Abstract
Nutrient enrichment of rivers and lakes has been increasing rapidly over the past few decades, primarily because of agricultural intensification. Although nutrient enrichment is known to drive excessive algal and microbial growth, which can directly and indirectly change the ecological community composition, the resulting changes in food web emergent properties are poorly understood. We used ecological network analysis (ENA) to examine the emergent properties of 12 riverine food webs across a nutrient enrichment gradient in the Manawatu, New Zealand. We also derive Keystone Sensitivity Indices to explore whether nutrients change the trophic importance of species in a way that alters the resilience of the communities to further nutrient enrichment or floods. Nutrient enrichment resulted in communities composed of energy inefficient species with high community (excluding microbes) respiration. Community respiration was several times greater in enriched communities, and this may drive hypoxic conditions even without concomitant changes in microbial respiration. Enriched communities exhibited weaker trophic cascades, which may yield greater robustness to energy flow loss. Interestingly, enriched communities were also more structurally and functionally affected by species sensitive to flow disturbance making these communities more vulnerable to floods.
Highlights
The eutrophication of rivers, lakes, and groundwater is increasing rapidly, and among the most influential drivers of the global decline in aquatic biodiversity (Dudgeon, 2014; Dudgeon et al, 2006; Vorosmarty et al, 2010)
Dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) are the dominant culprits of eutrophication, both are limiting the growth of both autotrophs and heterotrophs (Camargo & Alonso, 2006; Conley et al, 2009; Ferreira et al, 2015; Smith & Schindler, 2009)
The total food web biomass was unaffected by nutrient enrichment, rather the per-capita throughflow of energy increased instead
Summary
The eutrophication of rivers, lakes, and groundwater is increasing rapidly, and among the most influential drivers of the global decline in aquatic biodiversity (Dudgeon, 2014; Dudgeon et al, 2006; Vorosmarty et al, 2010). While riverine community composition changes with nutrient enrichment (Camargo & Alonso, 2006; Smith & Schindler, 2009), the influence of those changes on food web emergent properties/functioning is not well understood (Boersma et al, 2008; Dodds, 2007; Friberg et al, 2011; Price et al, 2019). Analysis of un-weighted food webs suggested species connectivity had high influence on secondary species extinctions If food webs change in a way that means the most sensitive species to disturbance are keystone species, the food web will likely have lower stability to the said disturbance. We assembled weighted food webs for twelve rivers across a nutrient gradient within the Manawatu, New Zealand. We assessed whether these sensitivity indexes changed across the nutrient gradient
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