Comparison of light compensation points for two submersed macrophytes

A laboratory growth experiment indicated that the ribbon-leaved Potamogeton gramineus L. had a lower light compensation point for growth (LCP) than the thread-leaved P. pectinatus L. LCP and 95% confidence intervals (CI) were 0.86 M m(-2) day(-1) (95% CI, -0.006 to 1.540) and 3.87 M m(-2) day(-1) (95% CI, 3.12 to 4.63), respectively. When germinated propagules of each species were planted in a P. gramineus stand in the Byrnes Canal, P. pectinatus exhibited lower survival than similar plants placed in the canal but outside the P. gramineus bed. Mean plant weight for P. pectinatus grown within the plant bed was less than 10% of that for plants grown outside the P. gramineus bed. Neither survival nor mean dry weight of P. gramineus planted at the same sites differed among the sites. Estimates of light levels in the P. gramineus bed indicated that shading by P. gramineus reduced light to levels at or below the LCP for P. pectinatus but above the LCP for P. gramineus. These results suggest that P. pectinatus may be displaced from areas inhabited by P. gramineus due to reduced light availability.