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Recent reports suggest that the potential phototrophic role of plastidic ciliates in marine ecosystems may be considerable. A critical review of the literature demonstrates some confusion surrounding acceptance of the trophic position of even a well-established example of a photosynthetic c~liate, Mesodinium rubrum. Despite good evidence of obligate phototrophy from bloom studies, this species has, until recently, been omitted from the majority of routine phytoplankton counts, and has either been assigned to the microzooplankton or completely overlooked. Moreover, problems involved with sampling, enumeration and estimates of productivity for M. rubrum are also highlighted from the literature. These principally result from extremes of fragility, motility and vertical aggregation, which are commonly noted for this ciliate. Several recent studies, which have minimized some of these sampling problems and grouped the microplankton into more meaningful ecological categories, suggest that M. rubrum has an extremely widespread distribution and can be a very significant member of the phytoplankton. The combination of trophic and methodological difficulties appear to have compounded a serious underestimation of the contribution of M. rubrum to the primary productivity of coastal, estuarine and upwelling ecosystems, during both bloom, and perhaps more significantly, non-bloom conditions.

) decreased markedly from west to east, with minima in NO 3 2 and NH 4 1 in surface BS-CB waters, but relatively invariant urea-N concentrations across the entire region. In the BS-CB domain, low uptake rates of nitrate (qNO 32 ) and ammonium (qNH 4 1 ) were exceeded by uptake of urea (qUrea-N). Whereas average qNO 3 2 was highest in the BE-CH domain, qUrea-N was maximal in BB-LS. Average depth-integrated f-ratios ranged from 0.27 in the BS-CB domain to 0.57 in BE-CH, while chlorophyll a (chl a) and primary productivity (qC) were highest in BE-CH and BB-LS, and consistently low in the BS-CB domain. The >5 mm phytoplankton fraction dominated qC and qNO 3 2 in the BE-CH and CAA domains, whereas ESNP and BS-CB were dominated by the <5 mm fraction. In the BB-LS domain, the larger cells were responsible for $50% of qC, qNO 3 2 , and qUrea-N. This study highlights the contrast in ice-corrected average new production between the BE-CH (396 mg C m 22 d 21 ) and BS-CB (5.50 mg C m 22 d 21) domains in summer, and the larger contribution of urea-N uptake to total N uptake in central and eastern regions where NO 3 2 concentrations were lower.