Major nitrogen (N) pools and bacterial transformations of N were examined in carbonate sediments of 3 reefs in the central area of the Great Barrier Reef, Australia. Depth distributions of nitrate (NO3-) and ammonium (NH,+) and rates of NH4+ production, N2 fixation (nitrogenase activity by CzHz reduction) and denitrification were measured in muddy sediments of an inshore reef and in fine-. medium-and coarse-grained sediments at a midshelf and shelf edge reef. Ammonium efflux was estimated from pore water profiles. Estimates of potential rates of NH,' and NOs-utilization were made in the upper 2 cm of sediments at the midshelf and shelf edge reefs. Highest concentrations of NH,' (up to 70 pM at 8 cm) were observed in muddy carbonate sediments of inshore Pandora Reef, with somewhat lower concentrations (up to 20 vM) in fine-grained sands of the other 2 sites. Relatively small NH4+ pools, usually less than 10 btM, typified coarse-grained sediments. Nitrate was generally undetectable in these sediments. Rates of NH,+ efflux among sites ranged from 0 to 4 pm01 N m-2 h-', with highest fluxes associated with muds and fine-grained sands. Ammonification rates in the upper 2 cm ranged from 6 to 26 pm01 N m-' h-' among sites, generally increasing with depth. Nitrogenase activity was detected in all sediments examined, with hlghest rates near the surface. N2 fixation could account for more than 50 % of NH,+ production in the upper 2 cm of sediment at 3 of 4 sites. The potential in the upper 2 cm for NH4+ consumption (nitrification and assimilation) ranged from 10 to 60 pm01 N m-' h-', while NO3-reduction potenhal ranged from 10 to 80 pm01 N m-' h-' suggesting these may be quantitatively important pathways. Inhibitor experiments indicated that much of the NH,' uthzation might be by nitrification. Very high nitrification rates [up to 3.8 nmol N (g dry sed.)-' h-' or 70 pm01 N m-' h-'] were confirmed at 1 site by a "N isotope dilution method. Low denitrification rates were also detected in these environments, and in many cases under apparently oxic conditions. However, highest rates noted were less than 5 % of the rate of N o 3 -reduction. While shallow carbonate sands may be poor in organic material, they are active sites of bacterial N transformations. The NH,' and NOs-pools in the upper few cm appear to be highly dynamic, with estimated turnover times of substantially less than 1 d. It is also noteworthy that bacterial N:, fixation appears to account for a much larger fraction of NH4+ turnover than in shallow temperate zone sediments.

The relative importance of phytoplankton and heterotrophlc bacteria in the ut~lization of ammonium in a temperate coastal pelaglc environment (Long Island Sound. New York, USA) was examined uslng the short-lived radioisotope, I3N Uptake of ' 3~~: Into different slze fractions under simulated in situ temperature and light condihons was compared to size fractionations of bacterial abundance, chlorophyll, and uptake of tritlated thymidme and 14C-b~carbonate In January and Apnl, little ' 3~ entered the bactenal slze fract~on, but when bacteria were more metabolically a c t~v e in May and July, about '/3 of the total I3N uptake appeared in the size fraction conta~ning most of the bactena but httle of the chlorophyll (0 2 to 0 6 ,pm) Exposure to up to 8 mC1 I-' of I3N radioactivity d~d not ~n h~b~t bacterial or phytoplankton activity, and hlled controls showed virtually no uptake Ammonium uptake rates measured by I3N were comparable to those measured by the stable isotope 15N in 2 of 3 experiments The size fract~onation results were consistent wlth estimates of bactenal and phytoplankton demand tor nitrogen for growth Turnover rates for the dissolved a m m o n~u m pool ranged from 0 6 % h-' in April to 116 O/ O h-' in July In separate experiments .r\~th mlxed seawater cultures of bactena, the half-saturation constant (K,) for ammonium uptake was 1 0 1 uM, and ammonium uptake was reduced more by additions (100 to 1000 nM) of glutamine than by glutamate or othel ainlno a c~d s suggesting that glutamlne is directly ~nvolved in a m m o n~u m uptake In these bacteria

Academy of Natural Sciences of Philadelphia. 19th and the Parkway, Philadelphia. ~e n n s~l v a n i a 19103. USA ABSTRACT: The contribution of zooplankton to flows of dissolved organic carbon was studied for the food web of coastal plankton assemblages off Southern California, USA. Seawater samples including active macrozooplankton were labelled with I4CO2. The time course of I4CO2 incorporation into size fractions of the plankton and dissolved organic materials was determined. Changes in extracellular concentrations of free primary amines and dissolved saccharides were also measured. In addition, the time course of 3H-thymidine incorporation into bacteria was determined along with bacterial cell counts. By comparing carbon flows in samples with macrozooplankton and without macrozooplankton, we evaluated the role of macrozooplankton in the flux of carbon to bacteria and dissolved organic matter. One-half or more of theI4C-carbon in the bacterial size fraction could be due to small photosynthetic forms or the remains of microplankton disrupted by the filtration used in the size fractionation. Moreover, most of the carbon flowing to bacteria from the macrozooplankton was not labelled by I4C and was proportional to the number of macrozooplankton. Conservative estimates of this unlabelled carbon flux to bacterial biomass due to macrozooplankton were 3 and 10 % of the I4C assimilated photosynthetically. Potential artefacts resulting from experlmental manipulations are considered.

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