Impacts of Warming on the Structure and Functioning of Aquatic Communities

Abstract

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

Substratum‐dependent responses of ciliate assemblages to temperature: a natural experiment in Icelandic streams

“…Biofilm-grazing gastropods represent common consumers or competitors of ciliates, and temperature is known to increase both the feeding rate and the activity rate of gastropods (Yee & Murray 2004). In our study system, Radix balthica snails play a key role in the food web, being the dominant large grazer in Hengill streams (O'Gorman et al 2012). Radix balthica consumes the same resources (bacteria and diatoms) as many protists, and it is more abundant and exerts higher grazing pressure on the biofilm at high temperatures (O'Gorman et al 2012).…”

“…In our study system, Radix balthica snails play a key role in the food web, being the dominant large grazer in Hengill streams (O'Gorman et al 2012). Radix balthica consumes the same resources (bacteria and diatoms) as many protists, and it is more abundant and exerts higher grazing pressure on the biofilm at high temperatures (O'Gorman et al 2012). The increased abundance and grazing pressure of R. balthica at high temperatures may reduce the species richness and total biomass in ciliate assemblages, either by direct competition for resources, by physically displacing ciliates while grazing the biofilm, or by consuming them directly.…”

“…S1), displaying a gradient of mean water temperatures from 5.4 to 20.2°C. Although there are some minor chemical differences among the streams, these are independent of temperature, which is the major environmental gradient in the system (Table S1) (Woodward et al 2010a;Demars et al 2011;O'Gorman et al 2012;Adams et al 2013).…”

“…This was performed by approximating the cell shape of each OTU to standard geometrical forms (O'Gorman et al 2012, after Hillebrand et al 1999 and estimating the corresponding volume. Individual cell masses were obtained by multiplying their estimated volumes with their physical density.…”

Aquatic macrophytes alter productivity‐richness relationships in eutrophic stream food webs

“…Indeed, McAbendroth et al (2005) found that increasing macrophyte habitat complexity was v www.esajournals.org associated with a greater number of small-bodied individuals. Size structure in turn affects a system's capacity to support predator biomass due to reduced trophic transfer efficiencies associated with smaller prey (O'Gorman et al 2012). Consequently, despite increasing ecosystem productivity, eutrophication can actually reduce the amount of available energy for stream food webs, because the increased productivity is largely trapped in defended primary production (macrophytes), which in turn favors defended primary consumers, thus creating energy bottlenecks at multiple trophic levels (Fig.…”