Stream nutrient enrichment has a greater effect on coarse than on fine benthic organic matter

Abstract

Abstract. Nutrient enrichment affects bacteria and fungi associated with detritus, but little is known about how biota associated with different size fractions of organic matter respond to nutrients. Bacteria dominate on fine (,1 mm) and fungi dominate on coarse (.1 mm) fractions, which are used by different groups of detritivores. We measured the effect of experimental nutrient enrichment on fungal and bacterial biomass, microbial respiration, and detrital nutrient content on benthic fine particulate organic matter (FPOM) and coarse particulate organic matter (CPOM). We collected FPOM and CPOM from 1 reference and 1 enriched stream. CPOM substrates consisted of 2 litter types with differing initial C:nutrient ratios (Acer rubrum L. and Rhododendron maximum L.). Fungal and bacterial biomass, respiration, and detrital nutrient content changed with nutrient enrichment, and effects were greater on CPOM than on FPOM. Fungal biomass dominated on CPOM (,99% total microbial biomass), whereas bacterial biomass dominated on FPOM (,95% total microbial biomass). These contributions were unchanged by nutrient enrichment. Bacterial and fungal biomass increased more on CPOM than FPOM. Respiration increased more on CPOM (up to 300% increase) than FPOM (,50% increase), indicating important C-loss pathways from these resources. Microbial biomass and detrital nutrient content were positively related. Greater changes in nutrient content were observed on CPOM than on FPOM, and changes in detrital C:P were greater than changes in detrital C:N. Threshold elemental ratios analyses indicated that enrichment may reduce P limitation for shredders and exacerbate C limitation for collector-gatherers. Changes in CPOMdominated pathways are critical in predicting shifts in detrital resource quality and C flow that may result from nutrient enrichment of detritus-based systems.

Low‐to‐moderate nitrogen and phosphorus concentrations accelerate microbially driven litter breakdown rates

“…For example, macroinvertebrate shredder biomass in CWT streams increases with reduced litter C:N , and lower C:N has also been associated with greater invertebrate-mediated breakdown in other studies (Hladyz et al 2009). Likewise, lower litter C:P values observed in this study (Table 3) are consistent with predictions of reduced P-limitation of stream shredders (Tant et al 2013). Likewise, lower litter C:P values observed in this study (Table 3) are consistent with predictions of reduced P-limitation of stream shredders (Tant et al 2013).…”

A stream insect detritivore violates common assumptions of threshold elemental ratio bioenergetics models

“…We did not observe significant changes in A x and GGE x across diet nutrient levels as might be expected from ecological stoichiometry theory (Sterner and Elser 2002). For example, A N and A P did not increase on high C ∶ N or C ∶ P diets, perhaps because of greater digestion resistance of diet nutrients in low-nutrient litter with lower microbial biomass (Tant et al 2013). This mechanism is similar to how greater digestive resistance of P-limited algae can cause Plimited zooplankton to display counterintuitively lower A P Table 3.…”

“…TER theory assumes that the 2 elements under consideration (i.e., C and P) are the primary determinants of growth, and thus, TER models should be grounded in divergent growth across varying ratios of the 2 elements. Growth is implicit in the definition of the TER, but growth data across resourceratio gradients are limited for most taxa, and instead, ecologists use models based on bioenergetics terms, such as C and P use efficiencies and body stoichiometry (hereafter referred to as bioenergetics models), to calculate TERs (Frost et al 2006, Doi et al 2010, El-Sabaawi et al 2012, Tant et al 2013.…”

“…Inaccurate bioenergetics TER estimates could have farreaching consequences in ecology because bioenergetics models are used for diverse purposes, such as integrating ecological stoichiometry and metabolic theory (Allen andGillooly 2009, Doi et al 2010), assessing resource constraints on microbial C use efficiency (Sinsabaugh et al 2013), and predicting detritivore responses to aquatic nutrient enrichment (Hladyz et al 2009, Tant et al 2013. Compared to taxa with other feeding modes, detritivores may be particularly responsive to nutrient enrichment because they consume high C ∶ P and C ∶ N resources (Cross et al 2003).…”

Biochemical quality of basal resources in a forested stream: effects of nutrient enrichment

“…Many of these biomolecules play important structural and regulatory roles in organisms, and some are essential for consumers because they cannot be synthesised de novo (Brett and Müller-Navarra 1997;Dauwe and Middelburg 1998). Most studies focused on characterizing the composition of OM have mainly been limited to the dissolved fraction (Fellman et al 2010; however, see, e.g., Gremare et al 1997;Ledger and Hildrew 1998;Ylla et al 2010;Kolmakova et al 2013) but few (Tant et al 2013) have examined the changes in OM composition in response to nutrient additions.…”