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.

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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.

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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.

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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.

Chironomids: the great Icelandic farmers

Ecologists describe plants and algae as producers because, with the help of solar energy, they produce sugar and other complex carbohydrates from carbon dioxide in the process we call photosynthesis. Consumers, such as horses, chironomids and lions, eat producers or other consumers, using the chemical energy in their food’s complex molecules to fuel their own metabolic processes. Without photosynthesis there would be almost no life, as consumers would need to scavenge the organic molecules that happened to be lurking about in the environment.

You’ll notice I slipped in chironomids as consumers, because they are the diminutive heroes of this tale. There are actually more than 10,000 chironomid species, but the ones we are discussing lay their fertilized eggs into lake water where they sink into the sediment and hatch out as larvae. These larvae go through several growth stages, building tubes which serve as their houses during growth and development. Larvae then transform into pupae, which after a few days swim to the surface and metamorphose into winged adults, which mate and produce more fertilized eggs.

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Chironomid Larvae. Credit Michael Drake

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Larval tubes in sediment. Credit Cristina Herren

 

 

 

 

 

 

 

We usually think about consumers as having a negative effect on their food and/or prey. If we introduce a herd of sheep into a small pasture, we are confident that the grass will quickly disappear inside of sheep bellies. Similarly, if we introduce a few lions into a pen of sheep, we can be fairly confident that the sheep population will decline. But consumer effects on populations can act in strange and wonderful ways.

The subarctic Lake Mývatn in Iceland is host to an amazingly huge number of chironomid larvae; over 100,000 of these insects can squeeze into 1 m2 of lake bottom!

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Lake Myvatn. Credit Michael Drake

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Swarming adult chironomids. Credit Michael Drake

 

 

 

 

 

As Cristina Herren describes, Lake Mývatn has long been studied because of the dramatic changes in chironomid densities. Chironomid populations can increase 10-fold in each of four or five consecutive generations during the subarctic spring and summer. So if you began with a density of 10 chironomids/m2 in April, you might find 100/m2 in May, 1000/m2 in June, 10,000/m2 in July and 100,000/m2 in August. Herren was intrigued by this exponential growth, as it suggested that somehow chironomid food resources (primarily algae) were keeping pace with the exploding chironomid population, despite being eaten by this ever-expanding population. How could the algae keep pace with the furious chironomid growth rate? Herren wondered whether the chironomids were acting as farmers and somehow stimulating algal production.

Herren worked with several other researchers to figure out the answer to this puzzle. First, they wondered whether the larval tubes provided an awesome place for algae to live. Second, perhaps chironomids were pooping-out vast quantities of nutrient-rich waste products, which algae used to build their bodies. Either or both of these positive effects could more than compensate for the direct negative effect from chironomids eating the algae.

To investigate both hypotheses, the researchers set up 1-liter clear plastic containers with lake sediment and either 0, 50, 100, 200, 400, 600, 800 or 1200 chironomids (that must have been fun to count!). They placed six or seven containers with each population size into racks at the bottom of the lake.

The first task was to discover whether production actually increased with chironomid abundance. Oxygen is a waste product of photosynthesis, so scientists use oxygen production to measure photosynthetic rate, and also to infer algal abundance. They found that oxygen production actually increased with chironomid abundance. Note that chironomids (and the algae too) actually consume oxygen (just like you and I do) in the process of respiration, so this is a truly remarkable result. It indicates that production increased so much that it more than compensated for the oxygen used up by the chironomids and algae from respiration.

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The algae use chlorophyll a in their cells as the molecule for photosynthesis, so algal abundance can be estimated by measuring how much chlorophyll a is present in each bottle. Again, they found more chlorophyll a in the bottles with more chironomids, despite the fact that chironomids were eating all the algae they could find.

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In addition, the chironomids from more crowded conditions weighed more than those from sparsely-populated bottles. The researchers concluded that by eating algae, the chironomids were actually increasing algal abundance, thereby creating more food for themselves.

These findings fly in the face of our usual assumptions about consumers reducing the population size of the species they are consuming. The researchers confirmed two important indirect effects of chironomids on algae. First, the chironomid larvae do excrete vast quantities of concentrated nutrients (nitrogen and phosphorus) that are consumed by the algae. In addition, the chironomids tubes are indeed ideal surfaces for algae to hang-out on, as evidenced by finding much more chlorophyll a on the tubes than in the nearby sediment.

We citizens need to recognize that both direct and indirect effects operate within ecosystems. In this case, algae are clearly directly benefitting chironomids by providing them with food. Less obvious, the chironomid effects on algae are in two directions. Chironomids have a direct negative effect on algal populations by consuming vast quantities of algae. However, they have a stronger indirect positive effect on algal populations, by providing algae with high quality housing and abundant resources, so that the algal population grows dramatically over the year. The net effect of chironomids on algal populations (dashed arrow) is positive.

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Most ecosystems have many different direct and indirect effects operating between a large number of species. It’s a real challenge to identify and understand these diverse interactions, which is why we need to be cautious about changing ecosystems in significant ways. For example, when we dump our waste products into waterways or soils, the added nutrients can have pronounced direct and indirect effects on the species that live there. In many cases, we won’t be able to identify the unintended consequences of our actions on an ecosystem until the native species have gone extinct.

note: the paper that describes this research is from the journal Ecology. The reference is Herren, Cristina M., et al. “Positive feedback between chironomids and algae creates net mutualism between benthic primary consumers and producers.” Ecology 98.2 (2017): 447-455. 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.