PATTERNS OF ALGAL BIOMASS IN DOMINANT ODD-LINK VS EVEN-LINK LAKE ECOSYSTEMS

Foodchain models predict strong positive response of algal biomass to total phosphorus (TP) in odd-link ecosystems where grazers are controlled by planktivores and no significant response of algal biomass in even-link ecosystems where grazers are released from planktivore predation. On the other hand, resource- or ratio-dependent models predict proportional increase in algal biomass with increasing TP. These predictions were tested with field data from a large number of north temperate lake ecosystems exhibiting a wide range of TP. Instead of using the traditional approach of the number of trophic links present in an ecosystem, I used the presence vs. absence or near absence of large Daphnia and planktivorous fish to indicate the functional dominance of odd vs. even links. Results showed that algal biomass response to TP is stronger in dominant odd-link ecosystems than in dominant even-link ecosystems, which is consistent with the predictions of foodchain models. The significant positive slope of the relationship of algal biomass to total phosphorus for dominant even-link ecosystems is not consistent with foodchain models. In addition, a nonlinear response pattern of algal biomass to total phosphorus appears to be inconsistent with both foodchain and ratio-dependent models. Results also showed that even-link ecosystems produce proportionately lower algal biomass, demonstrating that grazers can modify ratio dependence of algal biomass. Overall, algal biomass appears to be both predator and ratio dependent because grazers are capable of modifying algal response to nutrients, and algae show positive response to nutrients even in the presence of large grazers.