Thesis Defense Seminar - Lucero Barraza Gomez

COASTAL AND MARINE SYSTEM SCIENCE PROGRAM
DEPARTMENT OF PHYSICAL AND ENVIRONMENTAL SCIENCES
TEXAS A&M UNIVERSITY-CORPUS CHRISTI

TITLE: A STUDY OF WATER QUALITY AND PHYTOPLANKTON DYNAMICS IN LOS OLMOS-LAGUNA SALADA TRIBUTARY OF BAFFIN BAY, TX

GRADUATE COMMITTEE: Dr. Michael Wetz (Chair), Dr. Jeffrey Turner, Dr. Joseph Felix

ABSTRACT
Baffin Bay is an important Texas estuary that supports a highly productive fishery. However, its water quality has declined due to excessive nutrient loadings and persistent harmful "brown tide" algal blooms caused by the phytoplankton species Aureoumbra lagunensis. Previous studies have identified one of Baffin Bay's three tributaries, the Los Olmos-Laguna Salada (LOLS) complex, as a potential incubator for brown tide. The primary purpose of this thesis was to assess the water quality and phytoplankton dynamics in the LOLS. A one-year ecological assessment was conducted at three sites from the upper to lower portion of the LOLS complex, utilizing biweekly water quality and phytoplankton sampling. The Los Olmos (LO) Creek sites exhibited high salinity conditions (50–130) that were higher than at the downstream Laguna Salada (LS1) site. The LO sites also had elevated nutrient concentrations that were generally higher than at LS1, with Nitrate + Nitrite ranging from 0.16 to 7.56 µM, NH₄⁺ from 0.88 to 123 µM, PO₄³⁻ from 0.19 to 6.01 µM, dissolved organic carbon from 1,365 to 4,667 µM, and dissolved organic nitrogen from 113 to 453 µM at the LO sites. Chlorophyll a concentrations and A. lagunensis biovolumes were both higher at the LO sites compared to LS1, with chlorophyll a ranging from 2.3 to 38.28 µg/L and A. lagunensis biovolumes from 6.20 × 10⁶ to 7.13 × 10⁷ µm³/mL. A. lagunensis biovolume peaked during the summer and decreased in the winter. However, phytoplankton diversity did not noticeably change after precipitation events, but the relative contributions of different taxonomic groups shifted. This study identified key characteristics of the LOLS that favor the proliferation of A. lagunensis. First, the LO sites are notably shallow, which likely enhances the nutrient exchange from sediments to the water column. Dissolved nitrogen levels were generally high, especially dissolved organic nitrogen that can be favorable for mixotrophic taxa such as A. lagunensis. The dissolved inorganic nitrogen to silicate (DIN:Si) ratio above 1 that occasionally occurred at the LO sites could foster a nutrient regime disadvantageous to other groups that require silicate, like diatoms, while creating conditions that are more suitable for A.lagunensis. Additionally, circulation within the LOLS is primarily influenced by wind; during calm periods, water exchange is limited, resulting in prolonged residence times. Moreover, all three LOLS sites exhibit hypersaline conditions, which are conducive to the survival of the salt-tolerant and slow-growing A. lagunensis over other phytoplankton species. By identifying the conditions that favor A. lagunensis proliferation, this study offers a scientific basis for watershed protection efforts aimed at reducing nutrient inputs and protecting the long-term health of Baffin Bay.