Marine microbenthic community structure regulated by nitrogen loading and grazing pressure

Abstract

Generalisations on the combined effects of consumers and resources on autotrophs in aquatic food webs largely rely on freshwater studies. In this study, we tested these general concepts with marine benthic microalgae, which are important components of coastal food webs. We manipulated nitrogen availability and herbivore presence in a factorial field experiment in the Western Baltic Sea. Moreover, we investigated how herbivore control varied among 3 sites and 2 seasons and tested for trophic cascades by enhancing demersal fish density at 2 sites. Nitrogen availability and herbivore presence had strong and antagonistic effects on microalgal biomass, species composition and diversity. Herbivores significantly reduced algal biomass, whereas nutrient enrichment led to an increase in biomass. Herbivore effects on microalgal biomass increased with increasing nitrogen availability, indicating a functional response of herbivores to nutrient enrichment. The response of microalgae at the species level suggested a trade-off between nutrient use and grazing resistance which appeared to be linked to algal growth form. Compared to other growth forms, large erect species were most responsive to both nitrogen loading and herbivory. Grazing reduced microalgal diversity at low nutrient supply, but enhanced it at high nutrient supply. Herbivore effects varied considerably among different sites and were stronger in spring than in summer. Manipulations of fish density during summer did not have any effects on microalgal community structure. In conclusion, our results demonstrate that herbivores and nutrients have strong and balancing effects on marine microbenthic community structure.

Grazing and UV radiation effects on an Antarctic intertidal microalgal assemblage: a long-term field study

“…These findings are in agreement with previous studies on benthic diatoms from marine (e.g., Hillebrand et al 2000), and freshwater (Steinman 1996;Hill et al 1997), habitats. In our study, the absence of grazers generally favoured the abundance of the diatom genus Cocconeis.…”

“…In our experiment, as observed by others (e.g., Hillebrand et al 2000), some inconsistent cage artefacts were found; a higher biomass but a lower abundance in half cages compared with open ones at the second and forth sampling, respectively. These findings are most likely a consequence of the high natural variability in these types of fieldexperiments.…”

“…Another reason is the low biomass in general. Grazers were shown to be ineffective in low biomass environments (Hillebrand et al 2000). In contrast to other studies, grazers were not affected by UV radiation in our study (Bothwell et al 1994).…”

Not attackable or not crackable—How pre‐ and post‐attack defenses with different competition costs affect prey coexistence and population dynamics

“…However, species optimizing one functional trait commonly have to pay costs regarding other traits due to physiological, energetic, and genetic constraints (Stearns, 1989). Trade‐offs between defense and competitiveness have been frequently found in nature and may explain the high diversity of strategies along the gradient of being defended or highly competitive (Agrawal, 1998; Coley, Bryant, & Chapin, 1985; Hillebrand, Worm, & Lotze, 2000). Predation may enable coexistence of competing species facing a trade‐off between defense and competitiveness.…”

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

“…A decrease in cell number of microphytobenthic or epiphytic communities in the presence of invertebrate grazers is a well-known phenomenon which has been detected in numerous studies (Sommer 1997;Hillebrand et al 2000;Schanz et al 2002;Jaschinski et al 2008). In our experiment, both algal species were grazed to a similar degree by the studied grazers indicating that no active choice of food components took place.…”