Biotic Habitat Complexity Controls Species Diversity and Nutrient Effects on Net Biomass Production

Abstract

Canopy-forming plants and algae commonly contribute to spatial variation in habitat complexity for associated organisms and thereby create a biotic patchiness of communities. In this study, we tested for interaction effects between biotic habitat complexity and resource availability on net biomass production and species diversity of understory macroalgae by factorial field manipulations of light, nutrients, and algal canopy cover in a subtidal rocky-shore community. Presence of algal canopy cover and/or artificial shadings limited net biomass production and facilitated species diversity. Artificial shadings reduced light to levels similar to those under canopy cover, and net biomass production was significantly and positively correlated to light availability. Considering the comparable and dependent experimental effects from shadings and canopy cover, the results strongly suggest that canopy cover controlled net biomass production and species diversity by limiting light and thereby limiting resource availability for community production. Canopy cover also controlled experimental nutrient effects by preventing a significant increase in net biomass production from nutrient enrichment recorded in ambient light (no shading). Changes in species diversity were mediated by changes in species dominance patterns and species evenness, where canopy cover and shadings facilitated slow-growing crust-forming species and suppressed spatial dominance by Fucus vesiculosus, which was the main contributor to net production of algal biomass. The demonstrated impacts of biotic habitat complexity on biomass production and local diversity contribute significantly to understanding the importance of functionally important species and biodiversity for ecosystem processes. In particular, this study demonstrates how loss of a dominant species and decreased habitat complexity change the response of the remaining assembly to resource loading. This is of potential significance for marine conservation since resource loading often promotes low habitat complexity and canopy species are among the first groups lost in degraded aquatic systems.

Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

“…Accordingly, earlier experiments at the study site demonstrated that the production of algal biomass is controlled by light availability in a quadratic relation (Pearson moment correlation of bottom light availability and the square root of the macroalgal biomass accumulated over summer on propagule-seeded bricks in 2004 and 2005: n = 42, t, p, R 2 = 0.58; Fig. 1) and that nutrients increase biomass production only at high light (Eriksson et al 2006(Eriksson et al , 2007. Thus, the square root of biomass development of already established macroalgal propagules should therefore provide a good relative measure of resource availability and primary production at our field site (Eriksson et al 2006), while the abundance of recruits should represent the fraction of the algal community sensitive to consumer control.…”

“…The Schlei Fjord is characterized by insignificant tides, and in the summer, the salinity ranges between 12 and 18 PSU (practical salinity units), nutrient concentrations fluctuate highly, and water temperatures range between 16 and 25°C (Worm and Lotze 2006). The sandy benthos scattered with rocks support a submerged macroalgal community dominated by the canopy-forming macroalgae Fucus vesiculosus (hereafter Fucus), which covers approximately 80% of the stone surface along with several crust-forming species and a number of fastgrowing green ephemerals (Eriksson et al 2006). The grazer community consists of small peracarid crustacean mesograzers (isopods and amphipods) and 1-to 2-cmsized snails (mainly periwinkles, Littorina littorea, and L. saxatilis).…”

“…However, many of the algae produce a variety of microscopic resting stages that overwinter on the surfaces of rocks (Chapman 1986;Eriksson and Johansson 2005). These microscopic lifestages act as an overwintering propagule bank from which the algae can regenerate; they thereby enable a significantly earlier onset of growth in spring and provide a seasonal escape from high levels of grazing later in the year (Kiirikki and Ruuskanen 1996;Worm et al 2001;Eriksson et al 2006). Once the algal propagules start to accumulate visible biomass, they are much less susceptible to grazers Worm et al 2001).…”

“…1) and that nutrients increase biomass production only at high light (Eriksson et al 2006(Eriksson et al , 2007. Thus, the square root of biomass development of already established macroalgal propagules should therefore provide a good relative measure of resource availability and primary production at our field site (Eriksson et al 2006), while the abundance of recruits should represent the fraction of the algal community sensitive to consumer control. It is also reasonable to assume that resource availability estimated from biomass development of the already established macroalgal propagules is also a good estimation of the resource availability for the newly established propagules.…”

“…Our study system was dominated by highly mobile invertebrate grazers and predators. There is also a strong relationship between vulnerability to grazing and life-stage, such that grazers negatively affect macroalgal recruitment by consuming smaller life-stages, while already established and larger macroalgae are mainly limited by nutrient and light availability Worm et al 2001;Eriksson et al 2006). Thus, we assumed that resource availability control the biomass production of established macroalgae and that consumers control new macroalgal recruits.…”