Dissolved organic nutrients at the interface of fresh and marine waters: flow regime changes, biogeochemical cascades and picocyanobacterial blooms—the example of Florida Bay, USA

Abstract

The availability of dissolved inorganic and organic nutrients and their transformations along the fresh to marine continuum are being modified by various natural and anthropogenic activities and climate-related changes. Subtropical central and eastern Florida Bay, located at the southern end of the Florida peninsula, is classically considered to have inorganic nutrient conditions that are in higher-than-Redfield ratio proportions, and high levels of organic and chemically-reduced forms of nitrogen. However, salinity, pH and nutrients, both organic and inorganic, change with changes in freshwater flows to the bay. Here, using a time series of water quality and physico-chemical conditions from 2009 to 2019, the impacts of distinct changes in managed flow, drought, El Niño-related increases in precipitation, and intensive storms and hurricanes are explored with respect to changes in water quality and resulting ecosystem effects, with a focus on understanding why picocyanobacterial blooms formed when they did. Drought produced hyper-salinity conditions that were associated with a seagrass die-off. Years later, increases in precipitation resulting from intensive storms and a hurricane were associated with high loads of organic nutrients, and declines in pH, likely due to high organic acid input and decaying organic matter, collectively leading to physiologically favorable conditions for growth of the picocyanobacterium, Synechococcus spp. These conditions, including very high concentrations of NH4+, were likely inhibiting for seagrass recovery and for growth of competing phytoplankton or their grazers. Given projected future climate conditions, and anticipated cycles of drought and intensive storms, the likelihood of future seagrass die-offs and picocyanobacterial blooms is high.

Florida’s Harmful Algal Bloom (HAB) Problem: Escalating Risks to Human, Environmental and Economic Health With Climate Change

“…These blooms ultimately resulted in a loss of 30% of Florida Bay's seagrass (Hall et al, 1999;Durako et al, 2002) when an extended bloom resulted in extensive seagrass die-offs and associated mortalities (Glibert et al, 2009b,c). These blooms have become a reoccurring feature of Florida Bay, taking place in 1991 and frequently thereafter (Boyer et al, 1999;Phlips et al, 1999;Stumpf et al, 1999;Richardson and Zimba, 2002;Glibert et al, 2004Glibert et al, , 2009bGlibert et al, ,c, 2010 through 2018 (Glibert et al, 2021). Although not directly toxic per se, these blooms reach such dense concentrations that light attenuation is diminished, affecting other primary producers such as seagrass.…”

“…Other harmful blooms also occur with either uncharacterized toxins (e.g., undescribed Karenia species) or biomass related impacts [e.g., Synechococcus sp. blooms in Florida Bay (Glibert et al, 2009b(Glibert et al, ,c, 2021, and Aureoumbra lagunensis D. A. Stockwell, DeYoe, Hargraves & P. W. Johnson blooms in the Indian River Lagoon (Gobler et al, 2013;Kang et al, 2015)].…”

“…Lake Okeechobee water levels are managed by the United States Army Corps of Engineers for human safety (Steinman et al, 2002), which in 2018 resulted in the release of Microcystis contaminated lake waters to both the eastern coast via the St. Lucie River and the west coast by the Caloosahatchee River (Krimsky et al, 2018). Indeed, South Florida is currently undergoing one of the largest flow restoration projects in the world, the Comprehensive Everglades Restoration Plan (CERP), which is impacting the amount, timing and distribution of freshwater flows and subsequent nutrient loading to the southern third of the state (Glibert et al, 2021). Overlying these natural features and anthropogenic stressors are the escalating effects of climate change.…”

Tracing Carbon and Nitrogen Microbial Assimilation in Suspended Particles in Freshwaters

“…The importance of organic matter in inland waters as nutrient and energy source for aquatic organisms (Yates et al 2016;Mackay et al 2020;Glibert et al 2021), in controlling light attenuation (Mayer et al 2006;Lu et al 2013), and regulating the transport and bioavailability of pollutants (Artifon et al 2018) and metals (Yamashita and Jaffé 2008) is widely acknowledged. A significant amount of organic matter in freshwater is processed in transit to coastal areas, preserved in soils and sediments or degraded by microorganisms (Brailsford et al 2019b, a).…”

“…Particleattached microorganisms are therefore in a dynamic equilibrium with dissolved compounds, from which they can uptake nutrients. These biotic interactions that occur in the water column likely play a central role in nutrient cycling (Simon et al 2002;Amalfitano et al 2017;Glibert et al 2021). Recently, Attermeyer et al (2018) suggested that the partial loss of dissolved organic carbon (DOC) in rivers is due to remineralisation by suspended particleattached microbes.…”