Article: The Dark Side of Rocks: An Underestimated High Quality Food Resource in River Ecosystems by Guo and colleagues (2021)
Background: Stream and river ecosystems have complex food webs that rely on algae as the foundational nutrient supply for consumers. Depending on geographic location and local climate, rivers and streams can have varying levels of flow (or even drought periods) where we would not expect algae to exist in large quantities. However, on the surface of rocks, leaf litter, and other larger debris, small structures called biofilms are widespread. Where there is adequate light, algae grows and produces biofilms, which contain organic matter that is required by many invertebrates to survive. Surfaces without light, such as the underside of rocks, contain “dark” biofilms which lack algae and the nutritional organic matter that they produce. Stream invertebrates, such as mayflies, live on the underside of rocks to avoid predation and to avoid drifting, so it is likely those invertebrates take nutritional advantage of dark biofilms. These invertebrates, like many others, are a major food source for fish such as trout. Most studies analyzing aquatic consumer nutrient availability take biofilm samples, yet they are not consistent or specific as to which type of biofilm they take samples from: the light or dark biofilm. Guo and colleagues attempt to distinguish light and dark biofilm nutrient content, the impact of light biofilms on dark dark biofilms, and the consumption of biofilms by invertebrates (e.g., mayflies).
Methods: Guo and colleagues sampled biofilms (light and dark) and invertebrates during summer, fall, and winter from the subalpine River Ybbs catchment in Austria. The biofilm and invertebrate samples were analyzed for fatty acid content, which is an important organic nutrient for consumer growth and survival.
Findings: The fatty acid content of dark biofilms was found to be significantly different than the fatty acid content of light biofilms. Also, dark biofilms contained less algae-produced fatty acids than light biofilms, and the dark biofilms contained fatty acids representative of fungi and terrestrial plants. Some types of fatty acids were found to be associated with light and dark biofilms from the same sample location, indicating that light biofilms might influence the nutrient content of dark biofilms. Further, some types of fatty acids found on light and dark biofilms were associated with the fatty acid content of nearby invertebrates. In other words, those invertebrates likely fed on the dark and light biofilms, since their own fatty acid contents were similar to that of the biofilms.
Conclusions: In perhaps the first study that differentiates the nutrient contents of dark versus light biofilms, Guo and colleagues found that dark biofilms are also important contributors of physiologically meaningful fatty acids to invertebrates. Further, we now have a better understanding of how nutrients (and energy) flow through river and stream food webs. Since dark and light biofilms are different in many aspects of nutrient content, energy flow, and invertebrate feeding, future studies should specify which types of biofilms they collect from. Increased human disturbances (e.g., removing rocks) in streams and rivers likely harms the availability of biofilm-derived nutrients to aquatic consumers, which would likely lead to a disturbance of the entire aquatic food web. Future studies should focus on the role of human disturbances, climate change, and other environmental factors on dark versus light biofilm nutrient contents and how this would influence upper level consumers (such as fish and humans).
Figure: Rock collected from subalpine River Ybbs catchment. Light side biofilm (left) and dark side biofilm (right) are shown.
Reference:
Guo, Fen, et al. "The dark side of rocks: an underestimated high‐quality food resource in river ecosystems." Journal of Ecology.
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