# Analyzing the Long-Term Phenological Trends of Salt Marsh Ecosystem across Coastal LOUISIANA
## 4 Discussion
### SOS and EOS Determination
#### 4.1.2 SOS and EOS Validation through Analysis of Fluctuations  
The median SOS date for the salt marshes of southeast and southwest Louisiana as derived from TIMESAT was \(\sim\)60 Julian days (Figures 4a and 5a), which corresponds with the onset of spring at the end of February and beginning of March. However, in the years 2000, 2010, and 2013, the salt  
Figure 8: Spatial distribution of the areas showing different trends for (**a**) GBM base value; (**b**) GBM max value; (**c**) GBM amplitude; and (**d**) GBM small seasonal integral for salt marshes in Louisiana.  
marshes of southeast LA displayed a late SOS. The year 2000 had witnessed an acute recurring dieback event that affected almost 100,000 ha of marshes in LA [71; 90]. This dieback might have resulted from extremely dry conditions for two consecutive years (1999 and 2000) [65]. The acute dieback event had most likely delayed the normal SOS date in 2000. The delayed start to the 2010 growing season was most likely due to the late onset of spring owing to colder than normal temperatures at the beginning of the season [65], a phenomenon also observed in the salt marshes of southwest LA.  
Finally, the delayed SOS in 2013 might be due to the aftereffects of the Deepwater Horizon oil spill (2010) and Hurricane Isaac (2012). The residual oil and the cleanup efforts had negative effects on the marsh patches in southeast LA, and the restoration efforts may have been affected by the landfall of tropical storm Bonnie in 2010, which brought additional oil into the shoreline marsh. The short-term degradation due to the oil spill, subsequent cleanup, and the efforts and stress induced by tropical storm Bonnie, all delayed the peak growth of the marsh patches in the area in 2011 [60]. In spite of this, the marshes had started to recover from oil spill effects in 2011 [91]. However, the recovery of the marsh patches was further damaged by the landfall of Hurricane Isaac in 2012, during the peak of the growing season. Although Hurricane Isaac was a Category 1 hurricane, the timing of the landfall had an adverse effect, as the salt marshes were unable to achieve full recovery from the stress induced by the oil spill. That could be the reason why we noticed another delayed dormancy onset in 2013. Our observations are consistent with those made by [PERSON] et al. [92]. Any residual oil effects in 2011 or 2012 might have been localized, as the marshes had started to recover or had already recovered from the effects of the oil spill. Those localized effects were not severe enough to have manifested in the MODIS data. Further, the biomass model has a 17% error, so a minor localized disturbance that produced a localized effect in the biomass level may have been below the error threshold, and thus not have been picked up by the model.  
The salt marsh growing season in LA normally ends around mid-November (Figures 4b and 5b), which was observed in both southeast and southwest LA, with the median EOS date being ~322 Julian days. However, the salt marshes in both these regions experienced an early end to the growing season in 2005 and 2008. In southeast LA, Hurricane Katrina made landfall in September as a major Category 3 hurricane that destroyed most of the salt marsh ecosystem [93], which was unable to recover in the same year. A similarly drastic EOS was observed in the year 2008, when Hurricane Gustav made landfall; however, the effects of the landfall were not as severe as Katrina. In southwest LA, landfalls and subsequent damages caused by Hurricane Rita (2005) and Hurricane Ike (2008) were the main reasons for the early EOS in those respective years. These results provide justification to the SOS and EOS thresholds chosen for the derivation of seasonality parameters, as they have been able to capture the environmental events that have affected the SOS and EOS of the salt marsh ecosystem during the 16-year time period.