Geospatial distribution of hypoxia associated with a Karenia brevis bloom
Introduction
The combination of global climate change and increasing cultural eutrophication will likely exacerbate the incidence and intensity of harmful algal blooms and hypoxic events (Anderson et al., 2002; Heisler et al., 2008; Paerl and Paul, 2012; Griffith and Gobler, 2020; Phlips et al., 2020). Coastal regions can be subject to hypoxic conditions due to intense harmful algal blooms (HABs) that result in high biological oxygen demand, particularly as blooms collapse and decompose (Howarth et al., 2011). Although hypoxia is widely understood to disrupt coastal ecosystems, research on the extent of hypoxic conditions remains limited and the role of hypoxia is often overlooked unless it impacts higher trophic levels (Diaz and Rosenberg, 2008). One of the more extensively studied examples of hypoxia associated with HABs is the nearshore region of the Mississippi River discharge in the northern Gulf of Mexico, often referred to as the “dead zone” (Rabalais and Turner, 1994; Rabalais et al., 2014, Zhang et al., 2010).
Along the broad continental shelf environment of the eastern Gulf of Mexico, another potential cause of hypoxia is a red tide bloom. These blooms can extend over thousands of square kilometers and impact coastal regions all along the west coast of Florida. Red tide blooms in the eastern Gulf of Mexico frequently occur during the late summer and are most often dominated by the toxic dinoflagellate Karenia brevis (Tester and Steidinger, 1997; Vargo et al., 1987; Heil et al., 2014a). Karenia brevis produces brevetoxins, which can impact marine fauna, leading to mass mortalities (Fire et al., 2007; Landsberg et al., 2009). In turn, these mortalities, bacterial decomposition of the biomass, and the presence of high K. brevis biomass interact to create opportunities for the formation of low oxygen zones, particularly during periods of vertical water column stratification. Periods of low winds and increased solar heating during the summer can cause vertical stratification which results in the formation of a thermocline, and enhanced potential for hypoxia (He and Weisberg, 2002). Two studies of nearshore benthic and reef communities in the Eastern Gulf of Mexico before and after red tide events attributed mortality not only to exposure to brevetoxins, but also the effects of oxygen depletion, hydrogen sulfide poisoning and bacterial infections (Smith, 1975). These events can disrupt climax communities and re-structure epibenthic and fish communities for 2–3 years. The expectation is that deoxygenation of the oceans will be exasperated by climate change in areas with recurring HABs (Gobler et al., 2017; Griffith and Gobler, 2020).
In this study, a widespread hypoxia event in 2018 associated with a red tide is described along the southwest coast of Florida. The impacted region is part of the shallow West Florida Shelf (WFS) which extends over 250 km from shore in the eastern Gulf of Mexico (Weisberg et al., 2009). Along the coast, red tides are concentrated in shallow water by wind and currents resulting in elevated plankton biomass levels (Vargo, 2009). The red tide was first detected in November 2017 and partially mapped with an autonomous sailing vessel (Beckler et al., 2019). Hypoxia was found in a large area in the Gulf of Mexico in August and September 2018. The primary goals of the study were to characterize physicochemical conditions, including the intensity and geospatial distribution of hypoxia in parallel with concentrations of the dominant species, K. brevis. Specific study objectives included (1) monitoring physicochemical covariates and K. brevis levels across multiple depths and locations in waters impacted by the red tide, and (2) quantifying the severity and extent of hypoxia as a function of K. brevis concentrations and physicochemical conditions. Findings from this work provide new insights into the ecological impacts associated with the interactions of harmful algal blooms and hypoxia.
Section snippets
Materials and methods
The study area encompassed approximately 1200 km2, with relatively shallow depths ranging from 5 m to 20 m (Fig. 1). Sampling was initiated in September 2018 in response to an unusual mass mortality of fish and benthic invertebrates. A network of water sampling sites was established to determine the extent and severity of hypoxia in the study region. The network included three major components:
- 1)
Three sampling cruises (09/26/2018, 10/29/2018, 09/17/2019) involving 21 sites (Fig. 1, solid circles)
Results
Water physical, optical, dissolved oxygen and phytoplankton species. The first detection of Karenia brevis was reported in October 2017 by the Florida Wildlife Research Institute (FWRI) through routine discrete sampling (https://myfwc.com/research/redtide/). The weekly FWRI monitoring program tracked the expansion of the bloom in low, medium and high (>106 cells L−1) concentrations throughout the event. High concentrations of Karenia brevis were detected at many locations in July 2018 and the
Discussion
Over the past two decades, much of the attention on marine hypoxia has focused on large-scale persistent or recurring phenomena, such as the 18,000 km2 “dead zone” associated with the Mississippi River discharge (Rabalais and Turner, 1994; Scavia et al., 2003; IWG-HABHRCA, 2016), coastal upwelling of deep low oxygen water, and recurring hypoxia associated with intense cyanobacteria blooms in lakes, such as Lake Erie (Scavia et al., 2014; Smith et al., 2015; IWG-HABHRCA, 2016; Watson et al., 2016
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the National Science Foundation award 1853041 to N. Nelson, E. Phlips, and E. Milbrandt. RECON operations and maintenance were supported by NOAA through a subaward from the Gulf of Mexico Coastal Ocean Observing Network (GCOOS) to E. Milbrandt. Additional support was provided by the R. Vince Family and Goldman Sachs Gives. Field and lab assistance were provided by student interns from the University of Iowa Department of Earth and Environmental Science. The
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2024, Journal of Environmental ManagementNitrogen-enriched discharges from a highly managed watershed intensify red tide (Karenia brevis) blooms in southwest Florida
2022, Science of the Total EnvironmentCitation Excerpt :This study empirically investigates the dynamics of coastal K. brevis blooms in southwest Florida and several watershed covariates along a major flow path to the coast, to identify persistent and systematic anthropogenic drivers of blooms as targets for management and policy intervention. We focus on K. brevis blooms between Charlotte Harbor and the Caloosahatchee River (Fig. 1), a region whose wildlife, public health, and economic activity have suffered adverse impacts (Court et al., 2021; Gravinese et al., 2020; Milbrandt et al., 2021; Sonak et al., 2018). Past empirical investigations of ‘the anthropogenic hypothesis’—that anthropogenic nutrient inputs intensify K. brevis blooms along Florida's Gulf Coast—support its mechanistic plausibility.
Relationships between blooms of Karenia brevis and hypoxia across the West Florida Shelf
2022, Harmful AlgaeCitation Excerpt :Examining 16 years of CTD data collected over the West Florida Shelf, we identified three hypoxic events in 2005, 2014, and 2018, that co-occurred with major HABs. Other studies have identified hypoxia associated with HABs on the West Florida Shelf in 2005 (Hu et al. 2006), 2014 (Driggers et al. 2016), and 2018 (Milbrandt et al. 2021); however, no study has examined this relationship across longer time scales and on a larger geographic scope in this region. We hypothesize that HAB-hypoxia events were driven by the temporal coincidence of HABs and associated climatological factors.