Diatom productivity

Silicic acid (H4Si04) is a necessary nutrient for diatoms, who build their shells from opal (Si02 H20). Whether silicic acid becomes limiting for diatoms in seawater depends on the availability of Si relative to N and P. Estimates of diatom uptake of Si relative to P range from 16 1 to 23 1. Dugdale and Wilkerson (1998) and Dunne et al. (1999) have shown that much of the variability in new production in the equatorial Pacific may be tied to variability in diatom production. Diatom control is most important at times of very high nutrient concentrations and during non-steady-state times, perhaps because more iron is available at those times. 

It leads to lower diatom productivity in the Atlantic relative to the Pacific. 

R. D. Levine The coherence that is being discussed by Profs. Troe and Zewail is due to a localized vibrational motion in the AB diatomic product of a photodissociation experiment ABC — AB + C. Such experiments have been done both for the isolated ABC molecule and for the molecule in an environment. As the fragments recede, effective coupling of the AB vibrational motion to the other degrees of freedom can rapidly destroy the localized nature of the vibrational excitation. 

Uptake and Release by Diatoms. Diatoms represent a major sink and transport vector for P in Lake Michigan. The importance of diatom production is shown by the changes in dissolved reactive Si levels that resulted from incorporation into diatoms. During the mixed period (late March to mid-June) dissolved reactive Si decreased by 30.7 g/m2, representing 36% of the spring concentration. An additional 11.4 g/m2 was re-... 

Reactions of this type are nearly as scarce as those in which the diatomic product is electronically excited, and presumably for the same reasons. Again they tend to be proposed as parts of complex mechanisms. A relatively simple possibility is... 

A problem which is even more difficult is to investigate the yield of vibrational energy accompanying three-body combination of atoms. The combination rate coefficients tend to be small compared to the fast (and second order) vibrational relaxation. Thus the stationary concentration of the diatomic product usually corresponds very closely to an ambient Boltzmann distribution. Callear58 observed S2 from S-atom combination, but concluded that the relaxation was too fast for any meaningful quantitative measurements to be made. 

The availability of DSi can regulate the composition of phytoplankton species (Kilham, 1971 Officer and Ryther, 1980 Egge and Aksnes, 1992). For example, diatom growth is dependent on the availability of DSi to nondiatom phytoplankton species which are Si independent. DSi uptake by diatoms can even occur in bottom waters or turbid surface water where light may be limiting (Nelson et al., 1981 Brzezinski and Nelson, 1989). However, once the supply of DSi has diminished diatom production will decrease and diatoms will be replaced by other phytoplankton species. Limitation of DSi in Chesapeake Bay resulted in a rapid decline in diatoms and an increase in cyanobacteria (Malone et al., 1991). Similarly, an estimated 50% decline in DSi from the 1950s to the 1980s in the Mississippi River (Turner and Rabalais, 1991) resulted in DSi limitation for diatoms on the... 

Decreases in the delivery of DSi loading of Mississippi River to coastal waters have been attributed to enhanced N loading in the river, which is believed to have increased diatom production and sedimentation in the watershed. 

Bianchi, T.S., and Rice, D.L. (1988) Feeding ecology of Leitoscoloplosfragilis. II. Effects of worm density on benthic diatom production. Mar. Biol. 99, 123-131. 

The reactions of alkaline earth atoms with alkali halide molecules are especially noteworthy because laser-induced fluorescence has been employed in these crossed-molecular beam experiments to measure the product internal state distributions as a function of scattering angle. For Ba + KC1 and Ca + NaCl, both the atomic and diatomic products were detected. 

The symmetry of the Iv(6) distribution about the 6 = 90° plane remains a diagnostic test for complex formation. Moreover, the anisotropy of the / ( ) distribution can be used [44] to determine the partitioning of final angular momentum between L and J and therefore the rotational energy of the diatomic product AB. Unsymmetric scattering of the product about 6 = 90° indicates a direct reactive mechanism, and observations of the preferred direction of recoil provide important evidence concerning the character of the reactive collisions. 

These experiments show conclusively that the available energy is released almost entirely as internal excitation of the products. The observation that the diatomic product can subsequently excite M atoms electronically demands a degree of excitation which precludes the formation of 2P1/2 halogen atoms, which requires 21.7 kcal/mole (0.94 eV) for I and 10.5 kcal/mole (0.46 eV) for Br. Where electric deflection analysis has been performed [34-36], the averaged rotational energy yield, is about 5 kcal/mole (0.22 eV),... 

As in M + X2, the reactive scattering is strongly anisotropic, but now the diatomic product is found predominantly in the backward hemisphere. The early experiments [2,43, 70], performed without velocity selection or velocity analysis, were difficult to interpret quantitatively because of the unfavorable kinematics, which arose because (a) mKI mcH, (b) considerable blurring was introduced by the spread in the incident velocities, and (c) the v lu f factor in the Jacobian distorts the lab distribution and cannot be... 

Despite the relatively small reactive cross section (aE 5-20 A2), the diatomic product is scattered predominantly forward and the angular distribution is not unlike that from the reaction of K + Br2. For... 

Water column biogeochemical processes, however, do not always leave an obvious record in the seabed. For example, Amazon shelf sediments have little biogenic silica accumulation despite extensive diatom production and abundance in estuarine surface... 

Despite the high silicic acid load associated with rivers (discharge-weighted average silicic acid concentrations have been estimated at 150 pM Treguer et al, 1995), there is ample evidence for biological removal in estuaries and coastal waters. Nelson and Dortch (1996) demonstrated biological removal of 80—99% of the silicic acid in the Mississippi River plume. MiUiman and Boyle (1975) reported a 25% drawdown of silicic acid within the Amazon River estuary, with further depletion in the nearshore ocean. Kimmerer (2005) demonstrated ca. 85% drawdown of silicic acid within San Francisco Bay, directly related to diatom production. In contrast, silicic acid concentrations are rarely less than 0.5 pM in much of the world s oceans ( Nelson and Dortch, 1996), despite the prevalence of diatoms and the dominance of diatom productivity in export flux. 

Given the importance of riverine input to Si cycling in the oceans, any changes in the source or nutrient ratios could have an impact on diatom production and... 

McDonald D., Pederson T., and Crusius J. (1999) Multiple Late Quaternary episodes of exceptional diatom production in the Gulf of Alaska. Deep-Sea Res. II46, 2993-3017. 

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