Role of feedback processes in estuarine submersed plant dynamics
Seagrasses and associated submersed aquatic vegetation (SAV) have been disappearing globally due to chronic reduction in available light associated with increasing nutrient loading and resultant excess algal production. Sometimes, declines are surprising due to internal feedback processes, which promote plant bed stability until a critical light threshold is crossed and the bed abruptly deteriorates. However, a large submersed plant bed in Upper Chesapeake Bay recently recovered after nearly 30 years of degradation and subsequently survived a major flood event. In theory, feedbacks can also facilitate unexpected resurgences, as resuspension of unvegetated sediments prevents plant growth until the light threshold is again crossed, allowing for rapid recovery. These processes may also promote resilience to disturbances by ameliorating ambient conditions despite poor water quality outside the bed. We analyzed and synthesized a suite of publically available environmental monitoring datasets together with our own field samples using relatively simple statistics (e.g., linear regression, analysis of variance) to explore whether feedback processes played a role in the sudden resurgence and apparent resilience to extreme weather. Our overall motivation was that a better understanding of the mechanisms behind this natural recovery could inform SAV conservation and restoration efforts in other systems.
We found that SAV abundance generally decreased during wet years and increased during dry years. Therefore, we inferred that several consecutive dry years preceding the resurgence likely triggered plant bed expansion. However, positive feedback processes, as indicated by differences in water quality measured inside and outside the plant bed, may have produced the observed nonlinear SAV dynamics. For example, turbidity, an indicator of water clarity, was the same if not greater inside versus outside the bed during the spring before plants sprouted, supporting the idea that in the absence of SAV, sediment resuspension creates light-limiting conditions. On the other hand, low turbidity inside the bed and in a plume emanating from the down-bay edge of the bed during peak SAV biomass likely minimized plant loss and promoted bed expansion, thus enhancing resilience. This work demonstrates how theory (e.g., stability and resilience) can guide case-study analyses and how monitoring data can yield detailed insights into the dynamics of any system.