The vertical distribution of plankton is described for 3 stations representative of stratified. frontal and vertically mixed regions of the western English Channel in summer. All components of organic carbon, representing dissolved and particulate organic carbon, phytoplankton, bacteria. protozoans, micro-and meso-zooplankton, were estimated independently. Major differences were found In the abundance and species composition of the phytoplankton, and in the relative proportions of different groups of heterotrophs. In the frontal region the phytoplankton (26.5 g C m-2) was composed of an essentially monospecific, surface population of the dinoflagellate Gj~odinium aureolum; by contrast, under well-stratified cond~tions small naked flagellates (0.42 g C m-2) forming a sub-surface chlorophyll maximum were dominant, and the tidally mixed waters were characterised by diatoms (7.91 g C m-'). At each station the estimated standing stock of heterotrophs was between 2.3 and 3.2 g C m-' , 10 to 30 % of which consisted of bacteria. Hence the phytoplankton was the dominant compartment in the frontal and mixed regions, whereas the zooplankton biomass considerably exceeded that of the phytoplankton in the well-stratified water. The ecological implications of these carbon distribution patterns are discussed.

Estimates of the numbers and biomass of bacteria as a function of depth in coastal and upwelling waters off the western approaches to the Engllsh Channel and in the southern Benguela upwelling region off the Cape Peninsula, South Africa, show that the numbers of bacteria are correlated with the standing stocks of phytoplankton as assessed by chlorophyll a concentration. Standing stocks of heterotrophic microflagellates in the size range 3 to 10 pm, amount to some 16.9 % on average, of bacterial standing stocks (mg C m-3) estimated by direct microscopy. Calculations of carbon flow through the microheterotrophic consumer community suggest that approximately 20 to 60 % of primary production, posslbly representing the dissolved components leaching out of, and lost from phytoplankton cells during zooplankton grazing, enters the microbial food chain. Much of this appears to be dissipated by bacteria, with some 5.2 to 8.1 % of the photoassimilated carbon being incorporated into bacterial carbon production. At least 66% of this is exploited by the heterotrophic n~icroflagellates leaving a maximum of 34 % of bacterial production for the larger bactivorous suspension feeders

Detritus from the dinoflagellates Scrippsiella (= Peridinurn) trochoidea and Isochrysis galbana and from the diatoms Skeletonema costaturn, Thalassiosira angstii and Chaetoceros tricornuturn incubated at 10 "C in seawater is colonised by a succession of micro-organisms. Primary microbial decomposers in the incubation experiments were bacterial rods and cocci which reached a peak standing stock carbon of 1.86 f 0.76 % of the carbon supplied to the incubation media by the third day of incubation. The bacteria were subsequently replaced by flagellates which attained a mean peak biomass of 12.5 f 3.58 % of the bacterial biomass by Day 6 before declining. Synchronous measurement of the utilisation of dissolved and particulate components from the incubation media shows that there is a well-defined initial sequence of aggregation of particulate matter to form bacterioparticulate complexes, much as have been recorded for natural waters. During this phase, carbon is mainly utilised from the dissolved component of phytoplankton cell debris, whilst the more refractory components including particulate debris is used more slowly. The dissolved organic component comprises a mean of 34 24 % of the total carbon in the debris and has a 50 % utilisation time of only 1.56 d (37.44 h), whereas the particulate component comprises 65 76 % of the total carbon and has a 50 % utilisation time of as much as 11.56 d (277.4 h). Bacterial carbon conversion efficiency (bacterial carbon/detrital carbon used X 100) during the initial phases of colonisation is 9.8 Sb -a value similar to that recorded for bacterial conversion of dissolved components of macrophyte debris. The results suggest that the carbon conversion budget for the decomposition of phytoplankton cell debris is 100 g carbon yielding 4 -644 g of bacterial carbon. This value for incorporation of carbon into bacteria from phytoplankton cell debris is much lower than might be anticipated from the absorption efficiency of selected labile components released in small quantities by living phytoplankton. The carbon conversion budget for whole phytoplankton debris thus suggests that as much as 30.8 % of the carbon is mineralised and returned to the environment within 3 d by the bacteria which initially colonise the material, whereas the more refractory 64.4 %, comprising carbon in the particulate components of the cell debris, is mineralised within approximately 11 d by bacteria characteristically associated with the decomposition phase of a phytoplankton bloom.

No abstract

The micro-organisms which colonise seawater incubated with mucilage from the kelps Ecklonia maxima or Laminaria pallida show a clear succession. The media are first colonised by bacterial cocci followed by rods which are subsequently replaced by flagellates and ciliates whose combined biomass reaches some 6-10 % of that of the bacteria. The maximal biomass of bacteria is achieved in 7-10 d incubation at 10 "C but is dependent both on the time of appearance and biomass of the flagellate and ciliate populations. Estimates for the rate of consumption of bacteria by flagellates of only 10 pm3 body volume suggest that mineralisation of bacteria by marine microflagellates may considerably exceed that in larger organisms at higher trophic levels.