1c: The interplay between biogeochemistry and ecological stoichiometry across ecosystems

1c:     The interplay between biogeochemistry and ecological stoichiometry across ecosystems

Conveners:     Savannah Sarkis (Netherlands Institute of Ecology)

                       Annegreet Veeken (Utrecht University)

                       Justine Lejoly (Netherlands Institute of Ecology)

1. Biogeochemical drivers of forage quality and herbivore diet selection in African savannas

Yuhong Li, University of Groningen

African savannas support diverse herbivores that coexist by partitioning plant resources of varying quality. However, the drivers of such resource variation across spatial scales, particularly the roles of variation between species versus plasticity within species, are poorly understood. We analyzed nutrient contents (phosphorus, nitrogen, sodium) in 121 leaf samples from 50 plant species across a rainfall and fire gradient in the Serengeti ecosystem, Tanzania. Phosphorus within species decreased strongly with rainfall and fire, likely due to nutrient loss from ash runoff, emphasizing intraspecific variation. In contrast, intraspecific nitrogen content remained stable across sites, likely due to nitrogen-fixing species, highlighting interspecific variation. Plant sodium content was influenced by both species and site. These results suggest that nutrient content variation in forage is driven by nutrient-specific cycling processes, and that herbivores must select specific plants and locations to meet dietary needs—a strategy best supported in landscapes with diverse environmental conditions.

2. Vertebrate scavengers can prevent the leakage of nutrients from carcasses into the soil

Elke Wenting, Patrick Jansen, Simon Burggraeve, Devon Delsman, Henk Siepel & Frank van Langevelde, Radboud University

The decomposition of carcasses by scavengers and microbes is an important component of biochemical cycling that can strongly alter the chemical composition of soils. We empirically tested the assumption that different scavenger guilds have different influences on the chemical elements that leak into the soil during carcass decomposition. To do this, we systematically excluded different subsets of vertebrate scavengers in a field experiment. We examined a wide range of elemental concentrations in the soil beneath and in the vegetation next to carcasses over time throughout the decomposition process. We found that fluxes of several elements into the soil showed distinct peaks when all vertebrate scavengers were excluded. Especially trace elements (Cu and Zn) appeared to be influenced by carcass decomposition. This implies that vertebrate scavengers take up the carrion-derived elements in their bodies, potentially redistributing them over larger areas, and thus linking trace elemental cycles from local to landscape scales.

3. The effect of phosphorus limitation on the soil microarthropod community

Joren Bruggink, Radboud University

P-limitation, as a result of nitrogen deposition, has been linked to the decline of invertebrate species richness in the Netherlands, but the effect on microarthropods are still little studied. In this presentation I will show the results of a comparison of old forest soil locations in the Netherlands with hedgerow locations. Hedgerows are remnants of old forest soils, but are adjacent to agriculture and therefore have a constant influx of phosphate due to fertilization. As expected, was the Olsen-P levels much higher in the hedgerow locations, which resulted in a higher density of large mite species and a higher diversity of herbivorous species. We expect that the P-limitation leads to an inefficient trophic transfer of energy and therefore the support of mostly small bodied fauna, while herbivorous species are most strongly affected due to the fact that plants have a less stable N:P ratio as fungi.

4. Seasonal succession of toxic cyanobacteria in three lakes with contrasting nutrient stoichiometry

Annemieke Drost, Savannah Sarkis, Chaohong (Harley) Lin, Els Faassen, Jef Huisman, Uwe John, Dedmer Van de Waal, Netherlands Institute of Ecology

Toxic cyanobacterial blooms threaten freshwater quality, and are exacerbated by climate change. The potential toxicity of these blooms depends not only on the amount of cyanobacteria, but also on the presence of potentially toxin-producing species and the variation in their toxin production. One of the most prevalent cyano-toxins is microcystin. Here, we studied three Dutch lakes with contrasting nutrient stoichiometry (nitrogen:phosphorus:carbon ratios – C:N:P ratios) to answer the question: what are the consequences of different C:N:P ratios on temporal cyanobacterial biomass and toxin (specifically, microcystin) concentrations? The cyanobacterial communities in all three lakes were dominated by potentially microcystin producing genera, and microcystin concentrations in the water were highest at the peak of each bloom. Although the microcystin concentration was driven by biomass, the microcystin contents in the cells (quota) were instead highest both at the onset and end of each bloom. We also observed stoichiometric control on (the nitrogen-rich) microcystin: its concentration increased at higher TN:TC ratios. Our results show that the theory of ecological stoichiometry offers a promising conceptual framework to increase our mechanistic understanding of cyanobacterial toxin concentrations, and help predict future changes in toxin concentrations in lakes and reservoirs.

5. Do nutrient-ecosystem feedbacks in lakes aggravate imbalanced N:P ratios?

Dianneke van Wijk, Jan Janse, Mengru Wang, Wolf Mooija, Annette Janssen, Netherlands Institute of Ecology

Worldwide, anthropogenic activities threaten surface water quality by aggravating eutrophication and increasing total nitrogen to total phosphorus (TN:TP) ratios. In hydrologically connected systems, water quality management may benefit from in-ecosystem nutrient retention by preventing nutrient transport to downstream systems. However, nutrient retention may also alter TN:TP ratios with unforeseen consequences for downstream water quality. We analyzed how in-lake nutrient retention may influence nutrient transport (ratios) to downstream systems for numerous Chinese lakes. Using the process-based lake ecosystem model PCLake+, we compared socio-economic development and climate change scenarios with different inflow TN:TP ratios for 2050 against 2012. We found that both the inflow TN:TP ratio and the ecosystem state influence in-lake nutrient retention and alter the water's TN:TP ratio. Therewith, nutrient load reductions to restore lakes may prevent downstream nutrient pollution but could also aggravate imbalanced TN:TP ratios. Thus, careful analysis of connected systems is needed to guide water quality management.