What drives benthic primary production in north-temperate lakes?
2014-11-03
2014-11-03
Godwin, S.C., Solomon, C.T., Weidel, B.C., Jones, S.E. 2014. Dissolved organic carbon concentration controls benthic primary production: Results from in situ chambers in north-temperate lakes. Limnology and Oceanography. 59(6):2112-2120. [PDF]
Extracting data from dissolved oxygen sensors
Primary production is one of those many processes that we generally value more on land than we do in aquatic environments. This perhaps unsurprising; it is far easier to appreciate the leafy greens that we eat and the trees that we see than the vague and nebulous concept of algal photosynthesis. But the importance of primary production to food webs, both terrestrial and aquatic, should not be underestimated.
Ignoring some exceptions, primary production is the process by which photosynthesizing organisms convert light, water, and carbon dioxide into chemical energy and oxygen. Primary producers are eaten by primary consumers, like deer in terrestrial systems or zooplankton in aquatic ones, which are in turn eaten by secondary consumers, and so on. It is through these interactions that chemical energy makes its way up the food chain and algae find themselves forming its essential base.
Historically, lake primary production was thought to be performed mainly by free-floating microalgae called phytoplankton. However, recent work has shown that algae attached to benthic structures - those on or near the bottom of the lake - contribute a substantial portion of whole-lake primary production and actually dominate production in shallow, clearwater lakes.
But what controls benthic primary production? Previous research gives us some insights, but leaves questions as well. High phosphorous and dissolved organic carbon (DOC) concentrations can reduce benthic primary production, as can steep lake floors, but the lakes in these studies represent fairly extreme lake types. Most lakes that are relatively untouched by humans have more intermediate nutrient availability than those in these studies, as well as larger temperature fluctuations and a wider range of DOC concentrations.
Ignoring some exceptions, primary production is the process by which photosynthesizing organisms convert light, water, and carbon dioxide into chemical energy and oxygen. Primary producers are eaten by primary consumers, like deer in terrestrial systems or zooplankton in aquatic ones, which are in turn eaten by secondary consumers, and so on. It is through these interactions that chemical energy makes its way up the food chain and algae find themselves forming its essential base.
Historically, lake primary production was thought to be performed mainly by free-floating microalgae called phytoplankton. However, recent work has shown that algae attached to benthic structures - those on or near the bottom of the lake - contribute a substantial portion of whole-lake primary production and actually dominate production in shallow, clearwater lakes.
But what controls benthic primary production? Previous research gives us some insights, but leaves questions as well. High phosphorous and dissolved organic carbon (DOC) concentrations can reduce benthic primary production, as can steep lake floors, but the lakes in these studies represent fairly extreme lake types. Most lakes that are relatively untouched by humans have more intermediate nutrient availability than those in these studies, as well as larger temperature fluctuations and a wider range of DOC concentrations.
A benthic chamber for measuring benthic primary production
We can calculate benthic primary production rates by measuring the change in dissolved oxygen concentration of the water just above the lake floor. During photosynthesis, algae produce oxygen; some estimate the algal contribution to global oxygen production to be somewhere around 70%. During the day, when there is plenty of light for algae to use for photosynthesis, dissolved oxygen concentrations go up in aquatic systems. During the night, when oxygen consumption by lake organisms outweighs oxygen production by algae, dissolved oxygen concentrations go down. By using sensors (above) inside of dome-like benthic chambers (left), we can measure changes in dissolved oxygen concentrations and calculate benthic primary production rates.
For this study, we measured benthic primary production rates across a suite of north-temperate lakes. By measuring benthic production at several depths in these lakes over two summers, we were able to estimate benthic primary production rates at the whole-lake level and evaluate which lake characteristics best predicted these rates. We found that DOC was by far the most important predictor of benthic primary production (below) and our model fit the data extremely well. This is fairly intuitive; as DOC increases, lakes get darker and benthic primary production rates decline.
For this study, we measured benthic primary production rates across a suite of north-temperate lakes. By measuring benthic production at several depths in these lakes over two summers, we were able to estimate benthic primary production rates at the whole-lake level and evaluate which lake characteristics best predicted these rates. We found that DOC was by far the most important predictor of benthic primary production (below) and our model fit the data extremely well. This is fairly intuitive; as DOC increases, lakes get darker and benthic primary production rates decline.
Benthic primary production decreases with DOC. Each pointrepresents one lake.
This is an interesting result given the current trend of increasing DOC in Northern Hemisphere lakes. In order to predict how lake processes will respond to global shifts such as lake browning (i.e. increasing DOC), climate change, and nutrient availability, we need to understand how these processes respond to environmental change. To take the first step towards this understanding for the benthic algae and DOC interaction, we tested whether deeper algal assemblages were more efficient with their use of light during photosynthesis and found that indeed they were. To our knowledge, this is the first evidence for photoadaptation of lake algae.
To summarize, we found that DOC likely drives benthic primary production in relatively pristine north-temperate lakes and that deeper algal assemblages are better adapted to photosynthesizing in low-light environments. These results are important in the context of the browning of lakes globally and the key role that benthic algae plays in lake food webs.
To summarize, we found that DOC likely drives benthic primary production in relatively pristine north-temperate lakes and that deeper algal assemblages are better adapted to photosynthesizing in low-light environments. These results are important in the context of the browning of lakes globally and the key role that benthic algae plays in lake food webs.