# Analyzing the Long-Term Phenological Trends of Salt Marsh Ecosystem across Coastal LOUISIANA
## Materials and Methods  
### Study Area  
LA has the largest salt marsh ecosystem (>6500 km\({}^{2}\)) in the United States (US) [63] (Figure 1). The ecosystem is mostly dominated by smooth cordgrass (_Spartina alterniflora_), and salt meadow cordgrass (_Spartina patens_). Apart from that, patchy distributions of American Glasswort (_Salicornia virginica_), saltwort (_Batis maritima_), and seashore saltgrass (_Distichlis spicata_) are also encountered. The distribution of black needle rush (_Juncus roemarianus_) is limited along the brackish regions. The high salinity and anoxic soil do not allow the growth or survival of non-specialized plant species; hence, floral diversity is remarkably low [64]. The climate of the region is tropical to sub-tropical; it is characterized by hot and humid summers, with occasional tropical storms and moderately cold winters. The salt marshes stretch across four state climatic divisions: southwest, south-central, southeast, and east-central [65]. The majority of the salt marsh ecosystem falls within the southeast (\(\sim\)4000 km\({}^{2}\)) and the southwest (\(\sim\)2000 km\({}^{2}\)) climatic zones. The average annual temperature in this region varies between 15-25\({}^{\circ}\)C, while annual precipitation ranges from 80 cm to 100 cm [66]. Since 2000, the salt marsh ecosystem in LA has experienced the landfall of major hurricanes, such as Lili in 2002 (Category 1), Katrina and Rita in 2005 (Category 4 and 3, respectively), Gustav in 2008 (Category 2), and Isaac in 2012 (Category 1) [67]. The marsh ecosystem in LA is also home to more than 160,000 oil and gas wells [68] that account for almost 18% of the oil and 24% of the natural gas production in the US, and are valued at $6.3 billion and $10.3 billion, respectively [69]. Hence, these ecosystems have been subjected to intense dredging and channelization for transportation as well as significant groundwater removal, which has led to soil and marsh erosion and localized subsidence. Further, the sediment flow from the Mississippi River, which typically provides nutrients and substrate for salt marshes, has been extensively trapped and diverted through the excessive construction of levees across the LA coast. Therefore, it is increasingly difficult for the salt marshes to sustain themselves and fight against sea level rise. Disturbed salt marsh ecosystems in the region have been invaded by common reed (_Pirragmites australis_) at certain places [69].  
Figure 1: Salt marsh extent in coastal Louisiana spread across four climatic zones.  
Over the past decade, there has been an increasing number of reports of salt marsh \"dieback\" in the US. In 2000 and 2007, LA experienced a sudden and acute dieback event (termed \"brown marsh\") that affected over 100,000 ha of _Spartina alterniflora_-dominated salt marsh throughout the Mississippi River deltaic plain [(70; 71)]. In addition, although the long-term impact is still unknown, the Deepwater Horizon oil spill in 2010 had a severe short-term impact on the health of several fringe and interior salt marsh patches of LA, which was characterized by the loss of chlorophyll and biomass, and subsequently a reduction in photosynthetic capacity [(72; 60; 73)].