Dynamic changes in an estuary over the past eleven years

Abstract:

Extreme climatic events such as drought, 100-year floods, etc. can change the state of an ecosystem that is already in flux. The Sacramento-San Joaquin Delta in California is one of the most modified estuaries in the world. The hydrology of the Sacramento-San Joaquin Delta has been extensively altered by a network of levees, dams, and canals implemented through federal and state reclamation projects. These changes have made its ecosystem vulnerable to invasion by eliminating the competitive advantage of low-nutrient brackish-adapted native species via nutrient loading and reduced salinity which favor opportunistic species. The extreme drought in California since 2010 has likely increased shallow habitat with slow moving water, and the mild winters and lack of large storms have also favored the spread of non-native species.

Six years of hyperspectral data of the Delta spread over a period of 11 years (HyMap data from 2004 to 2008, and AVIRIS-NG data in 2014) offer a unique opportunity to study the invasive species distribution in the Delta. In 2004, two aggressive freshwater invasive species were flourishing in the Delta: the floating species, Eichhornia crassipes (water hyacinth) and the submerged species, Egeria densa (Brazilian waterweed). A third invasive floating genus Ludwigia spp. (water primrose) was just beginning to establish itself in the northwest portion of the Delta. We classified these invasive species for all six years of data. Our results show that from 2004 to 2008, floating and submerged vegetation decreased overall. Among floating species, Eichhornia crassipes was co-dominant with Hydrocotyle spp., a native floating macrophyte. From 2008 to 2014 however, the total area covered by floating macrophytes in the Delta increased many-fold from 320 to 2610 hectares. The composition of this functional group also changed. Native Hydrocotyle cover reduced drastically and Ludwigia replaced it as the co-dominant comprising a third of all floating macrophyte cover. Submerged macrophyte cover stayed relatively constant and dominated by Egeria densa. There might be multiple reasons in addition to drought for this shift in the distribution and abundance of the floating macrophyte community. More studies are needed to establish the relative importance of the California drought as compared to other factors.