Abstract
Seagrasses are foundation species that provide essential ecological services to coastal ecosystems, yet they are increasingly threatened by anthropogenic and climate-driven stressors. This study uses the SeagrassNet global monitoring protocol to assess environmental and biological indicators of seagrass condition at two back-reef sites in Turneffe Atoll Marine Reserve, Belize, the largest coral atoll in the Mesoamerican Barrier Reef System, between 2022 and 2025. Over the study period, we documented a steady decline in Thalassia testudinum and a shift in species dominance toward Syringodium filiforme. By 2025, seagrass had disappeared entirely from the shallow transects at both sites. Declines in abundance and the shift in species composition were associated with low light availability, elevated phosphate concentrations, and frequent exposure to temperatures exceeding the thermal optimum for these species. The observed loss of seagrass habitat indicates that even protected seagrass meadows within marine reserves remain vulnerable to shifting environmental conditions and raises broader concerns about the status and resilience of meadows throughout the Mesoamerican Barrier Reef System. Moreover, it underscores the broader ecological implications, as seagrass health is closely linked to the resilience of other coastal habitats including adjacent coral reef systems.
Introduction
Seagrasses are foundation species that thrive in shallow coastal and estuarine waters, forming extensive meadows that can be found growing along every continent except Antarctica (Green and Short, 2003; Jayathilake and Costello, 2018; Orth et al., 2006). They support diverse ecosystem services, including water-quality improvement, biodiversity enhancement, sediment stabilization, and nutrient cycling (Terrados and Duarte, 2000; Nordlund et al., 2016; Duffy et al., 2015). They also serve as globally significant carbon sinks, storing large quantities of carbon in both plant biomass and sediments (Fourqurean et al., 2012; Arias-Ortiz et al., 2018). Beyond these roles, seagrasses reduce pathogen exposure for humans and marine organisms such as coral reefs (Lamb et al., 2017; Reusch et al., 2021), sustain productive fisheries and food-web structure (Heck et al., 1997; Mumby et al., 2004; 2003; Whitfield, 2017; Unsworth et al., 2018), and enhance ecosystem resilience through biodiversity and functional redundancy (Duffy et al., 2015; Hughes et al., 2020).
Despite their importance, seagrasses are among the most threatened ecosystems on Earth (Orth et al., 2006; Waycott et al., 2009; UNEP, 2020; Dunic et al., 2021). Waycott et al. (2009) estimated that at least 29% of the known global extent of seagrass meadows has been lost since 1879. The greatest anthropogenic stressors have been eutrophication and sedimentation, which decrease the amount of light available to seagrasses for photosynthesis (Rogers, 1990). In addition, in systems with high nutrient loadings, epiphytes and fast-growing macroalgae outcompete seagrasses since they uptake nutrients more rapidly and have relatively lower light requirements to sustain growth (Short and Wyllie-Echeverria, 1996; McCook, 1999; Waycott et al., 2009; Wilson and Lotze, 2019; Heisler et al., 2008). Climate change, including thermal stress due to rising SST and heatwaves, has also been linked to large-scale losses of seagrass meadows (Smale et al., 2019; Plaisted et al., 2022; Lucey et al., 2023).
SeagrassNet is a scientific global monitoring program that documents the status and threats to seagrass resources worldwide. The program uses a standardized protocol to detect changes in seagrass habitat and capture both seagrass parameters and environmental variables. The program started in 2001 in the Western Pacific and now includes over 180 sites worldwide, with 21 sites in Belize, although most have not been actively monitored since 2018 (SeagrassNet, 2001). Since SeagrassNet’s establishment, the program has been an effective tool for documenting changes in seagrass species diversity and distribution and for identifying areas requiring conservation measures before significant losses occur (Short et al., 2006; Freeman et al., 2008; Fourqurean and Rutten, 2003; Plaisted et al., 2022).
In the following study, we collected data from 2022 to 2025 at two newly established SeagrassNet sites in Turneffe Atoll Marine Reserve to assess the status of meadows in the area. Turneffe Atoll Marine Reserve encompasses 1,369 km2 and is the largest coral atoll in the Mesoamerican Barrier Reef System. It is monitored and protected jointly by the Turneffe Atoll Sustainability Association (TASA) and the Government of Belize. Because seagrass meadows support reef resilience by maintaining water clarity, regulating nutrients, sustaining fish communities, and stabilizing sediments, their decline could increase coral vulnerability. Tracking changes in seagrass condition at Turneffe therefore provides insight into local conservation needs and early warning indicators for other reef-associated meadows throughout the Mesoamerican Barrier Reef System. Given the reserve’s remote location, protected status, and limited coastal development, one would hope that seagrass populations on the atoll would be stable. However, in recent years, widespread coral bleaching and mortality due to elevated sea surface temperatures have been observed across the Mesoamerican Barrier Reef System. Additionally, the increasing inundation of Caribbean coastlines by pelagic Sargassum has been a growing concern, as elevated Sargassum biomass can represent a potential threat to light availability and contribute to eutrophication of seagrass communities in this area (Jouanno et al., 2025; van Tussenbroek et al., 2017). Thus, we hypothesized that seagrass meadows in Turneffe are being affected by changing environmental conditions.
Methods
Site descriptions
Two SeagrassNet sites were established and monitored between 2022 and 2025 (Figure 1). The first site is on Calabash Caye (17.281626 N, 87.811231 W), located offshore of the Calabash Marine Field Station and the TASA Marine Headquarters. The second site is located at Blackbird Caye (17.305076 N, 87.804588 W), an uninhabited coastline located within patch reefs of the back reef zone of the Turneffe Atoll Marine Reserve. The seagrass meadows at both sites are directly adjacent to coral colonies that occupy the surrounding patch reef. In multiple areas, seagrass and coral occur in immediate proximity, forming a continuous landscape rather than distinct or isolated zones.
Figure 1
Each SeagrassNet site consisted of two fixed parallel 50-m cross transects, with their mid-points on a transect laid out seaward, perpendicular from the shore. Sampling activities occurred within 0.5 m2 quadrats at predetermined random locations along transects A and B on the landward side of the transect tape (Supplementary Figures 1, 2). Seagrass and environmental data were collected at both sites in November of 2022–2024 and in December of 2025.
Seagrass abundance
Seagrass abundance (percent cover, shoot density, and canopy height) was assessed using standardized SeagrassNet methods during November and December of 2022–2025 (SeagrassNet, 2001). Sampling occurred in 0.5 m2 quadrats placed at 12 predetermined random locations along each cross transect at each site (Supplementary Figure 2). Percent cover by species was visually estimated along with the canopy height of Thalassia testudinum within the 0.5-m2 quadrat, while shoot density was determined within a 0.25-m2 subsection. Each quadrat was also photographed to provide a permanent record of site condition.
Water quality
To assess seagrass habitat quality, we characterized environmental conditions by collecting information on water quality (i.e., ammonia, nitrate, nitrite, phosphate, and salinity). In 2022, samples were collected at Calabash Caye (CB) from the 13th to the 20th and at Blackbird Caye (BB) from the 16th to the 20th. In 2023, samples were collected from November 9th to the 17th, while in 2024, samples were collected from the 9th to the 13th due to weather constraints. In 2025, water samples were collected in December, from the 8th to the 14th. In 2025, isolated water sampling was also conducted at five additional exploration sites around the atoll to measure nitrate, nitrite, and phosphate levels (Supplementary Tables 2A, B;Supplementary Figure 3). Nutrient data were analyzed using the Hach DR 900 from the HACH Company, and all corresponding procedures for ammonia (HACH Company, 2015), nitrate (HACH Company, 2019a), nitrite (HACH Company, 2019b), and phosphate (HACH Company, 2017) were conducted. Salinity was measured using a portable refractometer.
Light and temperature data
Light and temperature data were collected at transects A and B at both sites in November 2022–2024 and December 2025 using Onset (HOBO) loggers. The loggers were placed at the 25-m mark (midpoint) of each transect, with an additional logger located on land that was used to calculate the percent surface light reaching the canopy. The loggers were positioned facing south, and light and temperature measurements were taken every 10 min. Light data between 1000 and 1400 h was analyzed and used to calculate the percent surface light reaching the canopy.
Sea surface temperature reference data from 2006 to 2026 were also assessed to determine sea surface trends in the region. The buoy data were obtained from NOAA NDBC buoy 42056, which is located in the Yucatan Basin (19.820 N, 84.980 W), approximately 411 km from the Turneffe Atoll (NOAA NDBC, 2026) (Supplementary Figure 6B). We also compared the buoy data with the HOBO logger data collected within the back-reef systems to assess how much warmer the back-reef environments were relative to offshore conditions. Historical standard meteorological records were downloaded for 2023–2025, and hourly to subhourly WTMP observations were aggregated to daily means. In situ HOBO logger temperatures were similarly aggregated to daily means for each field sampling date and compared to buoy daily SST on exact date matches only, resulting in 18 matched sampling dates. Primary comparison metrics were mean bias (HOBO minus buoy SST) and Pearson correlation.
Environmental benchmarks and thresholds
To aid in ecological interpretation of data, environmental thresholds for growth and survival for T. testudinum and Syringodium filiforme were included in Figures 2–4. The phosphate threshold of 0.1 mg L−1 represents a concentration associated with nutrient enrichment and documented declines or shifts in seagrass communities under eutrophic conditions (Burkholder et al., 2007). The threshold range of 0.1–0.5 mg L−1 for nitrate represents a concentration range associated with healthy nutrient levels and documented stability in seagrass communities in oligotrophic conditions (Lapointe, 1997; Burkholder et al., 2007). Light thresholds were based on reported minimum light requirements for persistence and productivity (20% to 25% surface irradiance), with higher requirements reported for T. testudinum than for S. filiforme (Short et al., 1990; Duarte, 1991; Ferdie and Fourqurean, 2004). Temperature thresholds were based on studies indicating physiological stress and reduced productivity when temperatures approach or exceed 30 °C to 33°C (Barber and Behrens, 1985).
Figure 2
Figure 3
Figure 4
Data analysis
For percent cover, canopy height, and shoot density, the unit of analysis was the individual quadrat (12 quadrats per transect). Quadrat-level observations were retained to preserve within-site variability. For nutrient concentrations, the unit of analysis was the individual water sample collected from each site. For seagrass abundance and nutrient variables, separate statistical comparisons across sampling years were conducted within each site. Light and temperature analyses were based on logger observations aggregated by year. For seagrass abundance and nutrient variables, separate statistical comparisons across sampling years were conducted within each site. For the transect-level percent cover analysis of both T. testudinum and S. filiforme, the temporal trend across years within each transect comparison was used.
For each response variable, normality of residuals was evaluated using the Shapiro–Wilk test (Shapiro and Wilk, 1965), and homogeneity of variance was evaluated using the Brown–Forsythe test (Brown and Forsythe, 1974). Raw data were assessed first. When both assumptions were met, one-way ANOVA followed by Tukey’s honestly significant difference post hoc tests was used (Tukey, 1949). When raw data did not meet assumptions, transformations were screened (percent cover: logit or arcsine-square root; count data: log1p or square root; positive continuous variables: log or square root). If only normality was met after transformation, Welch’s ANOVA followed by Games–Howell post hoc tests was used (Games and Howell, 1976). If only variance homogeneity was met, ANOVA with Tukey HSD was used. If neither assumption was met, Kruskal–Wallis tests were conducted on raw data, followed by Dunn–Bonferroni pairwise comparisons (Kruskal and Wallis, 1952).
Percent cover trends at the transect level were additionally evaluated using linear mixed-effects models, with year as a fixed effect and quadrat as a random intercept to account for repeated sampling of the same quadrats across years (Pinheiro and Bates, 2000). Because quadrats within the same transect are spatially proximate, and annual observations represent repeated measures through time, some degree of spatial and temporal autocorrelation may remain in site-level analyses. Accordingly, results with small effect sizes or marginal significance were interpreted cautiously.
Statistical analyses were conducted in R version 3.6.0 (R Core Team, 2020) within RStudio (RStudio Team, 2020). Data manipulation, visualization, and statistical workflows were performed using the packages readxl (Wickham and Bryan, 2019), dplyr (Wickham et al., 2023), ggplot2 (Wickham, 2016), car (Fox and Weisberg, 2019), rstatix (Kassambara, 2023), and nlme (Pinheiro et al., 2023). Significance was set at alpha = 0.05, and results were reported as means ± standard errors.
Results
Seagrass abundance
Overall, total percent cover decreased significantly for both species at each site (BB: χ2(3) = 43.43, P < 0.001; CB: χ2(3) = 23.22, P < 0.001). Thalassia testudinum percent cover decreased significantly over time at each site (BB: Kruskal–Wallis, χ2(3) = 46.33, P < 0.001; CB: χ2(3) = 28.44, P < 0.001), while S. filiforme percent cover increased significantly (BB: χ2(3) = 39.24, P < 0.001; CB: χ2(3) = 35.90, P < 0.001) (Figure 5A).
Figure 5
Along the shallow transects at each site (BBA and CBA) and at the deeper transect at Blackbird Caye (BBB), T. testudinum percent cover decreased significantly between 2022 and 2025 (linear mixed-effects models: BBA, F(1,35) = 266.66, P < 0.001; BBB, F(1,35) = 8.42, P = 0.006; CBA, F(1,35) = 62.12, P < 0.001; CBB, F(1,35) = 1.90, P = 0.177). There were no significant changes in S. filiforme percent cover during that time period along each transect (BBA, F(1,35) = 0.005, P = 0.942; BBB, F(1,35) = 2.40, P = 0.130; CBA, F(1,35) = 0.041, P = 0.841; CBB, F(1,35) = 0.206, P = 0.653) (Figure 5B).
Shoot density and canopy height
Overall, total shoot density decreased significantly for both species at each site between 2022 and 2025 (BB: χ2(3) = 64.69, P < 0.001; CB: Welch’s ANOVA, F(3,43.50) = 46.12, P < 0.001). Thalassia testudinum and S. filiforme shoot density decreased significantly over time at each site (Tt, BB: Kruskal–Wallis, χ2(3) = 64.33, P < 0.001; Tt, CB: Welch’s ANOVA, F(3,49.34) = 12.62, P < 0.001; Sf, BB: χ2(3) = 60.30, P < 0.001; Sf, CB: χ2(3) = 59.14, P < 0.001) (Supplementary Figure 4). Canopy height of T. testudinum increased significantly across years at both sites (BB: Welch’s ANOVA, F(3,34.87) = 51.85, P < 0.001; CB: one-way ANOVA, F(3,80) = 56.76, P < 0.001) (Supplementary Figure 5).
Water quality and nutrient analysis
In 2022, all phosphate samples fell below the 0.1 mg L−1 stress threshold (Burkholder et al., 2007). From 2023 to 2025, mean phosphate concentrations at all sites exceeded this threshold, indicating an increase in phosphate availability across the monitoring period although they varied between years (BB, Welch’s ANOVA on square-root-transformed data, F(2,7.54) = 4.89, P < 0.05; CB, one-way ANOVA on square-root-transformed data, F(3,21) = 4.07, P < 0.05) (Figure 2). All the other environmental parameters (nitrate, nitrite, ammonium, and salinity) were not elevated, and significant differences among years were not observed (Supplementary Table 1).
Light and temperature information
Overall, temperatures varied between 2023 and 2025 at all sites (Kruskal–Wallis, χ2(2) = 743.89, P < 0.001). In 2024, all transects exceeded the stress threshold for T. testudinum, but remained below the thresholds for S. filiforme (BBA, mean temperature = 31.41 °C; BBB, mean temperature = 31.32°C; CBA, mean temperature = 30.98 °C; BBB, mean temperature = 30.69 °C) (Figure 2). In 2023, mean temperatures along the deeper transects at each site (BBB and CBB) remained below the stress threshold of 30 °C for T. testudinum and 32 °C for S. filiforme (BBB, mean temperature = 29.97 °C; CBB, mean temperature = 29.62 °C) (Figure 2). However, along the shallow transect at Blackbird Caye (BBA), mean temperatures exceeded the stress threshold for T. testudinum (BBA, mean temperature = 30.28 °C) (Figure 2). In 2025, all transects fell below the stress threshold of both T. testudinum and S. filiforme (BBA, mean temperature = 29.22 °C; BBB, mean temperature = 28.92 °C; CBA, mean temperature = 29.21 °C; BBB, mean temperature = 29.33 °C) (Figure 2).
Between 2006 and 2026, buoy data showed a significant increase in sea surface temperature (linear regression, R = 0.814, P < 0.001) (Supplementary Figure 6A). For comparison of matched sampling dates, a strong agreement in temporal variation was observed between buoy and HOBO data; however, the shallow back reefs recorded higher mean sea surface temperatures (mean difference = +0.92 °C; Pearson correlation, r = 0.82) (Supplementary Figure 6B).
Light levels varied between 2023 and 2025 (Kruskal–Wallis, χ2(2) = 108.50, P < 0.001). In 2025, mean light levels were significantly lower than in the previous years. The only transect to remain above the minimum light levels for both species was Blackbird Caye’s shallow transect (BBA; mean % irradiance = 38.88%) (Figure 4). All other transects (CBA, CBB, and BBB) fell below the light threshold for T. testudinum, while only the deep transect of Blackbird Caye (BBB) and the shallow transect at Calabash Caye (CBA) fell below the stress threshold of S. filiforme (CBA: mean = 21.49%; CBB: mean = 3.84%; BBB: mean = 17.09%) (Figure 4). In 2023, both transects at Blackbird Caye (BBA and BBB) exceeded the minimum light threshold (BBA: mean = 33.43%; BBB: mean = 23.45%), while the deeper transect at Calabash Caye (CBB) fell below the threshold (Figure 4). In 2024, no significant changes were observed in mean light levels from the previous year (Figure 4). The shallowest transect at Blackbird Caye (BBA) was the only transect with mean irradiance remaining above the minimum light threshold for both species (mean = 33.48%) (Figure 4). Both transects at Calabash Caye (CBA and CBB) and the deeper transect at Blackbird Caye (BBB) fell below the threshold for T. testudinum, while both transects at Calabash Caye fell below the threshold for S. filiforme (CBA: mean = 16.77%; CBB: mean = 14.64%; BBB: mean = 20.94%) (Figure 4).
Discussion
Seagrasses provide numerous ecosystem services to coastal environments and adjacent habitats, including coral reefs. Monitoring the status and threats to seagrass meadows is important for identifying areas requiring conservation measures before significant losses occur. Between 2022 and 2025, our study monitored two SeagrassNet sites located within a back-reef system on Turneffe Atoll, a remote marine protected area along the Mesoamerican Barrier Reef System. We observed a steady decline in the dominant species T. testudinum and a shift in species composition toward the pioneer species S. filiforme. By 2025, seagrass had disappeared entirely from the shallow transects at both sites, and the structural characteristics of the meadows had changed. During monitoring efforts, light availability was below levels considered optimal for seagrass growth, while both temperature and phosphate concentrations were elevated, suggesting multiple stressors are associated with declines. This is the first study to provide quantitative data that links multiple environmental parameters to seagrass losses in Belize and raises broader concerns about the status and resilience of meadows throughout the Mesoamerican Barrier Reef System.
Changes in seagrass meadow characteristics
Seagrass losses have been documented throughout the Caribbean (Short et al., 2006; van Tussenbroek et al., 2014; Ávila-Mosqueda et al., 2025). However, data from Belize show relatively persistent, healthy, seagrass meadows with only a few sites reporting declines (TNC, 2015; Short et al., 2006; Short and Chi, 2014; Gaston et al., 2009). Thus, the overall decrease in T. testudinum from 65.12% in 2022 to 23.85% in 2025, the increase in S. filiforme from 7.35% in 2022 to 22.27% in 2024, and the complete loss of seagrass in the shallow portions of the meadow at both SeagassNet sites in this relatively isolated marine reserve were disheartening (Figure 5B) and will likely have cascading effects. Thalassia testudinum typically forms dense, structurally complex meadows with substantial belowground biomass that stabilize sediments, provide nursery habitat, and store carbon. They also provide water clarity-buffering functions that protect nearby reefs from resuspension and turbidity (Terrados and Duarte, 2000; Unsworth et al., 2012). A loss or shift to S. filiforme in the vegetated portions of the meadow will likely reduce canopy complexity, alter faunal assemblages, and reduce sediment retention, and long-term carbon storage relative to T. testudinum dominated meadows (Terrados and Duarte, 2000; Micheli et al., 2008; Fourqurean et al., 2012).
Light levels, nutrients, and temperature
There were multiple environmental parameters associated with the observed changes in the seagrass meadows at our sites. Light availability declined over the study period, with most transects falling below the minimum threshold required to support seagrass productivity, particularly in 2024 and 2025 (Figure 4). Across years, all transects except transect A at Blackbird Caye fell below the minimum light threshold for T. testudinum of approximately 25% surface irradiance, while multiple transects fell below the lower threshold for S. filiforme of approximately 20% surface irradiance. Low-light availability directly constrains seagrass growth by limiting photosynthetic carbon fixation, reducing the energy available for leaf production, rhizome expansion, and carbohydrate storage (Short et al., 1990; Burkholder et al., 2007). When light falls below species-specific thresholds, respiration can exceed photosynthesis, resulting in a negative carbon balance, depletion of stored reserves, and eventual shoot mortality (Short et al., 1990). Seagrasses can attempt to acclimate to low light by increasing shoot size and reducing shoot density, or self-thinning, in order to optimize light capture by the canopy (Enríquez et al., 2019). This response was observed in our study, further suggesting that light limitation is one factor contributing to overall changes at our sites (Supplementary Figures 4, 5).
In tropical systems, light limitation is caused by dissolved organic molecules and suspended particles. High concentrations of phytoplankton, sediment, and colored dissolved organic matter all attenuate light reaching the benthos (Burkholder et al., 2007; Howarth et al., 2011; Tzortziou et al., 2007). In 2024 and 2025, we observed Sargassum Brown Tides (SBTs) at Calabash, which likely reduced light availability and contributed to eutrophication at this site. SBTs form when beached Sargassum decomposes and releases particulate organic matter (POM), excess nutrients, heavy metals, and arsenic into nearshore waters, turning them brown (van Tussenbroek et al., 2017; Rodríguez-Martínez et al., 2019; Vásquez-Elizondo and Vázquez-Delfín, 2021). Sargassum and its decomposition products, therefore, represent a potential pathway for both light attenuation and nutrient enrichment at Calabash. Sargassum accumulates on the shoreline at Calabash but has not been observed at Blackbird Caye. This assessment is based on near-continuous observations by TASA personnel and University of Belize field station staff, who reside at Calabash for much of the year and regularly monitor conditions across both sites (Martinez, pers. comm.). This suggests that the complete loss of seagrass in the shallow transect of Blackbird Caye is not driven by reduced light availability but by other stressors.
In addition to seeing low light levels, we also observed elevated phosphate concentrations at both sites, exceeding typical background levels for carbonate tropical systems where phosphorus is often the limiting nutrient (Short et al., 1990). Mean phosphate concentrations were approximately 0.8–1.0 mg L−1, with values frequently exceeding 2.0 mg L−1 and reaching as high as 6.54 mg L−1 (Figure 2), compared to the low background phosphate concentrations typically reported for oligotrophic Caribbean systems (Lapointe, 1997; Burkholder et al., 2007). These values represent increases on the order of ~30–100× above background conditions. In contrast, other nutrients were not similarly elevated. Nitrate concentrations remained low (~0.02–0.04 mg L−1), within typical oligotrophic ranges and well below levels associated with eutrophication, indicating that nitrogen enrichment was unlikely to be a primary driver of observed changes (Lapointe, 1997; Burkholder et al., 2007; Jickells et al., 2017).
The causes of high phosphate concentrations are unknown. Elevated phosphate can alter seagrass growth allocation, including changes in shoot density and belowground biomass (Williams, 1987). Potential common pathways for local nutrient enrichment could include wastewater leakage, graywater discharge, organic waste accumulation, or runoff during rainfall events. However, these sources would seem unlikely given the low level of development in this area, other nutrients (nitrogen) were not elevated, and high phosphate levels have been observed at multiple locations across the atoll (Supplementary Figure 1). Regional factors that may contribute to high concentrations in Turneffe atoll include basin-scale oceanographic change. Previous studies have suggested that enhanced equatorial Atlantic upwelling can increase the supply of phosphorus to surface waters (Wang et al., 2019). Another possible mechanism is storm-driven sediment resuspension, which can mobilize sediment-associated phosphorus and release it into the water column (Zhang et al., 2004). Recent years have seen elevated storm activity across the Gulf of Mexico and western Caribbean, including the unusually active 2023 Atlantic hurricane season (Klotzbach et al., 2024). Lastly, biological processes may also contribute to phosphate enrichment across the atoll. In May 2023, a severe thermal anomaly triggered widespread coral bleaching and mortality in Belize, which may contribute nutrients through decomposition of organic tissue and through deposition of calcareous material that can later be resuspended (Lubofsky, 2024; Iijima et al., 2021).
In addition to low light levels and high phosphate concentrations, water temperatures frequently reached levels associated with thermal stress in T. testudinum (Figure 3). In 2024, mean temperatures at all transects exceeded the commonly reported thermal stress threshold of approximately 30 °C for T. testudinum, while remaining below the higher threshold reported for S. filiforme (~32 °C). In contrast, most transects in 2023 and all transects in 2025 remained below these thresholds. Regional buoy data also indicated warming across the broader Yucatan Basin, suggesting that elevated temperatures observed within the back-reef sites occurred within a broader pattern of western Caribbean warming. High sea surface temperatures affect seagrass physiology by disrupting the balance between photosynthesis and respiration. As temperatures rise, respiration rates increase more rapidly than photosynthetic capacity, leading to reduced net carbon gain and limiting the energy available for growth and maintenance (Barber and Behrens, 1985; Lee et al., 2007). Prolonged exposure to elevated temperatures can impair enzymatic function, reduce photosynthetic efficiency, and increase susceptibility to oxidative stress, ultimately resulting in declines in shoot density, canopy structure, and overall biomass. These effects can impact both species observed in this study. However, T. testudinum is generally more vulnerable due to its slower growth and reliance on stable environmental conditions, whereas S. filiforme may better tolerate short-term thermal stress through faster turnover and greater physiological flexibility (Ferdie and Fourqurean, 2004). In many cases, thermal stress does not act alone but compounds other environmental pressures, such as light limitation, further reducing seagrass resilience. Similar patterns have been observed in Florida seagrass meadows, where elevated summer temperatures were associated with reduced productivity of both T. testudinum and S. filiforme (Barber and Behrens, 1985). These patterns suggest that increasing temperatures likely imposed physiological stress on seagrass communities and contributed to the observed declines and shifts in species composition.
Overall, our data suggest that seagrass losses and shifts in species composition were associated with multiple stressors that varied by site and transect. At Calabash, reduced light availability was likely an important driver, as both transects fell below species-specific light thresholds in 2024 and 2025, coinciding with elevated phosphate and declines in seagrass abundance (Figures 2, 4). At Blackbird, however, the complete loss of seagrass in the shallow transect was not explained by light limitation, as this transect remained above minimum light thresholds (Figure 4). Temperature thresholds for T. testudinum were exceeded at the shallow transect of Blackbird Caye, which could provide an explanation for the complete loss observed for that species (Figure 3). Although elevated temperatures may have contributed to stress in T. testudinum, temperatures did not exceed the reported tolerance range for S. filiforme, making temperature alone an insufficient explanation for the loss of both species (Figure 3). Instead, the disappearance of S. filiforme may indicate the influence of another shared stressor, such as elevated phosphate or phosphate-associated changes in water quality. Together, these patterns indicate that seagrass decline at Turneffe Atoll was not driven by a single stressor, but by interacting effects of nutrient enrichment, light reduction, and thermal stress that differed across local conditions.
Limitations
Water quality measurements were collected during annual 2-week sampling windows rather than through continuous year-round monitoring, so these data should be interpreted as seasonal snapshots rather than comprehensive measures of temporal variability. Although environmental monitoring was limited to two permanent sites, additional visual surveys, one-time water quality sampling, and SeagrassNet observations across the atoll provided preliminary spatial context and highlighted the need for expanded long-term monitoring. Establishing additional permanent monitoring sites across Turneffe Atoll would allow year-to-year comparisons across several back-reef habitats and help confirm whether the patterns observed here are localized or more widespread.
Conclusion
SeagrassNet proved effective in detecting rapid ecological change at both sites within Turneffe Atoll Marine Reserve. Between 2022 and 2025, rising phosphate levels, reduced light availability, a shift from T. testudinum to S. filiforme, and localized die-offs were observed, collectively suggesting early-stage eutrophication. Because T. testudinum provides greater structural complexity and ecological value, its decline may reduce habitat quality, carbon storage, and overall ecosystem resilience. Without the annual monitoring provided by the SeagrassNet protocol, these shifts in species composition, meadow health, and back-reef environmental conditions may have gone undetected. These findings are particularly relevant as Belize considers expanding seagrass monitoring efforts at a national scale. The Nature Conservancy has identified SeagrassNet as a potential standardized approach, and the results of this study demonstrate its effectiveness in detecting rapid ecological change and identifying early signs of meadow degradation before irreversible loss occurs.
Although Turneffe Atoll represents one part of a broader network of remote islands and atolls extending from Nicaragua to the Yucatán Peninsula, the patterns documented here may be relevant throughout the wider Mesoamerican reef region. This highlights the growing need to expand monitoring efforts across back-reef systems to identify at-risk seagrass meadows and associated communities before severe degradation occurs. Our findings can help guide nutrient management and habitat protection strategies within Turneffe Atoll’s Marine Protected Areas, allowing targeted interventions before further biodiversity loss and declines in ecosystem services. Managing local nutrient inputs while maintaining the water-quality benefits provided by seagrass meadows may therefore be important for sustaining both seagrass habitats and the resilience of adjacent back-reef coral communities.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
KF: Data curation, Formal Analysis, Investigation, Project administration, Validation, Visualization, Writing – original draft, Writing – review & editing. KS: Writing – review & editing. AN: Conceptualization, Funding acquisition, Methodology, Supervision, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. Field support and logistics were provided through the Boston University Marine Program’s Tropical Seagrass Ecology course and the Boston University Undergraduate Research Opportunities Program.
Acknowledgments
We would like to thank the Boston University Marine Program’s Tropical Seagrass classes from 2022, 2023, 2024, and 2025 for their assistance with data collection in the field, with a special thanks to Mason Burns, Tiare Sierra Rivera, James Schipfer, and Logan Rajah. We would also like to thank Cat Hauser and Ninon Martinez for field assistance.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
Caribbean, eutrophication, light availability, marine protected area (MPA), sea surface temperature, SeagrassNet, Syringodium filiforme, Thalassia testudinum
Citation
Fietz KA, Scavo Lord K and Novak AB (2026) Seagrass composition shifts and declines associated with reduced light, high phosphate, and elevated water temperatures in Turneffe Atoll Marine Reserve, Belize. Front. Mar. Sci. 13:1842116. doi: 10.3389/fmars.2026.1842116
Received
29 March 2026
Revised
16 June 2026
Accepted
24 June 2026
Published
13 July 2026
Volume
13 - 2026
Edited by
Melanie McField, Healthy Reefs for Healthy People, United States
Reviewed by
Marcus W. Beck, Tampa Bay Estuary Program, United States
Joanna Smart, The University of Queensland, Australia
Updates
Copyright
© 2026 Fietz, Scavo Lord and Novak.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Kelly A. Fietz, kfietz@bu.edu; Alyssa B. Novak, abnovak@bu.edu
†ORCID: Kelly A. Fietz, orcid.org/0009-0005-5386-8131; Karina Scavo Lord, orcid.org/0000-0002-6257-6270; Alyssa B. Novak, orcid.org/0000-0003-3699-6180
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.