Towards an Understanding of the Interactions between Freshwater Inflows and Phytoplankton Communities in a Subtropical Estuary in the Gulf of Mexico

Abstract

Subtropical estuaries worldwide face increased pressure on their ecosystem health and services due to increasing human population growth and associated land use/land cover changes, expansion of ports, and climate change. We investigated freshwater inflows (river discharge) and the physico-chemical characteristics of Galveston Bay (Texas, USA) as mechanisms driving variability in phytoplankton biomass and community composition between February 2008 and December 2009. Results of multivariate analyses (hierarchical cluster analysis, PERMANOVA, Mantel test, and nMDS ordination coupled to environmental vector fitting) revealed that temporal and spatial differences in phytoplankton community structure correlate to differences in hydrographic and water quality parameters. Spatially, phytoplankton biomass and community composition responded to nutrient loading from the San Jacinto River in the northwest region of the bay (consistent with nutrient limitation) while hydraulic displacement (and perhaps other processes) resulted in overall lower biomass in the Trinity River delta (northeast region). The influence of inflows on phytoplankton diminished along a north to south gradient in the bay. Temporally, temperature and variables associated with freshwater inflow (discharge volume, salinity, inorganic nitrogen and phosphorus concentrations) were major influences on phytoplankton dynamics. Dissolved inorganic nitrogen: phosphorus (DIN:DIP) ratios suggest that phytoplankton communities will be predominately nitrogen limited. Diatoms dominated during periods of moderate to high freshwater inflows in winter/spring and were more abundant in the upper bay while cyanobacteria dominated during summer/fall when inflow was low. Given the differential influences of freshwater inflow on the phytoplankton communities of Galveston Bay, alterations upstream (magnitude, timing, frequency) will likely have a profound effect on downstream ecological processes and corresponding ecosystem services.

Spatiotemporal Distributions of Sr and Ba along an Estuarine River with a Large Salinity Gradient to the Gulf of Mexico

“…Estuarine systems face a variety of threats ranging from regional anthropogenic factors, such as river basin land use change [1,2], riverine nutrient enrichment [3,4], and coastal industrial pollution [5], to global stressors such as climate change and sea level rise [6,7]. This is especially true for lowland estuarine rivers where any rise in sea level can have a far-reaching impact on the community.…”

Phytoplankton pigment specific growth and losses due to microzooplankton grazing in a northern Gulf of Mexico estuary during winter/fall

“…The data in this study suggest that during times of low river flow (high salinity) or when sampling further from the JR, dinoflagellates and cryptophytes grew faster and were grazed at a higher rate than the other phytoplankton taxa. In Galveston Bay a greater biomass of diatoms was observed near the riverine inputs while dinoflagellates were found in areas of the bay where hydrologic displacement or nutrient loading from the river were not important (Dorado et al 2015). Previous studies in the BSL have shown that the DIN concentrations were highest during times of high river flow (Sawant 2009;Cai et al 2012;Camacho et al 2014).…”

“…Previous studies in the BSL have shown that the DIN concentrations were highest during times of high river flow (Sawant 2009;Cai et al 2012;Camacho et al 2014). As discussed by Dorado et al (2015), it is possible that diatoms utilize these nutrient inputs, while they may be unimportant to dinoflagellates and cryptophytes since these organisms may be able to utilize alternative methods to acquire nutrients (i.e., mixotrophy, phagotrophy). Mesocosm studies of estuarine phytoplankton communities have shown that phytoflagellates were more abundant in static waters than waters which were being actively mixed, supporting hydrologic displacement as a factor adversely affecting cryptophytes and dinoflagellates (Pinckney et al 1999).…”

Influence of Major Storm Events on the Quantity and Composition of Particulate Organic Matter and the Phytoplankton Community in a Subtropical Estuary, Texas

“…Although cyanobacteria tend to have slow growth rates compared to other phytoplankton taxa, they have been shown to respond more strongly to temperature in field and laboratory studies (Juhl and Murrell, 2005;Reynolds, 2006;Paerl et al, 2014a). Three of the major phytoplankton taxa in the MAE, diatoms, dinoflagellates, and cyanobacteria, have growth optima ranges of 15-18 • , 20-23 • , and 25-30 • C, respectively (Paerl et al, 2014a;Dorado et al, 2015). In addition, zeaxanthin shows stronger temperature dependence than other accessory pigments in the New River Estuary, North Carolina (Hall et al, 2013).…”

“…Phytoplankton community composition has been studied extensively in temperate estuaries with high freshwater inflows, but estuaries with low inflow and salinities that vary broadly over time have been less studied (Murrell et al, 2007). Phytoplankton community composition in temperate estuaries is commonly regulated by nutrients, flushing time, temperature, and light availability, whereas environmental controls in estuaries with episodic flooding events can vary drastically from year to year (Bianchi et al, 1997;Pinckney et al, 1998;Paerl et al, 2010;Dorado et al, 2015). Increases in terrestrially derived inorganic nutrients transported by freshwater inflows often cause a shift toward larger-celled phytoplankton, such as diatoms, which have relatively faster growth rates and reduced consumption by grazers (Cloern and Dufford, 2005;Reynolds, 2006).…”