Significance of microheterotrophs in the decomposition of phytoplankton: estimates of carbon and nitrogen flow based on the biomass of plankton communities

A study of the standing stocks of consumer organisms in waters off the western approaches to the English Channel during late August shows that the microheterotrophic community of protozoans and bacteria comprise as much as 46 % of the total consumer carbon at mixed water station (M), 60 % at frontal station (F) and 21 % at a deeper stratified water station (E5). Values for biomass have been combined with literature values for carbon budgets of representatives of each of the major components of the heterotrophic community to arrive at an estimate of carbon flow through the consumer assemblage as a whole. A depth-integrated carbon supply of 3.1 to 4.9 gC m-' d-' would be required to sustain the consumer community if all components were actively heterotrophic. An average gross primary production of 1000 g C m-' d-' would be capable of supporting the larger zooplankton and only 17 % of the bacteria in an actively heterotrophic state. In offshore or steady-state situations, therefore, carbon flow through the microheterotrophic community may be considerably lower than might be anticipated from their high biomass and relatively small cell size; this may account for the low values for bacterial production under natural conditions which are commonly reported in the literature. It is suggested that in coastal waters during late summer, heterotrophic activity of a greater proportion of the microheterotrophic community may be supported by consumption of both production and biomass of senescent phytoplankton cells; this may account for the rapid decline and dissipation of carbon at the end of phytoplankton blooms at this time of the year. Under these conditions, as much as 50 to 60 % of total carbon flow is estimated to enter the bacteria, 34 to 4 1 % being subsequently respired by the bacteria and a further 9 to 12 % by heterotrophic microflagellates. Nitrogen regeneration by the heterotrophic community as a whole during this phase may amount to as much as 2.8 to 5.3 mg N mP3 d-I or 63 to 72 % of the nitrogen entering the consumer community. This may be of significance in supporting further primary production after the decomposition phase of phytoplankton blooms in shallow water. The low carbon flow available to support the microheterotrophic community in offshore steady-state situations suggests, however, that nitrogen regeneration by bacteria and protozoans may be of less importance in offshore pelagic waters than in shallow coastal systems.