Tiny grazers love to eat their greens

This blog is from a mesocosm experiment (giant (5m by 8m deep) ‘bags’ placed into environments and filled with natural water) conducted during May 2017 (find more info here). It was written by myself & Sean Anderson (a graduate student at the Skidaway Institute, University of Georgia, USA).

Members of the Harvey Lab get really excited about phytoplankton and for good reason. Phytoplankton are super important, as they form the base of food webs and govern the transport of nutrients and carbon in the ocean. Phytoplankton populations fluctuate, sometimes growing high in numbers if they receive plenty of sunlight or nutrients. Other times, phytoplankton experience mortality, mainly from hungry grazers called microzooplankton (slightly larger carnivorous plankton) or from infectious viruses. In the Harvey lab, we are especially interested in the balance between phytoplankton life and death and the implications these life dynamics have on the food web. The mesocosms offer us a rare opportunity to assess growth and grazing over time and under different sunlight and nutrient conditions.

Every other day we conduct a phytoplankton grazing experiment. A typical day begins at 0600, rain or shine. After a much-needed slug of gull (Norwegian for gold) coffee, we pack up a small boat with collection bottles and head out to the mesocosm raft for sampling. The sampling is intensive and takes 2 and ½ hours to collect surface water (around 80 liters total) from all 12 mesocosm bags. We did find a local Norwegian radio station that blasts sweet sounds of the 80’s, which helps keep our early morning moral high. The water collected from each mesocosm bag is screened through a 200 µm mesh, which ensures we retain phytoplankton and their dominant microzooplankton grazers.

The Harvey lab heading out to sample on a rare sunny day in Bergen (top). Our boat (bottom) ready for a 6am sampling trip! 

Back on land, the fun truly begins. Some of the water from each mesocosm treatment is mixed with normal filtered seawater, that is free of any plankton. This technique allows us to dilute the natural community (phytoplankton, grazers and viruses) and assumes that phytoplankton growth rates remain unchanged, while mortality rates vary proportional to dilution. Whole and diluted samples from each mesocosm treatment are filled into 1-liter bottles (54 total bottles) and placed in an outside seawater tank for 24 hours. The next day we retrieve bottles and filter them for various parameters. Chief among them is chlorophyll, which is found in all photosynthetic cells (the green color) and is used as a proxy for phytoplankton in the water. By comparing changes in daily chlorophyll in both the whole and diluted samples, we can directly measure phytoplankton growth, grazing and viral lysis rates!

Sean collecting water using a niskin bottle deployed into the mesocosm bags (left). Our on land incubation tank (right). We use a screen on top to mimic the light algae would receive at 1m depth.

There are also important zooplankton grazers larger than 200 µm (called mesozooplankton), which can be small crustaceans or giant jellyfish. We are interested in mesozooplankton, as they can induce what is called a “trophic cascade”. They do this by consuming the microzooplankton grazers of phytoplankton, freeing them from predation pressure and allowing them to rapidly grow. To study the impact of mesozooplankton we use two main methods. One is to not screen the water coming from the mesocosms, this allows us to see how the natural community (including larger grazers) changes over 24 hours. The other is to collect zooplankton using a plankton net, and then carefully pick zooplankton under a microscope and add them to our bottles. This can be quite challenging when the zooplankton are < 1 millimetre in size.

Sean and I using a zooplankton net to collect mesozooplankton from the mesocosm bags

By comparing the results from all our experiments, for micro- and mesozooplankton, and viral mortality we can begin to see how phytoplankton mortality is divided. In the case of mesozooplankton, we can also observe if, and how much of a trophic cascade they may be inducing. These observations will help us to understand the dynamics of phytoplankton populations, and their associated impacts on the marine food web.

Sean Anderson & Kyle Mayers

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