Spatial and Temporal Dynamics of Dissolved Oxygen on the Shallow Shelf in the Northeastern Gulf of Mexico

Ch 1: This study provides the first long term data set on coastal oxygen dynamics in the northeastern Gulf of Mexico (NEGOM), located about 600 km east of the large hypoxic zone that extends west of the Mississippi delta. Oxygen concentrations and environmental parameters affecting the production and respiration of dissolved oxygen were measured over a 4-year period in the water column, and over a 2-year period in the benthic boundary layer (0.5 m above seafloor), along a 29 km long transect line following a depth gradient from 5 m to 18 m in the Florida Big Bend region of the West Florida Shelf. We show decreasing rates of oxygen production and consumption from the shallowest station to the deepest station that we attribute to the coastal energy gradient, i.e. higher light intensities, higher turbulent mixing rates and greater nutrient supply (organic-rich terrestrial runoff close to shore). Vertical profiles showed that oxygen production and consumption were highest in the benthic boundary layer (0.5 m above the seabed) from shore down to a water depth of 5 m; farther offshore, highest production and consumption values were generally measured higher in the water column. Water column photosynthetic oxygen production at an irradiance of 200 μE m-2 s-1 ranged from 0.5 to 4.5 mmol O2 m-3 h-1, and dark consumption rates ranged from 0.2 to 2.6 mmol O2 m-3 h-1. The water column was well mixed except during relatively short calm periods in summer and winter when a pycnocline developed. Oxygen below the pycnocline in summer dropped to 74% air saturation but never to levels that could have influenced aerobic life. Oxygen supersaturation as high as 118% air saturation was recorded during August 2007, reflecting high primary productivity of the upper water column. High wind events influenced dissolved oxygen down to the bottom at all stations. Our study shows the influence of the shore on the oxygen dynamics in the NEGOM Shelf and emphasizes the role of the benthic boundary layer for production and consumption processes in this shallow region. Ch 2: The West Florida Shelf is shallow and light can penetrate to its bottom facilitating photosynthesis at the sediment surface. This paper presents the first long term study of the activity of the benthic primary producers in the northeastern Gulf of Mexico. Photosynthetic rates of microphytobenthos colonizing permeable sandy sediments were quantified along a 29 km long transect starting at 5 m water depth and ending at 18 m water depth 29 km off the Northeast Gulf of Mexico coast during the 3-year study. The sediment of this region is classified as sand and slightly gravelly sand, being highly permeable. Chlorophyll concentrations and photosynthesis rates of the microphytobenthos showed relatively high rates of productivity for sediment from 20 m water depth compared to rates reported in studies from similar environments. In spring, the decrease of net production with depth amounted to approximately 0.11 mmol m-2 h-1 per 10 m water depth increase and consumption decreased by about 0.34 mmol m-2 h-1 per 10 m depth increase. In winter, these trends were reduced to one fourth to one third of the spring values reflecting a proportionally stronger influence of the coastal effect during the season with strongest growth. Our findings support the hypothesis that permeable sands can be photosynthetically highly active despite low standing stocks of algal cells and low nutrient concentrations due to their biocatalytical filtration function that effectively can provide limiting nutrients to photoautotrophs. The observed decrease in productivity with distance from shore indicates that benthic primary production in this region continues to approximately 30 m depth and thereby affects a large fraction of the shallow shelf in the northeastern Gulf of Mexico. Ch 3: With measurements conducted with instruments installed on the seafloor at 3 stations (5, 10, 18 m depth) along a transect in the Northeastern Gulf of Mexico, and one deeper station (40 m depth) nearby, we show that oxygen concentrations in the benthic boundary layer (50 cm above sediment) oscillate on a daily time scale and that these oscillations decrease with water depth to reach zero near 42 m. These oscillations, caused by primary production and respiration processes were affected by microphytobenthos activities. Maximum oxygen concentrations at all Stations were reached close to sunset with saturation values up to 108 % oxygen air saturation. The buildup of oxygen concentrations during the day was followed by an approximately equal drop of oxygen after sunset continuing throughout the night. Minimum oxygen concentrations were reached near sunrise but never fell below an oxygen saturation value of 77 % oxygen air saturation. Because these oscillations were present at water depth < 40 m, this process may affect the majority of the West Florida Shelf. As oxygen is a primary electron acceptor, the observed oscillations in the benthic boundary layer may influence coupled nitrification/denitrification, organic matter degradation, food web dynamics and they may act as biological timer and trigger function.