In still lakes, microscopic algae called diatoms sink and settle in fine layers on the bed, and different diatom species flourish at different water temperatures, so a core of stacked layers records how a lake’s temperature shifted over time. Cores drilled from two nearby lakes, Turl and Vane, span the same past century. Turl’s core shows its diatom community changing steadily from cold-water species to warm-water species across that span. Vane’s core — from a lake fed by a deep spring whose outflow stays near-constant in temperature the year round — shows almost no change in its diatom species over the same layers. Because the two lakes sit in one regional climate and differ chiefly in that Vane is spring-fed while Turl is not, the steady diatom record in Vane most likely reflects ______
Which choice most logically completes the text?
