Evaluating Heat Stress, Disease, and Economic Value in Temperate Seagrass Meadows

Seagrass meadows provide vital ecosystem services, including carbon and nitrogen sequestration, shoreline protection, and habitat for fisheries, yet they face accelerating threats from climate change, disease, and other anthropogenic stressors. These impacts are especially severe for Zostera marina in the southern extent of its temperate range, where temperatures are already near its physiological limits and even small increases can surpass its thermal threshold, triggering major ecological changes. In this dissertation, I examine the interactions between heat stress, sulfide intrusion, disease dynamics, and ecosystem service economic valuation in Z. marina meadows of the Virginia Coast Reserve (VCR). Restoration of these meadows began with seeding in 2001, and they now span over 40 km², making the VCR a model system for large-scale seagrass restoration. Across multiple spatial and temporal scales, I integrate high frequency in situ measurements, plant production models, isotopic analyses of plant tissue, disease surveillance, and economic modeling to quantify both biological and physical thresholds and the societal value of these habitats. I first developed a fine-scale (centimeters to kilometers) heat stress framework using 16 metrics grouped into four categories: cumulative heating, nighttime cumulative heating, marine heatwaves, and days exceeding the thermal threshold of 28.6 °C for Z. marina previously determined for this location, to capture both the intensity and duration of thermal stress events. Maximum nighttime temperatures and annual days above the physiological threshold emerged as strong predictors of decreased shoot density, with warmer sites experiencing up to 95% reduction in productivity and five times longer stress periods than cooler sites within the same meadow. Early-fall marine heatwaves occurred annually and reduced daily productivity more than the heat stress recorded in June and July. I then investigated sediment heat stress and its influence on sulfide intrusion, known to decrease plant production, and found that threshold temperatures were often exceeded in shallow sediments but rarely penetrated beyond 5 cm sediment depth, indicating that roots and rhizomes deeper than 5 cm did not experience heat stress. Sulfide concentrations in plant tissues indicated that both water column and sediment warming can amplify physiological stress. Next, I conducted a large-scale survey of seagrass wasting disease (caused by the slime mold, Labyrinthula zosterae) across the entire >40 km² of restored meadows, combining quantitative polymerase chain reaction (qPCR) diagnostics with leaf lesion mapping. Wasting disease prevalence (proportion of diseased leaves out of the total leaves sampled) was pervasive (96% of plants infected), but there were no signs of impact on seagrass production. Severity (proportion of lesion area divided by leaf area) was strongly linked to anomalous heat stress and long water residence times. Disease severity was lower in sites experiencing higher water turnover. Finally, I applied the benefits transfer method and logistic growth modeling to estimate the monetary worth of four key ecosystem services provided by the restored seagrass meadows over time. Using key factors in natural capital assessment, such as the social cost of carbon (SCC), discount rates, time horizons, and seagrass loss scenarios, the restored VCR meadows were estimated at $30.4 million over 30 years. Collectively, these studies identify warming as a critical driver of physiological stress, sulfide exposure, and disease propagation in temperate seagrass meadows, while also quantifying their substantial economic value. By linking fine-scale environmental monitoring with biological and economic outcomes, this work advances our understanding of climate vulnerability in seagrass ecosystems and provides actionable thresholds and valuation tools for adaptive management in a warming world.