Global pressures, specific responses: effects of nutrient enrichment in streams from different biomes

Abstract

We assessed the effects of nutrient enrichment on three stream ecosystems running through distinct biomes (Mediterranean, Pampean and Andean). We increased the concentrations of N and P in the stream water 1.6-4-fold following a before-after control-impact paired series (BACIPS) design in each stream, and evaluated changes in the biomass of bacteria, primary producers, invertebrates and fish in the enriched (E) versus control (C) reaches after nutrient addition through a predictive-BACIPS approach. The treatment produced variable biomass responses (2-77% of explained variance) among biological communities and streams. The greatest biomass response was observed for algae in the Andean stream (77% of the variance), although fish also showed important biomass responses (about 9-48%). The strongest biomass response to enrichment (77% in all biological compartments) was found in the Andean stream. The magnitude and seasonality of biomass responses to enrichment were highly site specific, often depending on the basal nutrient concentration and on windows of ecological opportunity (periods when environmental constraints other than nutrients do not limit biomass growth). The Pampean stream, with high basal nutrient concentrations, showed a weak response to enrichment (except for invertebrates), whereas the greater responses of Andean stream communities were presumably favored by wider windows of ecological opportunity in comparison to those from the Mediterranean stream. Despite variation among sites, enrichment globally stimulated the algal-based food webs (algae and invertebrate grazers) but not the detritus-based food webs (bacteria and invertebrate shredders).Environ. Res. Lett. 8 (2013) 014002 J Artigas et al nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but that its magnitude and extent within the food web are complex and are strongly determined by environmental factors and ecosystem structure.

Nutrient enrichment effect on macroinvertebrates in a lowland stream of Argentina

“…In this sense, McCormick et al (2004) did not register variations for species richness between control and fertilized sites in an experimental study carried out in Florida (USA). Artigas et al (2013) did not detect reduced species richness but a change in the dominance of taxa in favor of the more tolerant.…”

“…Twelve in-water bags were distributed along the treatment reach, and replaced 2 or 3 times a week during the fertilization period to maintain a constant fertilization rate (Feijoó et al, 2014). Water samples in triplicate also were collected and analyzed during enrichment period every 5 to 10 d to better capture changes in nutrient concentrations (Artigas et al, 2013).…”

“…However, while tolerance levels of a wide range of taxa have been established along gradients of organic pollution, little is known about how macroinvertebrates respond to elevated nutrient levels (Friberg et al, 2010). While nutrient studies in lowland streams showed impacts on primary producers (Acuña et al, 2011) and on benthic microbial community , researches showing direct and indirect effects on higher trophic levels have been less conclusive due to the complexity of food webs that can buffer the potential transference of bottomup effects after nutrient addition (Artigas et al, 2013).…”

“…Thus, changes in the runoff patterns and a greater discharge of water in the streams and rivers (Hulme & Sheard, 1999) would lead to a greater input of nutrients, especially nitrogen and phosphorous, and contaminants into streams and rivers (Rodrigues Capítulo et al, 2010;Ferreiro et al, 2011). This situation may have harmful effects on the whole ecosystem with accompanying economic repercussions (Dodds et al, 2009;Artigas et al, 2013).…”

Trophic relationships between macroinvertebrates and fish in a pampean lowland stream (Argentina)

“…However, prairie and pampean streams exhibit some differences. On the one hand, Pampean streams have high basal nutrient concentrations (Artigas et al, 2013). Moreover, they contain macrophytes that are usually native and play a key role in structuring the biological community (Giorgi et al, 2005), unlike many of the streams in the plains of New Zealand where the invasive species Egeria densa has become dominant (Champion & Tanner, 2000).…”