Pelagic Biomass Production

Primary and secondary production, as generally conceived, correspond to Lenz's "classical food web" (Lcnz 1992). In fact, the food web in the open ocean can hardly be called "classical" from the standpoint of a terrestrial ecologist. Primary producers in oceanic food webs for the most part cannot root themselves into physical structure, and thus have not evolved into the massive and long-lived primary producers found in terrestrial ecosystems (angiosperms) and shallow marine environments (kelp and coral). Instead, primary production in the open ocean is dominated by microscopic algae and bacteria (Figures 16.7 and 16.8). Because differences in body size are accompanied by differences in longevity, turnover rate, buffering capacity, etc. (Peters 1983), the ecological scaling effects of dominance by microprodu-cers are substantial. The difference in body size of the primary producers in terrestrial and marine environments has been suggested to be one key area where terrestrial and marine ecosystems operate differently (Steele 1991). In the open ocean, primary producers exhibit a higher turnover rate than their terrestrial counterparts. They are grazed more heavily, and are found at lower densities than is usual for other ecosystems (C'yr and Pace 1993). Remarkably, the standing crop of primary producers in planktonic communities is often lower than that of primary consumers. Upwelling systems tend to be species-poor and dominated by relatively short food chains and a larger standing crop of primary producers, whereas tropical gyres arc species-rich, have longer food chains and have a lower primary producer to herbivore biomass ratio (Valiela 1984). Over large spatial and temporal distances, there is relatively high temporal resource predictability (i.e. at the metacommunity level), although the lack of small-scale predictability limits the ability of organisms to specialize. These generalists with highly

Herbivores

Primary producers

Secondary

Consumers

Microbial Loop

Primary consumers

Dissolved Organic V Malter >

Inorganic C, N,

Mesupeiagic Consumers

Oiganic aggregates

Consumers

Deadfall

Microbial Loop

Benthic Sediment

Scavengers and Detritivorcs

Atmospheric gases COt . O2 . DMS

Sea Surface

Figure 16.7 A simplified food web of the open ocean. "Real" oceanic food webs derive complexity from the broad diets of many species, often profound ontogenetic and seasonal shifts in diet, and the inherent cyclicity of the system. For example, the eggs or larvae of many fish become food for zooplankton, which themselves become food for the adult fish, and cannibalism by the higher trophic links (e.g. cod) on their young is not uncommon

10 nm lO'am

103im lO'Vm

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