Application of a seagrass nutrient pollution indicator to tropical seagrasses from Puerto Rico: Preliminary results
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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 availability. A critical factor 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. Our objective was to evaluate the efficacy of using this NPI metric in tropical estuarine systems. We intend to apply the NPI metric to tropical seagrasses (Thalassia testudinum, Syringodium filiforme, Halodule wrightii, and the non-native Halophila stipulacea) and compare resulting NPI values with environmental nitrogen levels. In June 2022 we surveyed seagrass beds within the Jobos Bay National Estuarine Research Reserve (NERR, 5 sites), Guánica Bay (3 sites) and Laguna del Condado (2 sites) in Puerto Rico. In each estuary system, we sampled across an expected water column nutrient gradient. We collected water column and sediment nutrient samples and made morphological measurements on seagrass samples (leaf areal mass, mg dry weight cm -2). We also collected seagrass (n= 90), water (n= 96) and sediment (n= 28) samples for stable isotope analysis (%N and δ15N) to evaluate potential nitrogen sources. Stable isotope analyses of plant tissue and environmental samples are ongoing. We expect to have a complete data set later in the summer to present preliminary data on tropical seagrass NPI as well as isotopic information on potential nitrogen sources. During sample processing and stabilization at the Jobos Bay NERR we also documented the presence of a phytomyxid endoparasite occurring on Halophila stipulacea. The impact of phytomyxid infection on H. stipulacea ecology and carbon and nitrogen metabolism has not been studied. Development and application of a sensitive seagrass nutrient indicator for tropical and sub-tropical seagrass species may help to conserve and protect these important habitats and the ecosystem services they provide.