Award: OCE-1144670

In June of 2011, the size of the Northern Gulf of Mexico "dead zone" was predicted to be 26,000 km2 (the largest "dead zone" found since 1985 was 22,000 km2 in 2002) based on historic rainfall and flooding of the Mississippi River during May of 2011. The "dead zone" is a region of low oxygen bottom water, the result of excess production that is largely driven by riverflow and runoff that contains high nutrients. These nutrients lead to blooms of algae (unicellular plants that use nutrients and sunlight to create sugars). The algal blooms are large and are not eaten by other organisms, thus they die and sink to the bottom where they decompose. The decomposition of this algal mass by bacteria uses up the oxygen in the water column faster than it can be replaced by surface mixing. This leads to "hypoxic" conditions, defined as water with oxygen concentrations below 2 mg per liter, which causes severe direct and indirect impacts on living resources in the system. Our team proposed an NSF RAPID project to compare zooplankton measurements in 2011 with previous work carried out in the same region in previous years, in an effort to determine whether the historic flooding altered the zooplankton abundance, composition, and size spectrum. Zooplankton are key organisms in aquatic food webs, linking primary producers like algae with economically and ecologically important fish. Our earlier work showed that hypoxic conditions can alter zooplankton communities with decreased abundance and smaller individuals compared to fully oxygenated water, but we did not know how they community would respond to such historic flooding. We participated in a research cruise (http://www.gulfhypoxia.net/Research/Shelfwide%20Cruises/2011/) to map the dead zone, in which we collaborated with the team mapping hypoxia and collected zooplankton samples and measured particle size distribution. Our hypothesis was that the extreme flooding event would greatly alter the zooplankton community. The actual measurement of the 2011 "dead zone" area was 17,500 km2 significantly lower than was forecast, and near the long-term average of 15,400 km2. It has suggested that the passage of Tropical Storm Don in late July, just prior to the cruise, reduced the overall size of the "dead zone" due to mixing of the water column and aeration of the bottom waters. We found lower salinities in 2011 than we had measured in any previous year (Figure 1). In addition, the zooplankton community in 2011 was similar to that in 2010; data from these years appear to differ from previous zooplankton data. One possible explanation is a change in sampling methods during 2010 that may be driving much of the observed differences and we are still investigating that finding. The biomass size spectra from transect line "C" in 2011 matched those of previous years for some locations (Figure 2), but there was a distinct absence of small size sized individuals. This is unlike any observations we have made in this or in Chesapeake Bay hypoxic waters and may be a result of the historic nature of the conditions in NGOMEX that occurred in 2011. Overall, our data suggest that hypoxia alters aquatic food web dynamics whenever it occurs. The patterns of size distribution are affected in a consistent manner in areas that experience hypoxia, but the magnitude of the changes in those patterns appears closely correlated to the magnitude of hypoxia that occurs in the system. Thus, more severe hypoxia results in larger changes to the system, but any hypoxia can initiate the change. Last Modified: 10/29/2013 Submitted by: James J Pierson