Here is a food web for Lake Michigan:
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- What would happen if there was a change in the population size for any one of the producers (either an increase or a decrease)? How could these changes impact other producers and organisms on other trophic levels?
I had to edit your multiple questions down to the single related set seen above. The link you provide is a PDF file showing a food web for Lake Michigan. There are different levels of trophic groups shown including producers (phytoplankton), herbivores (zooplankton and macroinvertebrates), primary predators (foraging fish), and secondary predators (fish eaters). In terms of identifying the most important producer, the second page of the PDF file says that green algae are the most important producers in the summer food web so they would be the best choice. If there was a change in the population size of any of the producers it would have a large affect on the entire food web. A lowering of one of the producer populations would put a strain on the food supply for the rest of the consumer population in the web. This could also have the effect of allowing one or more of the other producers to increase in size due to lesser competition.
In terms of human impact, humans can have a huge affect on lake ecosystems and their food webs. If humans are putting large amounts of organic or mineral wastes into the lake water, this can cause algae blooms since they feed off this waste. A surge in algae can deplete the water of oxygen too fast and put a major stress on fish populations. Also, if humans over fish the waters for sport, they can remove important links in the food web, thus throwing the entire cycle into imbalance.