Whole-System Nutrient Enrichment Increases Secondary Production in a Detritus-Based Ecosystem

Abstract

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.

Environment shapes invertebrate assemblage structure differences between volcanic spring-fed and runoff rivers in northern California

“…Most stream ecosystems are naturally nutrient limited, and small increases in a limiting nutrient can enhance primary production with cascading effects on consumers (Hart and Robinson 1990, Slavik et al 2004, Cross et al 2006. We found that nutrient levels were significantly enhanced in spring-fed rivers and probably contributed to increased invertebrate density at these sites.…”

Comparison of Fungal Activities on Wood and Leaf Litter in Unaltered and Nutrient-Enriched Headwater Streams

“…This was caused by wood and leaf litter standing crops twofold lower in the nutrient-enriched than in the reference stream, presumably a result of the disappearance of organic matter in the treatment stream due to faster decomposition rates brought about by elevated microbial activity during 5 years of enrichment (21,24). Increased fungal biomass buildup and nitrogen immobilization in plant litter following nutrient addition (24,25) resulted in an enhanced food quality for consumers and higher invertebrate biomass and production of some invertebrate groups in this stream (8,9,21). At the same time, the availability of detrital resources decreased, which should have an opposite effect on some consumers over longer periods (8).…”

Flood disturbance and predator–prey effects on regional gradients in species diversity

“…BPOM and seston, which is derived from BPOM, are food resources for collector-gatherers/shredders and collector-filterers, respectively. Dissolved nitrogen can enhance the growth of microbes that colonize BPOM (Gulis et al 2004), resulting in high-quality BPOM (e.g., Cross et al 2006). A high quantity and quality of food resources may have the same function as high productivity (Yee and Juliano 2007), and thus promote an increase in community density, leading to an increase in taxonomic richness.…”