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

https://doi.org/10.1016/j.ecss.2017.02.001Get rights and content

Highlights

  • Baffin Bay (Texas) is displaying multiple symptoms of eutrophication.

  • Organic nitrogen concentrations are 2–3 fold higher than other Texas estuaries.

  • Long-term chlorophyll increase coincides with blooms of Aureoumbra lagunensis.

  • Increasing organic nitrogen, salinity and water temperature favor A. lagunensis.

Abstract

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 (<2–3 m) subtropical estuary located in a semi-arid region that has agriculture as the dominant (44%) watershed land use. Chlorophyll a averaged 28–30 μg/l in Baffin Bay from 2003 to 2013 and total Kjeldahl nitrogen (TKN) concentrations were also very high (116–120 μM), with concentrations of both variables exceeding those of most other Texas estuaries by 2–5 fold. More recent field sampling (2013–2015) showed that dissolved organic nitrogen concentrations in Baffin Bay (62 ± 14 μM) were >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.

Introduction

Estuaries provide critical habitat for important fish and shellfish species and play a vital role in the economy of coastal states, including Texas (Lacson and Lee, 1997, Jones and Tanyeri-Abur, 2001). Water quality is a major determinant of the health of estuaries, and consequently affects the ability of these systems to support healthy fisheries (Whitfield and Elliott, 2002, Eby et al., 2005). Texas has seven primary estuarine systems from its eastern border with Louisiana to its southern border with Mexico, along with numerous smaller bays within these systems. Texas’ coastal counties have experienced human population growth over the past two decades, with future growth expected to contribute to an additional 26–82% increase by 2050 (Texas State Data Center, http://txsdc.utsa.edu/Data/TPEPP/Projections/Index.aspx). Along with this growing urban footprint on coastal land use coverage, agriculture remains a significant land use in many Texas watersheds (NOAA Coastal Change Analysis Program). Worldwide, both urban and agricultural land uses have been shown to contribute to water quality degradation through pollutant (i.e., nutrients, bacteria) inputs to receiving waterbodies (Hopkinson and Vallino, 1995, Handler et al., 2006).

Despite the pronounced presence of urbanization and agriculture in Texas coastal watersheds, widespread negative effects on water quality have not been reported to date. Wetz et al. (2016) found localized negative effects of municipal wastewater discharge on a small secondary bay system (Oso Bay), but the ecosystem impacts were unclear. Thronson and Quigg (2008) summarized fish kill records for Texas estuaries from 1970 to 2006 and found that low dissolved oxygen during warmer months was believed to be an important cause of fish kills, yet the driver(s) of this low dissolved oxygen, whether natural or human-induced, were not determined. In the most recent National Estuarine Eutrophication Assessment, five of nine Texas estuaries assessed had moderate or moderate to low eutrophication condition, while four others lacked sufficient data for assessment (Bricker et al., 2007).

Although widespread water quality degradation is not apparent on the Texas coast, localized concerns have arisen over the past few decades. One area of concern is the Baffin Bay-Upper Laguna Madre complex. Since 1990, Baffin Bay and adjacent Upper Laguna Madre have experienced large, prolonged blooms of the “brown tide” phytoplankton species, Aureoumbra lagunensis (Buskey et al., 1998, Buskey et al., 2001). The first reported bloom began in early 1990 in tributaries of Baffin Bay, but eventually spread to both the Upper and Lower Laguna Madre (Stockwell et al., 1993), resulting in seagrass die-off (Onuf, 1996, Onuf, 2000). Remnants of this bloom persisted for nearly seven years in the Baffin Bay-Laguna Madre complex (Buskey et al., 2001). Since then, several additional brown tide blooms have occurred in the same area (unpubl. Texas Parks & Wildlife reports; Wetz et al. unpubl. data). Previous studies have attributed development of the 1990 brown tide bloom to both lack of top-down control and a concurrent pulse of nutrients that stimulated growth. Microzooplankton grazer abundances as well as benthic filter feeder abundances were very low during bloom development, coincident with a period of drought and hypersaline conditions (Montagna et al., 1993, Buskey et al., 1997). As for the nutrient pulse, it was speculated that this was derived from agricultural runoff and/or decay of dead fish (e.g., Stockwell et al., 1993, Whitledge, 1993), but the main source has never been identified. Likewise, virtually nothing is known about the nutrient source(s) that have allowed for brown tide bloom persistence and/or redevelopment in subsequent years. Experimental studies have determined that A. lagunensis is a mixotroph that relies on reduced nitrogen (ammonium, organic nitrogen) for growth, but it cannot utilize nitrate (DeYoe and Suttle, 1994, Muhlstein and Villareal, 2007, Agostoni and Erdner, 2011).

Brown tide is not the only issue of concern in terms of water quality in Baffin Bay. For example, hypoxia has been documented, including on several occasions that coincided with large fish kills over the past decade (unpubl. Texas Parks & Wildlife reports). In addition, a large fish kill occurred in late 2010 coincident with hypoxia as well as a bloom of the dinoflagellate Pyrodinium bahamense (unpubl. Texas Parks & Wildlife reports). P. bahamense is known to produce toxins under certain conditions (Phlips et al., 2015), and has been implicated in low dissolved oxygen events that led to fish kills elsewhere (e.g., Morrison and Greening, 2011). Recent studies suggest that P. bahamense is typically most competitive under relatively high nutrient input conditions (Phlips et al., 2015). As with the prevalence of brown tide blooms, it is unclear as to whether the hypoxic events as well as P. bahamense bloom represent symptoms of larger-scale water quality degradation or simply natural variability.

Here we report results from an assessment of water quality conditions in Baffin Bay, Texas. The primary focus is on the spatial extent of, and long-term changes in, select water quality indicators. Differences are also highlighted between Baffin Bay and other estuarine systems along the Texas coast that have not experienced brown tide blooms, offering new insight into potential drivers of brown tide bloom formation and persistence.

Section snippets

Baffin Bay characteristics

Baffin Bay is a shallow (<2–3 m) subtropical estuary located in a semi-arid region (Fig. 1). Land use in the Baffin Bay watershed is dominated by agriculture (44%), with urban contribution limited to small cities such as Kingsville (pop. 26,213) and Alice (pop. 19,576) (NOAA Coastal Change Assessment Program). The semi-arid climate coupled with high evaporation rates in the region can lead to hypersaline conditions in Baffin Bay. Winds can be relatively strong in the region and are thought to

Spatial patterns

Broad spatial patterns in water quality variables along the Texas coast were determined by calculating the 10-year average (from 2003 to 2013) of each relevant variable from all available SWQM estuarine sites. There is a strong salinity gradient on the Texas coast, with lower salinities in the northern estuaries and much higher salinities to the south (Fig. 2). This coincides with well-documented precipitation and freshwater inflow gradients (Montagna et al., 2013). Chlorophyll a concentrations

Discussion

For the central-south Texas coast as a whole, symptoms of water quality degradation were generally limited to a few small systems (e.g., Carancahua Bay and Oso Bay) and a site adjacent to the Arroyo Colorado outflow in the Lower Laguna Madre. In fact, most of the estuarine systems examined in this analysis displayed low nutrient and chlorophyll concentrations. Baffin Bay was a notable exception, as it is displaying multiple symptoms of eutrophication including very high organic carbon and

Acknowledgements

We thank the dedicated citizen scientist volunteers who were integral in the Baffin Bay data collections, including: Jim Atkins, Frank Baumann, Jerry Bjork, Jan Cannamore, Al Gordon, Michael Henry, Carl Hooker, Thomas Mullenix, Scott Murray, David Rowsey, Ron Ryon, Jim Scoggins, Charles Sellers, Diane Sellers, John Sutton and Paul Wimberly. We also thank the following individuals for assistance with sample processing: Victor Batres, Kalman Bugica, Kelsey Fisher, Anne-Marie Gavlas, Cory Staryk,

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