Preliminary assessment of a nutrient pollution indicator for application to tropical seagrasses of Puerto Rico
Seagrass health is often used as an indicator of water quality and plant tissue nitrogen content has long been used as an indicator of nitrogen availability. However, traditional tissue nitrogen content has not been a particularly sensitive early indicator of nutrient over enrichment. A critical characteristic for an early indicator is that it can detect increased nutrient availability before seagrasses die. In 2004, Lee, Short & Burdick proposed a new nutrient pollution indicator (NPI) for seagrass that integrates leaf tissue nitrogen content and leaf areal mass to track nutrient loading in temperate seagrass systems. Here, we evaluate the efficacy of applying this NPI metric to three tropical seagrasses Thalassia testudinum, Halodule wrightii, and the non-native Halophila stipulacea. Further we assess the underlying assumptions associated with the NPI metric. In June 2022 we sampled ten seagrass sites across three estuarine systems in Puerto Rico. In each estuary, we sampled across an expected water column nutrient gradient. We collected water column and sediment nutrient samples as well as samples for stable isotope analysis (%N and δ15N) and we made morphological measurements on seagrasses (leaf areal mass, mg dry weight cm -2). We found that T. testudinum sheath material, H. wrightii and H. stipulacea met the assumption of a negative correlation between areal leaf mass and tissue N content, suggesting that the NPI metric developed for a temperate species can be applied to these tropical species. Weak gradients in nutrients precluded clear patterns of detection of nutrient loading issues. However, incorporation of δ15N did identify several distinct N sources in different systems and provides new resolving power to the NPI metric. Specifically, in Jobos Bay isotopically light δ15N values suggest that either local N fixation or import of allochthonous N fixation from Sargassum mats are the dominant sources of N. Alternatively, in Guánica Bay, heavy δ15N values near the head of the estuary suggest either wastewater inputs or internal recycling of N are the dominant source. Additional research is required, but the weight of evidence suggests that the NPI methodology can be applied to some tropical seagrasses. Further, the NPI metric coupled with δ15N provides a sensitive seagrass nutrient indicator for tropical and sub-tropical seagrass species that may help to conserve and protect these important habitats and the ecosystem services they provide.