Terrestrial subsidies to lake food webs: an experimental approach.

Cross-ecosystem movements of material and energy are ubiquitous. Aquatic ecosystems typically receive material that also includes organic matter from the surrounding catchment. Terrestrial-derived (allochthonous) organic matter can enter aquatic ecosystems in dissolved or particulate form. Several studies have highlighted the importance of dissolved organic carbon to aquatic consumers, but less is known about allochthonous particulate organic carbon (POC). Similarly, most studies showing the effects of allochthonous organic carbon (OC) on aquatic consumers have investigated pelagic habitats; the effects of allochthonous OC on benthic communities are less well studied. Allochthonous inputs might further decrease primary production through light reduction, thereby potentially affecting autotrophic resource availability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clear-water lake. Corn starch (a C(4) plant) was used as a POC source due to its insoluble nature and its distinct carbon stable isotope value (δ(13)C). The starch carbon was closely dispersed over the bottom of the enclosures to study the fate of a POC source exclusively available to sediment biota. The addition of starch carbon resulted in a clear shift in the isotopic signature of surface-dwelling herbivorous and predatory invertebrates. Although the starch carbon was added solely to the sediment surface, the carbon originating from the starch reached zooplankton. We suggest that allochthonous POC can subsidize benthic food webs directly and can be further transferred to pelagic systems, thereby highlighting the importance of benthic pathways for pelagic habitats.

Declines in predatory fish promote bloom-forming macroalgae.

In the Baltic Sea, increased dominance of ephemeral and bloom-forming algae is presently attributed to increased nutrient loads. Simultaneously, coastal predatory fish are in strong decline. Using field data from nine areas covering a 700-km coastline, we examined whether formation of macroalgal blooms could be linked to the composition of the fish community. We then tested whether predator or nutrient availability could explain the field patterns in two small-scale field experiments, by comparing joint effects on algal net production from nutrient enrichment with agricultural fertilizer and exclusion of larger predatory fish with cages. We also manipulated the presence of invertebrate grazers. The abundance of piscivorous fish had a strong negative correlation with the large-scale distribution of bloom-forming macroalgae. Areas with depleted top-predator communities displayed massive increases in their prey, small-bodied fish, and high covers of ephemeral algae. Combining the results from the two experiments showed that excluding larger piscivorous fish: (1) increased the abundance of small-bodied predatory fish; (2) changed the size distribution of the dominating grazers, decreasing the smaller gastropod scrapers; and (3) increased the net production of ephemeral macroalgae. Effects of removing top predators and nutrient enrichment were similar and additive, together increasing the abundance of ephemeral algae many times. Predator effects depended on invertebrate grazers; in the absence of invertebrates there were no significant effects of predator exclusion on algal production. Our results provide strong support for regional declines of larger predatory fish in the Baltic Sea promoting algal production by decreasing invertebrate grazer control. This highlights the importance of trophic interactions for ecosystem responses to eutrophication. The view emerges that to achieve management goals for water quality we need to consider the interplay between top-down and bottom-up processes in future ecosystem management of marine resources.

Biotic habitat complexity controls species diversity and nutrient effects on net biomass production.

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.