Photosynthesis-irradiance relationships of epilithic algae measured in the laboratory and in situ

Abstract

Complementary laboratory and in situ techniques were developed to measure photosynthesis-irradiance relationships of epilithic algae in the littoral zone of an oligotrophic lake in the Experimental Lakes Area, northwestern Ontario, Canada. SCUBA divers placed colonized granite plates in acrylic chambers and returned them to a nearby laboratory. Changes in dissolved inorganic carbon (DIC) in water overlying the benthic sample were monitored in response to a gradient in intensity of artificial light. The field technique isolated natural communities on rock, and attenuated available light using diffusers. In both methods, dark chambers were monitored in parallel.

Both laboratory and in situ observations conformed well to Michaelis-Menten description of the dependence of photosynthesis upon irradiance (r²>0.94) permitting estimation of the maximum rate of photosynthesis ($C_{\max }^{gross}$) and the half-saturation constant (I_0.5). Carbon-14 uptake estimates of ($C_{\max }^{gross}$) and (I_0.5) were 85% (±5%, SD) and 141% (±30%, SD) respectively, of corresponding DIC uptake estimates. In the light ¹⁴C values were significantly different from and intermediate between net and gross DIC uptake (P<0.001). The discrepancy between ¹⁴C and net DIC uptake decreased loglinearly with increasing irradiance (r = −0.76, P<0.001). Calculated depth profiles indicated substantial ¹⁴C uptake below the DIC-predicted compensation depth.