Investigating the isotopic composition of reactive nitrogen in a South Texas estuary (Baffin Bay)

Harmful algal blooms (HABs) have the potential to adversely affect the water quality of estuaries and, consequently, their ability to support healthy and diverse ecosystems. Since 1989, Baffin Bay, a semi-arid south Texas estuary, has experienced harmful algal blooms. This work investigates the stable isotopic composition of reactive nitrogen (Nr) (δ15N-DON, δ15N-NH4+, and δ15N-NO3-) in samples collected monthly at nine stations over the period of one year, and provides insight into Nr sources and processing in Baffin Bay. The following seasonal stages summarize the influences affecting δ15N values throughout the study: 1) Elevated δ15N-DIN values (4.9‰ ± 5‰) in the winter indicate the influence of a source of DIN with a relatively high δ15N such as wastewater or septic effluent, which may also contribute to elevated DON concentrations (46.4 µM ± 10 µM) and δ15N-DON values (9.5‰ ± 2‰). 2) The increase of NH4+ concentrations in the spring from run off concurrent with steady δ15N-DIN values (3.6‰ ± 6 ‰) implies phytoplankton assimilation of DON as evidenced by high δ15N-DON values (12‰ ± 6‰) and low DON concentrations (36µM ± 13.2 µM). 3) Evidence of photo-ammonification is observed throughout the summer due to elevated δ15N-DON values (10.5‰ ± 3‰) and low DON concentrations concurrent with low δ15N-DIN (-1‰ ± 5‰) and elevated NH4+ concentrations. 4) The accumulation of DON concentrations in the fall are consistent with phytoplankton detritus, which is supported by the decrease in δ15N-DON value averages in the fall and winter (9.8‰ ± 2 ‰). Remineralization during this stage is supported by low δ15N-DIN values (-2.3‰ ± 4.4‰). Additionally, salinity gradients and cross plots of δ15N values and concentrations support the idea that while both mixing and processing can influence δ15N values and patterns observed throughout the year, Nr processing (i.e. photo-ammonification, phytoplankton uptake, bacterial mineralization) may be the dominant mechanism for N cycling in Baffin Bay. Overall, this study increases constraints on the Nr isotope budget in Baffin Bay and offers insight into the role of DON in the N-cycle in a south Texas estuary.