We postulate that microbial metabolism and production in cold waters are hmited by the ability of bacteria to transport and/or assimilate substrates at the low concentrations usually present. We measured rates of microbial activity in the water colunln and benthos in Conception Bay, Newfoundland and adjacent coastal waters, during 3 spring blooms. Chlorophyll abundance and distribution, photosynthesis, benthic respiratory rate and remineralization, bacterioplankton abundance, and production of bacterial blomass were measured in the water column, and the respuatory rate of benthic sediments was measured in situ by a free vehicle containing bell jars equipped w t h oxygen electrodes and syringe samplers. During 3 spring seasons, bacterioplankton numbers during the spring phytoplankton bloom exceeded 5 X lo5 ml-' only 15 % of the time. These numbers rank at the lower end of the range of bacterial numbers for the world's ocean. Bacterial productivity measured by 3 methods suggested average generation times of 30 to 86 d, although some samples in the chlorophyll maximum layer showed short generation times. Bacterial production and respiration, averaged over the entire water column, plus benthic aerobic respiration and denitrification, accounted for 3 % of primary production during the early bloom and 28 % of primary production during the late bloom. The difference between the early and late bloom is related to observed changes in primary production, not to increased microbial activity. Unless bacterial assimilation efficiency was very low, much less than half of the organic production of the spring diatom bloom was used by microbial processes dunng the early, highly productive phase of the bloom, while somewhat more than half of organic production was used in the short term during the later, less productive phase of the spring bloom. To test the hypothesis that bacteria require hlgher substrate concentrations at low temperature, water samples were amended with glucose and proteose-peptone and incubated for 2 wk at -1 to + 15 "C. Respiratory rate, measured at intervals of 1 to 2 d , increased with increasing temperature andlor substrate concentration. Analysis of vanance showed significant effects of temperature and substrate in all cases. In 3 of 4 experiments there were also significant effects due to the interaction of temperature and substrate

The quantitative significance of microbial heterotrophs in overall plankton metabolism was investigated in estuarine and coastal waters off the Georgia coast (USA). Three size classes were examined: < 208 pm, < 10 pm and < 1 p m Respiration was measured by monitoring the rate of oxygen consumption in water samples incubated at in situ temperatures in the dark. Rates by all size classes of plankton were highest in the upper reaches of the Duplin h v e r estuary during summer (52.4 pg O2 1-l h-') and lowest 10 km offshore during winter (10.1 pg O2 1-l h-'). Respiration by organisms passing a 1 pm screen exceeded rates in 208 pm and 10 pm filtered samples, but this enhancement decreased w~t h increasing distance offshore. This increased bacterial (< 1 pm) respiration could not be attributed to artifacts of the filtration procedure or to bacterial growth. Respiration results were consistent with concurrently conducted studies of bacterioplankton growth and bactivorous microprotozoan grazing. Static models of carbon and nitrogen flow between bacterioplankton and heterotrophic protozoa suggest that the C/N ratio of the bacterial food source could largely dictate organic carbon conversion efficiency of planktonic bacteria, the relative role of bacteria and microprotozoans as recyclers of nitrogen and the rate of organic carbon utilization. The relative importance of bacterial nitrogen remineralization decreases as the C/N ratio of the organic substrate increases. Results of this study are discussed in context of the microbial loop and the trophic flow leading to higher organisms.

No abstract

Rates of oxygen uptake, sulphate reduction, carbon dioxide production, ammonium turnover, nitrogen fixation and denitrification were measured in sediments in a transect across a mangrove swamp in Jamaica. Carbon and nitrogen mineralisation increased as the transect entered the mangrove, indicating a greater availability of organic matter, probably by root excretion. Time course measurements of sulphate reduction indicated a subsurface pool of labile carbon. Carbon dioxide production within the mangrove exceeded oxygen flux across the sediment surface, indicating a subsurface source of oxygen of 28 to 179 mm01 m-' d-' The turnover time for the sedimentary sulphide pools decreased as the transect entered the mangrove. The production of ammonium from organic nitrogen, measured by I5NH,+ turnover, also indicated active subsurface organic matter mineralisation. The source of this organic nitrogen is unknown. Measured rates of nitrogen fixation were too low to supply the required organic nitrogen. The estimated net availability of ammonium to the plant roots was 10 mm01 m" d-' This would give a productivity of 2000 g C m-2 yr-'.

In the Abrolhos Islands, site of a high-latitude coral reef, the net rate of community calcification is high but growth of frame-building corals is reduced. Our studles suggest that the latitudinal limits of coral reef development are often set by competition of macroalgae with corals, and that macroalgae are favoured at these latitudes by high nutrient concentrations, moderate water temperatures (for macroalgae) and possibly by reduced grazing pressure, while coral growth is reduced by temperatures which, for reef corals, are low.