The Bonnet Carré Spillway (BCS), located about 28 miles northwest of New Orleans, was constructed by the US Army Corp of Engineers in the early 1930s as part of an integrated flood-control system for the lower Mississippi River (MR). The BCS control structure consists of 350 individual bays that can be opened to divert water from the river to Lake Pontchartrain to relieve pressure on downstream levees. Lake Pontchartrain (LP) is hydrologically connected to the Mississippi Sound (MS Sound) and the Gulf of Mexico and is more accurately characterized as an estuarine embayment. BCS openings have occurred twelve times prior to 2019 because of high Mississippi River stages, typically in late spring. In 2019, the spillway opened twice in one year, first opened from February 2019 and closing April 2019, and then reopened one month later because of a second high-water peak in May 2019. Monitoring the spillway openings and the effects of diverted water on water quality in LP and the MS Sound is vitally important to resource managers in Louisiana and Mississippi. These resources provide habitat for many species of fish, shellfish, crabs, seagrass, and other marine mammals, as well as provide recreation activities and commercial fishing.
The introduction of nutrient-rich, fresh river water into the nutrient-poor, brackish LP is known to substantially change the chemistry and ecology of the lake (Mize and Demcheck, 2009). The historical 2019 BCS openings, two openings in one year, have never been recorded since the BCS was constructed. Unprecedented, large volumes of MR river water were diverted into LP and the MS Sound where lake and sound waters changed abruptly from a brackish-estuarine system to a freshwater dominated system with some areas maintaining low salinity for about 2 to 3 months. In the past, these spring openings have been associated with algal blooms later in the spring and summer. Although not normally an acute health hazard, these blooms can substantially reduce the access of the lake and sound waters for commercial and recreational uses. However, the timing and magnitude of the 2019 diversions provided nutrient flux and freshwater inflows that resulted in algal accumulations that included harmful algal bloom species with possible algal toxins in coastal zones in Mississippi. Especially hard hit was the Mississippi shoreline/beach area, where bloom status for several species of algae was recorded along with elevated algal toxin levels. The Mississippi Department of Environmental Quality issued water contact advisories for 21 different beach locations from June until October 2019 (MDEQ, 2019). The U.S. Geological Survey (USGS) completed synoptic sampling of LP and the MS Sound in summer and fall of 2019 to assess the water-quality effects and subsequent formation, persistence, and accumulation of algal blooms and their associated toxins in shoreline/beach and nearshore inflows (outlet areas) as well as open-water sampling from the MS Sound in relation to the introduction of freshwater from both the Mississippi River by way of the BCS and local coastal drainages. Water quality-results are reported in the USGS National Water Information System (NWIS). Phytoplankton and harmful algae bloom species (HAB) within benthic algae (periphyton) community samples and algal toxins samples were collected at the time of water-quality sampling within the MS Sound and the data are available in this data release. Phytoplankton samples were collected from the photic zone within the water column at 27 locations along the shoreline/beach and open water locations in the MS Sound. Periphyton samples were collected from 10 depositional zones along the shoreline/beach and outlets from Rigolets Pass, LA to the Pascagoula River, MS. Both phytoplankton and periphyton samples were collected using National Water Quality Assessment Program protocols (Moulton and others, 2002). Phytoplankton and periphyton samples were preserved with a 0.25-0.50% glutaraldehyde solution and sent to Phycotech in St. Joseph, Michigan for taxonomic analysis. Research-grade microscopes ranging from 40-1,000x were used to identify samples to the most practical taxonomic levels (normally, species or genus level).
Two different methods were used to identify toxins from water samples collected at the surface. Discrete, unfiltered water samples were collected from shoreline/beach and open water locations and filtered samples (intracellular filters) were only collected at 15 shoreline/beach areas. Discrete water samples were analyzed for total anatoxin, microcystin, and saxitoxin by the USGS Organic Geochemistry Research Laboratory (OGRL), Lawrence, Kansas. All cyanotoxin samples were lysed using a three-fold freeze and thaw procedure and filtered using a 0.7-micrometer syringe filter (Loftin and others, 2008) prior to cyanotoxin analyses. Enzyme-linked immunosorbent assays (ELISA) were used to measure cyanotoxin totals. Although many cyanotoxins have congeners, the ELISA test is not specific to these individual congeners, and therefore, concentrations are reported as totals in the USGS NWIS. A total volume of 300-400 mL of whole water was collected and filtered onto GF/F filters (47mm, 0.7 µm), two filters per station for intracellular analysis of microcystin at the Ecotoxicology Lab at the Dauphin Island Sea Lab (DISL). Microcystin concentrations were determined following EPA method 546: “Determination of Total Microcystins and Nodularins in Drinking Water and Ambient water by Adda Enzyme-Linked Immunosorbent Assay” (USEPA, 2016; ELISA; Abraxis). Some differences among the algal toxin (microsystin) results between the OGRL and DISL results are expected due to the differences in sample collection, processing, laboratory methodologies, and matrix effects. Although microcystin was not detected in most of the OGRL samples, microcystin was detected in DISL samples that were concentrated for the intracellular procedure.
Loftin, K.A., Meyer, M.T., Rubio, F., Kamp, L., Humphries, E., and Whereat, E., 2008, Comparison of two cell lysis procedures for recovery of microcystins in water samples from Silver Lake in Dover, Delaware, with microcystin producing cyanobacterial accumulations: U.S. Geological Survey Open-File Report 2008–1341, 9 p.
Mize, S.V., and Demcheck, D.K., 2009, Water quality and phytoplankton communities in Lake Pontchartrain during and after the Bonnet Carré Spillway opening, April to October 2008, in Louisiana, USA: Geo-Marine Letters, 29, pp.431-440.
Mississippi Department of Environmental Quality (MDEQ), 2019, issues water contact advisories as part of Mississippi Beach Monitoring Program (http://opcgis.deq.state.ms.us/beaches/).
Moulton II, SR., Kennen, J.G, Goldstein, R.M., and Hambrook, J.A., 2002, Revised protocols for sampling algal, invertebrate, and fish communities as part of the National Water-Quality Assessment Program: US Geological Survey Open File Report 02-150, 75 p.
U.S. EPA, August 2016, Method 546: Determination of total microcystins and nodularins in drinking water and ambient water by Adda Enzyme-Linked Immunosorbent Assay