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, but it is not a 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. The seagrass nutrient pollution indicator (NPI) integrates leaf tissue nitrogen content and leaf areal mass to track nutrient loading. We assess the underlying assumptions associated with the NPI metric and explore the application of the metric to three tropical seagrasses Thalassia testudinum, Halodule wrightii, and Halophila stipulacea. In June 2022 we sampled ten seagrass sites across three embayments in Puerto Rico. We used water column and sediment nutrient measurements, seagrass stable isotope analyses (%N and δ¹⁵N) and we made morphological measurements (leaf areal mass, mg dry weight cm ^-2) to evaluate the NPI relative to anticipated environmental gradients. T. testudinum sheath material, H. wrightii and H. stipulacea met the assumptions of the NPI method, suggesting that the NPI metric may be applied to these tropical species. Jobos Bay sites had isotopically light δ¹⁵N values suggesting that either local N fixation or import of allochthonous N fixation are dominant sources. In Guánica Bay, heavy δ¹⁵N values suggest either wastewater inputs or internal recycling of N are the dominant source. Additional research is required, but the evidence suggests that with further development and validation the NPI metric with δ¹⁵N could be appropriate for tropical seagrass species.