Observations on the ecological importance of salt marshes in the Cumberland Basin,a macrotidal estuary in the Bay of Fundy

The Cumberland Basin, a 118 km² estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m^?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin.     

Mapping sediment acoustic impedance using remote sensing acoustic techniques in a shallow-water carbonate environment

 Real-time trackline plots of surficial sediment acoustic impedance delineate several sedimentary facies off Garden Key in the Dry Tortugas. The sea floor within a 6×6 km surveyed area consists of carbonate muds (silts), sands and shell, rock, and live corals. The 4-kHz acoustic data supports this finding by providing a pictographic representation of the distribution and structure of several sediment facies types. Plotting the gridded acoustic data with commercial mapping software (Surfer) provides a three-dimensional (3D) perspective of the bottom topography with a color contour map of surficial sediment impedance (upper 0.4 m) draped over the 3D surface.     

Interlaboratory calibration experiments using the polychaetous annelid Capitella capitata

An interlaboratory calibration experiment was conducted at three laboratories to test two sources of variation associated with bioassay experiments, variation due to the experimenter and to the natural seawater. Twenty-eight day static (with frequent media renewal) bioassays exposing the polychaete Capitella capitata to cadmium were conducted with synthetic and natural seawaters. Test results varied between the three laboratories; however, the variations are most probably explained by the shipment of the experimental animals to the participating laboratories.     

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km² and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km.     

Solar system isotopic anomalies: Supernova neighbor or presolar carriers?

I evaluate several nuclear and chemical problems related both to the recent scenario suggesting that the known isotopic anomalies in the solar system have resulted from a supernova near the protosolar nebula and to the model of extinct presolar carriers. Major features include: (1) Large quantities of extinct ²⁴⁸Cm and ³⁶Cl are predicted from the Cameron-Truran model of a minor injection about 10⁶ yr before condensation; (2) an extinct-carrier model of ²⁶Mg is set forth in detail with a solid chemistry picture of the early solar system; (3) a major thermonuclear supernova responsible for ²⁶Al, ²⁴⁴Pu, and ⁴⁰K would have to have occurred several million years (3 m.y.) before condensation and contributed a large fraction of the major stable chemical elements; (4) carbon isotope families are to be expected if the oxygen isotope families are due to a late injection of ¹⁶O; (5) the Earth and E meteorites may have condensed primarily in a carbon-rich nebula existing before admixtures of a major late ¹⁶O-rich mixture; (6) the extinct-presolar-carrier model is the single best explanation of all anomalies.     

Temperature and its control of isotope fractionation by a sulfate-reducing bacterium

A synthesis of previous results, which we dub the “standard model,” provides a prediction as to how isotope fractionation during sulfate reduction should respond to physiological variables such as specific rate of sulfate reduction and environmental variables such as substrate availability and temperature. The standard model suggests that isotope fractionation should decrease with increasing specific rates of sulfate reduction (rate per cell). Furthermore, the standard model predicts that low fractionations should be found at both high and low temperatures whereas the highest fractionations should be found in the intermediate temperature range. These fractionation trends are controlled, as a function of temperature, by the balance between the transfer rates of sulfate into and out of the cell and the exchange between the sulfur pools internal to the organism. We test this standard model by conducting experiments on the growth physiology and isotope fractionation, as a function of temperature, by the sulfate-reducing bacterium Desulfovibrio desulfuricans (DSMZ 642). Our results contrast with the “standard model” by showing a positive correlation between specific rates of sulfate reduction and fractionation. Also by contrast with the standard model, we found the highest fractionations at low and high temperatures and the lowest fractionations in the intermediate temperature range. We develop a fractionation model which can be used to explain both our results as well as the results of the “standard model.” Differences in fractionation with temperature relate to differences in the specific temperature response of internal enzyme kinetics as well as the exchange rates of sulfate in and out of the cell. It is expected that the kinetics of these processes will show strain-specific differences.