Epiphyte load and seagrass performance are decoupled in an estuary with low eutrophication risk

Seagrass can decline due to blooms of algae following either loss of top-down control or increase in bottom-up nutrient delivery. Macroalgae negatively affect seagrass especially at high ratios of algal biomass to seagrass size. In contrast, for microalgae, epiphyte loads that affect seagrass performance are not well synthesized. Competition has been demonstrated primarily following nutrient or food web manipulations, which increase epiphyte load from background levels, rather than through experimental removal of epiphytes. Although the Willapa Bay (Washington, USA) is an estuary with low eutrophication risk, microalgae reach large biomass in summer, up to 60% of the mass of their intertidal seagrass host (eelgrass, Zostera marina). Based on a novel field technique, experimental reduction of epiphytes did not accelerate growth (g g(-1) d(-1)) of Z. marina. Neither did two other manipulations that were expected to improve resource access: removing nearby shoots or older, epiphytized leaves. Instead, removing older leaves slowed growth from 2.3% d(-1) to 1.8% d(-1), and removing lightly-epiphytized younger leaves slowed growth to 1.7% d(-1). To evaluate top-down and bottom-up causes of epiphyte load, two widespread methods were used. Observationally over time, epiphyte load was positively correlated with eelgrass leaf emergence rate and unrelated to mesograzer density; however, a crossed experiment reducing crustacean mesograzers and adding nutrients indicated top-down control, since chlorophyll-a in epiphytes increased with grazer reduction and no added nutrients (deterrent x nutrient interaction). At current epiphyte loads in the Willapa Bay, intertidal Z. marina tolerates seasonally-abundant epiphytes coinciding with rapid leaf emergence, thus producing fresh leaves where microalgal competition cannot be detected. (C) 2016 Published by Elsevier B.V.