Sediment distribution and accumulation in lagoons of the Southern Mediterranean Region (the MELMARINA Project) with special reference to environmental change and aquatic ecosystems

Surface sediments and sediment cores were collected from coastal lagoons and lakes located in the Southern Mediterranean Region (SMR) as part of the MELMARINA Project which involved integrated eco-hydrological monitoring and modelling. This study uses surface sediments and sediment cores to infer spatial characteristics and temporal changes at the MELMARINA primary sites, Merja Zerga in Morocco, Ghar El Melh in Tunisia and Lake Manzala in Egypt. In addition, surface sediment sampling was undertaken at Egyptian Lake Bardawil and sediment cores were collected from the Lagune de Nador (Morocco). Sediment distribution patterns are investigated using GIS with georeferenced sample locations to facilitate display and resurvey. Major variations in sedimentary organic matter and, particularly, carbonate content, occur within and between sites. Local landscapes combined with hydrological and biogeochemical processes influence the distributions of sediment bulk components (carbonates, organic material and clastic matter) and molluscan shells and shell debris are an important source of sedimentary carbonate at all three primary sites. Sediment cores were dated using natural (²¹⁰Pb) and artificial (¹³⁷Cs) radionuclides, and sediment accumulation rate changes indicate that sources of sediment supply varied markedly through the twentieth century but have generally diminished after the mid-1960s. Sedimentary siliceous microfossils (diatoms) were generally poorly preserved, but mollusc shell remains were well represented. Sediment chronologies and sediment bulk composition allow discussion of some recent changes in bulk, minerogenic and biogenic sediment accumulation patterns in the SMR lagoons. Sediment accumulation rates also varied between sites and multiple cores from Lake Manzala indicated that rates showed considerable spatial variability. Low-level sediment contamination by fossil fuel combustion particulates and trace metals was demonstrated for Ghar El Melh and Lagune de Nador where Pb and Zn accumulation rates were highest in twentieth century sediment. It is emphasized that sediment quality and quantity have strong influences on lagoon ecosystem function and sedimentation is relevant to hydromorphology and to concepts of ecological quality. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: J. R. Thompson & R. J. Flower Hydro-ecological Monitoring and Modelling of North African Coastal Lagoons     

Sediment accumulation in Lakes Lilla Ullfjärden and Stora Ullfjärden,Sweden

X-ray radiography is a superior method of showing structures in recently deposited sediments. Annually laminated sediments have been proved to exist in Lakes Lilla Ullfjärden and Stora Ullfjärden using this method.From X-ray radiographs of laminated sediment cores sampled nearly every year since 1977 in Lake Lilla Ullfjärden and since 1978 in Lake Stora Ullfjärden, it is possible to distinguish the seasonal variation in sediment deposition in the lakes. The variation in time and space in sediment accumulation is possible to estimate. The rate of sediment accumulation has been calculated to be 320 g solids/m² · y^–1 for the deepest part of Lake Lilla Ullfjärden and 920 g solids/m² · y^–1 for the central part of Lake Stora Ullfjärden for the period 1978 to 1983. It is also possible to trace the effect of eutrophication in time and space. An estimation of the appearance of the oxygen deficit in Lilla Ullfjärden gives a time around 1885.     

Nuisance cyanobacteria in an urbanized impoundment: interacting internal phosphorus loading, nitrogen metabolism, and polymixis

Severe nuisance blooms of cyanobacteria, mainly Aphanizomenon and Microcystis, historically have plagued polymictic Ford Lake, one of the most productive warm-water sport fishing lakes in Michigan, U.S.A. Biomass development in the lake is known to be limited by phosphorus. Alternative theories ascribed nuisance conditions either to allochthonous inputs or to internal loading of phosphorus from anoxic sediments. From 2003 to 2009, there was a strong linear relationship between allochthonous total phosphorus income and phosphorus retained within the basin. The relationship had a negative intercept, implying negative retention, or positive export, of phosphorus from the lake from May to September. Mass balance calculations at lake inlet and outlet were consistent with rates of sedimentary phosphorus accumulation measured from sediment cores. Release rates of soluble reactive phosphorus from anoxic sediments were half that of allochthonous inputs. However, severe declines in nitrogen to phosphorus ratio developed in the anoxic, nitrate-poor hypolimnion because accumulation of ammonium was only tenfold that of phosphate. The result was a steep decline in ratio of total nitrogen to total phosphorus during July and August throughout the lake after episodic mixing events, followed by and coinciding with development of heterocystous Aphanizomenon populations. Lake sediment composition determined by X-ray fluorescence in addition to results of sediment core experiments indicates that phosphorus release is governed by an iron trap mechanism such that phosphate and iron are released only when both oxygen and nitrate are depleted.     

The chemistry of lake sediments in time and space

Five short cores (1.0–1.5 m) representing different depositional zones of an isolated bay of Lake Minnetonka, Minnesota (USA), were independently dated by ²¹⁰Pb and pollen analysis and were analyzed stratigraphically for elemental chemistry (following sediment fractionation) and sedimentary pigments (including myxoxanthophyll and oscillaxanthin). Because of good dating control, short-interval time-stratigraphic units could be traced across the basin, and lake-wide accumulation rates and concentrations could be estimated. The accumulation and concentration of each component at a core-site relative to basin-wide averages provide new indices, relative accumulation and relative concentration, that are used to evaluate three major processes controlling sediment deposition: (1) the basin-wide flux, (2) the physical redistribution of bulk sediment, and (3) the selective bio-geochemical transport and fossilization of individual sedimentary components within the lake.     

The palaeolimnological potential of diatom assemblages in floodplain lakes of the Danube Delta, Romania: a pilot study

Within the framework of a palaeolimnological pilot study, the potential of sedimentary diatom assemblages for the reconstruction of the eutrophication history was studied in short sediment cores from five shallow lakes located in the Romanian Danube Delta. A total of 234 diatom taxa representing 57 genera could be identified but diatom assemblages are generally predominated by a few species only. Loss on ignition percentages, total diatom abundances and diatom downcore successions largely vary within the single sediment cores and between all five lakes. This suggests the presence of various lake environments within the Danube Delta, each characterized by lake-specific geomorphological, sedimentological, hydrological, and biological conditions. Highly variable depositional conditions are also confirmed by radiocesium measurements in two of the five cores suggesting distinct inter-lake differences in the sedimentation rate. Causes for the great intra-lake and inter-lake variability in diatom downcore successions are difficult to specify and may include changes in the physico-chemical conditions, aquatic vegetation cover, water depth, river influence, turbidity and inter-biotic interactions. However, based on this pilot study, it is likely that the spatial and temporal distribution of sedimentary diatom assemblages in Danube Delta lakes were not triggered by the trophic state. The delta lakes likely became meso- to eutrophic long before 1950, possibly during late-Holocene times.     

Sedimentary diatom concentrations and accumulation rates as predictors of lake trophic state

Diatom concentrations in surface sediments are positively correlated with limnetic chlorophyll a concentrations in Florida (USA) lakes. Using this relationship, I examine models that provide quantitative inferences for trophic state in historical applications.The best model predicts chlorophyll a trophic state index (TSI) values from log-transformed diatom concentrations and explains approximately half the variance in the dependent variable. Diatom accumulation rates are not better than sedimentary diatom concentrations as predictors of TSI. The entire diatom assemblage is as sensitive an indicator of TSI as are the planktonic diatoms alone. A model that considers the ecological preferences of specific taxa was found to be a better predictor than the model based on total diatom concentration.The sedimentary diatom concentration model provides a useful method for assessing historical changes in primary productivity, except in lakes where factors (e.g., silica limitation, blue-green bacterial inhibition) limited diatom production, or post-depositional changes removed sedimentary diatoms. TSI inferences are presented for sediment cores from two Florida lakes, one of which demonstrates a problematic application, and the other of which does not.     

The effect of a shift from macrophyte to phytoplankton dominance on phosphorus forms and burial in the sediments of a shallow hard-water lake

In shallow lakes, increasing phosphorus (P) loading has often been accompanied by a shift from a clear-water, macrophyte-dominated state to a turbid state featuring phytoplankton dominance. The effect of a regime shift on P burial and P fractions in lake sediments, however, is poorly understood. We used sediment cores from a eutrophic hard-water lake (Lake Gollinsee, Germany) that had undergone a regime shift (in approximately 1917) to investigate the effect on the accumulation rate of P and on changes in P forms. The cores were dated using Hg contents and radioisotopes (²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am). A combination of total organic carbon to total nitrogen ratios (TOC:TN), δ¹³TOC values, X-ray fluorescence calcium (Ca) counts, and sediment colour clearly distinguished sediment layers that were deposited during periods of macrophyte or phytoplankton dominance. The accumulation rate of total P (TP) in the sediments was 1.8 times higher after the regime shift and was associated with changes in the distribution of P fractions. The proportions of loosely-(NH₄Cl-extracted TP) and Ca-(HCl-extracted TP) bound P decreased significantly, whilst the proportions of biogenic P (NaOH-extracted NRP) and aluminium-bound P (NaOH-extracted SRP) increased significantly. A higher dry mass deposition rate, reduced burial of stable Ca-P complexes, and increased contents and proportions of the mobile iron-bound (BD-extracted TP) and biogenic P fractions in the near-surface sediment layers are assumed to have enhanced the internal cycling of P and hence to have helped to maintain a state of phytoplankton dominance.

     

An Integrated Case Study for Evaluating the Impacts of an Oil Refinery Effluent on Aquatic Biota in the Delaware River: Sediment Core Studies

The objectives of this study were to (1) determine if historical exceedences of oil and grease concentrations above the limits allowed in the National Pollution Elimination Discharge System permit for the wastewater treatment facility of Motiva Enterprises LLC Refinery, Delaware City, Delaware, could be determined from the sedimentary records of the Delaware River and, if so, (2) were the concentrations of contaminants high enough to be toxic to aquatic biota. Eighty-four surficial sediment samples, both within and outside the influence of the Refinery's discharge plume, were initially evaluated for their appropriateness for coring. Seven stations were ultimately selected for the historical core studies. Based on sediment type, radionuclide (Pb-210, Cs-137 and Be-7) geochronologies, and proximity of the cores to the Refinery, two cores were selected for more detailed polycyclic aromatic hydrocarbon (PAH) analyses. The rapid accumulation area of one core (located in the near-field of the discharge plume) had maximum total PAH (TPAH) concentration peaks at depths of 4 cm (1997; 3100 ng/g dw) and at 8.5–9 cm (1993; 3200 ng/g dw); the second core (located on the far-field periphery of the plume) had a maximum TPAH peak at 6.5 cm (1997; 3200 ng/g dw). In all cases, the maximum TPAH concentrations were below NOAA's Effects Range-Low concentration of 4022 ng/g dw for sediment biota toxicity. A chemical fingerprinting analysis of the PAHs in the two cores showed, however, that the PAHs present in the cores were predominantly pyrogenic PAHs consistent with Delaware River background PAHs. The core results are consistent with historic sediment PAH inputs in an industrial river system. PAH from the Refinery, if present, exist as a non-detectable increment to a high PAH baseline from many sources.     

Sedimentary microbial community dynamics in a regulated stream: East Fork of the Little Miami River,Ohio

A field study was conducted in the Lower East Fork of the Little Miami River, a regulated stream in Clermont county, Ohio, to determine how changes in streamflow, water temperature and photo-period affect sediment microbial community structure. Surface sediment cores were collected from sampling stations spanning 60 river kilometers three to four times per year between October 1996 and October 1999. During the final year of the field study, water temperature, water depth, conductivity, total suspended solids, dissolved organic carbon, instantaneous streamflow velocity, sediment grain size and sediment organic matter were determined. Total microbial biomass was measured using the phospholipid phosphate technique (PLP) and ranged between 2 and 134 nmol PLP * g(-1) dry weight sediment with a mean of 25 nmol PLP * g(-1). Microbial community structure was determined using the phospholipid fatty acid analysis and indicated seasonal shifts in sedimentary microbial community composition. January to June sedimentary microbial biomass was predominately prokaryotic (60% +/- 2), whereas microeukaryotes dominated samples collected during the late summer (55% +/- 2.4) and fall (60% +/- 2). These changes were correlated with stream discharge and water temperature. Microbial community structure varied spatially about a reservoir with prokaryotic biomass dominant at upstream stations and eukaryotic biomass dominant at downstream stations. These findings reveal that sedimentary microbial communities in streams are dynamic responding to the seasonal variation of environmental factors.     

Spatial distribution of denitrifying activity in a stream draining an agricultural catchment

SUMMARY 1. A field study was made of the spatial distribution of denitrification activity in the sediment of the River Dorn, Oxfordshire, England.
2. An assay of denitrifying enzyme activity was used to examine the distribution of denitrification with depth in cores of sediment representative of the types found in the stream. The maximum activity recorded in a predominantly silt sediment core was 5 times greater than that recorded in a sandy gravel core. In both fine sand and silt cores, peaks in denitrifier enzyme activity were shown to correspond to the limit of the nitrate diffusion front. At this depth the redox potential dropped rapidly from + 300 mV to 0 or less. Denitrifying enzyme activity in the stream water was negligible.
3. In situ denitrification activity (I DA) measurements were carried out in an 800 m reach of the Dorn using the acetylene inhibition technique on small sediment cores. Concurrent measurements were also made of stream depth and velocity, nitrate concentration in the interstitial water, and the wet bulk density, loss on ignition, mineraliz- able carbon and total nitrogen contents of the sediment. Mineralizable carbon was the variable which showed the best correlation with I DA. Highest IDAs were associated with accumulations of fine-grained sediment at meander bends. Mean IDAs measured under flood conditions were significantly higher ( P <0.05) than those measured under baseflow. It was estimated that denitrification reduced the nitrate load in the River Dorn by 15% under summer baseflow conditions     

Recent Increases in Sediment and Nutrient Accumulation in Bear Lake, Utah/Idaho, USA

This study examines historical changes in sediment and nutrient accumulation rates in Bear Lake along the northeastern Utah/Idaho border, USA. Two sediment cores were dated by measuring excess ²¹⁰Pb activities and applying the constant rate of supply (CRS) dating model. Historical rates of bulk sediment accumulation were calculated based on the ages within the sediment cores. Bulk sediment accumulation rates increased throughout the last 100 years. According to the CRS model, bulk sediment accumulation rates were <25mgcm^?2year^?1 prior to 1935. Between 1935 and 1980, bulk sediment accumulation rates increased to approximately 40mgcm^?2year^?1. This increase in sediment accumulation probably resulted from the re-connection of Bear River to Bear Lake. Bulk sediment accumulation rates accelerated again after 1980. Accumulation rates of total phosphorus (TP), total nitrogen (TN), total inorganic carbon (TIC), and total organic carbon (TOC) were calculated by multiplying bulk sediment accumulation rates times the concentrations of these nutrients in the sediment. Accumulation rates of TP, TN, TIC, and TOC increased as a consequence of increased bulk sediment accumulation rates after the re-connection of Bear River with Bear Lake.     

Marine subsurface eukaryotes: the fungal majority

Studies on the microbial communities of deep subsurface sediments have indicated the presence of Bacteria and Archaea throughout the sediment column. Microbial eukaryotes could also be present in deep-sea subsurface sediments; either bacterivorous protists or eukaryotes capable of assimilating buried organic carbon. DNA- and RNA-based clone library analyses are used here to examine the microbial eukaryotic diversity and identify the potentially active members in deep-sea sediment cores of the Peru Margin and the Peru Trench. We compared surface communities with those much deeper in the same cores, and compared cores from different sites. Fungal sequences were most often recovered from both DNA- and RNA-based clone libraries, with variable overall abundances of different sequence types and different dominant clone types in the RNA-based and the DNA-based libraries. Surficial sediment communities were different from each other and from the deep subsurface samples. Some fungal sequences represented potentially novel organisms as well as ones with a cosmopolitan distribution in terrestrial, fresh and salt water environments. Our results indicate that fungi are the most consistently detected eukaryotes in the marine sedimentary subsurface; further, some species may be specifically adapted to the deep subsurface and may play important roles in the utilization and recycling of nutrients.     

Distribution and Characteristics of Dissolved Organic Matter in Mangrove Sediment Pore Waters along the Coastline of French Guiana

Mangroves represent a major environment of tropical coasts. They are highly productive, and act both as a source and a sink of organic carbon. Concentrations and characteristics (fluorescence and hydrophobic–hydrophilic fractions) of dissolved organic matter (DOM) were investigated in relation to the organic content of sediments and to the chemistry of pore waters along the coastline of French Guiana. The pore waters studied were extracted (centrifugation, soil moisture sampler) from sediments cored beneath A. germinans mangrove stands representative of development stages: pioneer, mature and senescent. In order to asses the effects of seasonal changes, two cores were performed in each location, just after dry and wet seasons, respectively. Dissolved organic carbon (DOC) concentrations in pore waters of the upper sediment were found to increase, from 0.7 mmol l⁻¹ under the pioneers to 9 under senescent mangroves. The evolution of sedimentary organic carbon (SedOC) in the same sediment paralleled that of DOC, increasing from 0.7 to 28%. On the contrary, in the lower parts of sediment cores SedOC and DOC displayed contrasting vertical trends: SedOC decreased sharply with depth while DOC increased, reaching concentrations up to 30 mmol l⁻¹ at 50 cm in the older, senescent mangroves. In addition, the Fluorescence/DOC ratios and the hydrophobic contents of DOC were higher at greater depths in most cores, expressing changes in the DOC composition. These results suggest that the DOC of the upper layers originated directly from the SedOC of the enclosing sediment, while the hydrophobic and fluorescent DOC accumulated in the anoxic bottom layer. The mechanisms responsible for this accumulation at depth requires additional research to be fully understood. However, the anoxic conditions and high pH values prevailing in the lower sediment, by lessening DOM sorption and enhancing SedOC dissolution, may be partly responsible for the high DOC concentrations and fluorescences at depth. In addition, seasonal variation may be involved. During the rainy season, water sources were mixed resulting in lower DOC concentrations in the upper sediment, whereas during the dry season, increased evapotranspiration concentrate salts and DOC, which are transported vertically with percolating water.     

Paleolimnological studies of annually-laminated sediments in Loe Pool,Cornwall, U.K.

The sediments of Loe Pool, a eutrophic coastal lake in south west England, consist largely of laminated clays and clay-gyttjas. Studies of the diatom microstratigraphy of frozen sediment cores from the Pool indicate that the laminations are annual, and that they contain pairs of light and dark bands formed by seasonal variations in the supply of sediment to the Pool from its catchment. Analysis of the magnetic properties of individual laminations demonstrates the presence of physical and mineralogical microstratigraphic variations, which may also be related to seasonality. A varve chronology, which is confirmed by ¹³⁷Cs analysis and historical records, has been used to provide a timescale for the interpretation of data from other paleolimnological studies. A close agreement between variations in the abundance of sedimentary Sn, and the history of mining in the catchment, has been found. Similarly, analysis of total organic matter, total phosphorus, sedimentary chlorophyll a, sterols, diatoms and Cladocera in the uppermost sediments all indicate eutrophication of the Pool in the period AD 1940 to the present.     

Measurement of in situ rates of nitrification in sediment

A method has been developed for the measurement of nitrification rates in intact sediment cores without disturbing the concentration gradients of oxygen and ammonium. N-serve (2-chloro-6-trichloromethyl-pyridine), a specific inhibitor of the autotrophic ammonium oxidation, was injected into a 0–2 cm surface layer of the sediment (20 ppm) and added to the water column of sediment cores (5 ppm). N-serve in these concentrations was sufficient to inhibit nitrification, but did not change the rate of ammonium production or incorporation in sediment suspensions, which were incubated aerobically and anaerobically. The ammonium accumulation in cores injected with N-serve was thus equal to the amount of ammonium which was oxidized to nitrate in the control cores. Nitrification rates were in the range of 0–3 mmol N m^{–2 –1}     

Changes in Sedimentary Redox Associated with Mussel (Mytilus edulis L.) Farms on the West-Coast of Scotland

Aquaculture is growing rapidly in response to an increasing demand for protein and the over-exploitation of wild fisheries. Mussel (family Mytilidae) production has doubled over the last decade and currently stands at 1.5 million tonnes production per annum. Mussels produce organic biodeposits which are dispersed around the production site and, potentially, impact the receiving environment in a number of inter-linked ways. The reported benthic impacts that occur, primarily through the accumulation of these biodeposits and associated organic enrichment, vary widely between studies. The objectives of this research were to determine the nature of the relationship between sediment redox (a proxy for oxygenation) and farm-proximity and covariables whilst accounting for, and quantifying, differences in redox between sites. Sediment cores (N = 159) were taken remotely around a random sample of mussel farms, redox was measured at 10 mm sediment depth and linked to farm-distance and sediment organic/shell content and particle size, using an additive, mixed, weighted regression model. Redox varied considerably between sites and there was a highly significant reduction (50 mV) in redox adjacent to the mussel lines. Redox increased non-linearly with distance, rising rapidly at >7 m from the farm edge. The modest reduction in sediment oxygenation in close proximity to mussel farms reported here suggests that farms located over sediments characterised by pre-existing oxygen stress are likely to exacerbate benthic species impoverishment associated with reducing sedimentary conditions whilst those located over highly oxygenated sediments are likely to increase benthic productivity.     

Denitrification Rates in a Marine Sediment as Measured by the Acetylene Inhibition Technique

A method has been developed for measurement of denitrification activity in sediments by application of the acetylene inhibition technique. Acetylene-saturated water was injected, at close intervals, into sediment cores which were then incubated for a few hours at the in situ temperature. Frozen segments of the cores were assayed for accumulation of N₂O by a combined gas extraction and detection system. The segments were thawed under a stream of helium from which N₂O (and other gases) was collected in a liquid N₂ trap, and the quantity of N₂O was measured by gas chromatography. The maximum rate of denitrification in a coastal marine sediment was 35 nmol of N per cm³ of sediment per day at 2.5°C, and the rate of denitrification for the total sediment was 0.99 nmol of N per m² per day.     

Long-term retrospection on mangrove development using sediment cores and pollen analysis: A review

Mangroves are biogenic systems that accumulate sedimentary sequences, where cores can provide records of mangrove species variation in distribution with past climate change and sea-level change. Fossil evidence used for palaeoecological reconstruction is based on organic remains that preserve identifying features so that they can be identified to generic levels at least. This includes macrofossils such as fruit, flowers, wood or leaves, or microfossils particularly pollen. Anaerobic conditions in mangrove sediment allow the long-term preservation of these fossil records. Fossil pollen from core samples is concentrated for microscopic examination by use of standard chemical treatments, but refinements of these are necessary for the peculiarities of mangrove peat. Pollen diagrams are expressed in concentrations, or more usefully in mangrove environments as proportions relative to others, as this has been shown to demonstrate the depositional environment actually underneath the mangrove forest. Radiocarbon dating of sedimentary sequences is used to date palaeoecological successions shown by fossil sequences, or long-term sedimentation rates. Sediment accretion in the last 50–200 years can been analysed better using Cs¹³⁷ and Pb²¹⁰ analyses. From pollen and macrofossils mostly recovered from stratigraphic cores of sedimentary rock and more recent sediment, the evolution and dispersal of mangroves through geological time has been reconstructed. While reconstruction of actual temperatures in these earlier records is associative to the fossil types present, it is apparent that mangroves have always been tropical species, extending to higher latitudes only during global warm periods. Many sedimentary records show mangroves deeper than the present lower limit of mangrove growth at mean sea-level. These indicate sea-level rising over time, and mangroves keeping pace with rising sea-level. Stratigraphic dating shows accretion rates of 1 mm a⁻¹ for low island locations, and up to 1.5 mm a⁻¹ in high islands/continental margins. Sedimentary records can also show die-off of mangroves with more rapid sea-level rise and replacement by open water during rising sea-level, landward retreat of mangrove zones, or replacement of mangroves by freshwater forest with sedimentary infill. The causes of mangrove community changes identified in the palaeoecological record can only be inferred by comparison with ecological studies in the modern environment, the link between the two that may be possible through long-term mangrove monitoring being poorly established.     

Measurement of nitrification rates in lake sediments: Comparison of the nitrification inhibitors nitrapyrin and allylthiourea

A method for measuring rates of nitrification in intact marine sediment cores has been modified and adapted for use in freshwater sediments. The technique involves subsampling a sediment core into minicores. Half of these cores are treated with an inhibitor of chemolithotrophic nitrification and, after incubation, differences in ammonia and nitrate concentration between inhibited and uninhibited systems are calculated. The within-treatment variability of ammonia and nitrate concentrations could be reduced by storing the cores overnight prior to subsampling. Estimates of the nitrification rate using the difference in ammonia concentrations between the inhibited and uninhibited mini-cores were always greater than the rate estimate using the difference in nitrate concentrations. Comparison between the results using the nitrification inhibitors allylthiourea (ATU) and nitrapyrin (N-Serve) indicated that the former appeared to give larger values for the nitrification rate than did the latter. Differences in the efficiency of these inhibitors in the control of nitrification under the conditions used partly explain these results. Data are also presented on the effect of N-Serve and ATU on some other nitrogen transformations affecting ammonia and nitrate concentrations.     

A Decline in Benthic Foraminifera following the Deepwater Horizon Event in the Northeastern Gulf of Mexico

Sediment cores were collected from three sites (1000–1200 m water depth) in the northeastern Gulf of Mexico from December 2010 to June 2011 to assess changes in benthic foraminiferal density related to the Deepwater Horizon (DWH) event (April-July 2010, 1500 m water depth). Short-lived radioisotope geochronologies (²¹⁰Pb, ²³⁴Th), organic geochemical assessments, and redox metal concentrations were determined to relate changes in sediment accumulation rate, contamination, and redox conditions with benthic foraminiferal density. Cores collected in December 2010 indicated a decline in density (80–93%). This decline was characterized by a decrease in benthic foraminiferal density and benthic foraminiferal accumulation rate (BFAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera, including the dominant genera (Bulimina spp., Uvigerina spp., and Cibicidoides spp.). Cores collected in February 2011 documented a site-specific response. There was evidence of a recovery in the benthic foraminiferal density and BFAR at the site closest to the wellhead (45 NM, NE). However, the site farther afield (60 NM, NE) recorded a continued decline in benthic foraminiferal density and BFAR down to near-zero values. This decline in benthic foraminiferal density occurred simultaneously with abrupt increases in sedimentary accumulation rates, polycyclic aromatic hydrocarbon (PAH) concentrations, and changes in redox conditions. Persistent reducing conditions (as many as 10 months after the event) in the surface of these core records were a possible cause of the decline. Another possible cause was the increase (2–3 times background) in PAH’s, which are known to cause benthic foraminifera mortality and inhibit reproduction. Records of benthic foraminiferal density coupled with short-lived radionuclide geochronology and organic geochemistry were effective in quantifying the benthic response and will continue to be a valuable tool in determining the long-term effects of the DWH event on a larger spatial scale.