How climate change-induced early snowmelt may change the structure of the stream food web?

From:

UMR INRAE/UPPA Ecobiop: Mathieu Buoro (PI), Charlotte Evangelista (Postdoctoral researcher), Stephane Glise (Technician), & Tatiana Tronel (Master student, UMR Ecobiop)

UC Berkeley Freshwater group: Albert Ruhi (PI), Stephanie Carlson (PI) & Kyle Leathers (PhD student)

Context

The Earth’s ecosystems are facing major threats from global environmental change [1]. This is particularly true for freshwater ecosystems, which tend to be disproportionately affected by warming and altered hydrologic cycles (e.g., reduced snowpack). Altered patterns of streamflow variation, and aseasonal drought in particular, can induce mass mortality of organisms, and shifts in their spatial distribution and/or temporal eco-evolutionary dynamics [2]. Despite abundant research on the impacts of climate-induced flow change on riverine populations and communities, whole-ecosystem responses remain difficult to anticipate [3]. This is largely because the different trophic levels of a river food web (e.g., algal producers, invertebrate consumers, and fish predators) can respond in different ways to flow variation, and interaction strengths among species in these complex networks may change over time [4]. Understanding how climate-altered flow regimes affect not only individual species, but also the interactions among them, is key to anticipating climate change impacts on river ecosystems–as well as on the adjacent riparian ecosystems that benefit from them.

Setting up the collaborative effort

Over a year ago, we started discussing different approaches to study climate change in river ecosystems, motivated by rapidly changing flow regimes in both Europe (Southern part) and California. Those conversations led to the creation of a consortium to study the Management and Climate Impacts on Freshwater Ecosystems. The INRAE/UPPA group has deep experience in assessing the eco-evolutionary effects of human induced perturbations, such as fishing and climate change, at various levels of organization – from gene to (meta)populations. In turn, the UC Berkeley team has experience in the combination of observational, experimental, and model-based approaches to understand the direct and indirect effects of drought and flow intermittency at the community level. By joining our complementary expertise and facilities, this international collaboration has the potential to advance current understanding of the effects of climate change on stream ecosystems, focusing on the mechanisms at play and the scaling of responses across levels of biological organization—from populations to fluxes that may span ecosystem boundaries.

Toward an experimental approach to study responses to climate change

Experimental approaches are widely used to gain a mechanistic understanding of river ecosystem responses to climate change [5]. By definition, experiments simplify reality and represent a trade-off between levels of control (treatments and replication) and realism (size and complexity of the ecosystem captured). Hence, various experimental designs can be used to examine the same question, from short-term laboratory microcosm/flume experiments, to multi-year large-scale field experiments. This raises the question, what is the optimal design to study potential shifts in food-web structure in response to climate-altered river flow regimes? And, can we combine evidence from experiments at multiple spatio-temporal scales? Answering these questions may show when spatio-temporal scales may be too small to make inferences about the behavior of real food webs in nature–or adequate, but not replicated enough, to parse out what may be generalizable responses of food webs (“signal”) from patterns that are contingent on stochastic phenomena, such as random colonization and priority effects (“noise”). 

First, we have initiated a new collaboration to critically examine the different trade-offs in past stream climate change experiments, via a literature review and meta-analysis (in preparation) led by Dr. Charlotte Evangelista. Based on what we learned, we developed a collaborative experiment via large-scale, flow-through mesocosms in the Sierra Nevada at one of the University of California Natural Reserves. This experiment aimed to show how climate change-induced flow alteration due to reduced snowpack and early snowmelt may alter high-mountain river food webs–and the terrestrial consumers that depend upon them.

A collaborative experiment in the Sierra Nevada

Then, we initiated a 3-month experiment in Spring-Summer 2022 using stream mesocosms available at the Sierra Nevada Aquatic Research Lab (SNARL) in Mammoth Lakes, California. This experiment was led by Dr. Charlotte Evangelista in collaboration with Dr Albert Ruhi, Pr. Stephanie Carlson, Dr. Mathieu Buoro with the invaluable field assistance of Kyle Leathers (PhD student at UC Berkeley) and Tatiana Tronel (Master student at Ecobiop).

Experimental channels of the Sierra Nevada Aquatic Research Lab, SNARL (Valentine Eastern Sierra UC Reserve, California).

The experimental system consists of nine flow-through channels (50 × 1 m) that are composed of pools interspersed with straight rifles, sourced by a local river (Convict Creek) and naturally colonised by invertebrates, thus mimicking natural headwater streams. In these channels, via adjustable gates and a diversion system we simulated current vs. end-of-century flow regimes in the Sierra Nevada, when decreased snowpack and more prolonged low flows are expected. Using a nested design of 3 subsections per channel (for a total of 24 experimental units), we also manipulated the presence of a top predator fish species (brown trout, Salmo trutta) that feeds on benthic invertebrates, and for which both feeding behavior and growth are expected to change with low-flow induced stress. These additions would improve our mechanistic understanding of why and how mountain stream food webs may change under warmer climates, thus increasing the overall impact of our study.

We heavily instrumented the channels (with dissolved oxygen and water level sensors, and replicated temperature sensors), and we surveyed trout (using underwater camera and mark-recapture) to monitor behavior and individual growth trajectory as we manipulate flow levels.

Capture of benthic invertebrates, waterflow and benthic algal concentrations measurements (credits photos: Mathieu Buoro & Charlotte Evangelista)

We also periodically collected invertebrates living on the stream (benthic), focusing on the aquatic insect subset, as they complete their cycle and emerge into the adult, flying stage (via sticky and emergence traps). 

Sticky and emergence traps designed by our colleague Stephane Glise (UMR Ecobiop) (credits photos: Charlotte Evangelista & Albert Ruhi)

Monitoring the timing and magnitude of the flux of emerging insects is meaningful because past research has shown that riparian and terrestrial predators (e.g., Brewer’s black birds, various lizard species) in alpine meadows are “subsidized” by stream organisms. Thus, this experiment will show how climate change-induced early snowmelt may change the structure of the stream food web–and whether important cross-ecosystem subsidies that connect riverine with riparian habitats may be disrupted as a consequence. 

In particular, we hypothesize that:

1. Earlier, longer low-flows and the concomitant increase in water temperature will increase brown trout metabolic rates, depressing invertebrate prey abundance in treatment, but not control channels;
2. Treatment channels will show reduced numbers of insects emerging, and smaller sizes at emergence, as insects in larval (aquatic) stages will be subject to increased predation pressure, and taxa with plastic life histories may accelerate their life cycle to escape brown trout.

Stay tuned for the results!

References

[1] IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. [Masson-Delmotte et al. (eds.)]. Cambridge University Press. In press. 

[2] Rolls, R.J., Leigh, C. and Sheldon, F., 2012. Mechanistic effects of low-flow hydrology on riverine ecosystems: ecological principles and consequences of alteration. Freshwater Science, 31(4), pp.1163-1186.

[3] Palmer, M. and Ruhi, A., 2019. Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration. Science, 365(6459).

[4] Power, M.E., Parker, M.S. and Dietrich, W.E., 2008. Seasonal reassembly of a river food web: floods, droughts, and impacts of fish. Ecological Monographs, 78(2), pp.263-282.

[5] Stewart, R.I., Dossena, M., Bohan, D.A., Jeppesen, E., Kordas, R.L., Ledger, M.E., Meerhoff, M., Moss, B., Mulder, C., Shurin, J.B. and Suttle, B., 2013. Mesocosm experiments as a tool for ecological climate-change research. Advances in ecological research, 48, pp.71-181.