Assessing seagrass ecosystem status and condition : multi-scale applications of a long-term monitoring program




Congdon, Victoria Marie

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Fluctuations in seagrass abundance and distribution often signify changes in abiotic conditions, including irradiance, temperature, salinity, and nutrient concentrations that can have long-term effects on coastal ecosystems. Stress responses by seagrasses trigger physiological effects that modify morphology and if conditions persist, alters structure at the meadow-scale. Since seagrasses are important indicators of ecosystem condition, long-term monitoring can help us identify factors that influence seagrass habitats through time and space. Such relationships between seagrass structure (i.e., plant physiology, architecture) and environmental conditions can better inform resource managers on the status and trends of seagrass ecosystems. This work investigates the applicability of long-term seagrass monitoring programs in Texas and Florida to: (1) estimate organic carbon stores along the Texas coast; (2) assess seagrass responses to a major disturbance in Texas; (3) characterize seagrass edge effects; and (4) evaluate the effectiveness of ecological indicators in identifying changes in seagrass condition within Florida and Texas. Areas with greater carbon stocks (up to 400 g C m⁻² in living biomass) corresponded to increases in cover and biomass; moreover, greater carbon storage capacity was associated with Thalassia testudinum, a more physically robust species (i.e., wide leaves, thick belowground tissues), than Halodule wrightii or Syringodium filiforme. Interestingly, despite the robust architecture of T. testudinum, this late successional species was more sensitive to a Category 4 hurricane than the prolific pioneer species (H. wrightii) as measured by greater reductions in cover and blade length in mixed (−16 vs. +1 %) and monospecific (−20 vs. +2 %) beds. We identified 11 metrics contributing to the dissimilarity between edge and interior habitats, with greater leaf widths, leaves shoot⁻¹, δ¹⁵N values, and epiphytes specific to edges. Importantly, many of the same metrics overlap as indicators for assessing ecological condition. Cover, shoot allometry and species composition were sensitive indicators of large-scale climatic disturbances (i.e., droughts, hurricanes). Our findings illustrate the breadth of long-term monitoring data in assessing differential responses to carbon stores and disturbances arising from distinct physiology and structure. Moreover, the use of common ecological indicators acquired from long-term monitoring programs highlight the prospect of broad-scale applications which can be used to develop seagrass management and conservation initiatives



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