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Temporal and Spatial Changes in Vallisneria americana Michaux (Tape-grass) Beds in the Lower St. Johns River, Florida, from 2002–2011
Nisse A. Goldberg, Tiffany Trent, and John Hendrickson

Southeastern Naturalist, Volume 17, Issue 3 (2018): 396–410

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Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 396 2018 SOUTHEASTERN NATURALIST 17(3):396–410 Temporal and Spatial Changes in Vallisneria americana Michaux (Tape-grass) Beds in the Lower St. Johns River, Florida, from 2002–2011 Nisse A. Goldberg1,*, Tiffany Trent2, and John Hendrickson2 Abstract–Vallisneria americana (Tape-grass) in the lower St. Johns River, FL, is exposed to variability in salinity and turbidity. From 2002 to 2011, we compared mean blade length, plant depth, and bed width between residential and natural shorelines, the western and eastern sides, and river sections of 64–80 km, 81–96 km, and 97–112 km from the river mouth. Leaf blades of eastern plants were 27.0 cm longer and were found in 0.57 m greater depths, a pattern possibly related to seasonal westerly winds. We observed no differences with land use. Blades were 20.0 cm longer in the farthest section where salinity concentrations were 1.35 ppt lower than in the 64–80 km section. Following hurricanes, resilience depended on pre-storm bed health and post-storm water quality. Introduction Vallisneria americana Michaux (Tape-grass) is a rooted aquatic angiosperm that can grow in fresh to brackish waters with a distribution encompassing southern Canada and 42 states in the continental US (USDA NRCS 2017). In Florida, Tapegrass’ southern distribution reaches Miami-Dade County, extending northward to Duval County and westward to Santa Rosa County (Wunderlin et al. 2017). In northeastern Florida, Tape-grass beds occur between 43 km and 161 km from the mouth of the St. Johns River (SJR), a river with headwaters in east-central Florida flowing 500 km northward to discharge into the Atlantic Ocean (Dobberfuhl 2007, Sagan 2007). Tape-grass beds represent >60% cover of submerged aquatic vegetation (Dobberfuhl 2007, Sagan 2007), and the canopy provides refuge and food for crabs, fish, and marine mammals (Barnett and Schneider 1974, Camp et al. 2012, Hauxwell et al. 2004, Laughlin 1982). Although recorded in depths to 7 m (Catling et al. 1994), this aquatic plant grows in depths of less than 1 m in the lower SJR due to low-light conditions associated with the high-tannic waters (Gallegos 2005, Sagan 2007). Populations growing in Florida have blades present year-round and do not produce dormant winter buds as occurs in northern latitudes (Dobberfuhl 2007, Doering et al. 2001). Leaf length and biomass (aboveground and belowground), plant depth, and bed width are parameters used by researchers to describe health and persistence of Tape-grass as a function of exposure to salinity, light availability, and turbulence (Blanch et al. 1998, Boustany et al. 2007, Carter et al. 1996, Dobberfuhl 2007, Doering et al. 2001, Gurbisz et al. 2016, Kreiling et al. 2007). 1Department of Biology and Marine Science, Jacksonville University, 2800 University Boulevard North, Jacksonville, FL 32211. 2Bureau of Water Resources, St. Johns River Water-Management District, PO Box 1429, Palatka, FL 32178-1429. *Corresponding author - ngoldbe@ju.edu. Manuscript Editor: John Dilustro Southeastern Naturalist 397 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 Sustained and periodic exposure to salinity concentrations >1 ppt affects Tapegrass growth and survivorship (Boustany et al. 2010, Dobberfuhl 2007, Doering et al. 2001). For example, Boustany et al. (2010) observed mortality after a 70-d– exposure treatment of 8 ppt in mesocosm experiments. In a survey of 4 permanent stations on the lower SJR, Tape-grass populations declined in cover by 50% following exposure to 11 ppt in 2006 and by 80% following exposure to 17 ppt in the summer of 2002 (Morris and Dobberfuhl 2012). Recovery is possible from below-ground energy stores if salinity returns to ≤1 ppt, as observed in mesocosm experiments (Boustany et al. 2010) and in situ monitoring studies (Morris and Dobberfuhl 2012). Tape-grass can photosynthesize in 0.5–35% surface irradiance in the SJR (Dobberfuhl 2007). In shallower waters, Tape-grass can fix carbon efficiently during the summer growing season, despite reduced water clarity and self-shading in the blade canopy (Titus and Adams 1979). By comparison, root:shoot biomass increased 2.4- fold for plants growing at 145-cm depth relative to those growing at 14-cm depth, a response that is likely due to ambient low-light conditions at depth and the plant’s inability to reach the surface (Blanch et al. 1998, Titus and Adams 1979). Competition with phytoplankton for light in the water column and epiphytes growing on the surface of shoots and blades can also result in reduced productivity, as recorded from mesocosm experiments (Boustany et al. 2010). Coastlines developed with residential units are associated with nutrient enrichment in the adjacent waterways and phytoplankton blooms, which contribute to low-light conditions that can limit Tape-grass growth (Dennison et al. 1993, Rossi et al. 2010). Physical disturbance from tropical storms and hurricanes can affect Tape-grass growth and overall bed health due to the combined effects of increased water motion and salinity, and decreased light availability from suspended sediments in the water column (Gurbisz et al. 2016, Morris and Dobberfuhl 2012). Turbulence from storm surge contributes to bed thinning during extreme storm events (Gurbisz et al. 2016). The inner region of a bed is more likely to be protected from storm surge as turbulence is attenuated across the bed and energy is deflected away from the edge of the bed. Recovery rates depend on pre-storm bed health and post-storm water-clarity conditions (Carter et al. 1996, Gurbisz et al. 2016, Morris and Dobberfuhl 2012). The overall objective of this study, conducted from 2002 to 2011, was to investigate differences in Tape-grass blade length, depth that the plant was present, and bed width at permanent sites that differed in adjacent land use (residential vs. natural shorelines), sides of the river (western and eastern sides), and distance from the river mouth. We examined land use because nutrient inputs from runoff along shorelines populated by residential units may contribute indirectly to reduced water clarity (i.e., plankton blooms) and thus negatively impact growth of Tape-grass (Carpenter et al. 1998). Differences in Tape-grass distributions between the eastern and western sides of the river have been reported in the lower SJR basin, but not investigated with the factors tested in this study (SRR 2015). We compared paired sites located directly opposite each other to further test whether sides of the river contributed to variability in blade length and plant depth. We also compared blade Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 398 length, plant depth, bed width, and environmental variables of salinity and turbidity among river sections 64–80 km, 81–96 km, and 97–112 km from the river mouth, a region in the lower SJR where Tape-grass was recorded throughout the study period (Dobberfuhl 2007, Sagan 2007). We predicted that blade length, plant depth, and bed width would differ because salinity is typically lower in sections furthest from the river mouth (Blanch et al. 1998, Dobberfuhl 2007, Sagan 2007, Titus and Adams 1979). To investigate distributional patterns with respect to drought or extreme storm events, we compared environmental variables (rainfall, turbidity, and salinity) to patterns in Tape-grass during the study period. Understanding Tape-grass temporal and spatial changes in the lower SJR will assist in the management of this ecologically important submerged aquatic plant. Methods Site description The lower St. Johns River (SJR) has a mean width of 1.6 km and a mean depth of 3.3 m. Within the study area (30°7'35.4"N, 81°41'36.6"W–30°23'36.07"N, 81°44'35.19"W), mean salinity is 1–7 ppt (Fig. 1; Lowe et al. 2012). During extended periods of droughts, salinity can be 0 ppt and during episodic tropical storms or hurricanes, salinity can spike to 20 ppt (Fig. 1; Lowe et al. 2012, Morris and Dobberfuhl 2012). Substrate type was typically muddy sand and vegetated predominantly by Tape-grass with occasional Ruppia maritima L. (Widgeon-Grass), Potamogeton pusillus L. (Small Pondweed), Micranthemum sp. (a mudflower), Sagittaria subulata (L.) Buchenau (Awl-leaf Arrowhead), Ceratophyllum demersum L. (Hornwort), and Charophyta spp. (stoneworts). Shorelines were developed with residential units or vegetated with mixed-hardwood wetlands and upland forested areas. Sampling We utilized data collected annually by the St. Johns River Water-Management District (SJRWMD) at permanent sites from 2002–2011. Sampling occurred from June to August of each year, with most sites sampled in late June to July. Permanent sites had a designated inshore location from which a transect tape would be laid out perpendicular to shore and extend outward from the shore to where Tape-grass was last recorded. We defined the extent of the bed-width as the location of the last record of the plant. Along the transect tape, we recorded depth (to the nearest 0.01 m) and presence of Tape-grass. We recorded data from points that were equally spaced along the transect and crossed the extent of the bed. If visual confirmation of Tapegrass was not possible due to reduced water clarity, a researcher collected a voucher specimen by hand to confirm identification. In addition, we recorded blade length (measured to the nearest cm) from ~10 collections taken along the same intervals of each transect. We compared Tape-grass blade length, plant depth, and bed width per year to test for differences between sides of the river and land-use. We randomly selected sites from the SJRWMD database of permanent sites where Tape-grass had been recorded from 2002 to 2011. Of 29 possible sites, 80% were located between 64 Southeastern Naturalist 399 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 Figure 1. Map of extent of study site, lower St. Johns River, FL, indicating river sections (A = 64–80 km, B = 81–96 km, and C = 97–112 km from the river mouth). Inset showing the state of Florida with box delimiting the lower St. Johns River. For the site category: river section denotes sites used to compare annual differences with river sections, side of river denotes paired sites used to compare sides of river, and land use denotes the sites used to compare land use and sides of the river. Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 400 km and 112 km of the river mouth, and therefore, we focused our study within this stretch of the SJR. To ensure a balanced design in the analyses, 10 sites were located along the eastern side, and another 10 sites were located along the western side (n = 20 sites; Table 1). Five sites per side of the river were adjacent to a residential land-use category of less than 2 dwellings per acre, and another 5 sites per side were adjacent to a natural land-use category of mixed wetland hardwoods and upland forested areas. The east-shore and west-shore sites used in this analysis were not necessarily located directly opposite each other. To further examine the effect of side of river, we compared differences in plant depth and blade length (2 parameters that were significant from the previous analysis) between 6 pairs of sites located directly opposite each other (Table 1). Nine of the 12 sites were used in the previous analysis testing the effects of land use and sides of the river (Table 1). We calculated estimates for mean plant depth and blade length from annual measurements collected over the 10-y period. We compared water quality and the 3 Tape-grass parameters among the 3 river sections. We compiled environmental variables from fixed Water-Body Identification Number (WBID) stations located nearest to a permanent Tape-grass Table 1. Site, coordinates, side of river, land-use category, and river section (km) from mouth of the lower St. Johns River, FL. #denotes sites used for the land use and side of river comparison with time (n = 20 sites), *denotes sites used for paired comparisons between sides of river with time (n = 12 sites), and +denotes sites used for comparison among river sections (n = 5 sites). River Site Latitude Longitude Side of river Land use section (km) #*091 30°4'51.6''N 81°41'29.04''W Western Residential 64–80 #*+140 29°48'16.2''N 81°34'42.96''W Western Residential 97–112 #*149 29°46' 57.36''N 81°33'47.16''W Western Residential 97–112 #082 30°7'4.8''N 81°41'26.52''W Western Residential 64–80 #103 30°0'52.2''N 81°41'41.28''W Western Residential 81–96 #+077 30°7'35.4''N 81°41'36.6''W Western Wetland hardwoods 64–80 #*+113 29°57'51.12''N 81°36'46.44''W Western Wetland hardwoods 81–96 #125 29°57'51.12''N 81°36'46.44''W Western Wetland hardwoods 97–112 #*135 29°51'58.68''N 81°36'52.2''W Western Wetland hardwoods 97–112 #129 29°53'32.28''N 81°36'57.96''W Western Wetland hardwoods 97–112 #074 30°7'59.52''N 81°39'36.72''W Eastern Residential 64–80 #119 29°56'57.84''N 81°35'3.12''W Eastern Residential 81–96 #+123 29°54'15.84''N 81°35'23.28''W Eastern Residential 97–112 #*141 29°47'51.36''N 81°32'11.76''W Eastern Residential 97–112 #137 29°48'57.96''N 81°33'3.96''W Eastern Residential 97–112 #*131 29°52'23.16''N 81°34'37.2''W Eastern Upland forested 97–112 #104 30°1'35.04''N 81°39'34.56''W Eastern Upland forested 81–96 #081 30°1'35.04''N 81°38'29.04''W Eastern Wetland hardwoods 64–80 #*112 29°58'58.8''N 81°34'47.28''W Eastern Wetland hardwoods 81–96 #*146 29°47'18.96''N 81°31'40.08''W Eastern Wetland hardwoods 97–112 *087 30°5'6.36''N 81°38'52.44''W Eastern Residential 64–80 *157 29°45'35.64''N 81°33'55.8''W Western Residential 97–112 *161 29°45' 8.64'' N 81°32'40.2''W Eastern Wetland hardwoods 97–112 +021 30°4' 6.6'' N 81°41'31.92''W Western Residential 64–80 Southeastern Naturalist 401 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 collection site within each of the river sections: 64–80 km, 81–96 km, and 97–112 km from the river mouth. In the 64–80 km and 97–112 km sections, 2 permanent sites were equidistant from the WBID station (Table 1). For these 2 pairs of sites, we calculated the means per year and used these estimates for analyses. For a qualitative comparison, we calculated the means per river section for the 20 sites used in the land-use and side-of-river investigation. We calculated annual estimates of each environmental variable from monthly measurements recorded from 1 July to 30 June and downloaded salinity and turbidity (recorded in less than 0.6 m depth) data from the STORage and RETrieval database system (Florida Department of Environment Protection 2017). Total annual and mean monthly rainfall values were compiled from the closest weather station at the Jacksonville Naval Air Station (Wunderground 2017). Data analysis We compared mean bed-width, plant depth, and blade length per site with the 3 fixed factors: year (2002–2011), shore location (western vs. eastern side of the LSJR), and land-use category (residential vs. natural land-use). We conducted a 3-factor analysis of variance for bed width (log transformed) and plant depth with a significance level (α) of 0.05. Blade length data were not normal; thus, we employed Kruskal-Wallis non-parametric tests separately for comparisons among years, between sides of the river, and between land-use categories. We applied a Bonferroni correction to give a more conservative significance level (α = 0.017) because the same data set was used to test for the 3 Kruskal-Wallis non-parametric tests. We ran a paired t-test to test for differences in mean annual blade length and annual blade length between the 6 paired sites located on opposites sides of the river during the period 2002–2011. We conducted a one-way ANOVA or a Kruskal-Wallis non-parametric test to detect differences among the 3 river sections and annual estimates of salinity, turbidity, and the 3 Tape-grass parameters among the 3 river sections from 2002 to 2011. Results Comparisons between sides of river and land use Of the 3 parameters measured, Tape-grass mean plant depth and blade length per site were significantly different among years, and mean plant depth was significantly different between sides of the river (Table 2; Figs. 2, 3). From 2002 to 2011, mean plant depth was 0.58 ± 0.02 m per site with greatest recorded depths in 2008 (0.68 ± 0.04 m per site) and shallowest recorded depths in 2004, 2006, and 2009 (0.49 ± 0.03 m per site). Mean annual blade length was 18.7 ± 2.0 cm per site with the longest blades recorded in 2004 (30.9 ± 4.9 cm per site) and the shortest blades recorded in 2006 (11.2 ± 3.8 cm per site). Mean plant depth per site was 5 cm deeper along the eastern shore (Table 2, Fig. 2). By comparison, bed width was not significantly different among years or between sides of the river due to variability among replicate sites (Table 2, Fig. 4). We observed no significant differences with land use (Table 2). Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 402 The differences in plant depth and blade length that we observed for sites located randomly along the western and eastern sides of the river (Table 1; Figs. 2, 3) were also observed for sites located directly opposite each other. Plants from eastern sites were found in significantly greater depths than those from paired western sites (t5 = -6.36, P = 0.001) with a mean annual difference of 0.49 ± 0.04 m. In addition, blade lengths of plants growing on the eastern side were significantly longer relative to those from paired sites on the western side (t5 = -4.93, P = 0.004) with a mean annual difference of 27.0 ± 5.5 cm. Comparison among river sections: 64–80 km, 81–96 km, and 97–112 km from the river mouth From 2002 to 2011, the effect of river section (64–80, 81–96, and 97–112 km from the river mouth) on the 3 parameters studied was supported for bed width (H2 = 21.53, P < 0.001) and blade length (F2, 27 = 3.65, P = 0.04) (Fig. 5). Mean bed width per year was 85–95 m wider in the 64–80 km section, which was closest to the SJR mouth, compared to the other 2 sections (Fig. 5A). We also observed differences in bed width from the previous analysis, comparing side of the river and land use (64–80 km: 151.40 ± 27.78 m [n = 5 sites], 81–96 km: 55.95 ± 13.65 m [n = 5 sites], and 97–112 km: 65.85 ± 5.06 m [n = 10 sites]). Mean blade length per year was ~10 cm longer in the 97–112 km section, which was furthest from the mouth, compared to the other 2 sections (Fig. 5A). Similarly, blade length was greater in sites sampled furthest from the river mouth, as observed from the 20 sites sampled for the side of the river and land-use comparison (64–80 km: 13.8 ± 2.1 cm [n = 5 sites], 81–96 km: 13.3 ± 3.5 cm [n = 5 sites], and 97–112 km: 23.9 ± 2.8 cm [n = 10 sites]). Mean plant depth per year was not significantly different among the locations sampled (F2, 27 = 0.14, P = 0.87; Fig. 5A) during the study period. Mean depths among the river sections from the comparison with the broader survey showed a similar trend (64–80 km: 0.59 ± 0.03 m [n = 5 sites], 81–96 km: 0.57 ± 0.05 m [n = 5 sites], and 97–112 km: 0.57 ± 0.05 m [n = 10 sites]). Table 2. Statistical results comparing annual plant depth, bed width, and blade length of Tape-grass with land use (residential and natural) and side of the river (western and eastern) along the lower St. Johns River, FL, from 2002–2011. Plant depth and width examined with a 3-factor analysis of variance. Width was log-transformed. Blade lenth was analyzed with a Kruskal-Wallis tests using α = 0.0167; n = 5 sites per land use and shore location combination. Source df Plant depth (m) Width (m) Blade length (cm) Year 9 F = 4.81, P < 0.001 F = 0.28, P = 0.98 H = 37.08, P < 0.001 Land Use (LU) 1 F = 2.62, P = 0.11 F = 0.49, P = 0.48 H = 2.31, P = 0.13 Side of River 1 F = 5.17, P = 0.02 F = 0.00, P = 0.95 H = 4.05, P = 0.04 Year*LU 9 F = 0.69, P = 0.72 F = 0.09, P = 1.00 Year*Side of river 9 F = 1.18, P = 0.31 F = 0.28, P = 0.98 LU*Side of river 1 F = 2.43, P = 0.12 F = 0.31, P = 0.58 Year*LU*Side of river 9 F = 0.68, P = 0.72 F = 0.10, P = 1.00 Error 160 Southeastern Naturalist 403 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 Comparisons in salinity, turbidity, and rainfall We documented significant differences in salinity among sections of the SJR (F2, 27 = 4.96, P = 0.02), with >1.35 ppt per year difference between the 64–80 km and 97–112 km sections of the river (Fig. 5B). We detected no significant differences in annual turbidity estimates among the stations sampled in the 3 different river sections (F2, 27 = 2.22, P = 0.13; Fig. 5B). Over the 10-y period, mean salinity Figure 2. Comparison in mean plant depth (± SE) per site and year between (A) western (filled) and eastern (open) sides of the St. Johns River, FL, and (B) natural (filled) and residential (open) land use from 2002–2011. Mean plant depth (± SE) across years provided. n = 20 sites: 5 sites per land-use per side. Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 404 and turbidity were greatest in 2011 (4.85 ± 0.31 ppt, 7.20 ± 0.56 ntu, respectively) and lowest in 2003 (0.39 ± 0.06 ppt, 4.42 ± 0.58 ntu, respectively) (Table 3). Total annual rainfall was greatest in 2002 (147.96 cm) and 2009 (159.46 cm) as compared to low total rainfall in the years of 2004 (77.85 cm) and 2011 (76.05 cm) (Table 3). Mean rainfall per month was greatest in 2009 (13.28 ± 3.66 cm/mo) and lowest in 2006 (7.34 ± 1.93 cm/mo) and 2007 (6.78 ± 1.63 cm/mo). Figure 3. Comparison in mean blade length (± SE) per site and year between (A) western (filled) and eastern (open) sides of the St. Johns River, FL, and (B) natural (filled) and residential (open) land use from 2002–2011. Mean blade length (± SE) across years provided. n = 20 sites: 5 sites per land use per side. Southeastern Naturalist 405 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 Discussion A mean annual Tape-grass bed width of 85.0 ± 1.6 m per site from 2002 to 2011 in the lower St. Johns River (SJR) provided evidence that a persistent population was present to recover from physical disturbances despite reduced cover (SJRWMD, Palatka, FL, unpubl. data). However, changes in depth at which plants were present were not necessarily associated with extreme storm conditions as observed in Chesapeake Bay, MD (Gurbisz et al. 2016). For example, a mean plant Figure 4. Comparison in mean bed width (± SE) per site between (A) western (filled) and eastern (open) sides of the St. Johns River, FL, and (B) natural (filled) and residential (open) land use per year from 2002–2011. Mean bed width (± SE) across years provided. n = 20 sites: 5 sites per land use per side. Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 406 depth of 0.68 ± 0.04 m per site was recorded following the hurricanes and tropical storms in August and September of 2004, indicating sufficient light was available for photosynthesis. In contrast, mean plant depth was shallower (0.52 ± 0.03 m per site) following the August and November storms and relatively heavier rainfall of 2009. Dobberfuhl (2007) reported a 3–6-month lag in response to light availability. Perhaps, the combined effects of turbulence from the hurricanes and turbidity from suspended sediments prevented growth at greater depths. In support of suggested Figure 5. Comparison among locations that differed with distance from the river mouth in (A) mean blade length (black), mean plant depth (white), and mean bed width (grey), (B) salinity (black), and turbidity (white) per year from 2002–2011. Blade length, plant depth, and bed width: annual estimates averaged from 2 sites for the 64–80 km and 97–112 km sections and 1 site for the 81–96 km section; salinity and turbidity estimates: 1 station per river section. Different letters above columns of same color indicate significant differences. Southeastern Naturalist 407 N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 light limitation, we observed shorter mean blade lengths of 15.36 ± 3.54 cm per site despite the relatively shallow mean plant depth for sampled individuals. Challenges in predicting the effects of light limitation were further demonstrated by our investigation between residential and natural shorelines. We observed greater blade lengths only in beds along natural shorelines in 2002 and 2011, despite these years having relatively high turbidity estimates. Perhaps, at these locations, the forested-wetland habitats reduced the amount of sediment-laden runoff relative to residential shorelines, as has been observed in small-scale riparian restoration efforts along the lower SJR (Rossi et al. 2010). Lower salinity in the farthest river section, irrespective of storm events and drought conditions, likely explained the greater mean blade lengths during the 10-y period. Salinity spikes of ≥8 ppt occurred on 2 d in 2002, and 5–8 d in 2006, 2007, and 2011 in the 64–80 km section following extreme storm events. By comparison, only 1 one day in 2007 had a record of ≥8 ppt in the 81–96 km section, and no days were recorded with >3.5 ppt in the 97–112 km section of the river. Dobberfuhl (2007) described the 97–112 km section as having less than 0.5 ppt annually from 1998 to 2004. Thus, the significantly lower mean salinity in the 97–112 km section supports the findings from mesocosm experiments that above-ground biomass increases with exposure to lower salinity (Boustany et al. 2010, Doering et al. 2001, Twilley and Barko 1990). In addition, longer blade lengths are associated with periods of relatively low rainfall, low salinity, and improved water-clarity (Carter et al. 1996). For example, the drought of 2004 was associated with longer mean blade lengths (30.87 ± 3.45 cm per site in mean depth of 0.57 ± 0.03 m per site) as compared to mean blade lengths of 18.7 ± 2.14 cm per site throughout the study period. During this period, turbidity was 5.94 ± 0.53 ntu per year. By comparison, in 2007, mean blade lengths were shorter (14.50 ± 2.27 cm per site in mean depth of 0.59 ± 0.05 m per site). For the previous 2 years, rainfall had been relatively low but turbidity was high (>6.20 ntu per year), which may have contributed to reduced light availability (e.g., Table 3. Environmental variables from July 2001 to June 2011. Rainfall estimates: Jacksonville Naval Air Station, FL (Wunderground 2017); salinity and turbidity (Florida Department of Environment Protection 2017). Variability is reported as standard error. Annual rain Monthly rain Year total (cm) mean (cm/mo ± SE) Salinity (ppt ± SE) Turbidity (ntu ± SE) 2002 147.96 12.32 ± 3.86 1.33 ± 0.43 5.01 ± 0.46 2003 123.09 10.26 ± 1.88 0.39 ± 0.06 4.42 ± 0.58 2004 77.85 11.56 ± 1.37 0.50 ± 0.07 5.94 ± 0.53 2005 138.02 11.51 ± 3.30 0.46 ± 0.07 6.95 ± 0.77 2006 88.11 7.34 ± 1.93 0.78 ± 0.21 6.90 ± 0.67 2007 81.48 6.78 ± 1.63 2.92 ± 0.47 6.20 ± 0.47 2008 116.28 9.70 ± 2.44 1.72 ± 0.30 6.29 ± 0.47 2009 159.46 13.28 ± 3.66 0.93 ± 0.19 5.82 ± 0.47 2010 112.98 9.42 ± 2.26 0.45 ± 0.04 5.47 ± 0.83 2011 76.05 12.32 ± 1.27 4.85 ± 0.31 7.20 ± 0.56 Southeastern Naturalist N.A. Goldberg, T. Trent, and J. Hendrickson 2018 Vol. 17, No. 3 408 less than 10.8% surface light; Dobberfuhl 2007) that limited photosynthesis resulting in low amounts of above-ground biomass. The seasonal westerly winds that blow during the Tape-grass growing season from June to August may explain the significantly greater depths at which the plant was present and longer blades that we recorded from beds located along the eastern shores. In the upper Mississippi River, Kreiling et al. (2007) observed greater above-ground biomass where wind fetch was >1500 m despite reduced waterclarity with sediment resuspension. These seasonal winds crossing a span >1000 m in the study area may provide sufficient turbulence for nutrient exchange and help to reduce epiphyte growth that increases drag and reduces light availability for photosynthesis (Dunn et al. 2008). No other study has investigated the impact of wind-generated turbulence on riverine Tape-grass. Summary Persistence of Tape-grass in the lower SJR region is dependent on the ability of healthy beds to baffle and attenuate energy from storm surge and flood waters (Gurbisz et al. 2016). Following a season with extreme storm events and associated runoff and storm surge, Tape-grass resilience depends on the degree of exposure to salinity spikes and reduced water clarity from suspended particulates and nutrient-fueled plankton blooms in the water column (Dobberfuhl 2007). Given the importance of subaquatic vegetation to riverine ecosystems, efforts to maintain water clarity is critical to ensure the resiliency of these beds with the predicted rise in sea level and extreme storm events (Linhoss et al. 2014). Acknowledgments We thank the anonymous reviewers, editors of the Southeastern Naturalist, and J. Heine for comments that significantly improved the paper. Literature Cited Barnett, B.S., and R.W. Schneider. 1974. Fish populations in dense submersed plant communities. Hyacinth Control Journal 12:12–14. Blanch, S.J., G.G. Ganf, and K.F. Walker. 1998. Growth and recruitment in Vallisneria americana as related to average irradiance in the water column. 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