Seagrasses are vital coastal ecosystems providing numerous ecosystem services, including nursery habitat, shoreline protection, and carbon sequestration. However, seagrass meadows are experiencing significant global declines (approximately 29% since the mid-1700s) due to various anthropogenic and environmental factors such as algal blooms, pollution, disease, drought, eutrophication, heat waves, invasive species, mechanical damage, and storms. These factors disrupt the plants' photosynthetic balance. Seagrasses have high light requirements due to their high respiratory demands. Sea level rise (SLR) poses a significant threat, as it reduces light penetration to seagrass beds. While global mean sea level has risen by 1–3 mm/year, the Gulf of Mexico is experiencing SLR rates exceeding 10 mm/year since 2010, amplified by local factors like Rossby waves and land subsidence. Increased water depth can cause seagrass distribution shifts, but shoreline hardening prevents colonization of newly submerged areas, resulting in net habitat loss. This study investigates the rapid decline of seagrasses in the relatively pristine Upper Laguna Madre, Texas, using long-term monitoring data to determine the impact of SLR on seagrass populations.

Existing literature highlights the ecological and economic importance of seagrass meadows and their vulnerability to various stressors. Studies have documented the impact of light limitation, altered salinity, and other environmental changes on seagrass growth and distribution. Previous research has investigated the effects of individual stressors such as algal blooms, pollution, and temperature changes on seagrass health. Some studies have explored the potential for seagrass expansion under SLR scenarios, but often lack long-term data to assess the actual impacts of rapid changes in water depth. The current study addresses this gap by utilizing long-term data to evaluate the impact of accelerated sea level rise on seagrass populations in a relatively undisturbed ecosystem.

This study combines data from two long-term monitoring programs in the Upper Laguna Madre, Texas. The first is a fixed deep-edge station (LM-151) monitored since 1989, providing monthly measurements of water temperature, irradiance, salinity, and water depth, as well as seagrass biomass and shoot density. The second is a basin-wide Tier-2 sampling program (144 sentinel stations) conducted annually from 2011 to 2022, assessing seagrass presence across the region. Water quality parameters, including total suspended solids, Secchi depth, dissolved oxygen, ammonium, and dissolved inorganic nitrogen were also measured. General additive models (GAMs) were used to analyze the relationships between environmental variables (water temperature, irradiance, water depth, salinity) and seagrass biomass and presence. Hierarchical clustering was used to group monitoring sites based on water depth. Coastal-wide sea level rise rates were assessed using data from 54 NOAA stations. Finally, three sea level rise scenarios were used to model future seagrass habitat changes.

At the LM-151 deep-edge station, *Halodule wrightii* and *Syringodium filiforme* exhibited a typical successional pattern until 2014. After 2014, a rapid increase in water depth (14 mm/year) coincided with a significant decline in seagrass cover. By 2018, both species had disappeared from LM-151. GAM modeling revealed that water depth and irradiance were the most significant predictors of seagrass presence, while temperature and irradiance influenced biomass. Basin-wide monitoring confirmed widespread seagrass loss, particularly in deeper areas. Analysis of 54 NOAA stations revealed significantly increased rates of sea level rise after 2014, ranging up to 21 mm/year. Modeling future seagrass habitat changes under three SLR scenarios (best-case, current, worst-case) indicated a potential net gain in suitable seagrass habitat; however, this was offset by a substantial increase in unsuitable habitat due to increased water depth.

The results strongly indicate that rapid sea level rise is the primary driver of seagrass loss in the Upper Laguna Madre. The significant decrease in light penetration due to increased water depth, even in a relatively pristine ecosystem, overwhelmed other factors and resulted in the loss of both *Halodule wrightii* and *Syringodium filiforme*. The fact that the decline happened concurrently with a rapid increase in sea level rise, and the strong correlations found between water depth and seagrass presence, provides strong evidence supporting this conclusion. While there is potential for seagrass expansion into newly submerged areas, this is severely limited by shoreline hardening. This highlights the importance of mitigating SLR and protecting natural shorelines to conserve seagrass habitats.

This study provides compelling evidence of the detrimental effects of rapid sea level rise on seagrass meadows. The findings underscore the vulnerability of even relatively pristine ecosystems to accelerated SLR and emphasize the need for effective strategies to mitigate climate change and protect coastal habitats. Future research should focus on further investigating the effects of SLR on other seagrass ecosystems and exploring strategies for restoring and conserving seagrass meadows in the face of rising sea levels.

This study focused on a single, albeit large, estuarine system. While the findings suggest that rapid sea level rise is a significant threat to seagrasses globally, further research is needed to validate these findings across a wider range of ecosystems and geographic locations. The models used for projecting future seagrass habitat changes rely on certain assumptions about shoreline dynamics and seagrass colonization rates, which might introduce uncertainties in the predictions.