THE INFLUENCE OF PHOTOINHIBITION AND ALGAL SIZE ON VERTICAL VARIATIONS IN CHLOROPHYLL-A SPECIFIC PHOTOSYNTHESIS

Rates of chlorophyll-a specific photosynthesis were measured at 0.5 me ter and 3.0 meter depths within Lake Erken (60 degrees N, Central Swed en) during the summer when the lake was seasonally stratified, and the upper meters of the water column were often diurnally stratified. The surface Values of both light limited (alpha(B)) and light saturated ( P(B)max) photosynthesis consistently decreased to 30-40% of that measu red at 3.0 meters. Two possible explanations for the decline in surfac e photosynthesis are possible: Photoinhibition by high surface irradia nces. Migration of large colonial cyanobacteria, especially Gloeotrich ia echinulata, to the surface. Because of their large size (100 - 2000 mu m) rates of chlorophyll-a specific photosynthesis are reduced, so that an increase in G. echinulata numbers will lead to decreases in wh ole water estimates of photosynthesis. To separate the effects of thes e two processes additional measurements were made on samples separated by filtration through a 90 mu m net which effectively removed the G. echinulata colonies from the remaining phytoplankton. In both of the s ize fractions declines in surface photosynthesis of up to 50% were fou nd which could be attributed to photoinhibition. The chlorophyll-a spe cific rates of photosynthesis in the > 90 mu m size fraction were cons istently lower than the < 90 mu m size fraction. Measurements made dur ing a G. echinulata bloom suggest that size related reductions in surf ace photosynthesis on the order of 30% can be expected.