Water-quality and phytoplankton data for Lake Pontchartrain and the western Mississippi Sound associated with operation of the Bonnet Carre Spillway, 2008-2020

The Bonnet Carré Spillway (BCS), located about 28 miles northwest of New Orleans, Louisiana, was constructed in the early 1930s as part of an integrated flood-control system for the lower Mississippi River Plain. The BCS is designed to divert water from the Mississippi River (MSR) into Lake Pontchartrain (LP), which then flows into Lake Borgne and the Mississippi Sound (MS Sound), thus relieving pressure on levees downstream. Opening of the spillway occurs when measured streamflow in the MSR at New Orleans exceeds approximately 1.25 million cubic feet per second, which normally occurs once a year in late spring. In 2019, for the first time, the spillway opened twice in one year; the first opening occurred between February 27th and April 11th and the second occurred between May 10th and July 22nd (U.S. Army Corps of Engineers, 2022). Monitoring the quality of estuary surface waters that receive inflows from the MSR diverted through the BCS is of vital importance to public and natural resource managers in Louisiana and Mississippi. These waterbodies provide habitat for many species of fish, shellfish, crabs, seagrass, and marine mammals, and are used for recreational activities and commercial fishing (U.S. Geological Survey, 2020).

During the 2008–2020 BCS openings, MSR water entered LP and changed the brackish-estuarine system to a freshwater-dominated system, with some areas maintaining low salinity for 2 to 3 months. The introduction of nutrient-rich fresh river water into nutrient-poor brackish LP is known to substantially change the chemistry and ecology of the lake (Mize and Demcheck, 2009). Except for large openings during 2011 and 2019, algae blooms appear to be generally relegated to LP, particularly the northwest part of the lake, and in some instances, originating from Lake Maurepas.  Although not normally an acute health hazard, these blooms can substantially limit the use of lake and sound waters for commercial and recreational activity. The U. S. Geological Survey (USGS), Lower Mississippi-Gulf Water Science Center, in cooperation with the U.S. Army Corp of Engineers (USACE) New Orleans District, sampled water from LP and the western MS Sound prior, during, and after the seven BCS openings that occurred between 2008 and 2020. Water samples were analyzed for major ions, nutrients, inorganic plus organic particulate carbon, total suspended solids, chlorophyll a, and algal toxins; results are available on the USGS National Water Information System (NWIS; U.S. Geological Survey, 2022). Vertical water column profiles of field water quality parameters, phytoplankton community sample results from 2008, 2011, 2013, 2016, 2018, 2019, and 2020, and oxygen and hydrogen stable freshwater isotopic composition sample results from 2019 and 2020 are reported in this data release.

Field water-quality measurements were collected using a water quality sonde equipped with a depth transducer for measuring water depths and sensors for measuring water temperature, specific conductance (salinity), pH, dissolved oxygen, and oxygen percent saturation. Profile measurements were collected from 0.5 m above the water-sediment interface at the bottom, at mid-depth, and 0.5 m below the approximate water surface to help determine whether any water quality stratification is occurring, including salinity stratification and hypoxia in bottom waters that serve as habitat for bottom dwelling organisms, such as oysters. In addition, near-surface habitats, those found at the top of the water column, can exhibit elevated pH and oxygen saturation during the day when phytoplankton blooms are concentrated near the surface in the euphotic zone. Maximum depths in the euphotic zone, where photosynthesis occurs, were estimated from Secchi depth measurements. These measurements were used to guide sample collection depths for chlorophyll, phytoplankton, and algal toxins.

Phytoplankton samples were collected using National Water Quality Assessment Program protocols (Moulton II et al., 2002). Samples collected in 2008 and 2011 were preserved with a 1% Lugol’s solution and sent to the Academy of Natural Sciences in Philadelphia, PA for taxonomic analysis, according to Charles et al. (2002). Samples collected in 2013, 2016, 2018, 2019, and 2020 were preserved with a 0.25-0.50% glutaraldehyde solution and sent to Phycotech, Inc. in St. Joseph, MI for taxonomic analysis. Research-grade microscopes ranging from 40-1,000x magnification were used to identify phytoplankton in samples to the most practical taxonomic levels (normally, species or genus level).

Oxygen and hydrogen stable isotopic (ẟ 18O and ẟD) compositions were determined from samples collected in 2019 and 2020. The combination of salinity and isotope results were used to distinguish proportions of MSR water, water from local drainages, and seawater in LP and MS Sound throughout the sampling period. Water samples and salinity measurements were collected from the surface of the water column and filtered with a 0.45 µm syringe filter and stored in glass amber bottles with lids that were securely covered with parafilm to prevent evaporation. Isotopic analysis was performed using isotopic ratio infrared spectroscopy (Sanial et al., 2019).

This data set provides profile measurement, phytoplankton, and oxygen and hydrogen stable freshwater isotopic composition data for Lake Pontchartrain and the Mississippi Sound collected between 2008 and 2020. "Table_1_Station_Data.txt" contains profile data (latitude, longitude, station name, etc) for all sites sampled and an overview of data available for each site by year. "Table_2_Field_physiochemical_profile_data_2008_2019.txt" contains physiochemical data (temperature, specific conductance, salinity, etc) for all sites sampled. "Table_3_Phytoplankton_Community_Data_2008_2020.txt" contains taxonomic data for all sites sampled. "Table_4_Salinity_and_stable_water_isotope_2019_2020.txt" contains oxygen and hydrogen stable isotopic composition data for all sites sampled.