The Answer is b. First, we expect less C-13 in the fish tissue. Degraded systems tend to have too much phytoplankton because phytoplankton production is stimulated by the addition of nutrients from lawns, sewage plants and excess run-off (this is called eutrophication). Much of the algae sinks to the bottom of the wetland without being consumed, causing large amounts of respiration and sometimes low dissolved oxygen – called hypoxia – or no dissolved oxygen – called anoxia. The respired carbon is low in C-13 and is used by algae for growth, causing them to have depleted C-13 signatures. This signal is then transferred up the food chain to young fish.
Second, we expect more N-15 in the fish because the nitrogen associated with human waste is very high. Why? Apparently microbes associated with waste degradation cause the N-15 content to increase. This N-15 enriched nitrate is added from leaky septic systems and sewer plants and enters our rivers and lakes through groundwater and direct discharge. Once in the water body, it is used by growing phytoplankton. Once again, this heavy N-15 signal is then passed up the food chain to young fish.