Many of the aquatic organisms are omnivorous, complicating application of the concept of trophic levels. Figure 15.15 shows a food web for a small lake with organisms or groups arranged by trophic level. This lake is managed for fish production and thus receives an artificial energy input. Notice the large loss to the system from the emergence of chironomid insects.
As has been mentioned, most of the nutrients in the photic zone of lacustrine and marine ecosystems tend to be tied up in living things. This would place a severe limitation on further growth except for the existence of an efficient detrital recycling process. The
process known as the microbial loop involves the rapid utilization by bacteria and protozoans of organic matter liberated as waste products by all of the other aquatic organisms (Figure 15.16). This loop cycles nutrients largely independently of the phytoplankton, enabling growth to continue in the summer when the algal blooms have disappeared. The waste products are organics excreted by zooplankton and fish.
Another important source of soluble organic matter for the microbial loop are the extracellular products of photosynthesis (ECPP). These are organics such as glycolic acid that are produced by photosynthesis and leaked continuously by phytoplankton. The leakage has been measured to average from 7 to 41% of the carbon fixation rate. Nutrients are also recycled by bacterial and fungal degradation of dead organisms. These mechanisms are very important in the low-productivity areas of oligotrophic lakes and in the open ocean.
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