Near-shore or open-water food? Fish like it all!

Vander Zanden, M. J. and Vadeboncoeur, Y. (2002), FISHES AS INTEGRATORS OF BENTHIC AND PELAGIC FOOD WEBS IN LAKES. Ecology, 83: 2152–2161.

doi:10.1890/0012-9658(2002)083[2152:FAIOBA]2.0.CO;2

Walleye (Sander vitreus) from the U.S. Fish and Wildlife Service (Wikimedia Commons)

Near-shore (benthic) vs. open-water (pelagic) habitats, what’s the difference?

Historically limnolgists (scientists who study lakes and other inland aquatic systems) have focused primarily on the importance of open-water (pelagic) food webs, and have treated pelagic and near-shore, lake-bottom (benthic) habitats as separate systems.

These habitats are distinct because of the types of primary producers that form the basis of their food webs. Primary producers are organisms that convert sunlight into energy which is then used by all other organisms. Pelagic habitats in the open-water zones of lakes are dominated by free-floating phytoplankton (primary producer) while benthic habitats found near-shore are dominated by periphyton (primary producer), which are attached to substrate such as sediment and rocks.

Recent evidence has contradicted the historic perspective of considering pelagic and benthic habitats as separate systems with no overlap. An increasing number of studies have recently highlighted the importance of viewing food webs from a whole-lake perspective, which includes often overlooked benthic, periphyton-dominated energy pathways. Contrary to traditional thinking, these studies posit that benthic and pelagic food webs can be interlinked through many processes. For instance, foraging fishes often move between the two habitats in search of food. Of special importance are piscivores (fish that feed on other fish) like walleye that are known for being opportunistic predators. They are not picky about habitat or prey and feeding behavior is generally driven by availability.

Fish feed from both habitats, what’s the evidence?

Based on this more recent perspective, Vander Zanden et al. (2002) conducted an empirical investigation to understand how important benthic food sources are to whole-lake ecosystems: Are the two food webs linked? Are these links energetically important? They did this by analyzing the role of benthic and pelagic prey in supporting common fish communities found in lakes common in North America. They utilized both stable isotopes and diet analysis for this. Stable isotopes of carbon are a great way to identify where a fish’s food comes from since carbon values are retained along food webs. It is easy to distinguish between pelagic sources of food, which are usually depleted in heavier carbon, and benthic sources, which are usually enriched in heavier carbon. Diet analysis, on the other hand, is done by directly observing and quantifying stomach contents of fishes. This dual approach was advantageous for scientists because stable isotope data provide an indication of consumption patterns over a longer time period, while stomach content analysis provides a snapshot indication of items recently eaten by the fish.

Overall, data from the two techniques agreed quite well and confirmed that most fish populations rely on a mix of benthic and pelagic food sources. Stomach content analysis showed an average 64% reliance on benthic energy across species. This value reflects both fish that directly consume benthic prey, and fish that indirectly rely on benthic prey (because their prey fish that are consuming benthic species). The degree to which fish relied on benthic and pelagic prey was species-specific, with some species such as rock bass exhibiting consistent reliance on benthic food sources, while species such as yellow perch relied on both habitats. By comparison, analysis of stable isotope results showed that, on average, fish relied on benthic energy for 43-59% of their diet, with open-water species such as cisco relying less on benthic food sources compared to near-shore species like pike.

Future lake food-web studies, what’s the impact?

Evidence from this study supports the whole-lake perspective when studying lake food webs, rather than treating benthic and pelagic habitats as discrete food webs. The productivity of benthic habitats is dependent on many factors, with small (shallow) lakes exhibiting high benthic production due to larger near-shore areas. Regardless, results from this study point to a general trend of common fishes relying on a mix of benthic and pelagic energy across a variety of lakes in North America.

Knowing fish reliance on benthic and pelagic habitats may help us anticipate the potential impacts of land-use and invasive species on fish communities.  Through runoff, land-use practices such as intensive agriculture provide much needed nutrients to phytoplankton. Resulting in phytoplankton population booms but suppressing periphyton populations. Invasive species such as zebra mussels drastically restructure benthic habitats and deprive fishes of food. Factors such as these can result in fish communities shifting from one diet to another. Ultimately, diet shifts usually impact size, health, and reproductive ability of fishes, with direct impacts on food security and the fishing industry. It will be interesting to study the reliance of fish on benthic food sources in other types of lakes, such as prairie lakes and tropical lakes, and compare results to these North American lakes to evaluate the universality of fish reliance on benthic habitats.

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Lushani Nanayakkara

Lushani Nanayakkara

I completed my PhD at the University of Regina in Saskatchewan, Canada. I study both the human dimensions (via stakeholder surveys) and ecological dynamics (via ecosystem surveys and stable isotopes) of aquatic ecosystems. Prior to this I completed my MSc in Environmental Sciences and Policy at Johns Hopkins University. I currently live in Ottawa, and in my spare time I love hanging out with my dog Piper, travelling, cooking and listening to podcasts. Find me on Twitter @SciPoliBoundary

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