Potential effects of elevated atmospheric carbon dioxide on benthic autotrophs and consumers in stream ecosystems: a test using experimental stream mesocosms

Abstract

Elevated atmospheric carbon dioxide (eCO 2 ) has been shown to have a variety of ecosystem-level effects in terrestrial systems, but few studies have examined how eCO 2 might affect aquatic habitats. This limits broad generalizations about the effects of a changing climate across biomes. To broaden this generalization, we used free air CO 2 enrichment to compare effects of eCO 2 (i.e., double ambient $ 720 ppm) relative to ambient CO 2 (aCO 2 $ 360 ppm) on several ecosystem properties and functions in large, outdoor, experimental mesocosms that mimicked shallow sand-bottom prairie streams. In general, we showed that eCO 2 had strong bottom-up effects on stream autotrophs, which moved through the food web and indirectly affected consumer trophic levels. These general effects were likely mediated by differential CO 2 limitation between the eCO 2 and aCO 2 treatments. For example, we found that eCO 2 decreased water-column pH and increased dissolved CO 2 in the mesocosms, reducing CO 2 -limitation at times of intense primary production (PP). At these times, PP of benthic algae was about two times greater in the eCO 2 treatment than aCO 2 treatment. Elevated PP enhanced the rate of carbon assimilation relative to nutrient uptake, which reduced algae quality in the eCO 2 treatment. We predicted that reduced algae quality would negatively affect benthic invertebrates. However, density, biomass and average individual size of benthic invertebrates increased in the eCO 2 treatment relative to aCO 2 treatment. This suggested that total PP was a more important regulator of secondary production than food quality in our experiment. This study broadens generalizations about ecosystem-level effects of a changing climate by providing some of the first evidence that the global increase in atmospheric CO 2 might affect autotrophs and consumers in small stream ecosystems throughout the southern Great Plains and Gulf Coastal slope of North America.

Effects of experimental warming and increased ultraviolet B radiation on the Mediterranean plankton food web

“…Recently several studies also focused on another stressor linked to global changes, the increase of atmospheric CO 2 and consequent ocean acidification, but very few have been dedicated to elucidating the effects of this stressor on the entire plankton food web (Riebesell et al 2007) or benthic food web (Hargrave et al 2009). It has been shown that interactions among multiple stressors due to global change better explain their combined effect on ecosystem functioning, compared with the sum of the individual effects of each stressor (Christensen et al 2006).…”

In‐stream structure alters density‐dependent fish effects in stream ecosystems

“…Previous studies have shown flow, temperature, pH and alkalinity to be similar between the mesocosms and Harmon Creek (see Hargrave et al 2009). The mesocosms consisted of a single channel (1 m wide 9 4 m long) with ã 16-cm-thick layer of sand substrate designed to simulate small, fairly shallow, sand-bottom streams commonly found in the southern Great Plains and Gulf coastal slope region of North America.…”

Direct and indirect food web regulation of microbial decomposers in headwater streams

“…Specifically, total fish biomass per mesocosm averaged 40.3 ± 2.8, 42.5 ± 2.9 and 40.3 ± 2.2 g for blackstripe topminnow, bullhead minnow and blacktail shiner, respectively. Differential rates of insect inputs into the mesocosms could have affected water column nutrient loads (Hargrave et al 2009), influencing our results above. Thus, we examined variation in terrestrial input across treatments and found no significant effect of time or treatment on terrestrial input in this experiment (Table 1), which averaged 144 ± 27 mg m −2 d −1 (mean ± 1 SD), 165 ± 37 mg m −2 d −1 , and 160 ± 14 mg m −2 d −1 on days 0, 15 and 30, respectively.…”

“…Streams remained unaltered until 3 June 2007 (13 days) to allow establishment of a biofilm assemblage and to allow colonization of aquatic insect larvae by ovipositing adults prior to implementing the fish treatments. Invertebrates that colonized the stream mesocosms were primarily early succession species, such as chironomid and dragonfly larvae, and resembled density and biomass of the invertebrate assemblages found in sand‐bottom habitats common to Harmon Creek (Hargrave et al 2009).…”

“…The mesocosms were designed to mimic small, shallow, sand‐bottom headwater streams throughout the Gulf coastal slope of North America. Specifically, each mesocosm was 4 m long by 1 m wide, had a ∼10 cm thick sand substrate, an average water depth of ∼0.3 m, and similar water chemistry to streams in the region (see Hargrave et al 2009 for a comparison in abiotic and biotic characteristics between a second order Gulf coastal stream (Harmon Creek) and experimental mesocosms). Mesocosms were housed under a shade structure to simulate a dense canopy cover common to small headwater streams.…”