FIG. 2. Trophic flows in the Peruvian ecosystem, averaging the periods 1964-1971 and 1973-1981. The size of the boxes is proportional to the biomass of the corresponding components in the models, if the boxes are imagined as cubes instead of squares. Boxes are arranged along the vertical axis according to their trophic level, defined as 1 for primary producers and detritus and, for consumers, as 1 plus the mean trophic level of the prey items, weighted according to their fraction in the consumer's total diet. Flows of wet mass are in units of 103kg・km-2・yr-1. Flows leave boxes on the upper half and enter them on the lower half. The width of the line indicates the order of magnitude of trophic flows. Flows of at least 10kg・km-2・yr-1 were rounded to integers, those less than that, to one digit. Trophic flows of < 0.01% of the total consumption in the system, corresponding to 〜3×103kg・km-2・yr-1 during 1964-1971 and to 〜0.9×103kg・km-2・yr-1 during 1973-1981, were omitted for clarity. Biomass of detritus is a rough estimate.
the ecosystems' characteristics they are based on is given in Table 2.
RESULTS
Trophic flow diagrams
Examples of trophic flow diagrams are given for the Peruvian upwelling ecosystem for the two different regimes in Fig. 2. While primary producers and detritus are situated on trophic level 1 by definition, the planktonic and benthic invertebrate groups were located at trophic levels 2-2.5. The planktivorous fish were situated only fractions of a trophic level higher than the invertebrate consumers as a consequence of their mixed diets. Anchovy, sardine, and the "other pelagics" occupied trophic levels 2.5-3, and the predatory fish, as well as marine birds and mammals, were placed between trophic levels 3 and 4. Large scombrids, mammals, and birds were the top predators in the system The fishery was situated at trophic level 3.5 during the earlier period, acting almost as pure anchovy predator and on trophic level 3.7 during the later period, when sardine and hake became more important target groups.
Quite unsurprisingly for pelagic ecosystems, the major biomass flows (i.e., the largest flows entering the compartments) occurred among the planktonic invertebrates. Other important flows in the Peruvian ecosystem comprised anchovy and the benthic invertebrates, as well as sardine during the later regime. Flows towards anchovy were reduced by a factor of > 4 between the two periods, while. those to sardine increased by a factor of 7 towards a similar order as those to anchovy.
