What are the two ways that harmful algal blooms can negatively impact an ecosystem?

The two harmful ways are:

Local asphyxia of the environment (impact on marine biodiversity): The algae would constitute a sufficiently thick film to prevent the light from penetrating and thus limit the photosynthetic activity of the vegetation during a period of growth. The algae would therefore limit the extension of salt meadows. In addition, algae are degraded by aerobic bacteria causing significant oxygen consumption and the production of sulfur compounds (hydrogen sulphide in particular) resulting in a decrease in the biodiversity of the marine marsh. It can no longer play its purifying role, thus favoring the arrival of other pollutants in the marine environment.

Impacts on fauna (for example, birdlife): During periods of green tides, ulva cover areas exploited by birds in search of food. For example, passerines may have less access to insects usually found in the marine marsh and waders with burrowing shells and worms buried in the sand.

poor water quality affects most estuarine organisms, including commercially important fish and shellfish. the pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses.