Temporal variability in potential connectivity of Vallisneria americana in the Chesapeake Bay
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Submersed aquatic vegetation (SAV) performs water quality enhancing functions that are critical to the overall health of estuaries such as the Chesapeake Bay. However, eutrophication and sedimentation have decimated the Bay’s SAV population to a fraction of its historical coverage. Understanding the spatial distribution of and connectedness among patches is important for assessing the dynamics and health of the remaining SAV population.
We seek to explore the distribution of SAV patches and patterns of potential connectivity in the Chesapeake Bay through time.
We assess critical distances, from complete patch isolation to connection of all patches, in a merged composite coverage map that represents the sum of all probable Vallisneria americana containing patches between 1984 and 2010 and in coverage maps for individual years within that timeframe for which complete survey data are available.
We have three key findings: First, the amount of SAV coverage in any given year is much smaller than the total recently occupied acreage. Second, the vast majority of patches of SAV that are within the tolerances of V. americana are ephemeral, being observed in only 1 or 2 years out of 26 years. Third, this high patch turnover results in highly variable connectivity from year to year, dependent on dispersal distance and patch arrangement.
Most of the connectivity thresholds are beyond reasonable dispersal distances for V. americana. If the high turnover in habitat occupancy is due to marginal water quality, relatively small improvements could greatly increase V. americana growth and persistence.
KeywordsGraph theory Submersed aquatic vegetation Fragmentation
The authors thank the Chesapeake Bay Programs, the Virginia Institute of Marine Science and the National Oceanic and Atmospheric Administration for making data freely available on the internet. T. Schultz, R. Lloyd, and the reviewers for comments on earlier versions of this manuscript. M. Kweskin for assistance with software installation.
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