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Recent studies have shown that Lake Pontchartrain Estuary in Louisiana experiences frequent harmful cyanobacterial blooms (cyanoHABs). In 2019, the Bonnet Carré Spillway (BCS) that diverts Mississippi River water into the estuary opened twice in the same year for the first time in history to prevent flooding in New Orleans. Short-term water quality monitoring was conducted in shoreline areas with high public use for the presence of cyanoHABs and cyanotoxins to assess the public health risks. Field sampling methods and satellite imagery were used to determine water quality and quantify bloom intensity and toxicity across time and space. Long-term BCS opening created a fresh (salinity < 0.2) and nutrient-rich estuary that supported several cyanoHABs in warmer months during and after the second BCS closure. Cyanobacterial biomass ranged from 35 to 4972 µg PC L−1, while toxin microcystin ranged from undetected to 8.41 µg MC L−1. The highest biomass and toxin were detected on June 25 at the north shore, station LP8, Mandeville Beach, dominated by Microcystis and Dolichospermum species. CyanoHABs occurred mostly in the northern part of the estuary, where tributary discharge is also a strong influence. Some of these blooms exited the estuary and were transported to the Gulf of Mexico following passage through Lake Borgne and then Mississippi Sound. Modifications in the timing and duration of river diversion operations can create prolonged cyanobacterial blooms that can cause environmental and public health risks, especially in warmer months, and this may intensify due to a changing climate. [ABSTRACT FROM AUTHOR]

Estuarine and coastal geomorphology, biogeochemistry, water quality, and coastal food webs in river-dominated shelves of the Gulf of Mexico (GoM) are modulated by transport processes associated with river inputs, winds, waves, tides, and deep-ocean/continental shelf interactions. For instance, transport processes control the fate of river-borne sediments, which in turn affect coastal land loss. Similarly, transport of freshwater, nutrients, and carbon control the dynamics of eutrophication, hypoxia, harmful algal blooms, and coastal acidification. Further, freshwater inflow transports pesticides, herbicides, heavy metals, and oil into receiving estuaries and coastal systems. Lastly, transport processes along the continuum from the rivers and estuaries to coastal and shelf areas and adjacent open ocean (abbreviated herein as "river-estuary-shelf-ocean") regulate the movements of organisms, including the spatial distributions of individuals and the exchange of genetic information between distinct subpopulations. The Gulf of Mexico Research Initiative (GoMRI) provided unprecedented opportunities to study transport processes along the river-estuary-shelf-ocean continuum in the GoM. The understanding of transport at multiple spatial and temporal scales in this topographically and dynamically complex marginal sea was improved, allowing for more accurate forecasting of the fate of oil and other constituents. For this review, we focus on five specific transport themes: (i) wetland, estuary, and shelf exchanges; (ii) river-estuary coupling; (iii) nearshore and inlet processes; (iv) open ocean transport processes; and (v) river-induced fronts and cross-basin transport. We then discuss the relevancy of GoMRI findings on the transport processes for ecological connectivity and oil transport and fate. We also examine the implications of new findings for informing the response to future oil spills, and the management of coastal resources and ecosystems. Lastly, we summarize the research gaps identified in the many studies and offer recommendations for continuing the momentum of the research provided by the GoMRI effort. A number of uncertainties were identified that occurred in multiple settings. These include the quantification of sediment, carbon, dissolved gasses and nutrient fluxes during storms, consistent specification of the various external forcings used in analyses, methods for smooth integration of multiscale advection mechanisms across different flow regimes, dynamic coupling of the atmosphere with sub-mesoscale and mesoscale phenomena, and methods for simulating finer-scale dynamics over long time periods. Addressing these uncertainties would allow the scientific community to be better prepared to predict the fate of hydrocarbons and their impacts to the coastal ocean, rivers, and marshes in the event of another spill in the GoM. [ABSTRACT FROM AUTHOR]

Water management operations can drive long-term water quality patterns in nutrient-enriched lakes and reservoirs. This study identified multidecadal trends of rainfall, flow, water levels, total phosphorous (TP), total nitrogen (TN), and Chlorophyll-a in the water column and eight (dis)tributaries during four specific operation schedules (1974–1990, 1991–1999, 2000–2007, and 2008–2019) in Lake Okeechobee (Florida). We found an increasing trend in water discharges towards the Gulf of Mexico (via the Caloosahatchee River) during the dry season for the entire 46 years, and a decreasing trend in discharges towards the Atlantic Ocean (via the St. Lucie Canal) during 2008–2019. Higher flows, combined with increasing TP concentrations in the lake and the Caloosahatchee, led to significant increases in monthly TP mass loads to the Gulf of Mexico by up to 122%–158% during 2008–2019. Higher flows in the Caloosahatchee also resulted in a net TN load increase to the Gulf, despite system-wide decreases in TN concentrations. Overall, results from this study demonstrate that reservoir operations could have long-term effects on nutrient status and exports; thus, modifying operations should be considered as a potential nutrient management tool. [ABSTRACT FROM AUTHOR]

Floodplains are some of the most productive and diverse ecosystems on Earth. The Usumacinta River Basin, in the Southern Gulf of Mexico, hosts several floodplain lakes, whose integrity is threatened by water resource development. We aimed to investigate how food webs in these ecosystems vary in relation to their hydrological connectivity and seasonal flood cycles. To do this, we combined analysis of water quality and stable isotopes in two lakes with permanent connection to the Usumacinta River and two isolated lakes during dry, early rainy and late rainy season. Significant differences in chlorophyll-a, nutrients and suspended solids were reflected in distinct trophic dynamics among lakes. Stable isotope analysis revealed that fish from all lakes relied predominantly on autochthonous primary production, but while phytoplankton was the dominant resource for fish in the isolated lakes, aquatic macrophytes made the highest contribution to consumers' production in the connected lakes. Fish tended to have broader isotopic niches during the early rainy season, likely reflecting consumption of a wider range of resources following the first flow pulses of the year. Our study highlights the importance of hydrological connectivity and natural flow regimes to maintain the outstanding biodiversity and fishery yield of these floodplain ecosystems. [ABSTRACT FROM AUTHOR]

Agricultural non-point sources of nutrients and sediments have caused eutrophication and other water quality issues in aquatic and marine ecosystems, such as the annual occurrence of hypoxia in the Gulf of Mexico. Management practices have been implemented adjacent to and in agricultural drainage ditches to promote their wetland characteristics and functions, including reduction of nitrogen, phosphorus, and sediment losses downstream. This review: (1) summarized studies examining changes in nutrient and total suspended solid concentrations and loads associated with management practices in drainage ditches (i.e., riser and slotted pipes, two-stage ditches, vegetated ditches, low-grade weirs, and organic carbon amendments) with emphasis on the Lower Mississippi Alluvial Valley, (2) quantified management system effects on nutrient and total suspended solid concentrations and loads and, (3) identified information gaps regarding water quality associated with these management practices and research needs in this area. In general, management practices used in drainage ditches at times reduced losses of total suspended solids, N, and P. However, management practices were often ineffective during storm events that were uncommon and intense in duration and volume, although these types of events could increase in frequency and intensity with climate change. Studies on combined effects of management practices on drainage ditch water quality, along with research towards improved nutrient and sediment reduction efficiency during intense storm events are urgently needed. [ABSTRACT FROM AUTHOR]

The acquisition of reliable and accurate data to assess environmental changes over large spatial scales is one of the main limitations to determine the impact of eutrophication, and the effectiveness of management strategies in coastal systems. Here, we used a continuous in situ Chl-a fluorometry sensor and L8/OLI satellite data to develop an algorithm and map Chl-a spatial distribution to assess the impact of freshwater diversions and associated high nutrient loading rates in the Barataria Basin (BB) complex, a coastal system in the northern Gulf of Mexico. We collected water quality samples at 24 sampling stations and high-frequency continuous fluorometry in situ [Chl-a] data along a ~87 km transect from 2019–2020. Field [Chl-a] values were highly correlated (r = 0.86; p < 0.0001) with continuous in situ [Chl-a] fluorometry values. These continuous in situ [Chl-a] values were significantly related to a surface reflectance ratio ([B1 + B4]/B3) estimated using L8/OLI data (exponential model; R2 = 0.46; RMSE = 4.8, p < 0.0001). The statistical model replicated [Chl-a] spatial patterns across the BB complex. This work shows the utility of high-frequency continuous Chl-a fluorometry sampling coupled with L8/OLI image analysis to increase the frequency and number of field data sets to assess water quality conditions at large spatial scales in highly dynamic deltaic regions. [ABSTRACT FROM AUTHOR]