Scientists studying cause of major underwater grass comeback in Chesapeake Bay
Scientists at the University of Maryland Center for Environmental Science are conducting a new study to explore the resurgence of the Susquehanna Flats near the mouth of the Susquehanna River in the Chesapeake Bay, which began to mysteriously return in the early 2000s.
CAMBRIDGE, MD, Sept. 5, 2014 -- Scientists at the University of Maryland Center for Environmental Science are conducting a new study to explore the resurgence of the Susquehanna Flats, a large bed of underwater grasses, or submerged aquatic vegetation (SAV), near the mouth of the Susquehanna River in the upper Chesapeake Bay, which began to mysteriously return in the early 2000s after disappearing following Tropical Storm Agnes more than 40 years ago. Today, the bed is one of the largest and healthiest in the Bay, spanning some 20 square miles.
SAV is important to the Bay because it provides habitat for juvenile fish and enhances water clarity by trapping and removing sediment from the water. Historically extolled by trophy fisherman and waterfowl enthusiasts as prime wildlife habitat, researchers believe that the underwater grass beds at the shallow Susquehanna Flats began to decline in the 1960s when polluted runoff from a rapidly developing watershed overwhelmed the Bay's waters with nutrients, causing algae blooms that blocked out much-needed sunlight for underwater plants.
With SAV already stressed by nutrient pollution, it was Tropical Storm Agnes in 1972 -- a three-day weather event in June that dumped up to 19 inches of rain on the region and an estimated 30 million tons of sediment into the Chesapeake Bay -- that was ultimately destroyed the bed. A torrent of polluted floodwaters and sediment overwhelmed the beds, and the submerged plants virtually disappeared for nearly three decades. That is until the early 2000s when SAV rapidly recolonized nearly the entire region.
It was clear that the extreme flood event following Agnes triggered the historic demise of the grasses at Susquehanna Flats; however, the extended absence of SAV for over 30 years -- and the rapid comeback in the last decade -- was puzzling to scientists. Researchers analyzed recent and historical data to try to develop a model that explains this resurgence. Monitoring programs throughout the years provided a wealth of information on underwater grasses (since 1958), water quality (since 1984) and even climate-related variables, such as temperature and river discharge, dating back to the late 1800s.
Researchers found that modest reductions in nutrient pollution to the Bay beginning in the late 1980s had led to long-term improvements to water clarity and the amount of light available for plants to grow underwater. A dry period from 1997 to 2002 combined with the absence of major storm events provided ideal conditions for new plant growth, and a critical threshold for the amount of light reaching the plants was crossed. As a result, the bed began to expand and colonize deeper water.
The plants then took over with a process called positive feedback. That is, once given an opportunity, grass beds can improve their own growing conditions by helping sediment drop to the bottom and stay there (increasing the amount of sunlight that can reach their leaves), and using excess nutrients in the water to grow. The researchers found lower nitrogen concentrations and less turbidity in the grass beds than the surrounding waters.
These feedbacks also affect a plant bed's resilience, or its ability to resist disturbances such as storms and rebound after they pass. The researchers noted that in the decade before Agnes, the bed was deteriorating. As a result, feedbacks were not very strong, and the bed was unable to stand up to the storm. The present bed is, evidently, more resilient. When major floodwaters flowed from the Susquehanna River in Fall 2011, a portion of the bed was lost. However, the remaining bed has continued to thrive and expand.