Final answer:
The advanced search technique used to refine the results is Boolean operators.
Step-by-step explanation:
The advanced search technique being used to refine the results in the keyword search is Boolean operators. By using the operators 'AND', 'NOT', and 'OR', the search query is able to combine or exclude specific terms to generate more accurate and relevant results. In this case, 'ocean acidification AND plankton NOT coral' will retrieve information about ocean acidification and plankton, excluding any references to coral.
Learn more about Advanced search techniques in research