Daniel M. Perkins

Climate change and freshwater ecosystems: impacts across multiple levels of organization

Fresh waters are particularly vulnerable to climate change because (i) many species within these fragmented habitats have limited abilities to disperse as the environment changes; (ii) water temperature and availability are climate-dependent; and (iii) many systems are already exposed to numerous anthropogenic stressors. Most climate change studies to date have focused on individuals or species populations, rather than the higher levels of organization (i.e. communities, food webs, ecosystems). We propose that an understanding of the connections between these different levels, which are all ultimately based on individuals, can help to develop a more coherent theoretical framework based on metabolic scaling, foraging theory and ecological stoichiometry, to predict the ecological consequences of climate change. For instance, individual basal metabolic rate scales with body size (which also constrains food web structure and dynamics) and temperature (which determines many ecosystem processes and key aspects of foraging behaviour). In addition, increasing atmospheric CO2 is predicted to alter molar CNP ratios of detrital inputs, which could lead to profound shifts in the stoichiometry of elemental fluxes between consumers and resources at the base of the food web. The different components of climate change (e.g. temperature, hydrology and atmospheric composition) not only affect multiple levels of biological organization, but they may also interact with the many other stressors to which fresh waters are exposed, and future research needs to address these potentially important synergies.

Warming alters community size structure and ecosystem functioning

Global warming can affect all levels of biological complexity, though we currently understand least about its potential impact on communities and ecosystems. At the ecosystem level, warming has the capacity to alter the structure of communities and the rates of key ecosystem processes they mediate. Here we assessed the effects of a 4°C rise in temperature on the size structure and taxonomic composition of benthic communities in aquatic mesocosms, and the rates of detrital decomposition they mediated. Warming had no effect on biodiversity, but altered community size structure in two ways. In spring, warmer systems exhibited steeper size spectra driven by declines in total community biomass and the proportion of large organisms. By contrast, in autumn, warmer systems had shallower size spectra driven by elevated total community biomass and a greater proportion of large organisms. Community-level shifts were mirrored by changes in decomposition rates. Temperature-corrected microbial and macrofaunal decomposition rates reflected the shifts in community structure and were strongly correlated with biomass across mesocosms. Our study demonstrates that the 4°C rise in temperature expected by the end of the century has the potential to alter the structure and functioning of aquatic ecosystems profoundly, as well as the intimate linkages between these levels of ecological organization.

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

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.

Global change and food webs in running waters

Riverine habitats are vulnerable to a host of environmental stressors, many of which are increasing in frequency and intensity across the globe. Climate change is arguably the greatest threat on the horizon, with serious implications for freshwater food webs via alterations in thermal regimes, resource quality and availability, and hydrology. This will induce radical restructuring of many food webs, by altering the identity of nodes, the strength and patterning of interactions and consequently the dynamics and architecture of the trophic network as a whole. Although such effects are likely to be apparent globally, they are predicted to be especially rapid and dramatic in high altitude and latitude ecosystems, which represent ‘sentinel systems’. The complex and subtle connections between members of a food web and potential synergistic interactions with other environmental stressors can lead to seemingly counterintuitive responses to perturbations that cannot be predicted from the traditional focus of studying individual species in isolation. In this review, we highlight the need for developing new network-based approaches to understand and predict the consequences of global change in running waters.

Higher biodiversity is required to sustain multiple ecosystem processes across temperature regimes

Biodiversity loss is occurring rapidly worldwide, yet it is uncertain whether few or many species are required to sustain ecosystem functioning in the face of environmental change. The importance of biodiversity might be enhanced when multiple ecosystem processes (termed multifunctionality) and environmental contexts are considered, yet no studies have quantified this explicitly to date. We measured five key processes and their combined multifunctionality at three temperatures (5, 10 and 15 °C) in freshwater aquaria containing different animal assemblages (1–4 benthic macroinvertebrate species). For single processes, biodiversity effects were weak and were best predicted by additive-based models, i.e. polyculture performances represented the sum of their monoculture parts. There were, however, significant effects of biodiversity on multifunctionality at the low and the high (but not the intermediate) temperature. Variation in the contribution of species to processes across temperatures meant that greater biodiversity was required to sustain multifunctionality across different temperatures than was the case for single processes. This suggests that previous studies might have underestimated the importance of biodiversity in sustaining ecosystem functioning in a changing environment.

Biodiversity, Ecosystem Functioning, and Services in Fresh Waters: Ecological and Evolutionary Implications of Climate Change

Abstract Freshwater ecosystems are particularly vulnerable to the different components of climate change, yet we still have a limited understanding of the consequences of these environmental drivers and their interactions with other stressors, especially at the higher, multispecies, organizational levels. We review the current state of the field and identify several key areas where rapid progress has already been made, as well as those that still remain largely uncharted: although the jigsaw puzzle of our understanding of ecological and evolutionary responses to climate change is taking shape at an accelerating rate, huge gaps still need to be filled before we can start to see the bigger picture. In the future a more integrated “multiplex” approach is needed to forge the currently missing links between organizational levels, across scales in time and space, and among ecological and evolutionary phenomena—some of this can be achieved by redirecting existing research in a more coordinated fashion, but other areas will require entirely new approaches to both how research is funded and how it is done.

Groundwater flooding: Ecosystem structure following an extreme recharge event

Abstract 1) Aquifers are recharged by surface water percolating through soil and rock and by connections with surface streams and rivers. Extreme rainfall can cause extensive flooding of surface waters and, eventually, of groundwaters. However, how the resultant changes in nutrients impact groundwater organisms and the structure of groundwater food webs is largely unknown. 2) We monitored abiotic (nutrients, temperature and more) and biotic (all organismal groups except viruses) conditions in eight groundwater boreholes in two locations in a chalk aquifer over the course of 25 weeks (ten sampling occasions), following an extreme rainfall- and groundwater-flooding event in the UK. 3) We show that groundwater flooding can cause substantial nutrient fertilisation of aquifers – nutrient concentrations (especially dissolved organic carbon) in the groundwater were highest when we started the sampling campaign, directly following the flood event, and then decreased over time while groundwater levels also declined back to their baseline. 4) Bacteria in the open water (i.e. bacteria not associated with sediment) became more abundant as the water table and DOC concentrations decreased. Importantly their functional richness tracked the DOC patterns, illustrating that bacteria were responsible for respiring DOC. Microbial metabolic activity and bacterial respiration, measured using smart tracers, supported this finding; DOC and microbial respiration showed a positive correlation. 5) The other biota (protists, micro- and macro-metazoans) showed different abundance patterns over time, but importantly, the entire sediment community, ranging from bacteria to macrofaunal species, showed a strong community size structure (mean size spectra slope: −1.12). Size spectra changed gradually through time towards steeper slopes, except in the very deep aquifer. 6) Our approach allowed us to demonstrate that groundwater communities track extreme changes in their usually stable environment, highlighting that they potentially buffer environmental change, although we still do not know what the limits of this ‘service’ might be.