Experimental manipulation of top-down and bottom-up factors in a Northern Australian tropical river

Background/Question/Methods

Although most ecologists agree that both top-down and bottom-up factors (consumers and resource limitation, respectively) influence biomass of primary producers, few studies experimentally examine both in concert. Here the relative importance of top-down and bottom-up factors was experimentally examined in two reaches in a 4^th order oligotrophic river in the Australian wet-dry tropics during base flow conditions. The top-down factor (i.e. macroconsumer grazing) was examined using split-stream, large scale exclusion cages. The bottom-up factor (i.e. nutrient limitation) was manipulated using nutrient diffusing substrates with control, low N and P and high N and P addition treatments. 

Results/Conclusions

Macroconsumer grazing was found to be important but varied depending on reach and substrate tested. Chlorophyll a biomass on natural rocky substrates within exclusion cages was significantly greater than in controls but only in the downstream reach. On the nutrient diffusing substrates chl a biomass was significantly greater within exclusion cages compared to controls at the upstream reach only. Nutrient addition at the high N and P level significantly increased chl a biomass at both reaches compared to low N and P and control treatments. Additionally, we used log response ratios as the metric for effect sizes to compare the top down and bottom up factors and found both to be positive but nutrient addition was greater at both reaches. The interaction effect was negative implying that the combined effect of nutrient addition and macrograzer exclusion was less than the product of the two main effects. Also, the macroinvertebrate assemblage significantly differed between exclusion cages and controls at both reaches and this difference was mostly due to a greater overall abundance of macroinvertebrates in the exclusion cages. The strong effect size of the nutrient addition is consistent with other freshwater studies but the sub-additive interaction effect is not commonly found in oligotrophic lotic systems and the mechanism for this needs further investigation.