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