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Not sure if I'm reading this right but from the meetings I've been to on this topic. When Pennsylvania gets heavy rain in the Susquehanna basin this washes large amounts of nitrogent and phosphorus from fertilizers into the bay. This increase in nitrogent and phosphorus, called "nutrients", causing algal blooms which reduces sunlight in the water causing a reduction in the amount of oxygen in the water. This reduction in oxygen causes fish kills and reduces the amount of plant life. You will often see posting about dead zones in the Chesapeake caused lack of oxygen. Often getting worse in the summer as temperature also comes into play.
One problem being that the seasonal rains and heavy fertilizer use both occur in the spring. Since 1987 this has been a focus area for bay protection. Actions such as bans on detergents with phosphorus in them and improved sewage treatment are helping but it is still a big issue.
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