Microbiology of a former dredge spoil disposal area

A microbiological study of an historically active dredge spoil disposal area in Long Island Sound showed no obvious long-term effects on the bacterial populations of water and sediment. Observations were made 4 years after the last deposition of dredge spoils. Determinations included total and faecal coliforms and numbers, hydrolytic types, and taxonomic groupings of ‘total’ heterotrophic bacteria. Comparisons with water and sediment from other areas of the sound indicated that the dump site was not bacteriologically unique.     

Impact of Fertilization on a Salt Marsh Food Web in Georgia

We examined the response of a salt marsh food web to increases in nutrients at 19 coastal sites in Georgia. Fertilization increased the nitrogen content of the two dominant plants, Spartina alterniflora and Juncus roemerianus, indicating that added nutrients were available to and taken up by both species. Fertilization increased Spartina cover, height, and biomass and Juncus height, but led to decreases in Juncus cover and biomass. Fertilization increased abundances of herbivores (grasshoppers) and herbivore damage, but had little effect on decomposers (fungi), and no effect on detritivores (snails). In the laboratory, herbivores and detritivores did not show a feeding preference for fertilized versus control plants of either species, nor did detritivores grow more rapidly on fertilized versus control plants, suggesting that changes in herbivore abundance in the field were driven more by plant size or appearance than by plant nutritional quality. Community patterns in control plots varied predictably among sites (i.e., 17 of 20 regression models examining variation in biological variables across sites were significant), but variation in the effects of fertilization across sites could not be easily predicted (i.e., only 6 of 20 models were significant). Natural variation among sites was typically similar or greater than impacts of fertilization when both were assessed using the coefficient of variation. Overall, these results suggest that eutrophication of salt marshes is likely to have stronger impacts on plants and herbivores than on decomposers and detritivores, and that impacts at any particular site might be hard to distinguish from natural variation among sites.     

Why meter-wide dikes at oceanic spreading centers?

Numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre-diking horizontal stress is close to the Andersonian normal faulting stress and the stress is fully released in one dike intrusion. Dikes at slow-spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate. However, data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0.5 m to 1.5 m, and does not clearly correlate with plate spreading rate. Dike width is reduced if either the pre-diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike. Partial stress release during a dike intrusion is the more plausible explanation, and is also consistent with the fact that dikes intrude in episodes at Iceland and Afar. Partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions. Limited magma supply sets the upper limit on the width of dikes, and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress. The observation of dikes that are wider than a few meters (such as the recent event in Afar) indicates that large tectonic stress and large magma supply sometimes exist.     

Salt marsh litter and detritivores: A closer look at redundancy

Most primary production of angiosperms in coastal salt marshes enters the detritivore food web; studies of this link have predominantly focused on one plant species (Spartina alterniflora) and one detritivore species (Littoraria irrorata). In mesocosm experiments, we studied the rates and pattern of decomposition of litter derived from four plant species common in southeastern United States coastal salt marshes and marsh-fringing terrestrial habitats. Crustanceans and gastropods were selected as detritivores feeding on, and affecting degradation of, the litter of two monocotyledons and two dicotyledons. Despite interspecific similarities in consumption, detritivores exhibited species-specific effects on litter chemistry and on the activity of litter-colonizing microbiota. The chemical composition of feces depended upon both the litter type and the detritivores’ species-specific digestive capabilities. Growth rates and survival of detritivores differed among litter species. Different salt marsh detritivores are likely to have different effects on decomposition processes in the salt marsh and cannot be regarded as functionally redundant nor can the litter of different plant species be regard ed as redundant as food for marsh detritivores.     

Control of rheological stratification on rifting geometry: a symmetric model resolving the upper plate paradox

Numerical experiments reproduce the fundamental architecture of magma-poor rifted margins such as the Iberian or Alpine margins if the lithosphere has a weak mid-crustal channel on top of strong lower crust and a horizontal thermal weakness in the rift center. During model extension, the upper crust undergoes distributed collapse into the rift center where the thermally weakened portion of the model tears. Among the features reproduced by the modeling, we observe: (1) an array of tilted upper-crustal blocks resting directly on exhumed mantle at the distal margin, (2) consistently oceanward-dipping normal faults, (3) a mid-crustal high strain zone at the base of the crustal blocks (S-reflector), (4) new ocean floor up against a low angle normal fault at the tip of the continent, (5) shear zones consistent with continentward-dipping reflectors in the mantle lithosphere, (6) the mismatch frequently observed between stretching values inferred from surface extension and bulk crustal thinning at distal margins (upper plate paradox). Rifting in the experiment is symmetric at a lithospheric scale and the above features develop on both sides of the rift center. We discuss three controversial points in more detail: (1) weak versus strong lower crust, (2) the deformation pattern in the mantle, and (3) the significance of detachment faults during continental breakup. We argue that the transition from wide rifting towards narrow rifting with a pronounced polarity towards the rift center is associated with the advective growth of a thermal perturbation in the mantle lithosphere.     

Whole-basin,mass-balance approach for identifying critical phosphorus-loading thresholds in shallow lakes

Lake Lochloosa, Florida (USA) recently underwent a shift from macrophyte to phytoplankton dominance, offering us the opportunity to use a whole-basin, mass-balance approach to investigate the influence of phosphorus loading on ecosystem change in a shallow, sub-tropical lake. We analyzed total phosphorus (TP) sedimentation in the basin to improve our understanding of the forcing factor responsible for the recent shift to phytoplankton dominance. We measured ²¹⁰Pb activity, organic matter (OM), organic carbon (OC) and TP in short sediment cores from 20 locations to develop a comprehensive, whole-basin estimate of recent mass sedimentation rates (MSR) for bulk sediment, OM, OC and TP. The whole-basin sedimentation models provided insights into historic lake processes that were not evident from the limited, historic water quality data. We used Akaike’s Information Criteria to differentiate statistically between constant MSR and exponentially increasing MSR. An eightfold, exponential increase in TP accumulation over the past century provided evidence for the critical role of increased P loading as a forcing factor in the recent shift to phytoplankton dominance. Model results show increased TP retention and decreased TP residence time were in-lake responses to increased TP loading and the shift from macrophyte to phytoplankton dominance in Lake Lochloosa. Comparison of TP loading with TP retention and historic, diatom-inferred limnetic TP concentrations identified the TP loading threshold that was exceeded to trigger the shift to phytoplankton dominance.     

The Influence of Coastal Nutrients on Phytoplankton Productivity in a Shallow Low Inflow Estuary,Drakes Estero,California (USA)

Seasonal wind-driven upwelling along the U.S. West Coast supplies large concentrations of nitrogen to surface waters that drives high primary production. However, the influence of coastal upwelled nutrients on phytoplankton productivity in adjacent small estuaries and bays is poorly understood. This study was conducted in Drakes Estero, California, a low inflow estuary located in the Point Reyes National Seashore and the site of an oyster mariculture facility that produces 40 % of the oysters harvested in California. Measurements of nutrients, chlorophyll a, phytoplankton functional groups, and phytoplankton carbon and nitrogen uptake were made between May 2010 and June 2011. A sea-to-land gradient in nutrient concentrations was observed with elevated nitrate at the coast and higher ammonium at the landward region. Larger phytoplankton cells (>5 μm diameter) were dominant within the outer and middle Estero where phytoplankton primary productivity was fueled by nitrate and f-ratios were >0.5; the greatest primary production rates were in the middle Estero. Primary production was lowest within the inner Estero, where smaller phytoplankton cells (<5 μm) were dominant, and nitrogen uptake was dominated by ammonium. Phytoplankton blooms occurred at the outer and middle Estero and were dominated by diatoms during the spring and dry-upwelling seasons but dinoflagellates during the fall. Small flagellated algae (>2 μm) were dominant at the inner Estero where no blooms occurred. These results indicate that coastal nitrate and phytoplankton are imported into Drakes Estero and lead to periods of high new production that can support the oyster mariculture; a likely scenario also for other small estuaries and bays.