Soil legacy nutrients contribute to the decreasing stoichiometric ratio of N and P loading from the Mississippi River Basin.

Human-induced nitrogen-phosphorus (N, P) imbalance in terrestrial ecosystems can lead to disproportionate N and P loading to aquatic ecosystems, subsequently shifting the elemental ratio in estuaries and coastal oceans and impacting both the structure and functioning of aquatic ecosystems. The N:P ratio of nutrient loading to the Gulf of Mexico from the Mississippi River Basin increased before the late 1980s driven by the enhanced usage of N fertilizer over P fertilizer, whereafter the N:P loading ratio started to decrease although the N:P ratio of fertilizer application did not exhibit a similar trend. Here, we hypothesize that different release rates of soil legacy nutrients might contribute to the decreasing N:P loading ratio. Our study used a data-model integration framework to evaluate N and P dynamics and the potential for long-term accumulation or release of internal soil nutrient legacy stores to alter the ratio of N and P transported down the rivers. We show that the longer residence time of P in terrestrial ecosystems results in a much slower release of P to coastal oceans than N. If contemporary nutrient sources were reduced or suspended, P loading sustained by soil legacy P would decrease much slower than that of N, causing a decrease in the N and P loading ratio. The longer residence time of P in terrestrial ecosystems and the increasingly important role of soil legacy nutrients as a loading source may explain the decreasing N:P loading ratio in the Mississippi River Basin. Our study underscores a promising prospect for N loading control and the urgency to integrate soil P legacy into sustainable nutrient management strategies for aquatic ecosystem health and water security.

GLORIA - A globally representative hyperspectral in situ dataset for optical sensing of water quality.

The development of algorithms for remote sensing of water quality (RSWQ) requires a large amount of in situ data to account for the bio-geo-optical diversity of inland and coastal waters. The GLObal Reflectance community dataset for Imaging and optical sensing of Aquatic environments (GLORIA) includes 7,572 curated hyperspectral remote sensing reflectance measurements at 1 nm intervals within the 350 to 900 nm wavelength range. In addition, at least one co-located water quality measurement of chlorophyll a, total suspended solids, absorption by dissolved substances, and Secchi depth, is provided. The data were contributed by researchers affiliated with 59 institutions worldwide and come from 450 different water bodies, making GLORIA the de-facto state of knowledge of in situ coastal and inland aquatic optical diversity. Each measurement is documented with comprehensive methodological details, allowing users to evaluate fitness-for-purpose, and providing a reference for practitioners planning similar measurements. We provide open and free access to this dataset with the goal of enabling scientific and technological advancement towards operational regional and global RSWQ monitoring.

Photophysiological and light absorption properties of phytoplankton communities in the river-dominated margin of the northern Gulf of Mexico.

Spatial and temporal variability in photophysiological properties of phytoplankton were examined in relationship to phytoplankton community composition in the river-dominated continental margin of the northern Gulf of Mexico (NGOM). Observations made during five research cruises in the NGOM included phytoplankton photosynthetic and optical properties and associated environmental conditions and phytoplankton community structure. Distinct patterns of spatial and temporal variability in photophysiological parameters were found for waters dominated by different phytoplankton groups. Photophysiological properties for locations associated with dominance by a particular group of phytoplankton showed evidence of photoacclimation as reflected by differences in light absorption and pigment characteristics in relationship to different light environments. The maximum rate of photosynthesis normalized to chlorophyll ( PmaxB) was significantly higher for communities dominated (>60% biomass) by cyanobacteria + prochlorophyte (cyano + prochl). The initial slope of the photosynthesis-irradiance (P-E) curve normalized to chlorophyll ( αB) was not clearly related to phytoplankton community structure and no significant differences were found in PmaxB and αB between different geographic regions. In contrast, maximum quantum yield of carbon fixation in photosynthesis (Φcmax) differed significantly between regions and was higher for diatom-dominated communities. Multiple linear regression models, specific for the different phytoplankton communities, using a combination of environmental and bio-optical proxies as predictor variables showed considerable promise for estimation of the photophysiological parameters on a regional scale. Such an approach may be utilized to develop size class-specific or phytoplankton group-specific primary productivity models for the NGOM.

Changing Dynamics of Dissolved Organic Matter Fluorescence in the Northern Gulf of Mexico Following the Deepwater Horizon Oil Spill.

The characteristics of fluorescent components of dissolved organic matter (DOM) were examined using excitation emission matrix (EEM) fluorescence spectroscopy combined with parallel-factor analysis (PARAFAC) for seawater samples obtained from the northern Gulf of Mexico (NGoM) before, during, and after the 2010 Deepwater Horizon (DwH) oil spill. An EEMs PARAFAC modeling of samples collected within 16 km of the wellhead during the oil spill in May 2010, which included one typical subsurface sample with a PAH concentration of 1.09 µg/L, identified two humic-like and two previously reported oil-like components. Compared to prespill levels, however, there were order-of-magnitude higher fluorescence intensities associated with these components that are consistent with an oil-spill source. The spectral decomposition of the EEMs data using individual and combined data sets from coastal and offshore waters impacted by the DwH spill further revealed the changing nature of fluorescent DOM composition. Although the PAHs concentrations were at prespill conditions after the spill in 2012 and 2013 near the DwH site, the variable and anomalous levels of fluorescence intensities and DOC concentrations three years after the spill suggest the potential long-term persistence of the oil in the DOC pool in the NGoM.

The carbon dioxide system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico: 1. Distribution and air-sea CO2 flux.

River-dominated continental shelf environments are active sites of air-sea CO2 exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (pCO2) was measured, and the air-sea CO2 flux was calculated. Results show that CO2 exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO2 during spring and early summer, and it was neutral or a weak source of CO2 to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO2 in low-salinity zones (0≤S<17) to a strong CO2 sink in the middle-to-high-salinity zones (17≤S<33), and finally was a near-neutral state in the high-salinity areas (33≤S<35) and in the open gulf (S≥35). High pCO2 values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated pCO2 generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of pCO2 variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen-enhanced biological removal, along with mixing processes, dominated pCO2 variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore pCO2 distribution and to affect regional air-sea CO2 flux estimates. Overall, the study region acted as a net CO2 sink of 0.96 ± 3.7 mol m-2 yr-1 (1.15 ± 4.4 Tg C yr-1).

Large increase in dissolved inorganic carbon flux from the Mississippi River to Gulf of Mexico due to climatic and anthropogenic changes over the 21st century.

It is recognized that anthropogenic factors have had a major impact on carbon fluxes from land to the ocean during the past two centuries. However, little is known about how future changes in climate, atmospheric CO2, and land use may affect riverine carbon fluxes over the 21st century. Using a coupled hydrological-biogeochemical model, the Dynamic Land Ecosystem Model, this study examines potential changes in dissolved inorganic carbon (DIC) export from the Mississippi River basin to the Gulf of Mexico during 2010-2099 attributable to climate-related conditions (temperature and precipitation), atmospheric CO2, and land use change. Rates of annual DIC export are projected to increase by 65% under the high emission scenario (A2) and 35% under the low emission scenario (B1) between the 2000s and the 2090s. Climate-related changes along with rising atmospheric CO2 together would account for over 90% of the total increase in DIC export throughout the 21st century. The predicted increase in DIC export from the Mississippi River basin would alter chemistry of the coastal ocean unless appropriate climate mitigation actions are taken in the near future.

Satellite Assessment of Bio-Optical Properties of Northern Gulf of Mexico Coastal Waters Following Hurricanes Katrina and Rita.

The impacts of major tropical storms events on coastal waters include sediment resuspension, intense water column mixing, and increased delivery of terrestrial materials into coastal waters. We examined satellite imagery acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensor aboard the Aqua spacecraft following two major hurricane events: Hurricane Katrina, which made landfall on 29 August 2005, and Hurricane Rita, which made landfall on 24 September. MODIS Aqua true color imagery revealed high turbidity levels in shelf waters immediately following the storms indicative of intense resuspension. However, imagery following the landfall of Katrina showed relatively rapid return of shelf water mass properties to pre-storm conditions. Indeed, MODIS Aqua-derived estimates of diffuse attenuation at 490 nm (K_490) and chlorophyll (chlor_a) from mid-August prior to the landfall of Hurricane Katrina were comparable to those observed in mid-September following the storm. Regions of elevated K_490 and chlor_a were evident in offshore waters and appeared to be associated with cyclonic circulation (cold-core eddies) identified on the basis of sea surface height anomaly (SSHA). Imagery acquired shortly after Hurricane Rita made landfall showed increased water column turbidity extending over a large area of the shelf off Louisiana and Texas, consistent with intense resuspension and sediment disturbance. An interannual comparison of satellite-derived estimates of K_490 for late September and early October revealed relatively lower levels in 2005, compared to the mean for the prior three years, in the vicinity of the Mississippi River birdfoot delta. In contrast, levels above the previous three year mean were observed off Texas and Louisiana 7-10 d after the passage of Rita. The lower values of K_490 near the delta could be attributed to relatively low river discharge during the preceding months of the 2005 season. The elevated levels off Texas and Louisiana were speculated to be due to the presence of fine grain sediment or dissolved materials that remained in the water column following the storm, and may also have been associated with enhanced phytoplankton biomass stimulated by the intense vertical mixing and offshore delivery of shelf water and associated nutrients. This latter view was supported by observations of high chlor_a in association with regions of cyclonic circulation.

Use of hyperspectral remote sensing reflectance for detection and assessment of the harmful alga, Karenia brevis.

We applied two numerical methods to in situ hyperspectral measurements of remote sensing reflectance Rrs to assess the feasibility of remote detection and monitoring of the toxic dinoflagellate, Karenia brevis, which has been shown to exhibit unique absorption properties. First, an existing quasi-analytical algorithm was used to invert remote sensing reflectance spectra, Rrs(lambda), to derive phytoplankton absorption spectra, a(phi)Rrs(lambda). Second, the fourth derivatives of the a(phi)Rrs(lambda) spectra were compared to the fourth derivative of a reference K. brevis absorption spectrum by means of a similarity index (SI) analysis. Comparison of reflectance-derived a(phi) with filter pad measured a(phi) found them to agree well (R2=0.891; average percentage difference, 22.8%). A strong correlation (R2=0.743) between surface cell concentration and the SI was observed, showing the potential utility of SI magnitude as an indicator of bloom strength. A sensitivity analysis conducted to investigate the effects of varying levels of cell concentrations and colored dissolved organic matter (CDOM) on the efficacy of the quasi-analytical algorithm and SI found that a(phi)Rrs(lambda) could not be derived for very low cell concentrations and that, although it is possible to derive a(phi)Rrs(lambda) in the presence of high CDOM concentrations, CDOM levels influence the a(phi)Rrs(lambda) amplitude and shape. Results suggest that detection and mapping of K. brevis blooms based on hyperspectral measurements of Rrs are feasible.