Unexpected resurgence of a large submersed plant bed in Chesapeake Bay: Analysis of time series data

Abstract

An historically large (. 50 km 2 ) submersed plant bed in upper Chesapeake Bay virtually disappeared in 1972, following Tropical Storm Agnes. The bed experienced little regrowth until the early 2000s, when plant abundance rapidly increased. Here, we analyze a suite of recent (1984-2010) and historical (1958-1983) time series datasets to assess alternative explanations for the submersed plant resurgence. Change-point analysis showed that spring nitrogen (N) loading increased from 1945 to 1988 and decreased from 1988 to 2010. Analysis of variance on recent time series showed a significant difference in submersed aquatic vegetation (SAV) abundance percent change during wet years (27 6 11%) and dry years (53 6 20%), indicating that floods and droughts likely contributed to SAV loss and growth, respectively. In the historic dataset, however, increasingly poor water quality led to SAV loss despite an extended drought period, indicating that underlying water quality trends were also important in driving change in SAV abundance. Several water quality variables, including N concentration and turbidity, were lower inside the SAV bed than outside the SAV bed, implying the presence of feedback processes whereby the bed improves its own growing conditions by enhancing biophysical processes such as sediment deposition and nutrient cycling. Together, these analyses suggest that stochastic extremes in river discharge and long-term water quality trends synergistically facilitated sudden shifts in SAV abundance and that feedback processes likely reinforced the state of the bed before and after the shifts. Management efforts should consider these dynamic interactions and minimize chronic underlying stressors, which are often anthropogenic in origin.

Vegetation wakes and wake interaction shaping aquatic landscape evolution

“…The sudden growth transitions observed in Fig. 6B are similar to sudden increases in vegetation growth observed in the field (Gurbisz and Kemp 2014). These authors reconstructed a time series of submerged aquatic vegetation in Chesapeake Bay, which experienced no growth for 16 yr followed by a period of rapid growth for 10 yr.…”

Season‐specific trends and linkages of nitrogen and oxygen cycles in Chesapeake Bay

“…Analysis of time‐series data from Chesapeake Bay monitoring has revealed many significant trends for key ecological properties in different regions of the estuary. For example, apart from hypoxia volume and duration discussed above, trends have been detected (1) for phytoplankton biomass and community composition (Harding and Perry ; Harding et al a ), (2) for submersed aquatic vegetation areal cover and density (Orth et al ; Gurbisz and Kemp ), and (3) for all eutrophication‐relevant variables in small tributaries where substantial nutrient reductions have taken place (Boynton et al ). Similar analyses in other large, eutrophic coastal systems (Andersen et al ; Riemann et al ) indicate that a wide‐variety of estuaries have responded positively to nutrient loading reductions.…”

Resilience indicators support valuation of estuarine ecosystem restoration under climate change

“…Additionally, upper Chesapeake Bay SAV, which includes the Susquehanna Flats bed, surpassed the goal for that region in the early 2000s, likely due to decreased nutrient loading from the Susquehanna River (Orth et al. , Gurbisz and Kemp , Gurbisz et al. ).…”

“…Remarkably, the Susquehanna Flats SAV recently underwent a rapid (2005)(2006)(2007)(2008)(2009)(2010) and complete recovery and was able to withstand two subsequent extreme weather events (Hurricane Irene and flooding associated with the remnants of Tropical Storm Lee in 2011). Research by Gurbisz and Kemp (2014) suggested that recovery was initiated by gradual long-term reductions in nutrient loading coupled with calm conditions and exceptional water clarity during an extended dry period. Although the decreasing nutrient inputs led to gradual improvement in water quality (Orth et al 2010), the drought provided the extra push necessary to tip the system into a new normal of abundant SAV cover.…”