ABSTRACT-The stable isotope compositions (C, N, and S) of hve, senescent, and standing dead Spartina alterniflora were compared in order to determine the effects of aerial decomposition on the isotopic signature of aboveground S. dlterniflora tissue entering the food chain. Aerial decomposition of aboveground S. alterniflora resulted in a 6 to 18% increase in P S , and a 2 to 3 % decrease in 615N values; 613c values were unchanged. We describe mechanisms whereby the activity of fungi and epiphytic microorganisms may contribute to the observed shlfts in and 6I5N, respectively. The 613c value of salt marsh benthic and eplphytic microalgae ranged from -13.0% in summer to -17.6%0 in early spring. Average 6'" values of microalgae and standing dead S. altemiflora were -0 3 and l.?%, respectively, reflecting the activity of NZ-fixing microorganisms. The 615N values for omnivorous and herbivorous salt marsh macrofauna ranged between 7.5 and 2.2% and for predatory Fundulus spp. averaged 9.2%. Given a presumptive + 3 % trophic shift in N assimilation, these results suggest that N2-fixing microorganisms associdted with microalgal communities were an important source of N to salt marsh consumers. The 6% 6'"N, and 6"' s values of primary producers were compared to the values of the following consumers: Fundulus spp., Uca spp., Ilyanassa obsoleta, and Littoraria irrorata. 6I3C VS 6 1 5~ and 613C vs 63JS dual isotope plots demonstrated that microalgae and standing dead S. alternjflora are important food resources in the North Carolina (USA) marshes we sampled. In addition, a compilation of literature values suggests that this is true in salt marshes throughout the East and Gulf coasts of North America. Future isotope studies of marsh food webs should include detrital Spartina spp. material in analyses of trophic structure.

Rates of leucine lncorporahon have been suggested recently to be useful for estunating rates of protein synthesis and biomass product~on by bactena In natural water samples We examined 2 potential problems wlth this approach de novo synthesis of leucine and lntracellular protein turnover Rates of leucine and methionine biosynthesis were e s t~m a t e d from the incorporation of '4C-pyruvate and 35S0,', respectively Leucine inhibited '4C-pyruvate and 'H-glucose incorporahon and methionine inhibited "SO, lncorporation However rates of biosynthesis of leucine and lnethionine were still much higher than the maximum rate of exogenous amlno acld Incorporation This problem can be surmounted with emplncally determined conversion factors which relate rates of leucine incorporation to rates of proteln synthesis or biomass production The ratio of the emplncally determined factor to the theoretical factor is similar to the ratio of the rate of biosynthesis to the incorporation rate of exogenous leuclne The rate of intracellular proteln turnover as determined by the pulse-chase approach was large compared wlth net protein synthesis ~n only 1 out of 5 expcnments Leuclne lncorporation rates are at least an underestimate of rates of protein synthesis and In some environments may prove to b e a useful measure of bacterial biomass production Our results also Indicate that the supply of dissolved amino acids may affect the uptake and minerahzatlon of other dissolved compounds vla regulation of amino acid uptake and biosynthesls

We measured the effects of filtration by the Atlantic ribbed mussel Geukensia demissa (Dillwyn, 1817) on microbial abundance in water exposed to the salt-marsh surface. Various groups of living and non-living particles (large and small autotrophs, large and small heterotrophs, cyanobacteria, bacteria, non-living clay-organic floc) were removed with greatly differing effectiveness, ranging from 25 to 95 O/o of initial abundance per hour Phytoplankton would contribute an estimated 72 "10 of livlng microbial carbon removed by field populations of mussels (47 O/O of living microbial carbon contnbuted by small autotrophs). Although an equal biomass of sn~all mussels was more effective at removing particles than large mussels, medium to large mussels would account for over 90 % of mussel grazing in the field because of their greater abundance. Particle removal was not solely dependent on particle size, as large heterotrophs and bacteria were removed with low (25 to 5 6 % h-') efficiency, whereas ~ntermediate-sized living particles and non-living clay-organic particles (ranging widely in size) were removed with high efficiency (up to 9S0/0 h-'). D~fferential removal of small heterotrophs (high efficiency) versus bacteria (low efficiency) may perturb the balance between bacterial production and microbial bacterivory in salt-marsh systems. The presence of G. den~issa enhanced bacterial production rates slightly; this could not be solely attributed to nitrogen excretion by the mussels. Mussel filtration was sufficient to balance bacterial production during high tide excursions of water onto the marsh, and may account for the net importation of bacteria from tidal creeks to the intertidal marsh reported in other studies. Present data indicate that ingestion of microbial carbon is not sufficient to meet the carbon requirements of G. demissa on an annual basis. INTRODUCTIONThe feeding activity of suspension-feeding bivalve molluscs can have a profound influence on the abundance of water-colun~n microbiota in shallow water (Wright et al. 1982), and may be an important mechanism for coupling pelagic and benthic processes (Dame et al. 1980). Cloern (1982) suggested that the abundance of phytoplankton within the southern portion of San Francisco Bay is controlled by the effects of grazing by benthic bivalves, which could filter a water volume equivalent to the entire water column each day. Officer et al. (1982) considered the criteria which could lead to such control in south San Francisco Bay (shallow water, limited hydrodynamic exchange, dense benthic filter-feeding community) and identified 2 other estuarine systems which were also likely to exhibit benthic control of water-column microbial abundance. Peterson & Black (1987) concluded that benthic bivalves could potentially filter a large fraction of incoming tidal volume on sandflats of a subtropical embayment (Shark Bay, Australia). Sherr et al. (1986) found that in the Duplin h v e r estuary (Sapelo Island, Georgia, USA) much of bacterioplankton production was no...

ABSTRACT-Competition experiments were performed using precolonized leaves or leaf disks of red mangrove Rhizophora mangle with: (1) disks containing pure cultures of single species of marine true fungi or species of Halophytophthora (the principal genus of marine oomycotes); and (2) leaves bearing bacterial films. Preoccupied leaves were exposed to natural microflorae in mangrove creeks at 2 Cays in the Bahaina Islands, or placed in laboratory seawater enclosures wherein pairs of halophytophthoras were given equivalent opportunity to occupy fresh leaf material. The ubiquitous coastal-marme oomycote H. vesicula was found to be an able competitor versus true fungi and versus other halophytophthoras. Against other halophytophthoras, this was true for both primary and secondary resource capture. The one exception among the fungi was a species (Dendryphiella salina) common in decaying drift material in high-intertidal zones. H. spinosa was a weak competitor with true fungi and with H. vesicula, though it was not displaced by H. vesicula, and H. spinosa could depress the frequency of H. vesicula occupation when H. spinosa was well established. H. bahamensis did not routinely form sporangia, preventing identification and firm conclusions regarding compet~tiveness, other than that it could not block H. vesicula, but could block H. spinosa from entering its occupied arenas. When bacterial films were present on leaves prior to access by halophytophthoras, the occupation frequency of halophytophthoras was sharply depressed (by about 70 to 90% with 48 h bacterial films), including for H. vesicula, implying that in some types or parts of mangrove systems, submerged-leaf decomposition may sustain low levels of participation by halophytophthoras.