DOI: 10.5061/dryad.jh9w0vtbd
title: Submerged aquatic vegetation, water quality (pH, salinity, and turbidity) and waterfowl abundance data from 1991-2017 in Back Bay, Virginia

description: Back Bay, Virginia, has been documented as an important foraging area for
 waterfowl since at least the mid-1800s. Expansive submerged plant beds
 historically supported diverse assemblages of non-breeding waterfowl,
 however coastal development and other anthropogenic influences have since
 led to fluctuations in submerged aquatic vegetation (SAV) and an
 associated decline in waterfowl abundance in the bay. To gain insight into
 the effects of environmental drivers on waterfowl foraging guilds, our
 study explores the effects of SAV frequency and water quality on the
 abundance of dabbling ducks, diving ducks, and swans and geese in Back
 Bay. We use 8 years of SAV, water quality, and waterfowl monitoring data
 collected by state and federal agencies to model the effects of salinity,
 turbidity, pH, and percent frequency of SAV on the relative abundance of
 waterfowl by foraging guild in Back Bay. The appropriateness of the data
 and reasonability of the preliminary results were then evaluated through
 semi-structured interviews with 11 local informants representing state,
 federal, and non-governmental organizations. Quantitative results
 indicated that dabbling ducks are affected differently than other guilds
 by water quality and percent frequency of SAV. Thematic analysis of the
 interview data revealed a number of potential explanations for the model
 results, as well as highlighted areas of uncertainty in need of further
 research. In a test of face validity, participants demonstrated a
 significant degree of belief in turbidity, salinity, and SAV as drivers of
 waterfowl abundance, but were not convinced by the potential effects of pH
 as demonstrated by the model. This mixed methods study provides insights
 that could potentially influence the management and conservation of
 non-breeding waterfowl populations by challenging the assumption that
 particular environmental conditions serve all foraging groups equally.
description: STUDY AREA
 Back Bay, Virginia, USA
 (36°36’49” N, 75°56’40” W) is narrowly separated from the Atlantic Ocean
 by sand dunes and marsh impoundments to the east, bordered by emergent
 wetlands and maritime forest to the north and west, and connected to
 Currituck Sound through Knotts Island Channel to the south (Morton and
 Kane, 1994; USFWS, 2010) (Fig. 1). As Back Bay is situated approximately
 128 km north of the nearest marine inlet, Oregon Inlet, North Carolina,
 the waterbody experiences no lunar tidal action; instead, water levels are
 dictated by the prevailing wind direction and speed (Morton &
 Kane, 1994; USFWS, 2010). Wind tides coupled with precipitation and
 watershed inputs influence the salinity levels of the bay, which has been
 historically characterized as oligohaline, ranging 0-3 parts per thousands
 (ppt) (Norman, 1991; USFWS, 2010). WATERFOWL AERIAL
 SURVEYS Aerial surveys designed to estimate
 wintering waterfowl abundance and distribution were conducted by the VDWR,
 in January of each year between 1998 and 2017, and by the Back Bay
 National Wildlife Refuge biologist in 2011 (January, February, March,
 November, December), 2012 (January, February, March), and 2013 (November,
 December). In both the VDWR and USFWS aerial surveys, waterfowl numbers
 were recorded by species in generalized locations in and around Back Bay.
 Mid-winter inventory flight transects were designed to provide complete
 aerial coverage of the mid-winter inventory survey unit, however USFWS
 surveys placed a greater focus on refuge and state park property (see
 Settle & Schwab, 1991). Count data from the refuge and state park
 impoundments were excluded here to retain compatibility with the SAV and
 water quality datasets. Waterfowl counts by species and
 location were spoken into a handheld recorder (Sony ICD-BX140 4GB Digital
 Voice Recorder, San Diego, USA, or similar), and later reproduced onto
 handwritten datasheets. Scanned copies of the original datasheets were
 provided by the respective agencies and electronically transcribed.
 While pilots varied between surveys, the VDWR and USFWS
 surveys were conducted by the same two biologists, one state and one
 federal, respectively, thus reducing the probability of observer bias
 (Pearse et al., 2008). While Huesmann (1999) and Eggeman and Johnson
 (1989) caution against using aerial mid-winter survey data, the latter
 identify Virginia as an Atlantic Flyway state where surveys have been
 conducted with consistency in methods, personnel, route coverage, and
 survey effort. Waterfowl were categorized into guilds (dabbling ducks,
 swans and geese, and diving ducks) based foraging behavior (Table S1).
 Dabbling ducks normally feed by dabbling their bills or tipping forward in
 water ranging in depth from 5 to 30 cm (Guillemain et al., 2000; Sibley,
 2000; Nelms et al., 2007). Swans and geese feed similarly, by tipping up
 or grazing, foraging at water depths of 0 to 10 cm (Fredrickson &
 Reid, 1988; Sibley, 2000; Tatu et al., 2007; Gyimesi et al., 2011; Nelms
 et al., 2007). Diving ducks prefer water greater than 25 cm in depth,
 where they can dive for SAV as well as animal matter such as clams, fish,
 and various other invertebrates (Pöysä, 1983; Sibley, 2000; Nelms et al.,
 2007). WATER QUALITY MONITORING
 STATIONS Monthly water quality data for
 salinity, pH, and turbidity collected by the Virginia Department of
 Environmental Quality was downloaded from the National Water Quality
 Monitoring Council’s Water Quality Portal for nine stations in and around
 the perimeter of Back Bay (NWQMC, 2021; Fig. 1). Salinity was measured via
 the electrical conductivity method, and pH value in water was measured by
 potentiometry using a standard hydrogen electrode (NWQMC, 2020). Turbidity
 was measured in nephelometric turbidity units (NTU) by the nephelometric
 method (NWQMC, 1995). While water quality measurements were collected with
 consistent methodologies, variation in the time of day that the samples
 were taken, and the speed and direction of wind during sampling events may
 have introduced variability into otherwise standardized measurement
 procedures. SUBMERGED AQUATIC VEGETATION
 TRANSECTS Submerged aquatic vegetation surveys
 in Back Bay were established in 1958 (Sincock et al., 1965) and have been
 surveyed annually by the VDWR since 2009 following the methods of Schwab
 et al. (1991) (G.R. Costanzo, VDWR, unpublished data). During each survey
 event, vegetation samples were taken at 500 m intervals along eight
 transect lines traversing Back Bay (Fig. 1). At each 500 m interval, three
 two-square-foot bottom samples were taken using modified oyster tongs, and
 species of SAV was recorded along with a visual estimate of percent cover
 or density (trace, low, medium, and high) (Fig. S2). The one or two-day
 sampling events were conducted annually between September and November.
 Annual percent frequency of SAV in Back Bay was derived from dividing the
 number of samples with any SAV by the total number of samples taken each
 year. STATISTICAL
 ANALYSIS All quantitative analyses were
 conducted in the R statistical platform with the ‘lme4’, ‘MuMIn’,
 ‘lsmeans’ and ‘multcomp’ packages (Version 1.1.463, 2009-2018) (Hothorn et
 al., 2008; Bates et al., 2015; Lenth, 2016; Bartoń, 2019). As the data
 were not subject to overdispersion, we used a Poisson distribution model
 to investigate the impact of the selected environmental drivers on
 waterfowl abundance by foraging guild. The water quality and SAV variables
 were centered and standardized before analyses to allow for direct
 comparison of model coefficients between variables with different units
 (z-scores; Gelman, 2008). A Bayesian information criterion (BIC)
 comparison, which penalizes more complex models by excluding terms that
 explain only little variability in the data, was used to evaluate three
 models with three-way interactions between feeding guild, SAV percentage,
 and each water quality characteristic (pH, salinity, and turbidity) (Aho
 et al., 2014; Table 1). The first model comprises three, three-way
 interactions, including all additive terms and lower order interactions.
 The second model builds on the first by adding an offset for the number of
 waterfowl surveys per year. In addition to the added offset, the third
 model incorporates year as a random factor. The BIC comparison indicated
 that the first and second models were equally parsimonious. The second
 model, which includes three, three-way interactions between waterfowl
 guild, average water quality measurements, and annual vegetation
 frequencies from 2010 to 2017, as well as an offset for the number of
 waterfowl surveys per year, was chosen to evaluate the data
 (R<sup>2</sup><sub>Adjusted </sub>= 0.46; Table
 S2). A Tukey multiple comparison of means analysis was subsequently run to
 assess differences between abundances across feeding
 guilds.
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