Clasen, Jessica

The effects of global and local environmental changes are transmitted through networks of interacting organisms to shape the structure of communities and the dynamics of ecosystems. We tested the impact of elevated temperature on the top-down and bottom-up forces structuring experimental freshwater pond food webs in western Canada over 16 months. Experimental warming was crossed with treatments manipulating the presence of planktivorous fish and eutrophication through enhanced nutrient supply. We found that higher temperatures produced top-heavy food webs with lower biomass of benthic and pelagic producers, equivalent biomass of Zooplankton, zoobenthos and pelagic bacteria, and more pelagic viruses. Eutrophication increased the biomass of all organisms studied, while fish had cascading positive effects on periphyton, phytoplankton and bacteria, and reduced biomass of invertebrates. Surprisingly, virus biomass was reduced in the presence of fish, suggesting the possibility for complex mechanisms of top-down control of the lytic cycle. Warming reduced the effects of eutrophication on periphyton, and magnified the already strong effects of fish on phytoplankton and bacteria. Warming, fish and nutrients all increased whole-system rates of net production despite their distinct impacts on the distribution of biomass between producers and consumers, plankton and benthos, and microbes and macrobes. Our results indicate that warming exerts a host of indirect effects on aquatic food webs mediated through shifts in the magnitudes of top-down and bottom-up forcing.

The chapter, "The vulnerability of ecosystem trophic dynamics to anthropogenically induced environmental change: A comparative approach" was written by the listed authors including Jessica L. Clasen (Douglas College Faculty). The Ecological Dissertations in the Aquatic Sciences (Eco-DAS) symposia bring together 35-40 recent PhD recipients for one week in alternate years. Eco-DAS VIII was held in 2008. Eco-DAS is sponsored by the Center for Microbial Oceanography: Research and Education (C-MORE), the University of Hawai`i School of Ocean and Earth Science and Technology (SOEST) and its Department of Oceanography, and the Association for the Sciences of Limnology and Oceanography (ASLO). The Proceedings of Eco-DAS VIII includes nine chapters published in open access. <p>We employed a comparative approach to review the vulnerability of the trophic interactions within aquatic systems to global threats associated with anthropogenic activities. The goal of this chapter was to identify and characterize mechanisms by which human-mediated environmental threats may modulate trophic dynamics across aquatic ecosystems. Trophic dynamics include some of the most obvious and pervasive factors influencing ecosystems and were used as a metric because of their importance and commonality across all aquatic environments. Our use of trophic dynamics proved to be insightful, illustrating that the flow of energy through aquatic food webs will be (or already has been) altered by invasive species, land-use change, nutrient loading, exposure to ultraviolet radiation, overharvesting, acidification, and increasing global temperatures. The response of trophic dynamics to these threats was often similar across oceans, estuaries, lakes, and rivers. This similarity proved to be interesting given the differences in both the level of concern expressed by scientists and the predicted variability in environment-specific responses. As the trophic interactions of an ecosystem are at the root of its function and structure, examining trophic dynamics could be an informative method for evaluating the response of aquatic environments to global threats. If future analyses validate the use of trophic dynamics as a metric, it is our hope that trophic dynamics can be used by scientists and politicians to mitigate the effects of human actions.

Marine ecosystems contain an immense diversity of phages, many of which infect the cyanobacteria responsible for a portion of oceanic primary productivity. To add to the growing body of research on the dynamics and diversity of these cyanophages, we measured cyanophage abundance, diversity, and community composition monthly for 15 mo at 3 coastal locations in Southern California, USA. We characterized over 900 individual cyanophages, isolated on Syne- chococcus sp. WH7803, by amplifying and sequencing 2 different genes from each isolate: g20 (the portal protein gene) and psbA (a host-derived gene involved in photosynthesis). The taxo- nomic composition and diversity of isolated cyanophages were highly variable over time, with lit- tle variation across the 3 locations. Particular taxa dominated the community at distinct times of year, with more rapid turnover of dominant types and higher overall abundance and diversity dur- ing spring and summer. These temporal patterns as well as taxonomic designations were highly congruent for both genes, based on operational taxonomic units (OTUs) defined at 99% nucleo- tide similarity, despite a greater level of amino acid conservation within psbA as compared to g20. Overall, this suggests that either gene can serve as a useful marker for cyanophage diversity. However, we detected 1 myovirus taxon (representing about 2.5% of the isolates) of uncertain phylogenetic affiliation that was negative for psbA amplification. Further sampling is necessary to assess whether these temporal dynamics in cyanophage composition, diversity, and abundance are seasonally recurring.