Award: OCE-1357169

Spring algal blooms are important mechanisms to jumpstart biological production in marine and estuarine systems each year. Single-celled marine algae, collectively known as phytoplankton, create these blooms and are eaten by microscopic animals called zooplankton; in turn, zooplankton are eaten by fish. These trophic interactions determine marine food web productivity. However, some algae produce toxic chemicals to deter their grazers and suppress competitors for resources in the process called allelopathy Diatoms are an important group of phytoplankton that are known to produce these chemicals. They also are dominant bloom-forming organisms in the spring in many coastal and estuarine systems. Previous work has examined the toxicity of these chemicals, called polyunsaturated aldehydes, or PUA, on the development of marine invertebrate animals and in particular on tiny crustaceans called copepods. Copepods are probably the most numerous animal on earth, found in almost every aquatic environment. Much less work has been done to examine how these chemicals affect even smaller, single-celled zooplankton known as heterotrophic protists. Recent studies show that these microorganisms are the most important grazers of phytoplankton, responsible for more than half of the feeding on algae in the ocean, and they are also important food for copepods. The goal of this project was to determine how PUA affected the growth and feeding of the heterotrophic protists and the overall plankton food web including the algae, protists, and animal zooplankton. We hypothesized that when algae that produce the PUA were present, it would decrease the feeding by both the heterotrophic protists and the copepods, which would allow the algae populations to grow faster and create bigger blooms. To test our hypothesis, we conducted a number of field and laboratory experiments in and around the Chesapeake Bay, and in the Adriatic Sea (the Mediterranean). These locations were chosen because large blooms of diatoms known to produce PUA occur there regularly. We collected water and measured how much the algae and heterotrophic protists grew with and without PUA added to the water, and how much the heterotrophic protists and copepods ate. We were also able to look at swimming patterns of heterotrophic protists exposed to PUA, and we were able to conduct some experiments, where we also added not only the chemical PUA but also some diatoms that produced PUA that we grew in the lab. In addition, we examined food web interactions in the high Arctic, where diatoms and other algae also form dense blooms when the sea ice starts melting in the spring. Our findings show for the first time that when PUA is present, the heterotrophic protists eat fewer diatoms, more other algae, and they grew slower. In addition, in the presence of PUA copepods switch their feeding preference to eat more heterotrophic protists, although they did not stop eating the algae completely. Some of these patterns are specific to the species of heterotrophic protists and copepods, with some of them showing stronger responses than others to the addition of PUA. Overall these findings illustrate a means that algae may be affecting their grazers and competitors for resources in a way that is beneficial to the algae, allowing it to grow faster and create larger blooms (see the figure on the right). This collaborative project involved a diverse group of scientists at different stages of their careers, primarily from the University of Akron (UA) and the University of Maryland Center for Environmental Sciences (UMCES), but also from Woods Hole Oceanographic Institution and Rider University. The work in Italy was conducted with the help of collaborators from ISMAR CNR in Venice, SZN in Naples, and Marche Polytechnic University in Ancona, whereas the work in the Arctic was supported by our colleagues from the University of Troms? in Norway. The project has produced a number of peer-reviewed papers describing the findings and interpreting the data, and the findings from this work was included in K-12 STEM learning programs conducted at the UMCES and UA. Last Modified: 06/13/2019 Submitted by: James J Pierson