What is the name of a survey technique which counts numbers of indicator species in aquatic ecosystems, to give an idea of how polluted the water has become?

The survey technique you are referring to is known as biological monitoring or bioindication. Specifically, when counting the numbers of indicator species in aquatic ecosystems, it can also be termed biodiversity assessment or biological assessment.

This method involves analyzing the presence and abundance of certain species that are sensitive to pollution, such as certain macroinvertebrates, fish, and aquatic plants. The rationale is that certain species are more tolerant or sensitive to pollutants, making them good indicators of ecological health. For example, a high number of pollution-sensitive species might suggest cleaner water, while a predominance of pollution-tolerant species could indicate degraded conditions.

The use of indicator species allows environmental scientists and ecologists to assess the health of aquatic ecosystems and the effectiveness of pollution control measures without needing to analyze chemical parameters directly, thus providing a more comprehensive ecological perspective.