Diatom assemblages are controlled by light attenuation in oligotrophic and mesotrophic lakes in northern Ontario (Canada)

Abstract

Diatom communities are composed of three distinct water-depth-associated assemblages: a nearshore benthic assemblage, a deep-water benthic assemblage, and a primarily planktonic assemblage. Light attenuation with depth has been hypothesized to be the factor that controls benthic-to-planktonic (B:P) assemblage transitions. Previous studies, however, could not distinguish between the individual effects of light and temperature on diatom assemblage transition because the depth of the B:P transition occurred at or near the depth of the thermocline. This has caused confusion regarding which factor is the more influential variable on benthic-planktonic diatom assemblage transition in surface sediments along water-depth transects in lakes. Two boreal lakes in northeast Ontario were selected to examine this problem. The study lakes are sufficiently deep and clear (Secchi disk depths ~ 9–10 m; dissolved organic carbon ~ 3 mg/L), making it possible to distinguish between the depth of 1% light penetration and the depth of the thermocline. Surface sediment samples were collected along water-depth transects from these lakes to examine intra-lake diatom assemblage structure. In both lakes, the water depth of the diatom assemblage B:P transition was closely associated with the depth of the ~ 1% photosynthetically active radiation value, and was well below the depth of the thermocline, suggesting that light penetration has more influence on this transition than does the thermocline. Because temporal fluxes in nutrients and DOC concentrations can attenuate light, understanding how the light regime influences benthic diatoms in lakes can help inform interpretations of diatom assemblage changes in paleolimnological studies.