Phytoplankton dynamics in an urbanizing south Texas estuary, Corpus Christi Bay, Texas

Date

2021-12

Authors

Tominack, Sarah

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Abstract

Low inflow estuaries are thought to be susceptible to cultural eutrophication due to lack of flushing and increased residence times, allowing for accumulation of phytoplankton. Corpus Christi Bay is a low inflow estuary located on the south Texas coast that has a rapidly urbanizing watershed and is subject to near annual occurrence of K. brevis red tides. Increasing demands for freshwater and a shift to a warmer and drier climate predicted for this region have the potential to further decrease freshwater inflows to the estuary, resulting in decreased nutrient inputs, increased salinity, and increased water residence time. To date, however, there has been little work to quantify patterns in phytoplankton biovolume, community composition, or their environmental drivers. This dissertation provides an assessment of phytoplankton dynamics in Corpus Christi Bay and furthers our understanding of how urbanization and global climate change are likely to impact the estuary. Results of a two-year field study indicate that phytoplankton biomass in Corpus Christi Bay displayed relationships with nutrients, precipitation, and temperature. Accumulation of phytoplankton biovolume during the spring and summer were limited by the availability of nutrients, whereas hydraulic flushing and decreased temperatures were also important during the fall and winter, respectively. A site located in a man-made canal system demonstrated relatively high phytoplankton biovolume and the occurrence of high biovolume blooms following precipitation-derived nutrient inputs. In contrast, regions closer to freshwater sources demonstrated lower overall phytoplankton biovolume despite relatively high concentrations of precipitation-derived nutrients, likely due to flushing effects. This indicates that projected decreases in precipitation and increases in temperature may create an environment more susceptible to phytoplankton blooms and earlier timing of the spring bloom. Phytoplankton community composition also varied in relation to nutrients, precipitation, and temperature. Diatoms were dominant during periods of cooler temperatures and higher nutrients, followed by a shift to dinoflagellate and picocyanobacteria dominance as temperatures warmed and nutrients were depleted. Dinoflagellates and picocyanobacteria were also dominant in the man-made canal site, where higher residence times and recycled nutrients were important factors supporting dominance of these groups. Given projections for a drier, warmer climate and increased water residence times, these findings indicate that there is potential for a prolonged increase in mean phytoplankton standing crops and a potential shift to more persistent dominance by dinoflagellates and picocyanobacteria in Corpus Christi Bay. Nutrient addition bioassays showed that nitrogen was the primary limiting nutrient. In spring, summer, and fall, phytoplankton growth rates increased with nitrogen additions indicating that future increases in the availability of nitrogen due to increased urbanization will influence the accumulation of phytoplankton in Corpus Christi Bay. The short duration of the experiments tended to favor diatoms in comparison to other taxonomic groups. This may indicate that when nutrients are pulsed, as is common in this region due to the flashy nature of rainfall, the Corpus Christi Bay system is buffered against negative impacts of nutrient loading on community composition (i.e., dominance of dinoflagellates and HABs) that have been observed elsewhere. Lastly, this study quantified K. brevis red tide frequency, duration, and environmental drivers. Results show a long-term increase in K. brevis frequency in the Nueces Estuary, which correlated with increased salinity and decreased precipitation during non-El NiƱo periods and the negative phase of the North Atlantic Oscillation. Additionally, duration was inversely related to temperature and wind speed, with fall-like temperatures and calm water conditions conducive to prolonging red tides. Given projections for warmer and drier conditions in south Texas, these results indicate the potential for continued increases in the occurrence of red tides, a potential shift to occurrence later in the year, and the potential for blooms to extend longer into winter months. Together, this research improves our understanding of factors driving phytoplankton dynamics in a rapidly urbanizing, low-inflow estuary. Results suggest that Corpus Christi Bay may currently be buffered against the occurrence of large, nutrient-driven phytoplankton blooms due to the opposing influences of nutrient availability and increased flushing (during rain events). Evidence presented here suggests that future changes in climate patterns, such as overall decreased precipitation and warmer temperatures, are likely to result in changes in both the frequency, timing and composition of phytoplankton blooms, and this study provides the basis for additional hypothesis-based studies to address these issues.

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Bioassay, environmental monitoring, estuary, phytoplankton, Red Tide, water quality

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