Effects of grazing and fertilization on epiphyte growth dynamics under moderately eutrophic conditions: implications for grazing rate estimates

The effects of grazing and nutrients on epiphyte biomass in seagrass beds have received much attention, yet less is known about effects on other metrics of epiphyte growth dynamics, such as epiphyte productivity and biomass turnover rates. To help address this gap, we present here a number of mesocosm experiments in which we manipulate grazer presence and nutrient concentrations under initially moderate eutrophic conditions. We also examine the potential bias contained in estimates of epiphyte grazing that disregard the effects of grazing on epiphyte productivity. The effects of grazing and nutrient enrichment on the epiphyte growth metrics examined were disparate. Namely, the effects of grazing on epiphyte biomass predominated over those of nutrient enrichment, but grazing and nutrients had similar importance as controls of epiphyte productivity and biomass turnover rates. The results illustrate that the effects of environmental and biological factors on one given metric of epiphyte growth dynamics can be quite different from those on other metrics. Thus, we suggest that the biomass-centered view mostly used to date in studies of epiphyte dynamics should perhaps shift toward a broader approach including other metrics of epiphyte growth, such as productivity and turnover rates, to better understand epiphyte dynamics in coastal systems and cascading functional consequences in the system. Another risk of biomass-centered studies lies in the calculation of grazing rates since, as also shown here, estimates of grazing rates derived as the difference in biomass accrual rates between non-grazed and grazed conditions can be highly biased.