The primary effect of acid rain on freshwater ecosystems is the alteration of pH levels, leading to detrimental changes in water chemistry. This can harm aquatic life, especially fish and other organisms that are sensitive to pH fluctuations. Acid rain can lead to the leaching of toxic metals like aluminum into the water, further harming species and disrupting entire food webs. Ultimately, the damage to freshwater ecosystems can reduce biodiversity and affect the health of surrounding environments and human communities relying on these water bodies.