Some of these impacts include harmful algal blooms and fish kills. Outbreaks of the toxic microorganism Pfiesteria piscicida have been linked to excessive nutrients in the affected waters. Pathogens threaten human health and cause beach closures. The most frequent sources of disease-causing microorganisms, or pathogens, are sewage overflows, polluted storm water runoff, sewage treatment plant malfunctions, boating wastes, and malfunctioning septic systems. Swimming in or ingesting waters contaminated with pathogens can result in human health problems such as sore throat, gastroenteritis, or even meningitis or encephalitis.
Pathogens can also contaminate shellfish beds.
Land development activities within the coastal watershed can have serious consequences for beaches. Clearing of land for obtaining timber, the construction of homes and roads, and other development projects completed without properly revegetating the area can lead to excessive sediments being washed into the system. These sediments are deposited downstream along coastal shorelines. Not only do excessive sediments muddy the water and make it unappealing to swim in, but they can also carry excessive nutrients and pesticides, causing water quality problems. Land development directly on the beach can destroy the dune and beach ecosystem and increase erosion of beaches.