Abstract. The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, inc1uding cyanobacteria, and Archaea can utilize dinitrogen (N2) as an alternative N source. In the marine environment, these microorganisms can have profound effects on net community production processes and can impact the coupling of C-N-P cyc1es as weil as the net oceanic sequestration of atmospheric carbon dioxide. As one component of an integrated 'Nitrogen Transport and Transformations' project, we have begun to re-assess our understanding of (I) the biotic sources and rates of N2 fixation in the world's oceans, (2) the major controls on rates of oceanic N2 fixation, (3) the significance of this N2 fixation for the global carbon cyc1e and (4) the role of human activities in the alteration of oceanic N2 fixation. Preliminary resuIts indicate that rates of N2 fixation, especially in subtropical and tropical open oeean habitats, have a major role in the global marine N budget. Iron (Fe) bioavailability appears to be an important control and is, therefore, critical in extrapolation to global rates of N2 fixation. Anthropogenic perturbations may alter N2 fixation in coastal environments through habitat destruetion and eutrophication, and open ocean N2 fixation may be enhaneed by warming and inereased stratification of the upper water column. Global anthropogenie and c1imatie ehanges mayaiso affeet N2 fixation rates, for example by altering dust inputs (i.e. Fe) or by expansion of subtropieal boundaries. Some recent estimates of global ocean N2 fixation are in the range of 100--200 Tg N (1-2 x 10 14 g N) yr-I , but have large uncertainties. These estimates are nearly an order of magnitude greater than historieal, pre-I 980 estimates, but approach modern estimates of oceanic denitrification. 48

A conceptual basis is established to estimate the duration of cell cycle phases from a partially synchronized population. This information, together with the time course of cell cycle phase fractions, can be applied to estimate species-specific, in situ growth rates of phytoplankton. The model adopts the paired-nuclei method of McDuff & Chisholm (1982) with the substitution of the sum of S, G2, and M phases of the cell cycle as the terminal event. Quantitative fluorescence microscopy is used to measure unicellular DNA contents through a diel cycle. The model has the advantage of avoiding bottle incubations and allows the measurement of species-specific growth rates.

Abundance and composition of tintinnid and phytoplankton species were followed in central Long Island Sound from August 1979 to October 1980. In all, 28 tintinnid species were observed; the greatest diversity occurred between September and April. Highest tintinnid concentrations occurred in summer, with concentrations of 103 or more individuals I-' observed only when nanophytoplankton concentrations equalled or exceeded 1.3 X 105 cells I-'. Although necessary, the occurrence of small food, alone, was not a sufficient condition for high tintinnid densities. Tintinnids in central Long Island Sound exhibited the same order of magnitude yearly community ingestion rates as did the copepods. The tintinnids were responsible for removing approximately 27 % of the annual primary production from thls region. It is concluded that tintinnids are a n integral part of the Long Island Sound plankton community, equal in importance to copepods.

Natural populations of the planktonic cyanobacteria Synechococcus spp. exhibit a diel pattern in the frequency of dividing cells (FDC). Based on this observation, we tested the applicability of the FDC technique for estimating growth rate of a population grown on a IighWdark cycle. In laboratory experiments, the phycoerythrin-containing strains WH7803, WH8012, WH8107, and WH8108 were grown under various light and temperature conditions and the duration of division, td, a parameter necessary for the calculation of growth rate, was determined to be 3 h. Using this td value, growth rates of oceanic populations exhibiting diel patterns in FDC were found to range from 0.42 to 0.86 d-l, and these values exceed previous estimates of phytoplankton growth rates in oligotrophic regions. In experiments conducted at a Sargasso Sea station, growth rates calculated by the FDC technique were in close agreement with calculated instantaneous growth rates. In all cases, growth rates were higher for samplesincubated at surface light intensities than at 1 % surface light intensities. Using this technique under appropriate conditions, strain-specific growth rates can b e determined.

The 2 tropical cyanobactenal specles Trichodesmium thiebautii and 7: erythraeum had simlar photosynthetic characteristics in the southwestern Sargasso Sea and Canbbean Sea, with mean rates of hght saturated photosynthesis (uslng O2 electrode) of 42 (SD = 21.3) and 37 (SD = 18.4) mg 0, mg chl a -' h-' at 1410 FE m-2 S-', respectively over a 1300 n mlle cruise track. Rates of dark respiration were high, and the compensation point for both specles was 150 FE m-2 S-' (ca 55 m, m d d a y ) . Estimates of carbon doubllng times (using photosynthetic quotient) were from 3.0 to 3.8 d based on expected photosynthetic rates in the water column. The mean rate of nltrogenase actlvlty at 300 pE m-2 S-' by T thiebautu averaged 0.45 nmol ethylene colony-' h -' , 1.6 times that of 7: erythraeum (p < 0.01) as observed from samples collected on 3 cruises (64 paired observations). Furthermore, in a comparison of nltrogenase actlvitles, at light intensities between ca 500 and 2500 FE m-2 S-', 7: thiebautii was about t w~c e as actlve as T erythraeum. The phycoerythrin content of 7: erythraeum averaged 260 ng colony-', 4.4 times that of T thiebautii, and the mean PE : chl a ratios were 3.2 and 1.2, respectivelyOther pigments: (p-carotene, zeaxanthin, myxoxanthophyll, echinenone, and trace pigments) were s i d a r between the 2 specles. The organization of subcellular inclusions was distinctly different in these 2 species. The high abundance of T thiebautii relative to T erythraeum in many tropical seas may be due to higher rates of N2 fixation and a previously reported neurotoxin in the former specles.