Evidence of grazer control on nitrogen fixation by eelgrass epiphytes in a temperate coastal bay

Abstract

In this study, we present data to support the hypothesis that removal of epiphytes by grazers is an important control of nitrogen fixation in temperate seagrass meadows during the summer. Previous work in West Falmouth Harbor, Massachusetts, USA, found highest rates of epiphytic nitrogen fixation in the part of the harbor (Snug Harbor) with the greatest nitrogen load and the lowest phosphate concentrations, a somewhat paradoxical result suggesting that biogeochemical controls are not the major factor regulating this nitrogen fixation. Here we report that the density of invertebrate grazers on epiphytic algae (predominantly Bittiolum alternatum) was least in Snug Harbor, where nitrogen fixation rates were greatest. Reciprocal transplant experiments showed that seagrass shoots transplanted into Snug Harbor from the part of the harbor (Outer Harbor) where external nitrogen loading was lower but grazer densities were 4-fold higher, had a more than 5-fold increase in epiphytic nitrogen fixation after a 12 d incubation period. Shoots transplanted from Snug Harbor to Outer Harbor showed a large, rapid reduction in epiphytic nitrogen fixation rates after only 6 d, likely due to consumption of epiphytes. Our results suggest that trophic control is a potentially important determinant of epiphytic nitrogen fixation rates in temperate seagrass meadows.

Global Nitrogen Cycle: Critical Enzymes, Organisms, and Processes for Nitrogen Budgets and Dynamics

“…Another important control is phosphorus availability, which indirectly affects N 2 fixation by constraining organism growth. ,− ,− A high P requirement related to the large ATP demand of nitrogenase has been suggested to make N 2 -fixers more prone to P limitation than nonfixers . Inadequate moisture levels can also be problematic for terrestrial diazotrophs. ,, Finally, ecological interactions (e.g., grazing of diazotrophs, competition with nonfixers over nutrients and light, narrower range of growth conditions) are also important in explaining the distribution of diazotrophy in nature. ,,− Despite a variety of influencing factors, the dominant proximate control on diazotroph activity is fixed N availability (i.e., equivalent to supply minus demand). This dynamic is consistent with the genetic regulation, metabolic costs, and biological role of N 2 fixation.…”

Benthic nitrogen fixation in Zostera marina meadows in an upwelling-influenced coastal lagoon

“…As in the case of sediments, one of the factors stimulating fixation rates in the phyllosphere is labile C exuded by seagrasses, in this case through the leaves, since this C serves as substrate for diazotrophs (Capone et al 1979). The amount of N 2 -fixing epiphytes in the leaves can also determine the variation in fixation rates in the phyllosphere (Reynolds et al 2015). In this study we did not measure epiphyte biomass, but visually there were no differences in coverage between leaves 2 and 4 of the same shoot, nor between the leaves from different meadows.…”

“…Al igual que en los sedimentos, uno de los factores que estimulan las tasas de fijación en la filosfera es el C lábil exudado por los pastos marinos, en este caso a través de las hojas, ya que este C sirve como sustrato para los diazótrofos (Capone et al 1979). También la cantidad de epífitas fijadoras de N 2 en las hojas puede determinar la variación de las tasas de fijación en la filosfera (Reynolds et al 2015). Aunque en este trabajo no se midió la cantidad de epífitas, visualmente no se apreciaron diferencias en su cobertura entre las hojas 2 y 4 de un mismo haz, ni entre las hojas de diferentes praderas.…”

Ecosystem functioning of canopy and turf-forming algae: potential supply of invertebrate prey to pelagic consumers

“…Resource supply at the sampled habitats may well explain the differences observed on the abundance of these groups. Small gastropods, chiefly the very abundant genera Eulithidium Pilsbry and Bittiolum Cossmann, and specialized canopy-dwelling spider crabs are grazing animals that feed directly on the main algal host or on their epiphytic algae (Hines 1982, Reynolds et al 2015. Differently, decaying organic particles and interstitial meiofauna in turfretained sediments may be easily accessed by soft-bodied flatworms and polychaetes (e.g.…”

“…However, as very highly mobile organisms, amphipods tend to flee rather than engage in direct competition with more sedentary species (Neideman et al 2003). Conversely, gastropods (as grazers; Reynolds et al 2015) and ostracods (as deposit feeders; van Oevelen 2006) tend to remain near their foraging grounds, in the Sargassum case very likely their fronds where very small epiphytic algae and organic debris tend to accumulate as plants get older. Rather than optimizing the use of food resources, filter-feeding bivalves may become dominant because they may effectively clump to each other and remain attached to the substrate by different means, including the production of byssal threads (e.g.…”