Impacts of Hypoxia on Zooplankton Spatial Distributions in the Northern Gulf of Mexico

The northern Gulf of Mexico (NGOMEX) was surveyed to examine the broad-scale spatial patterns and inter-relationships between hypoxia (<2?mg?L^?1 dissolved oxygen) and zooplankton biovolume. We used an undulating towed body equipped with sensors for conductivity, temperature, depth, oxygen, fluorescence, and an optical plankton counter to sample water column structure, oxygen, and zooplankton at high spatial resolution (1?m??vertical; 0.25?C1?km??horizontal). We contrast the distribution of zooplankton during summer surveys with different freshwater input, stratification, and horizontal and vertical extent of bottom-water hypoxia. Bottom-water hypoxia did not appear to influence the total amount of zooplankton biomass present in the water column or the areal integration of zooplankton standing stock in the NGOMEX region surveyed. However, where there were hypoxic bottom waters, zooplankton shifted their vertical distribution to the upper water column during the day where they normally would reside in deeper and darker waters. When bottom waters were normoxic (>2?mg?L^?1 dissolved oxygen), the daytime median depth of the water column zooplankton was on average 7?m deeper than the median depth of zooplankton in water columns with hypoxic bottom waters. A reduction in larger zooplankton when there were hypoxic bottom waters suggests that if zooplankton cannot migrate to deeper, darker water under hypoxic conditions, they may be more susceptible to size-selective predation by visual predators. Thus, habitat compression in the northern Gulf of Mexico due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey.     

Hypoxia in the northern Gulf of Mexico: Does the science support the Plan to Reduce,Mitigate, and Control Hypoxia?

We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.     

Carbonate Chemistry Dynamics of Surface Waters in the Northern Gulf of Mexico

This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ω_aragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO₂ fugacity (fCO₂), total alkalinity (TA) and Ω_aragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity. However, inner shelf waters (<50 m depth) exhibited large variations in fCO₂, TA, and Ω_aragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters were supersaturated with respect to aragonite. Offshore waters had an average Ω_aragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ω_aragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ω_aragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological drawdown of CO₂. A dominant factor controlling Ω_aragonite distribution in offshore waters with relatively constant temperatures was fCO₂, with higher supersaturation occurring in areas with low fCO₂.     

Benthic Oxygen Fluxes Measured by Eddy Covariance in Permeable Gulf of Mexico Shallow-Water Sands

Oxygen fluxes across the sediment–water interface reflect primary production and organic matter degradation in coastal sediments and thus provide data that can be used for assessing ecosystem function, carbon cycling and the response to coastal eutrophication. In this study, the aquatic eddy covariance technique was used to measure seafloor–water column oxygen fluxes at shallow coastal sites with highly permeable sandy sediment in the northeastern Gulf of Mexico for which oxygen flux data currently are lacking. Oxygen fluxes at wave-exposed Gulf sites were compared to those at protected Bay sites over a period of 4 years and covering the four seasons. A total of 17 daytime and 14 nighttime deployments, producing 408 flux measurements (14.5 min each), were conducted. Average annual oxygen release and uptake (mean ± standard error) were 191 ± 66 and ?191 ± 45 mmol m^?2 day^?1 for the Gulf sites and 130 ± 57 and ?152 ± 64 mmol m^?2 day^?1 for the Bay sites. Seasonal variation in oxygen flux was observed, with high rates typically occurring during spring and lower rates during summer. The ratio of average oxygen release to uptake at both sites was close to 1 (Bay: 0.9, Gulf: 1.0). Close responses of the flux to changes in light, temperature, bottom current velocity, and wave action (significant wave height) documented tight physical–biological, benthic–pelagic coupling. The increase of the sedimentary oxygen uptake with increasing temperature corresponded to a Q₁₀ temperature coefficient of 1.4 ± 0.3. An increase in flow velocity resulted in increased oxygen uptake (by a factor of 1–6 for a doubling in flow), which is explained by the enhanced transport of organic matter and electron acceptors into the permeable sediment. Benthic photosynthetic production and oxygen release from the sediment was modulated by light intensity at the temporal scale (minutes) of the flux measurements. The fluxes measured in this study contribute to baseline data in a region with rapid coastal development and can be used in large-scale assessments and estimates of carbon transformations.     

Wetland loss in the Northern Gulf of Mexico: Multiple working hypotheses

I examined four hypotheses about causes for the dramatically high coastal wetland losses (0.86% yr^?1) in the northern Gulf of Mexico: an extensive dredged canal and spoil bank network, a decline in sediments in the Mississippi River during the 1950s, Mississippi River navigation and flood protection levees, and salinity changes. Natural factors contributing to these habitat changes include eustatic sea-level rise and geological compaction, which appear to have remained relatively constant this century, although variation does occur. These four hypotheses were tested using data on land-to-water changes in 15-min quadrangle maps inventoried for four intervals between the 1930s and 1990. Land loss rates were directly proportional to changes in wefland hydrology in time and space. A linear regression of the direct losses due to dredging versus the losses due to all other factors (indirect losses) had a zero intercept and a slope that increased with time. The ratio indirect:direct land loss was highest nearest the estuarine entrance. The coastwide patterns of land loss do not appear to be affected by riverine sediment reductions over the last 60 yr. The effects of changes in wetland hydrology from dredging human-made channels and forming dredged spoil banks appear to be the most efficacious hypothesis explaning these dramatic losses. The effects of extensive human-induced changes on this coast have apparently overwhelmed the causal linkages identified in the historical re-constructionist view of deltaic gain and loss that emphasizes the role of mineral sediments. A paradigm shift is therefore proposed that emphasizes a broad ecological view as contrasted to a mostly physical view emphasizing the role of sediment supply in wetland maintenance. In this view, plants are not an ancillary consequence of strictly geological dynamics such as sediment supply but are dominant agents controlling factors relevant to coastal restoration and management efforts.     

Examination of Winter Circulation in a Northern Gulf of Mexico Estuary

Numerical model experiments were conducted to examine how estuarine circulation and salinity distribution in the Calcasieu Lake Estuary (CLE) of southwest Louisiana respond to the passage of cold fronts. River runoff, local wind stress, and tides from December 20, 2011, to February 1, 2012, were included as input. The experiments showed an anticyclonic circulation in the eastern CLE, a cyclonic circulation in West Cove, and a saltwater conduit in the navigation channel between these circulation cells. Freshwater from the river and wetlands tends to flow over the shallow shoals toward the ocean, presenting a case of the conventional estuarine circulation with shallow water influenced by river discharge and with weak tidally-induced motion, enhanced by wind. The baroclinic pressure gradient is important for the circulation and saltwater intrusion. The effect of remote wind-driven oscillation plays an important role in circulation and salinity distribution in winter. Unless it is from the east, wind is found to inhibit saltwater intrusion through the narrow navigation channel, indicating the effect of Ekman setup during easterly wind. A series of north-south oriented barrier islands in the lake uniquely influenced water level and salinity distribution between the shallow lake and deep navigation channel. The depth of the navigation channel is also crucial in influencing saltwater intrusion: the deeper the channel, the more saltwater intrusion and the more intense estuarine circulation. Recurring winter storms have a significant accumulated effect on the transport of water and sediment, saltwater intrusion, and associated environmental and ecosystem effects.     

Summer Movements of the Gulf Killifish (Fundulus grandis) in a Northern Gulf of Mexico Salt Marsh

The Gulf Killifish (Fundulus grandis) is one of the most abundant nekton species in the US Gulf of Mexico (GOM) salt marshes, providing an important trophic link in these systems. Recently, the use of F. grandis as an indicator species of salt marsh health in the region has been suggested because its Atlantic coast congener, the Mummichog (Fundulus heteroclitus) has filled such a role due to its demonstrated high site fidelity and small-scale movements. Given the similar life histories between species, F. grandis was assumed to exhibit the same type of small-scale movements, although this has not been documented. During summer 2013, we collected and marked 1,719 fish from a northern Gulf of Mexico estuary, recapturing 959 (56 % recapture rate). Of these recaptured fish, only 31 moved from their original tagging location, and of these, 29 moved only 100 m between sites connected by salt marsh. Based on these results, F. grandis appears to exhibit high site fidelity and make only small-scale movements, similar to F. heteroclitus, supporting its role as an indicator species.     

Pyritization of Iron in Sediments from the Continental Slope of the Northern Gulf of Mexico

In sediments from the continental slope of the Northern Gulf of Mexico, generally,the degree of iron pyritization (DOP) is low (<0.1) and dissolved sulfide is belowdetection limits (5 M), whereas dissolved Fe is typically about 50 to100 M. Therefore, the dissolution of kinetically reactive iron minerals generallydominates over the rate of sulfide production in sediments throughout this region.However, in sediments where hydrocarbons have been added via seepage from thesubsurface, dissolved-Fe is undetectable, DOP can approach 1, and high concentrationsof dissolved sulfide (up to 11 mM) are commonly present. Even though thesesediments have high total reduced sulfide (TRS) concentrations (typically 150 to370 mol gdw^-1), their average C/S ratio is about 4 times that of normal marine sediments reflecting the major input of hydrocarbons. DOP is significantly (20%) higher when calculated using reactive-Fe extracted by citrate dithionite than by cold 1N HCl. This difference is primarily due to the greater extraction efficiency of the cold HCl method for silicate-Fe. TRS tends to rise to a maximum, and remains close to constant even at high (mM) dissolved sulfide concentrations. These TRS concentrations, therefore, represent the size of the ``kinetically' reactive-Fe pool during early diagenesis.     

Interactions between Northern-Shifting Tropical Species and Native Species in the Northern Gulf of Mexico

Niche models applied in the context of future climate change predict that as regional temperatures increase, the distribution of tropical species will shift poleward. While range expansions have been documented for a number of species, there is limited information on the ecological impacts of shifts on native species. Recently, abundances of tropically-associated gray snapper (Lutjanus griseus) and lane snapper (Lutjanus synagris) have increased in seagrass nurseries in the northern Gulf of Mexico (GOM), concurrent with regional increases in sea surface temperature. We investigated effects of increased abundances of these species on abundance and growth of pinfish (Lagodon rhomboides), the dominant native species. Because juvenile pinfish and snappers share common prey, predators, and habitat, the high degree of niche overlap suggests an equally high potential for competition. We used a multiple before–after control impact design to determine whether increased snapper abundances significantly affected abundance or growth of pinfish. Trawl surveys at six locations in the northern GOM in summer and fall 2010 were used to calculate pinfish and snapper abundances. We identified three locations with high snapper abundances and three locations with no snapper and compared pinfish abundance and otolith-determined growth rates in these locations before and after snapper recruitment. Paired t tests and two-way analysis of variance revealed no significant differences in pinfish abundance or growth in the presence of snappers compared to locations and seasons without snappers. We conclude that range expansions of tropically associated snappers have had no significant effect on abundance or growth of native pinfish in northern GOM seagrass habitats.     

Otolith Microchemistry Reveals Substantial Use of Freshwater by Southern Flounder in the Northern Gulf of Mexico

Southern flounder Paralichthys lethostigma is a recreationally and commercially important species along the western Atlantic and northern Gulf of Mexico coasts that can exhibit complex early-life habitat-use patterns. Herein, we used an otolith microchemical approach to test the conventional wisdom that juvenile southern flounder spend most of their early life in low-salinity areas of estuaries, focusing on the largely unstudied population in the Mobile-Tensaw Delta, AL. Analysis of strontium/calcium concentrations in otolith cores of age 0 juveniles demonstrated that 68% of these individuals hatched in high-salinity waters before moving into freshwater habitats, with the remaining individuals being spawned in or near freshwater habitat. Further, otolith edge Sr/Ca concentrations revealed that even juveniles used freshwater habitats, particularly during freshwater/oligohaline conditions in our study system. Otolith edge Sr/Ca ratios for fish collected during high-salinity periods differed significantly among collection regions, suggesting seasonal differences in patterns of habitat use between individuals collected upstream (i.e., freshwater habitats) vs. downstream (i.e., euryhaline habitats). These data support the hypothesis that early-life stages of a substantial portion of a coastal southern flounder population use freshwater habitat.     

Tide gauge records,water level rise,and subsidence in the Northern Gulf of Mexico

Long-term water level changes in the northern Gulf of Mexico were examined using tide gauge records for this century. Strong coherence exists between the annual mean water changes at Galveston, Texas, and (1) the relatively geologically-stable west coast of Florida, (2) global mean sea level, and 93) the subsiding Louisiana coast. Water levels at the Galveston gauge, one of the longest records (81 yr), have risen steadily, but not accelerated over the long-term. The apparent acceleration of water rise in the recent two decades is within the historical pattern, and is probably driven by regional or global, but not local climatic factors. Because eustatic sea level has risen steadily this century, the analysis supports the conclusion that regional geologic subsidence has not varied significantly over the tide gauge record (1909–1988). Variations in the estimates of subsidence in the surface layers are generally consistent with the generally accepted understanding of the geology of deltaic processes on this coast.     

Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico

Net fluxes of respiratory metabolites (O₂, dissolved inorganic carbon (DIC), NH₄ ⁺, NO₃ ^?, and NO₂ ^?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of³⁵S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.     

Spatial and temporal variations of photosynthetic parameters in relation to environmental conditions in coastal waters of the Northern Gulf of Mexico

On a series of eight cruises conducted in the northern Gulf of Mexico, efforts were made to characterize temporal and spatial variability in parameters of the photosynthesis-irradiance saturation curve (P _mas ^B , α^B, I_k) and to relate the observed variations to environmental conditions. Experiments to examine the importance of diel variation in upper mixed layer populations were conducted in July–August 1990 and March 1991. During July–August 1990, P _max ^B and I_k showed significant increases and α^B decreased during the photoperiod in both river plume and shelf-slope populations. During March 1991, no consistent covariance of P-I parameters with local time was found, although highest values of α^B in the river plume were observed in early morning. Seasonal variation in P _max ^B and α^B were correlated with temperature. Spatial variations of photosynthetic parameters in the upper mixed layer ranged from twofold to threefold within any given cruise. Variations of photosynthetic parameters in the upper mixed layer were related to principal components derived from environmental variables, including temperature, salinity, nutrients, mixed layer depth, attenuation coefficient, and daily photosynthetically available radiation (PAR). Greater than 70% of the variation in the environmental variables could be accounted for by two principal components; the majority of this variation was associated with the first principal component, which was generally strongly correlated with salinity, nutrients, mixed layer depth, and attenuation coefficient. Correlations of P _max ^B , α^B, and I_k with the first principal component were found to be significant in some cases, an indication that spatial variability in P-I parameters was related to river outflow. Variation of P-I parameters in relation to depth and PAR were evaluated by regressions with principal components derived from depth, temperature, and mean daily PAR. For most cruises, P _max ^B and I_k were negatively correlated with the first principal component, which was strongly positively correlated with depth and negatively correlated with daily PAR. This was consistent with a decrease in both P _max ^B and I_k with depth that could be related to decreasing daily PAR. Positive correlations of α^B with the first principal component for two cruises, March 1991 and April 1992, indicated an increasing trend with depth. In conclusion, relationships between P-I parameters and environmental variables in the region of study were significant in some cases, but variation between cruises made it difficult to generalize. We attributed this variation to the physically dynamic characteristics of the region and the possible effects of variables that were not included in the analysis such as species composition. Our findings do support the view that a limited set of observations may be adequate to characterize P-I parameter distributions in a given region within a restricted period of time.     

墨西哥湾北部深水区Wilcox沉积特征及沉积模式研究

墨西哥湾北部盆地的始新—古新统Wilcox是深水区重要的储集层和产油层,埋深大,分布广.前人对Wilcox的沉积类型认识存在不同的观点,本文通过岩芯观察、薄片分析,结合测井、录井资料和地震资料,对深水区的Wilcox砂岩的沉积环境和沉积特征进行了深入研究.研究表明,Wilcox砂岩为深水扇沉积,中等到好分选,细粒—极细粒,以岩屑砂岩和长石质岩屑砂岩为主.结合沉积物源分析和盐岩演化形变形成的微盆地对Wilcox沉积的影响,建立了Wilcox深水沉积模式,下陆坡区为受盐岩微盆控制的水道沉积,盆地区为深水扇沉积.     

Tropicalization of the Northern Gulf of Mexico: Impacts of Salt Marsh Transition to Black Mangrove Dominance on Faunal Communities

The tropically associated black mangrove (Avicennia germinans) is expanding into salt marshes of the northern Gulf of Mexico (nGOM). This species has colonized temperate systems dominated by smooth cordgrass (Spartina alterniflora) in Texas, Louisiana, Florida and, most recently, Mississippi. To date, little is known about the habitat value of black mangroves for juvenile fish and invertebrates. Here we compare benthic epifauna, infauna, and nekton use of Spartina-dominated, Avicennia-dominated, and mixed Spartina and black mangrove habitats in two areas with varying densities and ages of black mangroves. Faunal samples and sediment cores were collected monthly from April to October in 2012 and 2013 from Horn Island, MS, and twice yearly in the Chandeleur Islands, LA. Multivariate analysis suggested benthic epifauna communities differed significantly between study location and among habitat types, with a significant interaction between the two fixed factors. Differences in mangrove and marsh community composition were greater at the Chandeleurs than at Horn Island, perhaps because of the distinct mangrove/marsh ecotone and the high density and age of mangroves there. Infaunal abundances were significantly higher at Horn Island, with tanaids acting as the main driver of differences between study locations. We predict that if black mangroves continue to increase in abundance in the northern GOM, estuarine faunal community composition could shift substantially because black mangroves typically colonize shorelines at higher elevations than smooth cordgrass, resulting in habitats of differing complexity and flooding duration.     

Chemical variations in sedimentary facies of an inner continental shelf environment,Northern Gulf of Mexico

Four major sedimentary provinces of the inner continental shelf off the Louisiana coast have been recognized by textural studies (Krawiec, 1966): deltaic, non-deltaic, and relict sediments; and deltadestructional sands. Samples from these provinces have been analyzed for Rb, Sr, Ni, Fe,Mn, Ti, and Zr; trend surface analysis has been used to discern regional trends of these elements. These trends substantiate the previous division of shelf provinces. Although deltaic and non-deltaic sediments cannot be distinguished by chemical criteria, they are distinctive from relict sediments, which are low in Mn, Fe, Ti, Zr, Rb, and Ni, and high in Sr. The delta-destructional sands are distinctive from the other three provinces by their low contents of Mn, Fe, Rb, and Ni, and high Ti, Zr, and Sr.In an energetic environment such as this, processes subsequent to deposition tend to disperse Rb, Fe, Ni, and Mn, as these elements are closely associated with the clays; these processes may give rise to enrichment of Zr due to the high stability and density of the mineral zircon, or the enrichment of Sr by accumulation of organic remains.     

Influence of Grasshopper Herbivory on Nitrogen Cycling in Northern Gulf of Mexico Black Needlerush Salt Marshes

Herbivory is a common process in salt marshes. However, the direct impact of marsh herbivory on nutrient cycling in this ecosystem is poorly understood. Using a ¹⁵N enrichment mesocosm study, we quantified nitrogen (N) cycling in sediment and plants of black needlerush (Juncus roemerianus) salt marshes, facilitated by litter decomposition and litter plus grasshopper feces decomposition. We found 15 times more ¹⁵N recovery in sediment with grasshopper herbivory compared to sediment with no grasshopper herbivory. In plants, even though we found three times and a half larger ¹⁵N recovery with grasshopper herbivory, we did not find significant differences. Thus, herbivory can enhance N cycling in black needlerush salt marshes sediments and elevate the role of these salt marshes as nutrient sinks.     

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25?±?0.56?mmol?m^?2?d^?1 on anoxic bottoms, and ?0.01?±?0.08?mmol?m^?2?d^?1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69?±?0.26?mmol?m^?2?d^?1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean?Csea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.     

Hydrodynamic Response of Northeastern Gulf of Mexico to Hurricanes

The northeastern Gulf of Mexico in the USA is extremely susceptible to the impacts of tropical cyclones because of its unique geometric and topographic features. Focusing on Hurricanes Ivan (2004) and Katrina (2005), this paper has addressed four scientific questions on this area’s response to hurricanes: (1) How does the shallow, abandoned Mississippi delta contribute to the storm surge? (2) What was the controlling factor that caused the record-high storm surge of Hurricane Katrina? (3) Why are the responses of an estuary to Hurricanes Ivan and Katrina so different from the corresponding surges on the open coast? (4) How would the storm surge differ if Hurricane Katrina had taken a different course? Guided by field observations of winds, waves, water levels, and currents, two state-of-the-art numerical models for storm surges and wind waves have been coupled to hindcast the relevant hydrodynamic conditions, including storm surges, surface waves, and depth-averaged currents. Fairly good agreement between the modeled and measured surge hydrographs was found. The quantitative numerical simulations and simple qualitative analysis have revealed that the record-high storm surge of Hurricane Katrina was caused by the interaction of the surge with the extremely shallow, ancient deltaic lobe of Mississippi River. A hypothetical scenario formed by shifting the path of Hurricane Katrina to the observed path of Hurricane Frederic (1979) resulted in a much smaller surge than that observed in coastal Mississippi and Louisiana. However, this scenario did still result in a high surge near the head of Mobile Bay. One of the important lessons learned from Hurricane Katrina is that the Saffir–Simpson scale should be systematically revised to reflect the topographic and geometric features of a complex, heterogeneous coast, including the possible surge amplification in an estuary or a submerged river delta.     

Sedimentation of the northwestern Gulf of Mexico

Six years of intensive study of the shallow water sediments of the northwestern Gulf of Mexico have indicated that there are many criteria by which ancient sediments deposited under similar conditions can be recognized. Thus the presence of echinoid fragments and of glauconite-filled Foraminifera tests favors marine shelf deposition over bay deposits; calcareous aggregates and grains of gypsum suggest high salinity bay deposits; abundance of wood fibers, high mica content, ferruginous aggregates, and well laminated sediments all are suggestive of deposition near river mouths; greater roundness distinguishes dunes from adjacent beach sands; and coarser grain size distinguishes beach sands from shallow shelf sands in the vicinity.Faunal assemblages of Foraminifera, Ostracoda, and Mollusca all serve to distinguish between bay and shelf deposits. Within the bays the faunas show a close relationship to salinity conditions whereas on the shelf the faunas are arranged in bands parallel to the shore and to the depth contours. Certain generalizations concerning the relation of faunas to environment can be made without detailed knowledge of the species.Contribution from the Scripps Institution of Oceanography, New Series No. 997. Investigation supported by a grant from American Petroleum Institute, Project 51.