Dinoflagellate Cysts Track Eutrophication In The Northern Gulf Of Mexico

We examined organic-walled dinoflagellate cysts from one 210Pb-dated sediment core and 39 surface sediment samples from the northern Gulf of Mexico to determine the relationship between nutrient enrichment and cyst assemblages in this region characterized by oxygen deficiency. The core spans from 1962 to 1997 and its sampling location is directly influenced by the Mississippi River plume. Surface sediments were collected in 2006, 2007, 2008, and 2014 and represent approximately 1 to 4 years of accumulation. A total of 57 cyst taxa were recorded, and four heterotrophic taxa in particular were found to increase in the top section (1986–1997) of the core—Brigantedinium spp., cysts of Archaeperidinium minutum, cysts of Polykrikos kofoidii, and Quinquecuspis concreta. These taxa show a similar increasing trend with variations in US fertilizer consumption and Mississippi River nitrate concentrations, both of which increased substantially in the 1970s and 1980s. The same four heterotrophic taxa dominated dinoflagellate cyst assemblages collected near the Mississippi River Bird’s Foot Delta where nutrient concentrations were higher, especially in 2014. We propose that these cyst taxa can be used as indicators of eutrophication in the Gulf of Mexico. A canonical correspondence analysis (CCA) supports this proposition. The CCA identified sea-surface nutrient concentrations, sea-surface temperature, and sea-surface salinity as the most important factors influencing the cyst assemblages. In addition, cysts produced by the potentially toxic dinoflagellates Pyrodinium bahamense and Lingulodinium polyedrum were documented, but did not appear to have increased over the past 50 years

Historical Shifts in Benthic Infaunal Diversity in the Northern Gulf of Mexico since the Appearance of Seasonally Severe Hypoxia

Severe and persistent bottom-water hypoxia (≤2 mg O2 L−1) occurs on the Louisiana/Texas continental shelf from mid-May through mid-September over a large area (up to 23,000 km2 in mid-summer). Benthic infauna are less mobile than demersal organisms and become stressed by the low dissolved oxygen; benthic community composition, abundance, diversity, and biomass become altered. From the 1950s to the early 1970s, when sediment core indicators identified the initiation and subsequent worsening of dissolved oxygen conditions, there were no hydrographic data or benthic infaunal studies within the current area of frequent bottom-water hypoxia. This study highlights the impacts of severe hypoxia on benthic macroinfaunal communities and how they may have changed from less-hypoxic periods. Polychaetes were and are the dominant taxa in the available studies, but polychaete species richness in summer is now curtailed severely beginning with our 1985–1986 data. Species richness of polychaetes in summer hypoxia (1985–1986 and 1990–1991) was about 60% less than comparable taxa in 1972–1973. Abundance of polychaetes was much less in summer than spring, and recent infaunal biomass in summer was only 15% of what was found in spring. The result is less prey for demersal penaeid shrimp and fishes. Over the period of our comparison, infaunal feeding modes shifted from subsurface deposit feeders and surface deposit feeders to primarily surface deposit feeders (i.e., 95.5% of all polychaetes). Most were opportunistic, hypoxia tolerant, and recruited in high numbers following hypoxia abatement, some in fall and winter but most in spring. As benthic communities succumb to the stress of severe and continued seasonal low oxygen, they occupy the few upper centimeters of the sediment profile above the redox discontinuity layer with negative feedbacks to the water column by way of altered biogeochemical processes