Quantifying Heatwaves and Seagrass Recovery Dynamics in Aquatic Ecosystems

Increasing climatic variability has amplified the frequency of pulsed disturbance events, including extremes in temperature. These discrete periods of anomalously high temperature, referred to as heatwaves, have gained attention due to their destructive and lasting impacts relative to longer-term increases in mean temperature. Heatwaves in the open and coastal oceans have increased in frequency, duration, and magnitude in recent decades, occasionally producing mass mortality and regime shifts. While positive water temperature trends have been documented in many inland waterways, heatwaves in lotic systems have not been analyzed. In this dissertation, I quantified trends in heatwaves for rivers, estuaries, and coastal sediments. I also conducted a seagrass removal experiment to test seagrass resilience (i.e., recovery rate) to a heatwave-like disturbance event and synthesized related studies on seagrass recovery following disturbance.

Riverine heatwaves increased in frequency throughout the United States over the period 1996-2021. Lotic heatwaves varied based on atmospheric temperature, discharge, stream order, and position relative to a reservoir. There were no significant trends in estuarine heatwaves based on analyses of 17 sites from 1996 to 2019. However, estuarine heatwaves co-occurred with deleterious water quality conditions such as extreme low dissolved oxygen and acidic pH events. Within the Virginia Coastal Reserve (VCR) over the period 1994-2022, coastal sediment heatwaves increased in frequency at a depth of 5 cm and were tightly coupled with water column heatwaves.

Seagrasses were removed from replicated plots at two locations within a large VCR meadow. Seagrass recovery was faster at the meadow interior relative to the edge, likely due to greater hydrodynamic stress at the meadow edge. Seagrass recovered linearly, independent of position within the meadow, and were facilitated by lateral clonal growth and seedling recruitment. Lastly, the literature synthesis provided evidence that 47% of known seagrass species have been included in English language disturbance-recovery studies but that the majority of studies have occurred in monocultures of the genus Zostera. Experimental disturbances were most often conducted on relatively small spatial scales (median = 0.25 m2), likely overrepresenting the contribution of lateral clonal growth and underrepresenting the effect of hydrodynamics that larger disturbances experience.

This dissertation shows how climate change signals are manifesting in aquatic ecosystems. My findings provide an assessment of heatwaves in rivers, estuaries, and coastal sediments and the largest-scale experimental analysis of seagrass recovery. These studies suggests that key ecosystem services such as maintaining high water quality and high carbon sequestration could be reduced as heatwaves and disturbances become more frequent in coastal areas with a warming climate. Lastly, my dissertation highlights the value of long-term monitoring in establishing baseline assessments that inform ecosystem management.