Land-to-Lake Resource Subsidies

Studying how terrestrial resource subsidies and freshwater mussels influence lake ecosystems in Ontario, Canada

The Main Project

I spent the summers of 2016 and 2017 at the Vale Living with Lakes Centre in Sudbury, Canada, working on a large-scale research project run by Dr Andrew Tanentzap of the University of Cambridge’s Ecosystem and Global Change Research Group.

The idea behind studying land-to-lake resource subsidies is that terrestrial systems can input organic matter into lakes and affect how aquatic ecosystems function. Previous research by the same research team had shown that ‘forests fuel fish growth’ by inputting organic matter into freshwater deltas, which enhanced bacterial and then zooplankton biomass (Tanentzap et al. 2014). The Sudbury landscape was once so degraded due to mining in the area that it looked more like a moonscape. However, in recent years, mining remediation efforts have restored large swathes of the Sudbury landscape. With forests having appeared in an area that was previously barren, Sudbury provided an ideal location for investigating how forest regeneration affects nearby lakes.

In short, we collected leaves of different species of tree and mulched them into different sizes. We then used different mixtures of these leaves as terrestrial organic matter (tOM) and inorganic materials (clay, sand and gravel) to create artificial lake sediments of differing percentages of tOM. The sediments were loaded into mesocosms, which we placed in three different lakes.

In the side of each mesocosm we attached a syringe and clear tubing (with a float on top for ID), which allowed us to take water samples from the sediment of each mesocosm. Using a variety of methods and specialist equipment, including a TOC analyser, gas chromatograph and spectrophotometer, we were able to obtain measures of dissolved organic carbon (DOC), methane, nitrogen and phosphorus from the water samples. We also took surface phytoplankton measures using a BenthoTorch (a chlorophyll fluorometer) and sampled the zooplankton with emergence traps. All of these measures were used to determine how tOM additions affect lake ecosystems.

My Project

Alongside eastablishing and monitoring the main mesocosms, I got to design my own experiment for my final year undergraduate research project. Using the same methodology as above, I made mesocosms of 5% and 35% tOM but added freshwater mussels (Elliptio complanata) as well. By burrowing into sediments (bioturbation), depositing faeces and pseudofaeces, and filtering vast quantities of water, mussels can substantially modify the physical state of their habitat and surrounding biotic communities. I therefore wanted to know whether freshwater mussels, which are common in lake sediments, would alter how the addition of tOM to lakes affected sediment chemistry and benthic communities.

Freshwater mussel (Elliptio complanata) with its foot visible.

Methods: The experiment was set up with a cross-over design of the two tOM treatments (5% and 35%) and four mussel treatments. These four mussel treatments were live mussels at a low density, live mussels at a high density, ‘sham’ mussels at a low density and a mussel-free control. Sham mussels were empty shells filled with sand and were included to tease apart responses to the physical presence of shells versus the biological activity of mussels. Over a 1 month period I recorded the sediment chemistry (DOC, nitrogen and phosphorus), littoral organisms (benthic diatoms and zooplankton) and mussel growth.

Zooplankton emergence traps – inverted funnels that trap zooplankton as they migrate upwards off the sediment surface.

Results: At high mussel densities there was a 90%, 80%, 45% and 40% reduction in phosphorus, dissolved organic carbon, nitrogen and benthic diatoms, respectively, whereas at low mussel densities there was a 3-fold increase in zooplankton. These reductions were likely caused by bioturbation and trophic interactions between species. Benthic diatom concentrations were also reduced by 20% in sediments of 35% tOM, likely due to shading and competition with bacteria. Mussel growth increased at high mussel densities but was offset at high tOM, likely due to the organic matter interfering with filter feeding. These results suggest that mussels can alter the geochemical composition of sediments and abundances of associated littoral organisms, in some cases regardless of tOM quantity. Therefore, the dominant top-down control exerted by freshwater mussels may outweigh bottom-up effects of tOM additions.

We published the findings of this study in Science of the Total Environment in 2018, so you can read the full study: Mussels can both outweigh and interact with the effects of terrestrial to freshwater resource subsidies on littoral benthic communities.

Being the nature-lover that I am, it wouldn’t be right if I didn’t finish this page with some incredible wildlife sightings from my two summers in Canada:

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