Light levels limit lake phytoplankton response to fertilization

One might naively think that because we humans are land-dwelling creatures, our impact on aquatic ecosystems might be relatively minor. Unfortunately, this assumption is incorrect, as human activities are changing aquatic environments in profound ways that influence how aquatic species survive and interact. Global warming is increasing lake and river temperatures, uncontrolled development is causing some streams to run dry and others to flood, and agricultural practices are adding nutrients to many lakes and streams. Because these human impacts occur simultaneously, it is difficult to evaluate how each factor contributes to the observed changes in species relations.

In northern Sweden, lakes vary naturally in the amount of dissolved organic carbon (DOC) they contain. DOC comes from runoff of decaying plant matter, so lakes surrounded by substantial vegetation, or that experience a great deal of water input (runoff) from the surrounding area, would have higher DOC than other lakes. DOC is potentially very important to lakes, because DOC tends to discolor a lake, which reduces light penetration and slows down photosynthesis. On the positive side, carbon may bond to other molecules such as phosphorus and nitrogen, which are important nutrients that may be in short supply in these relatively infertile lakes.   Anne Deininger and her colleagues focused their studies on two factors: DOC and nitrogen. Most lakes have too much nitrogen, a result of excessive use of nitrogen fertilizers that run off into lakes, so these relatively low-nitrogen lakes provided the researchers with a unique opportunity to see how these two factors, DOC and nitrogen, interacted in a natural ecosystem.

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Low DOC control lake. Credit: M. Klaus

The researchers selected six lakes that varied naturally in DOC levels: two low (~7 mg DOC/liter), two medium (~11 mg DOC/liter), and two high (~20 mg DOC/liter). In 2011 they measured everything possible about each lake: abundance of all of the life forms, DOC, temperature, light levels, nutrients and photosynthetic rates. In 2012 and 2013, they supplemented one of each pair of lakes with nitrogen compounds every one to two weeks. The added nitrogen was equivalent to the higher nitrogen inputs that are experienced by lakes in southern Sweden. And, as you might expect, the researchers continued measuring all factors of interest in both the experimental (fertilized) and control (unfertilized) lakes throughout the year – at least until the lakes froze over.


Anne Deininger (in orange) and Sonja Prideaux collect samples from a lake. Credit: M. Deininger.

Deininger and her colleagues were most interested in differences in the abundance of phytoplankton – small free-floating photosynthetic organisms, because these are the primary producers – the organisms that produce the chemical energy (via photosynthesis) that enters food webs. There are many different types or groups of these phytoplankton; some are flagellated, with hair-like processes that allow them to navigate in the water column. Some are exclusively autotrophs, producing their own energy from photosynthesis, some are primarily hetrotrophic, eating other organisms or the remains of dead organisms, while others are mixotrophs, using both strategies to produce energy. Cyanobacteria are photosynthetic bacteria, while picophytoplankton are phytoplankton of unusually small size.

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Flagellated phytoplankton (Cryptomonas). Illustration by Anne Deininger.

Many important findings are summarized in the graph below. “B” represents the year before fertilization (2011), while “A1” is 2012 (after fertilization – 1st year) and “A2” is 2013 (after fertilization – 2nd year). Remember only the N-lakes were fertilized; the control lakes were simply monitored all three years. One finding is that in 2011, the high DOC lakes had the lowest phytoplankton abundance.  A second is that the low and medium DOC lakes had both flagellated and non-flagellated phytoplankton, while the high DOC lakes were dominated by flagellated phytoplankton.

Moving to the years after fertilization (A1 and A2), you can see that nitrogen fertilization increased phytoplankton abundance, but more so for the low-DOC lake. However, fertilization had little impact on the types of phytoplankton found in each lake; rather it simply increased the abundance of already existing groups.


Mean biomass of major phytoplankton groups in relation to DOC.  Recall that B refers to 2011 (the year before fertilization), while A1 and A2 refer to the two years after fertilization (2012, 2013).

The data can be organized so we can get a better view of what is happening quantitatively. Fertilization increases phytoplankton biomass, but much more for lakes with low DOC levels. In addition DOC appears to decrease phytoplankton abundance.


Deininger and her colleagues conclude that in these northern lakes, phytoplankton production is nutrient-limited at low DOC levels, but becomes limited by light availability in more murky waters. So adding nitrogen increases phytoplankton abundance to a greater extent in low DOC lakes. High DOC lakes have more flagellated autotrophs, as these species can swim to the top of the water column where there is more light for photosynthesis. As needed, flagellated phytoplankton can move lower in the water column where nutrients are more abundant.

The researchers emphasize that the nitrogen experiments were only conducted for two years. They don’t know if, for example, the types of species would change if fertilization continued for more than two years. They also don’t know if after 2013, the communities reverted to their pre-fertilization state, or if biomasses remained higher when nitrogen fertilization stopped. These types of questions are important to pursue because we humans are making drastic changes to most of our aquatic systems in a very uncontrolled manner. We need to understand the effects of these changes to the aquatic environment, and also how we can reverse the effects should they prove to be highly detrimental.

note: the paper that describes this research is from the journal Ecology. The reference is Deininger, A., Faithfull, C. L., & Bergström, A. K. (2017). Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon. Ecology98(4), 982-994. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Urban frogs shed no blood

Life is a series of tradeoffs. As one example, we humans have the opportunity (if we are fortunate enough to be given choices) to opt for an urban or rural existence. The urban life is quicker-paced, offers more cultural opportunities, and can be annoyingly noisy and polluted. The rural existence is more laid-back, has fewer cultural opportunities, and may provide a peaceful and relatively unpolluted environment. These different environments can profoundly affect how we feel, with some people being stressed-out by cities and others by farms. On a personal level, I was born in New York City and now live in a very small town in Virginia – I was one of the fortunate ones who was given a choice.


Panama City skyline. Credit: Mariordo (Mario Roberto Duran Ortiz)

Heat, light and noise pollution are common in and near cities, and can influence the distribution and behavior of individuals of many different species. But these factors don’t only operate individually; they can work interactively. In other words, someone might not be annoyed by flashing light nor by loud noise, but might find the combination of the two very disturbing. In addition, these factors might not only operate on individuals; they can also affect relationships, or interactions. For example, loud noises generated by natural gas wells have been shown to influence the abundance of seed predators and seed dispersers, ultimately reducing the number of newly-established pine trees.

Armed with this understanding of interactions and relationships, Taegan McMahon and her colleagues wondered how the combination of heat, light and noise pollution might affect urban túngara frogs (Engystomops pustulosus) in comparison to their more rural counterparts. McMahon had observed that urban frogs were not being swarmed by small Corethrella midges that bite them and suck their blood in more rural and forested areas. These midges carry parasites, and if her observation was correct, urban frogs might have lower exposure to some diseases than do their rural counterparts.


Corethrella midge biting a túngara frog. Credit: Taegan McMahon.

The researchers surveyed 49 túngara frog calling sites in urban (Panama City) and rural (near the small town of Gamboa) areas. At each site they counted the number of frogs, number of midges on or above the frogs, the number of frog egg masses (in foam nests), and measured the air temperature, and the light and sound intensity. As expected, urban calling sites were lighter, noisier and warmer. There were slightly more (statistically insignificant) frogs at the urban sites and considerably more egg masses at rural sites. But the dramatic finding was that there were no midges to be seen on or near any urban frogs. So it might have been hot, bright and noisy, but at least those urban frogs were unbitten!

Factor Urban Rural
Light intensity 0.16 ± 0.02 lx 0.11 ± 0.02 lx
Noise intensity 69.0 ± 0.80 dB 59.2 ± 1.00 dB
Temperature 27.6° ± 0.09°C 25.9° ± 0.04°C
Túngara frog abundance 6.09 ± 2.63 frogs/site 4.05 ± 1.11 frogs/site
Foam nest abundance 0.24 ± 0.23 nests/site 2.06 ± 0.74 nests/site
Frog-biting midge abundance 0.00 ± 0.00 midges/site 67.75 ± 43.27 midges/site

Values are means ± standard error.

Analysis of the field survey data showed that temperature did not influence midge abundance but that light and noise were both important. Interestingly, light and noise interacted with each other in an interesting way. At low sound levels (below 65 db) light was important, in that midge abundance decreased at higher light intensity (Figure A). But at high sound levels, it could be pitch black and you would still have no midges (Figure B).


Log(number of midges) in relation to light levels, in field surveys in which noise levels were (A) below 65 db or (B) above 65 db.

Did light and noise somehow influence a midge’s ability to locate a frog? The researchers set up an experiment to see whether midges were attracted to frog calls at low, medium and high light intensities, and low, medium and high sound intensities. The sounds were recordings of Panama City traffic noise. At the same time, the researchers also broadcast the mating calls of túngara frogs at their normal calling intensity (which is remarkably loud for a small animal). They then counted the number of midges attracted to these traps, which were positioned in a rural setting.


Two calling túngara frogs competing for a female’s attention. Credit: Taegan McMahon.

At low light intensities, many midges were attracted to the recorded frog calls, but city noise (low or high) greatly reduced this attraction. At medium light intensity, fewer midges were attracted to frog calls, and again city noise reduced this attraction. Finally, at high light, even fewer midges were attracted to frog calls, regardless of noise.


Number of midges attracted to recordings of túngara frog calls in relation to light and sound intensity.

McMahon and her colleagues conclude that city noise and light pollution work together to disrupt the frog-biting midges host-parasite interaction. However, the overall impact of urbanization on túngara frogs is unclear at this point. Frogs can lose up to 10% of their blood volume to midges in a night of active calling. Frog-biting midges can transmit blood parasites such as Trypanosoma tungarae to túngara frogs, so urban frogs may be liberated from this scourge. A midge-free existence may allow urban male túngara frogs to call louder and for longer periods of time, which would make them more attractive to females. However, loud and long calling has also been shown to attract the túngara frogs’s mortal enemy, the voracious frog-eating bat. The researchers call for more research on how urbanization can affect species interactions, and for greater consideration of how different forms of pollution can interact to influence ecosystem dynamics.

note: the paper that describes this research is from the journal Ecology. The reference is McMahon, T. A., Rohr, J. R., & Bernal, X. E. (2017). Light and noise pollution interact to disrupt interspecific interactions. Ecology, 98(5), 1290-1299. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.