Magnitude and causes of change in submerged aquatic vegetation in the Hudson River

Background/Question/Methods

Submerged plants in the littoral zone of aquatic ecosystems are widely recognized as providing important functions as habitat and in biogeochemical processes.  Additionally they are considered one of the best indicators of water quality as well as affecting multiple human uses. These plants are known to be highly variable in space and time, probably reflecting a diverse set of drivers of change.  Untangling various natural or anthropogenic causes of change would improve understanding of ecological processes as well as establishing links to potential management and restoration activities.  We use a hectare-scale analysis of change in plant cover over a ten-year period to relate fluctuations to local bathymetry, plant interactions and possible effects of adjacent shorelines.   

Results/Conclusions

Coverage by SAV is about 6% of the river bottom and varies dramatically over time. The predominant submerged plant is Vallisneria americana but there are also significant areas of an exotic species, Trapa natans.  Both these plants serve as important habitats for fishes and invertebrates but generally support different taxa.  There was an overall decline in SAV cover of about 30% during the interval 1997 to 2007 although there is evidence for a recent recovery.  At the whole-river and long-term scales change is most likely linked to light availability which is largely controlled by suspended sediments rather than phytoplankton.  Change in coverage at the meso-scale (hectares) appears related to the geographic context as well as possible effects of human-modified shorelines.  Quadrats adjacent to human-engineered, hardened shorelines showed greater declines over the interval than quadrats adjacent to natural shorelines.  While there are certainly cases of replacement of native Vallisneria by the exotic Trapa there has not been a net increase in extent of this invasive plant.  While most studies of change in SAV have focused on nutrient driven eutrophication and declines in water transparency we see large fluctuations likely driven by natural fluctuations in flow and sediment concentrations.  Human modification of adjacent shorelines has generally acted to increase the vulnerability of SAV to change.