As shown in figure 4-16 the energy bound in phytoplankton biomass can be 

 utilized in five ways. Some is utilized by the plants themselves for growth 

 and respiration. (This is indicated by a heat sink symbol in the figure.) 

 The four other pathways of utilization of phytoplankton energy result in 

 either direct or indirect energy transfers to higher trophic levels. Many 

 plant cells are grazed directly by zooplankton, filter-feeding benthic 

 organisms, and pelagic fishes. Some cells die and enter the pool of suspended 

 organic material. 



Zooplankton feed on living phytoplankton and suspended organic matter and its 

 associated bacteria and, in turn, are devoured by certain pelagic fishes and 

 filter-feeding benthic invertebrates. Through zooplankton death and normal 

 metabolism these organisms, as well as all groups of consumers, recycle 

 nutrients and contribute to the pools of nonliving organic material. 



The top consumers of the pelagic system are the fishes and squid, that feed on 

 one another as well as on species more closely associated with the bottom. 

 These mobile carnivores import or export energy to or from the system by their 

 migratory habits, and some of their biomass is removed from the system by 

 fishing. 



The suspended organic pool, with contributions from other systems, is utilized 

 by bacteria, which remineralize much of the nutrients, and by benthic filter 

 feeders. Another portion settles to the bottom, where it enters another part 

 of the benthic food chain. 



Much of the gross productivity of macroalgae is consumed by the macroalgae 

 themselves via respiration. At low light levels, or in dense populations with 

 little turnover, this value may be large. Net production in excess of 

 respiratory demands may provide considerable food for higher trophic levels. 



Benthic microalgae, including diatoms, in addition to small macroalgae, are 

 often directly consumed by grazers (such as various snails, limpets, and sea 

 urchins) and fishes. Large plants, especially their basal portions, also are 

 eaten directly. Generally, it is thought that the direct ingestion of live, 

 attached macroalgae accounts for a relatively small proportion of macroalgal 

 consumption. 



Marine macroalgae also may leach large amounts of dissolved organic matter 

 into surrounding waters. Although the extent of this process has not been 

 completely determined yet, the release of dissolved organic matter appears to 

 be greatest in plants under stress conditions (e.g., desiccation and 

 freezing). On a long-term basis, this might account for losses on the order 

 of a few percent of net production. The epiphytic bacterial, algal, and 

 invertebrate populations present on the surface of macroalgae are exposed to 

 high concentrations of these organic materials as they are released and 

 presumably use them as energy sources. Much of the dissolved organic matter 

 that escapes epiphytic consumption may be consumed by the high populations of 

 marine bacteria in coastal areas. 



Most of the net production of marine macroalgae only becomes available to 

 higher trophic levels after plants have been dislodged from their substrata by 

 grazing, ice scouring, or storms, or have totally or partially decomposed. 

 Although some of this plant material also becomes available for direct 



4-47 



10-80 



