experimental Mixed Phalar s G meaxima S.erecrum P.natans Other  199 1991 1992 1993 * 1994 Figure 2. Changes in floral cover (% omposition) at four test sites in the R. Tullamore Silver, 1990-1994. Site 1 so   2 EL m P+KIYM 1 1 -4 JA . Law 11111111116b.M11111111111 l  201 boat access for electrofishing and instream monitoring. A floral mapping was also carried out in 1994. Results Site I - Apium standard maintenance This was the shallowest site in cross-section with min-imal maintenance required. The bed slope to the left bank from mid channel was a consistent feature of all cross-sections measured (Figure 1). Prior to main-tenance the site had less than 50 floral cover com- posed mainly of Apium sp., Potamogeton natans L. and Sparganium erectum. The floral cover was substantial- ly reduced following maintenance and remained at a level lower than that of the pre-maintenance regime for two years. However, a small increase in the Apium cov-er, a reduction of P. natans and loss of both S. erectum and Phalaris arundinacea was noted in the mapping site. Three years after maintenance a dramatic expan- sion of Apium occurred, with full-width coverage, in much of the site, of submerged and marginal emergentmaterial (Figure 2). This emergent growth occurredalmost entirely on the left-hand-side bed slope area where water depth was lowest. Half of this Apium cov- er was lost 12 months later and large areas of barebed were observed. Increases in the cover of P. natans, S. erectum and Hippuris sp. were recorded. Sites 2, 3 - Sparganium standard maintenance Both sites showed evidence that maintenance, carried out from the left bank, had excavated material pri-marily from the right-of-centre of the channel leav- ing a uniform slope from mid-channel to the water surface on the left bank (Figure 1). Photographic evi- dence compiled during a fish stock survey three monthsafter maintenance showed a continuous band of Spar- ganium erectum, up to 2.5 m in width, growing on this partially-maintained slope. Two similar vegetationchanges occurred at both sites in the three-year peri- od after maintenance. Firstly, each showed an annualdecline in Sparganium erectum L. cover (Figure 2) andits replacement by various combinations of Glyceria maxima, Phalaris arundinacea or, where trampling by cattle had occurred, terrestrial grasses. In addition,low-growing fringing herbs, principally Apium sp., Mentha aquatica L., Rorippa sp., Veronica anagallis-aquatica L. and Myosotis sp. formed mixed or discretestands either as an understorey to the tall emergentgrasses or in open space. The emergent grasses grew on the landward side of the Sparganium. The moresubstantial cover of S. erectum at site 3 was related tothe more extensive shelf of shallower water on the left side of the channel, relative to that at site 2 (Figure 1). Trampling by cattle was considered responsible for the greater initial expression of fringing herbs at site 2 than site 3, where cattle did not access the channel.Percentage cover of S. erectum, P. arundinacea and G. maxima remained stable between 1993 and 1994 at site 2 whereas the cover of S. erectum continued todecline at site 3, being replaced by fringing herbs, pri- marily Rorippa sp. and Mentha aquatica L.. On someoccasions, it was not possible to identify individual floral elements due to heavy grazing pressure on some vegetated areas.. Such areas are referred to as 'Mixed Vegetation' in Figure 2. The status of the open water flora, comprising Potamogeton natans and P pectina- tus L., remained relatively constant in each site over the 1991-94 period. Site 4 - Sparganium control This site persistently showed a full channel width of S. erectum, fringed on both margins by Glyceria maxi- ma and Phalaris arundinacea. Shallow water adjacent to cattle slips harboured stands of Callitriche sp. and Veronica anagallis aquatica. Photographic evidence taken annually in September confirmed the consistent,uniform status of the flora here and only one mapping was compiled in the study period. The cross-section(Figure 1) ndicated the extent of the silt bar colonized by S. erectum. The small area of deeper water formed a narrow 'open-water' weed-free passage. Site 5 - Sparganium experimental maintenance The shallow marginal areas of bed clay created by maintenance were liable to inundation, depending on water level, (Figure 1) and were colonized by a range of fringing herbs within a year of maintenance. Spar- ganium erectum covered less than 25 of the site in this first year (Figure 2) and was confined to narrow marginal strips on the sides of the re-shaped channel.The contribution of S. erectum declined substantially in the following two years as colonization of the exten- sive secondary bank shelf on each side of the channel, by Phalaris and Apium (Figure 2), continued. Themore extensive shelf on the left side also harboured the fringing herb species found at sites 2 and 3. A wide,open-channel area remained free of nuisance emergent
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