Tiny grazers love to eat their greens

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, members of the Harvey and Whalen labs 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.

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!

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. 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

What do oceanographers do? They study tiny organisms!


I’ll be entering my senior year at Haverford College as an undergraduate in the biological sciences, and I’ve worked with Dr. Kristen Whalen since last summer right before she started as a biology professor. Under her guidance I have been studying compounds that marine bacteria produce (i.e. natural products) and their usefulness in combating antibiotic resistance in bacteria. So I don’t have any prior oceanographic experience and didn’t know much at all about phytoplankton before MesoHux2017. However, since coming to Norway I do know (and have learned the past couple of weeks) a little bit more about bacteria in the ocean!

In this mesocosm experiment, we are also interested in marine bacterial natural products, but for much different purposes than what I study in College. Bacteria produce a multitude of natural products, depending on bacterial species, where they live, and what kinds of stresses they are experiencing. Many of these are not essential for survival but are only produced under certain conditions, and sometimes we don’t know why bacteria produce them in the first place. They can produce things like antibiotics, molecules that scavenge for sparse nutrients such as iron, etc. and also infochemicals that allow them to communicate and coordinate with each other to enhance their survival as a community.

Bacteria like to colonize and live as communities of cells, so when they’re in the ocean they like to colonize on ocean surfaces – kind of like particle hitchhikers. One great example of a surface in the ocean is larger organisms such as phytoplankton! During bloom events, periods when these tiny algae are locally present in very large numbers, many bacteria associate with the phytoplankton and the two have a sort of love/hate relationship – they benefit from the recycling of nutrients from each other, but at the same time are competing for those same nutrients. These interactions between phytoplankton and bacteria are essential, since they mediate the marine food web structure as well as the cycling of carbon and other important compounds in the ocean.

Understanding the interactions of bacteria and phytoplankton and how bacteria can influence phytoplankton bloom development will give us more insight into how they might drive energy flow in the ocean and the cycling of compounds that influence climate change. These bacterial influences are not very well understood, so one thing that Liz Harvey and Kristen have been interested in is the sorts of chemicals that drive bacteria-phytoplankton interactions. Recently they discovered how HHQ, a known bacterial infochemical/quorum sensing molecule, can also control phytoplankton growth. You can read all about HHQ and how we will study its effects in the field over at Kristen’s blog post (link)!

For us to be able to study the questions that we are interested in, we need to go out on the boat early in the morning, collect a lot of water from the mesocosm bags, and filter, and filter, and filter to get the samples that we want (in our case, bacterial DNA and phytoplankton RNA from total of almost 500 liters of water!). I’ve never done any sort of field work before, and one thing that I did not expect is how much heavy lifting it involves. Oceanographers need to be tough and brave the elements for the sake of science. But even with this great courage we still have time for fun out on the dock – we always check up on our resident mesocosm seagull mom-to-be (pictured below), and some of us like to sing/dance along to music blasting from the tiny radio in the shed while others prefer to examine, identify, and take pictures of the jellyfish floating around in the water.

I’ve learned so much during my time at MesoHux2017, including (but not limited to) various new techniques in the lab, various new techniques in the kitchen, how to safely operate a motorboat and almost park it successfully, and, most importantly, the importance of scientific collaboration. Having the opportunity to participate on this team and attend the nightly science team meetings has given me insight on the process of real science and real collaboration. Since the nature of this research and phytoplankton bloom dynamics in general is so unknown, a lot of things happen that are unexpected and hard to explain, and it takes a lot of data sharing and constant exchange of ideas and questions to figure out what’s going on and decide on an often-changing game plan. It’s the sort of environment where when one person gets very encouraging data after a long time of hoping, she’s met by cheers and happy dances as she spreads the news around the lab. Watching this sort of real-time, collaborative problem solving has been eye-opening, and it’s really exciting to be a part of this project.


Anna Schrecengost