Phytoplankton dynamics in a eutrophying south Texas estuary

Low freshwater inflow estuaries are common worldwide, yet data is lacking on phytoplankton dynamics in these systems. This dissertation advances our understanding of phytoplankton dynamics in low-inflow estuaries through analysis of field data collected at multiple temporal scales throughout Baffin Bay, Texas, an estuary that is frequently hypersaline and that has been experiencing symptoms of eutrophication. These symptoms include long-term increases in nutrients and chlorophyll as well as recurring blooms of the “brown tide” phytoplankton, Aureoumbra lagunensis. Variability in phytoplankton biomass and community composition were related to climatic/hydrologic shifts associated with El Niño-Southern Oscillation. During drought conditions, high chlorophyll concentrations were observed (25.7 ± 11.7 µg L-1) and the phytoplankton community consisted of a near monoculture of A. lagunensis (87% of phytoplankton biovolume). These results point to the importance of regenerated nutrients in supporting phytoplankton growth during this time. When El Niño conditions developed, high rainfall took place and corresponded with increased dissolved inorganic nitrogen concentrations (ammonium, nitrate + nitrite) in the bay, but chlorophyll concentrations were lower during this time (16.2 ± 14.3 µg L-1). This suggests that the potential stimulatory effects of nutrient pulses were counter balanced by decreased residence times associated with increased inflow. This study also identified sub-monthly variability as a dominant timescale of chlorophyll variability in Baffin Bay. Wind speed was found to be an important factor related to short-term (timescales of days) bloom events, often positively correlating with chlorophyll concentrations. Results additionally demonstrate that traditional monitoring frequencies are often are not adequate to capture short-term process and bloom events. Lastly, this study quantified the distribution of, and potential controls upon, A. lagunensis in Baffin Bay. Persistently high abundances of A. lagunensis were observed in Laguna Salada, indicating that this tertiary bay may serve as a reservoir for A. lagunensis in the system. High abundances of A. lagunensis (> 1×106 cells mL-1) were found at salinities as low as 20, indicating that hypersalinity is not a prerequisite for A. lagunensis blooms. Instead, results suggest a variety of physical, chemical and biological factors (top-down controls, nutrient concentrations and forms, salinity) drive A. lagunensis population dynamics in the bay. This research improves our ecological understanding of low-inflow estuaries, specifically pertaining to the ecology of phytoplankton in them. Results also suggest that as climate pressures increase, causing systems like Baffin Bay to become warmer and drier, they will be more conducive for harmful taxa such as A. lagunensis. The finding that Laguna Salada may be a reservoir for A. lagunensis presents an opportunity for targeted bloom mitigation efforts.Physical and Environmental SciencesCollege of Science and Engineerin

Effects of Nitrogen Availability and Form on Phytoplankton Growth in a Eutrophied Estuary (Neuse River Estuary, NC, USA)

<div><p>Nitrogen availability and form are important controls on estuarine phytoplankton growth. This study experimentally determined the influence of urea and nitrate additions on phytoplankton growth throughout the growing season (March 2012, June 2011, August 2011) in a temperate, eutrophied estuary (Neuse River Estuary, North Carolina, USA). Photopigments (chlorophyll <i>a</i> and diagnostic photopigments: peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) and microscopy-based cell counts were used as indicators of phytoplankton growth. In March, the phytoplankton community was dominated by <i>Gyrodinium instriatum</i> and only fucoxanthin-based growth rates were stimulated by nitrogen addition. The limited response to nitrogen suggests other factors may control phytoplankton growth and community composition in early spring. In June, inorganic nitrogen concentrations were low and stimulatory effects of both nitrogen forms were observed for chlorophyll <i>a</i>- and diagnostic photopigment-based growth rates. In contrast, cell counts showed that only cryptophyte and dinoflagellate (<i>Heterocapsa rotundata</i>) growth were stimulated. Responses of other photopigments may have been due to an increase in pigment per cell or growth of plankton too small to be counted with the microscopic methods used. Despite high nitrate concentrations in August, growth rates were elevated in response to urea and/or nitrate addition for all photopigments except peridinin. However, this response was not observed in cell counts, again suggesting that pigment-based growth responses may not always be indicative of a true community and/or taxa-specific growth response. This highlights the need to employ targeted microscopy-based cell enumeration concurrent with pigment-based technology to facilitate a more complete understanding of phytoplankton dynamics in estuarine systems. These results are consistent with previous studies showing the seasonal importance of nitrogen availability in estuaries, and also reflect taxa-specific responses nitrogen availability. Finally, this study demonstrates that under nitrogen-limiting conditions, the phytoplankton community and its various taxa are capable of using both urea and nitrate to support growth.</p></div

Growth rates based on diagnostic pigments in March 2012 experiment.

<p>Bars represent standard deviation (<i>n</i> = 3). Asterisk (*) indicates statistically significant difference between N treatment and control, while ‘a’ indicates statistically significant difference between nitrate and urea treatments. Refer to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0160663#pone.0160663.t002" target="_blank">Table 2</a> for initial concentrations of pigments.</p

Initial abundances and growth rates (μ ± SD) of the major phytoplankton taxa in each experiment.

<p>Initial abundances and growth rates (μ ± SD) of the major phytoplankton taxa in each experiment.</p

Initial nutrient concentrations and physical conditions for each experiment.

<p>Initial nutrient concentrations and physical conditions for each experiment.</p

Growth rates based on diagnostic pigments in August 2011 experiment.

<p>Bars represent standard deviation (<i>n</i> = 3). Asterisk (*) indicates statistically significant difference between N treatment and control, while ‘a’ indicates statistically significant difference between nitrate and urea treatments. Refer to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0160663#pone.0160663.t002" target="_blank">Table 2</a> for initial concentrations of pigments.</p

Initial concentrations of diagnostic photopigments (μg L^-1) for each experiment.

<p>Initial concentrations of diagnostic photopigments (μg L^-1) for each experiment.</p

Exceptionally high organic nitrogen concentrations in a semi-arid South Texas estuary susceptible to brown tide blooms

Studies of estuarine eutrophication have tended to focus on systems with continually flowing rivers, while little is known about estuaries from semi-arid/arid regions. Here we report results from an assessment of water quality conditions in Baffin Bay, Texas, a shallow (2 fold higher than in three other Texas estuaries. In contrast, inorganic nitrogen (ammonium, nitrate) and phosphate concentrations were relatively low. Statistically significant long-term annual increases in chlorophyll a and salinity were observed in Baffin Bay, while long-term seasonal increases were observed for water temperature and TKN. Overall, Baffin Bay is displaying multiple symptoms of eutrophication, namely very high organic carbon, organic nitrogen and chlorophyll concentrations, as well as symptoms not quantified here such as fish kills and episodic hypoxia. Much of the increase in chlorophyll in Baffin Bay, at least since ∼1990, have coincided with blooms of the mixotrophic phytoplankton species, Aureoumbra lagunensis, which is thought to be favored under high proportions of organic to inorganic nitrogen. Thus the high and possibly increasing organic nitrogen concentrations, coupled with a long-term annual increase in salinity and a long-term seasonal increase in water temperature are likely to promote additional brown tide blooms in this system in the future