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Date Correct Total Accuracy
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11/26/2017 13 15 86.6%
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2/14/2018 10 13 76.9%
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2/17/2019 11 13 84.6%
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6/25/2019 11 13 84.6%
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2/4/2020 9 13 69.2%
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Total 54 67 80.6%
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significantly affected the size of the plume, explaining 63% of
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the deviance, and found no evidence of variation as a function
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of season (e.g., fall vs. spring satellite images; Figure 4 and
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Table 2). We detected a breakpoint in the sediment plume
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coverage of the region in November 2016 (95% CI: September
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2015–February 2018; Table 3). This date straddled the Fall 2015
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seagrass die-off and the Fall 2017 hurricane Irma. The average
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size of the sediment plume before the breakpoint was 163.5 km2
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(±26.8 km2
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), which increased to 223.5 km2
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(±19.9 km2
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) postbreakpoint–a 37% increase in area.
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Basin Specific Responses to
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Disturbances and Sediment Plume
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Coverage
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When considering our focal basins, Johnson and Rankin, we
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found differences in the magnitude, timing and duration of effects
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of the sediment plume. Both basins exhibited breakpoints, but
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the breakpoint was earlier, and resulted in more severe and
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longer lasting effects in the western basin, Johnson (Figure 5). An
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ANOVA conducted on the proportion of the basins covered by
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the plume, showed a significant basin and breakpoint interaction,
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with both Johnson and Rankin having a significantly higher
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proportion of basin covered by the plume after the change
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point compared to before (Tukey’s HSD < 0.05; Figure 6A
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and Table 4). In Johnson, there was a breakpoint in March
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2015 (95% CI: October 2014–September 2015; Figure 5 and
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Table 3), and sustained high sediment coverage through the last
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data point in the time series. Before the breakpoint, an average
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of 11.6% (±10.8%) of Johnson was covered by the sediment
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plume while 78.6% (±13.4%) of Johnson was covered by the
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plume after the breakpoint (Figure 6A). In contrast, for Rankin,
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there were two breakpoints. The first breakpoint was February
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2017 (95% CI September 2015–November 2017), and the second
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was November 2018 (95% CI February 2018–February 2020;
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Figure 5 and Table 4). Here, 0% of Rankin was covered by
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the plume before the first breakpoint, while 22% (±21%) was
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covered by the plume after the first breakpoint (Figure 6A). The
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second breakpoint in Rankin represents a sediment contraction,
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indicating the short term effects in this basin, and thus, was not
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considered in further analyses.
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Interaction Between Sediment Plume
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Coverage and Changes in Seagrass
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Cover
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Along with differences in the extent of the sediment plume,
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we also found differences in seagrass cover between basins, and
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a basin-specific interaction between plume extent and seagrass
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cover. For seagrass cover, the interaction between basin and
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before and after the change point was significant (Table 4).
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Seagrass cover in Johnson basin decreased significantly after its
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March 2015 breakpoint (Tukey’s HSD < 0.05; Figure 6B and
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Table 4). The average BB score in Johnson dropped from 3.6
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(±0.34; approximately 60% cover) to 2.5 (±0.38; approximately
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35% cover) after the breakpoint. In contrast, we found no
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change in seagrass cover as a function of its February 2017
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breakpoint in Rankin. The average BB score of Rankin before
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Frontiers in Marine Science | www.frontiersin.org 6 July 2021 | Volume 8 | Article 633240
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Rodemann et al. Sediment Plume and Seagrass Resilience
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FIGURE 4 | Temporal trend in the sediment plume size in Florida Bay between 2008 and 2020. The black line shows the fitted GAM model and 95% confidence
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interval (gray shaded area). The vertical red line shows the breakpoint in sediment plume size, with its 95% confidence interval (red shaded area). The vertical green
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line represents the 2015 seagrass die-off and the vertical blue line represents Hurricane Irma.
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the breakpoint was 2.7 (±0.61; approximately 40% cover) and
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was 2.8 (±0.05; approximately 45% cover) after the breakpoint.
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Further, in Johnson, there was a negative correlation between the
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proportion of basin covered by the sediment plume and seagrass
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cover (r = −0.75, p = 0.005; Figure 6C). There was no correlation
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between the proportion of basin covered by the sediment plume
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and seagrass cover in Rankin (r = 0.05, p = 0.88; Figure 6C).
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DISCUSSION
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Anthropogenic and natural disturbances have jointly contributed
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to the degradation of seagrass habitats worldwide, including
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in Florida Bay, which have resulted in two drought-induced
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seagrass die-offs (Fourqurean and Robblee, 1999; Hall et al.,
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TABLE 2 | Generalized additive models (GAM) used for the temporal assessment
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of sediment plume size.
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Term edf Ref.df F p Deviance
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explained
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AIC
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(a) Date Date 4.3 5.3 5.8 0.002* 62.8 229.8
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(b) Date and
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Season
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Date*Fall 2.2 2.8 7.3 0.002* 59.5 230.5
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Date*Spring 1.4 1.7 3.5 0.04*
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Two models were fitted: (a) without seasonality and (b) with seasonality. Models
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were based on a log link function. Shown are the smooth term effective degrees
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of freedom (edf), the test statistic of the model smooth terms (F), and the p-values
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for the null hypotheses that each smooth term is zero (p). Percentage deviance
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explained and Akaike information criterion (AIC) of the GAMS were used to
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determine the best model.
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Significant values are denoted with *.
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2016). The most recent seagrass die-off occurred in 2015, causing
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a potential localized regime shift from a densely vegetated
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state to a turbid, non-vegetated state (Hall et al., 2016; Hall
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