Patterns in iscoscapes and N:P stoichioscapes of the dominant seagrasses (Halodule wrightii and Thalassia testudinum) in the western Gulf of Mexico

Seagrasses assimilate carbon (C), nitrogen (N), and phosphorus (P) from the water column and sediment, making their tissue nutrient content an excellent bioindicator of long-term, system-wide environmental conditions. I examined the role of seagrasses as ecological indicators of water quality and nutrient loading in three Texas estuarine systems through examination of their tissue isotopic (δ¹³C and δ¹⁵N) and stoichiometric (N:P) ratios over a period of five years using maps of spatial patterns in isotopic and stoichiometric regimes (“isoscapes” and stoichioscapes”). Leaf tissue samples were collected from the dominant Texas seagrasses, Halodule wrightii and Thalassia testudinum, at 567 stations during annual sampling between 2011 and 2015. Tissues were analyzed for C, N, and P content and C and N isotopic composition. Data were used to develop interpolated maps of variations in seagrass δ¹³C and δ¹⁵N signatures and N:P ratios in the Mission-Aransas NERR, Corpus Christi Bay, and upper and lower Laguna Madre. Regions where seagrasses had significantly enriched δ¹⁵N signatures, depleted δ¹³C signatures, or elevated N:P ratios were often associated with areas of urbanization or development. This was supported by significant relationships between δ¹⁵N and δ¹³C clusters and distance from outlets draining high population watersheds. I also documented a distinct temporal shift in δ¹³C signatures and N:P ratios across the study areas. The change in δ¹³C signatures was particularly notable in H. wrightii in 2015, when δ¹³C signatures became more depleted and N:P ratios were elevated, following an influx of freshwater and nutrients ending a three-year drought in south Texas. The spatial and temporal variation in seagrass tissue C content reported here reflects inputs of freshwater and riverine DIC as well as changes in the benthic light environment, while the N and P dynamics reported suggest that N:P ratios and δ¹⁵N signatures of seagrasses on the Texas coast are accurate bioindicators of nutrient loading in these estuaries. These metrics may thus serve as early indicators of changes in water quality.