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

Abstract

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.

Short-term impacts of salinity pulses on ionic ratios of the seagrasses Thalassia testudinum and Halodule wrightii

“…To maintain ambient seawater temperature, the aquaria were submerged in an outdoor flow-through system with pumped seawater that flowed around the aquaria and covered about ¾ of their height. In general, the ambient conditions of temperature, salinity and nutrients between the water and seagrass collection sites were similar (Stutes et al, 2007, Cebrian et al, 2013. The water in the aquaria was oxygenated with air pumps and seagrasses were exposed to the following salinity treatments for six (T. testudinum) and seven (H. wrightii) days: 10, 23 (ambient salinity and control treatment), 30, 40, 50 and 70 psu with three replicated aquaria per treatment.…”

Shallow infaunal responses to the Deepwater Horizon event: Implications for studying future oil spills

“…However, the pattern is unequivocal, however, for Neritina usnea, which we observed only at the oiled site, even after the reoiling event. Moreover, while previous information on N. usnea in seagrass beds is scarce, densities at the oiled site greatly exceed published abundances for other N. usnea populations in Halodule wrightii beds from the northern Gulf of Mexico (Cebrian et al, 2013). This suggests that attention should be directed at evaluating the mechanisms driving this snail's positive response.…”

Does Ocean Acidification Benefit Seagrasses in a Mesohaline Environment? A Mesocosm Experiment in the Northern Gulf of Mexico

“…Aquaria were arranged randomly within each block and covered with screen to prevent excess light stress (Fig. 1; Cebrian et al 2013). Seawater was pumped from the bay (1 m depth) into header tanks, from where it was channeled into the aquaria to overflow into surrounding water bath and released back into the bay.…”