Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 3: Ecosystem Effects <strong>of</strong> Invasive Spartinaeach mudflat section we estimated <strong>the</strong> slope (assumed linear)by dividing <strong>the</strong> total change in elevation across <strong>the</strong> mudflat(MTL) by mudflat width (slope = rise/run). Then we createdtwo 3-m elevation grids based on <strong>the</strong> following equations,where each 3- m pixel value was equal to <strong>the</strong> elevation atthat point:(1) elevation 1 = slope * distance to MLLW; and(2) elevation 2 = MTL – (slope * distance to MTL).We averaged <strong>the</strong>se two grids to obtain <strong>the</strong> final 3-mmudflat elevation grid (DEM). Values greater than localMTL were redefined to be equal to local MTL.Tidal Inundation and Shorebird Habitat ValuePublished verified continuous (six-minute interval)water level data was available from NOS for only three <strong>of</strong> <strong>the</strong>seven tide station locations: Alameda (station ID 9414750,year 2001), Dumbarton Bridge (station ID 9414509, year1996), and Redwood City (station ID 9414523, year 2002).Assuming that <strong>the</strong> tidal inundation regime for each mudflatarea was most similar to <strong>the</strong> nearest tide station with availablecontinous water level data, we created a tide station gridlayer by allocating each 3-m pixel to a tide station (Fig. 1).Because our shorebird data collection efforts were centeredaround April and September, we generated monthly inundationcurves for <strong>the</strong>se months using cumulative water level datafrom each tide station using Stata 8.0 (2003) (Fig. 2).Water level values were in meters above MLLW. Topredict <strong>the</strong> monthly tidal inundation percent <strong>of</strong> each 3-mmudflat pixel, we performed separate polynomial regressionanalyses (Stata 8.0, 2003) for each tide station and eachmonth (April and September) using <strong>the</strong> appropriate NOS6 minute water level data. Resulting regression equations(Table 1) were used to calculate grids representing April andSeptember inundation. Separate equations were developedfor each tide station so grids for each <strong>of</strong> <strong>the</strong> three tide stationTable 1. Coefficients for regression equations used to predict inundationfrom elevation. Equations were based on inundation curves obtained from6-minute water level data from NOS tide stations and took <strong>the</strong> form y =ax + bx 2 + cx 3 + d, where y = percent <strong>of</strong> time inundated and x = elevationabove MLLW.Tide station Month a b c d R2Alameda April -27.9 -24.9 7.85 91.3 0.9974Alameda Sept -21.6 -28.4 7.92 96.4 0.9997DumbartonBridgeApril -19.7 -13.7 2.93 94.6 0.9990DumbartonBridgeSept -20.4 -12.1 2.36 98.2 0.9984RedwoodCityApril -20.4 -16.8 4.01 93.9 0.9974RedwoodCitySept -15.7 -18.2 3.79 97.9 0.9994areas could be calculated separately and <strong>the</strong>n merged to createone seamless 3-m inundation grid for each season. We <strong>the</strong>ngenerated grids representing April and September mudflatexposure (100 - monthly inundation percent), as indices <strong>of</strong>overall shorebird habitat value (Fig. 3).Potential Spartina SpreadCumulative water level data (m above MLLW) from eachtide station were used to generate monthly June inundationcurves using Stata 8.0 (2003). For each tide station, June dura-Fig. 2. Cumulative duration <strong>of</strong> tidal inundation curves for April based onwater level data from Alameda, Redwood City, and Dumbarton Bridgetide stations. Elevations corresponding to 60% inundation were 0.762 m,1.018 m, and 1.083 m, respectively.Fig. 3. April mudflat habitat availability, based on monthly mudflat exposureexpressed as 100% - monthly mudflat inundation. Color shadings representmudflat habitat quantiles, where 20% <strong>of</strong> <strong>the</strong> total area is contained in eachshading category and darker colors have higher value.- 177 -