Seagrass structural and elemental indicators reveal high nutrient availability within a tropical lagoon in Panama

Abstract

Seagrass meadows are valued coastal habitats that provide ecological and economic benefits around the world. Despite their importance, many meadows are in decline, driven by a variety of anthropogenic impacts. While these declines have been well documented in some regions, other locations (particularly within the tropics) lack long-term monitoring programs needed to resolve seagrass trends over time. Effective and spatially-expansive monitoring within under-represented regions is critical to provide an accurate perspective on seagrass status and trends. We present a comprehensive dataset on seagrass coverage and composition across 24 sites in Bahía Almirante, a lagoon along the Caribbean coast of Panama. Using a single survey, we focus on capturing spatial variation in seagrass physical and elemental characteristics and provide data on key seagrass bio-indicators, such as leaf morphology (length and width), elemental content (% nitrogen and phosphorus) and stable isotopic signatures (δ13C and δ15N). We further explore relationships between these variables and water depth (proxy for light availability) and proximity to shore (proxy for terrestrial inputs). The seagrass assemblage was mostly monospecific (dominated by Thalassia testudinum) and restricted to shallow water (<3 m). Above-ground biomass varied widely, averaging 71.7 g dry mass m−2, yet ranging from 24.8 to 139.6 g dry mass m−2. Leaf nitrogen content averaged 2.2%, ranging from 1.76 to 2.57%, while phosphorus content averaged 0.19% and ranged from 0.15 to 0.23%. These values were high compared to other published reports for T. testudinum, indicating elevated nutrient availability within the lagoon. Seagrass stable isotopic characteristics varied slightly and were comparable with other published values. Leaf carbon signatures (δ13C) ranged from −11.74 to −6.70‰ and were positively correlated to shoreline proximity, suggesting a contribution of terrestrial carbon to seagrass biomass. Leaf nitrogen signatures (δ15N) ranged from −1.75 to 3.15‰ and showed no correlation with shoreline proximity, suggesting that N sources within the bay were not dominated by localized point-source discharge of treated sewage. Correlations between other seagrass bio-indicators and environmental metrics were mixed: seagrass cover declined with depth, while biomass was negatively correlated with N, indicating that light and nutrient availability may jointly regulate seagrass cover and biomass. Our work documents the response of seagrass in Bahía Almirante to light and nutrient availability and highlights the eutrophic status of this bay. Using the broad spatial coverage of our survey as a baseline, we suggest the future implementation of a continuous and spatially expansive seagrass monitoring program within this region to assess the health of these important systems subject to global and local stressors.

Seagrass ecosystem multifunctionality under the rise of a flagship marine megaherbivore

“…To assess the impact of megaherbivore grazing intensity on ecosystem multifunctionality, we have experimentally manipulated seagrass biomass and grazing intensity to simulate three progressing grazing intensity scenarios, all of which can be found in three ocean basins (Figure 1b ): (1) no turtle grazing, representing the absence of turtles. The absence of turtle grazing (or intensive grazing by smaller herbivores) results in high seagrass biomass as observed in many current modern seagrass meadows where turtles remain ecological extinct (Gaubert‐Boussarie et al, 2021 ; Jackson, 1997 ; Jones et al, 2018 ; van der Laan & Wolff, 2006 ; e.g., Vonk et al, 2010 ); (2) intermediate turtle grazing, representing presence by turtles. Ecosystems were exposed to natural or intermediate grazing intensity resulting in intermediate seagrass biomass, with plenty of leaf biomass still present as observed in meadows with turtles (Christianen et al, 2019 ; Molina Hernández & van Tussenbroek, 2014 ); (3) intensive turtle grazing, representing the accumulation of turtles, resulting in very high grazing pressure and sometimes “overgrazing,” that is, when grazing rates exceed production rates, and very low seagrass biomass as observed in areas with turtle accumulation (Fourqurean et al, 2019 ; Gangal et al, 2021 ).…”

“…The absence of turtle grazing (or intensive grazing by smaller herbivores) results in high seagrass biomass as observed in many current modern seagrass meadows where turtles remain ecological extinct (Gaubert-Boussarie et al, 2021;Jackson, 1997;Jones et al, 2018;van der Laan & Wolff, 2006;e.g., Vonk et al, 2010) (Christianen et al, 2019;Molina Hernández & van Tussenbroek, 2014); (3) intensive turtle grazing, representing the accumulation of turtles, resulting in very high grazing pressure and sometimes "overgrazing," that is, when grazing rates exceed production rates, and very low seagrass biomass as observed in areas with turtle accumulation (Fourqurean et al, 2019;Gangal et al, 2021).…”

“…Green dots: global seagrass distribution (UNEP-WCMC & Short, 2021), blue: distribution of the green turtle, Chelonia mydas, (Kot et al, 2022). (Scenario 1) (Gaubert-Boussarie et al, 2021;Jackson, 1997;Jones et al, 2018;van der Laan & Wolff, 2006;Vonk et al, 2008)…”

“…Green dots: global seagrass distribution (UNEP‐WCMC & Short, 2021 ), blue: distribution of the green turtle, Chelonia mydas , (Kot et al, 2022 ). (Scenario 1) (Gaubert‐Boussarie et al, 2021 ; Jackson, 1997 ; Jones et al, 2018 ; van der Laan & Wolff, 2006 ; Vonk et al, 2008 ); (Scenario 2) (Ballorain et al, 2010 ; Christianen et al, 2019 ; Gulick et al, 2020 ; Molina Hernández & van Tussenbroek, 2014 ; Rodriguez & Heck, 2020 ; Scott et al, 2020 ); (Scenario 3) (Christianen et al, 2014 ; Fourqurean et al, 2019 ; Gangal et al, 2021 ).…”

Herbivores shape the seascape : Cascading effects of herbivores on the functioning of tropical seagrasses in a changing world

“…Green dots: global seagrass distribution (UNEP-WCMC and Short 2021), blue: distribution of the green turtle, Chelonia mydas,(Kot et al 2022). (Scenario 1)(Jackson 1997, van der Laan and Wolff 2006, Vonk et al 2008, Jones et al 2018, Gaubert-Boussarie et al 2021; Scenario 2)(Ballorain et al 2010, Molina Hernández and van Tussenbroek 2014, Rodriguez and Heck 2020, Scott et al 2020); Scenario 3), Fourqurean et al 2019, Gangal et al 2021).…”