Zooplankton Responses to Heatwaves and Plankton Invasions in Temperate Shallow Lakes

Abstract

Temperate freshwater shallow lakes are facing ecosystem level changes from climate change and biological invasions. Under IPCC’s future extreme warming scenarios, alien species that are introduced from lower latitudes could have a competitive advantage over the native species. This could instigate an irreversible impact on the biodiversity and ecosystem of these lakes. Therefore, scientific predictions based on these scenarios might facilitate and guide measures for preparedness, remediation, and adaptation. With this intention, the effects of heatwaves and biological invasions were experimentally assessed in an outdoor experiment consisting of 24 mesocosms (containing 400 L water from Lake Ringsjön) with 2x2 factorial design of four treatments: (i) control ambient conditions reflecting current regional climate (C); (ii) heatwaves of 0-8°C (average 4°C) above ambient (H), (iii) invasion (I); and, (iv) combined heatwaves and invasion (HI). For invasion, plankton were collected from Lake Müggelsee in Berlin, Germany (about 3°C warmer than lakes in southern Sweden). I particularly investigated the impact of these treatments on dominant zooplankton species in a three-trophic level food web. For this, I conducted biweekly samplings of zooplankton for which 5 L water was filtered using nylon mesh of 55 μm pore size. All individuals of cladocerans and copepods were counted and identified at 32× magnification while approximately 200 rotifers per sample were counted at 100x magnification. Mean abundance (abundance across sampling dates) and total abundance (cumulative abundance across sampling dates) of each taxon were calculated and a Kruskal-Wallis test was performed to test for treatment effects on the total abundance. Effect sizes (Cohen’s d), generalized linear models (GLM), and Spearman correlation test were also performed to evaluate the treatment effects. The analyses showed that heatwave-induced warming created a favorable environment for an invertebrate carnivore, Mesostoma to flourish. Daphnia populations diminished severely in the heated treatments due to Mesostoma predation. Among small-bodied cladocerans, Chydorus and Bosmina were negatively affected and as a result thermophilic Ceriodaphnia became the dominant cladoceran in the heated treatments. Abundance of rotifers and copepods were not significantly affected by warming but some rotifers showed species-specific responses with Filinia and Polyarthra potentially benefiting from warmer autumn. Nauplii were negatively affected from warming which in prolonged periods could affect calanoid populations. Negative effects on Bosmina were observed from effect sizes in the invasion treatments though other conclusive evidence to support this were lacking. Lastly, the interaction effects were weak and no introduced taxa benefitted from warming. The results obtained suggests that invasion effects in a warming environment might be dampened from invertebrate predatory control but on the other hand would result in severe population loss of bigger cladocerans populations.

Popular Abstract

Zooplankton are tiny invertebrates (with some species ranging upto 6 mm in size) that live in the open waters, floating in the water column of all water bodies. In shallow lakes (≤ 0.1 km2, approx. 3 m depth) these organisms play an important role in maintaining a clear water state by feeding on algae (also called ‘phytoplankton’) and controlling them from over-blooming and degrading the lake ecosystem. Zooplankton are also a major source of food for many invertebrates and fishes in the lake. Therefore, understanding how and why the populations of different zooplankton species are changing will enable us to protect our freshwater resources.

At present, the shallow lakes in the temperate region are steadily getting warmer from global warming. Scientists from the UN’s Intergovernmental Panel on Climate Change (IPCC) have predicted that under worst case scenarios, the Earth’s could rise as much as 4℃ by the end of 21rst century and extreme climate events like heatwaves could be more intense and frequent. Temperatures in the freshwater shallow lakes, which are the most dominant type of lakes in the temperate region, might also increase in a similar manner. Such warming and heatwaves will have a drastic impact on the lake biodiversity, including zooplankton. Furthermore, scientists have also theorized that as the planet gets warmer, species from the warmer lower latitudes will track their thermal preferences and move towards relatively colder higher latitudes. These newly introduced species might have a competitive advantage over the native species and the subsequent effect of this change could be devastating for freshwater ecosystems.

Hence, in this study, through experimental means, I aimed to predict how the dominant zooplankton populations in shallow temperature lakes might be influenced by climate warming and biological invasions. In order to test the individual and interactive effects of heatwaves and invasions, 24 barrels or ‘mesocosms’ containing 400 L of water and sediment from Lake Ringsjön were put outdoors where: (i) six mesocosms were artificially heated to assess impact of heatwaves; (ii) another six were treated with a plankton invasion (where 2 L of water was added from Lake Müggelsee in Berlin, Germany, which is about 3°C warmer than lakes in southern Sweden; (iii) another six mesocosms were heated and treated with a plankton invasion; and lastly (iv) six were not affected by any treatments (‘control’) and reflected the current regional climate conditions. Heating was introduced as heatwaves fluctuating from 0°C to 8°C, averaging 4°C above the ‘control’ mesocosms’ temperatures.

The results from various statistical analyses portray varying responses of the treatments in different dominant zooplankton taxa. The smallest zooplankton, rotifers (200 to 500 micrometers in size) had species-specific responses to warming with soft-bodied Filinia and Polyarthra showing signs of benefiting from warmer autumn temperatures. Major positive impact of warming was seen on an invertebrate carnivore Mesostoma (max 6 mm in size) while severe negative impact was observed on large-bodied cladoceran, Daphnia (0.2 to 6 mm in size) most likely due to increased predation from Mesostoma. Small-bodied cladoceran, Bosmina and Chydorus (0.2 to 0.6 mm in size) populations also decreased from warming. The other small-bodied cladoceran, Ceriodaphnia (max 1 mm in size) was not affected hence replaced Daphnia as the dominant cladocera in heated mesocosms. Adult copepods (max 2 mm in size) were not significantly affected from warming however their babies, nauplii were negatively affected which could have subsequent impact from prolonged warming conditions on the adult copepod populations.
Negative impact of invasion was observed on Bosmina though conclusive evidence to support this observation were lacking. Lastly, the interaction effects were weak and no introduced taxa benefitted from warming. The results obtained suggest that invasion effects in a warming environment might be dampened from invertebrate predatory control but on the other hand would result in severe population loss of bigger cladocerans populations. The loss of major herbivore zooplankton (which mainly includes large-bodied cladocerans species) could help harmful algae to proliferate and bloom, thereby causing severe negative effects on the shallow freshwater ecosystems.

Masters Degree Project 45 credits
Advisor: Romana Salis
Department of Biology, Lund University

• author: Devkota, Nischal
• supervisor: Romana Salis
• organization: Degree Projects in Biology
• course: BION02 20202
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9047403
• date added to LUP: 2021-05-31 14:14:20
• date last changed: 2021-05-31 14:14:20