Control of Microbenthic Communities by Grazing and Nutrient Supply

Abstract

In periphyton communities, autotrophic algae and prokaryotes live in close spatial proximity to heterotrophic components such as bacteria and micro-and meiofauna. In factorial field experiments, we manipulated grazer access and nutrient supply to periphyton communities and measured the effects on algal, ciliate, meiofaunal, and bacterial biomass. We tested whether grazing macrozoobenthos affects all periphytic components (generalist consumption), whether nutrient effects propagate through the community, and whether interactions between the different periphyton groups allow for indirect feedback mechanisms. The experiments were conducted during three different seasons in a mesoeutrophic lake in Sweden (Lake Erken) and at an adjacent coastal marine site (Väddö ) of similar productivity, but with contrasting grazer fauna. We found strong direct effects of nutrients and grazing on algae at both sites. Algal biomass increased in fertilized treatments and was significantly reduced when grazers were present. The algae clearly dominated the system quantitatively and were positively correlated to the biomass of ciliates and meiofauna. The effects of grazing and nutrients were more complex for heterotrophs than for algae. Generally, the presence of grazers tended to increase the biomass of bacteria, ciliates, and meiofauna. Thus, macrograzers were not generalist consumers of the entire community, but mainly reduced algae. Furthermore, the results suggested strong indirect effects of grazing, presumably through changes in nutrient supply and algal size structure. Nutrient enrichment had weak and inconsistent effects on bacterial, ciliate, and meiofaunal biomass. There was thus no complete propagation of bottom-up effects through the community, and strong internal feedback mechanisms within the periphyton mediated the effects of macroconsumers and nutrient enrichment.

Top-down and bottom-up control of infauna varies across the saltmarsh landscape

“…In addition to numerical responses, the body size of individual infauna may increase in response to nutrient addition (Posey et al, 2006). There have been fewer studies of nutrient addition effects on meiofauna but changes in community composition are more common than large changes in biomass or abundance (Widbom and Elmgren, 1988;Hillebrand et al, 2002). We found no fertilization effects on infauna at the extremes of the inundation gradient (i.e., mudflat and stunted S. alterniflora habitats) (Fig.…”

“…Interactions in salt marshes between fertilization and predator removal have been observed by (Posey et al, 2006) in a mudflat location for haustoriid amphipods, and our related work in PIE suggests that talitrid amphipods at the marsh edge and sediment algae respond to these treatments in a non-additive fashion (Deegan et al, 2007). Other studies (Foreman et al, 1995;Posey et al, 1999;Hillebrand et al, 2002) found no evidence for interactions between nutrient addition and predator reduction on infauna. Trophic cascades mediated by infauna on sediment algae were also not apparent from our study or work by Posey et al (1995; suggesting infauna are weak interactors with sediment algae, and that trait-mediated effects associated with top-down factors are functionally similar throughout the inundation gradient.…”

Effects of snails, submerged plants and their coexistence on eutrophication in aquatic ecosystems

“…While it has been suggested by Hillebrand et al (2002) that snails can increase nutrients levels and stimulate planktonic algae growth with the products of their metabolism and by enhancing release of sediment nutrient, other authors report a negative influence on nutrients and planktonic algae levels. The mechanism proposed by Wei and Pu (1999) was flocculation, while Han et al (2010) found that in rich planktonic algae conditions B. aeruginosa could adopt a filter feeding behaviour, and thereby make a notable impact on both biomass and nutrient levels.…”

Altered complementary feeding strategies of the consumers Hydrobia ulvae and Idotea emarginata via passive selectivity

“…This is not surprising since benthic microalgae are considered to be the major food source for benthic consumers in the euphotic zone (Sumner and McIntire 1982;Underwood and Thomas 1990;Hillebrand et al 2002) and thus, their abundance and species composition are crucial factors for studies in aquatic community ecology.…”

“…Complementary foraging strategies and selective resource use of competitors that allow coexistence are aspects which are gaining recognition for studies on grazer-microalgae interaction studies (Fenchel 1975;Hillebrand et al 2002;Aberle et al 2005). Wilson et al (1999) demonstrated that complementary feeding facilitates coexistence and that this is an aspect which commonly regulates consumer-resource systems.…”