Date on Master's Thesis/Doctoral Dissertation


Document Type

Doctoral Dissertation

Degree Name

Ph. D.



Committee Chair

Carreiro, Margaret Mary

Author's Keywords

Feeding rate; Temperature; Jellyfish; Nutrient effects; Daphnia


Freshwater zooplankton; Global warming; Nitrogen--Environmental aspects


Aquatic ecosystems face major transformations as humans increasingly alter their environment by introducing exotic species and changing the temperature regime and nutrient availability of freshwater systems. The impacts of such alterations of conditions, resources and species composition on zooplankton are important to consider because zooplankton link primary producers to higher trophic levels. Among recent exotic invaders of freshwater systems in Europe and the Western Hemisphere is Craspedacusta sowerbii. This freshwater cnidarian can impact zooplankton abundance and community composition, thus altering ecosystem energy transfer to higher trophic levels. In experiments described in chapters 2 and 3 of this dissertation, C. sowerbii was found to have a significant and differential effect on zooplankton taxa such as Bosmina sp., Daphnia sp., and copepods. While medusa greatly reduced abundances of these genera, other potential prey items, such as Keratella sp., did not show a response to medusa presence, indicating that C. sowerbii medusa differentially alter zooplankton communities. However, while C. sowerbii did not affect all potential prey items, some species such as the large D. magna were killed at high rates but not consumed. This indicates that the effects of C. sowerbii medusa presence are not only the result of prey consumption, but also extend to their killing effect on species they cannot ingest. The rate of global warming is predicted to accelerate in the coming decades and temperate freshwater ecosystems will face alterations in response to these changing thermal conditions. Simultaneously we can expect that nitrogen input to aquatic systems will increase through the deposition of fossil fuel combustion products, fertilizer runoff and sewage inputs. Such increased inputs are having and will have greatly affected primary production in freshwater ecosystems. As nitrogen levels increase so does the probability of both primary and secondary producers becoming phosphorus limited. These alterations can have dramatic effects on zooplankton such as Daphnia, a common and important genus of Cladocera in many lakes and rivers. Together warming temperature and increased nitrogen availability are expected to affect the growth and reproduction of aquatic organisms such as Daphnia, which are often the link between primary producers and higher trophic levels. The experiments contained in Chapter 4 reveal that juvenile growth rate and reproductive output of D. magna and D. pulex, two ubiquitous species of Daphnia, can indeed be directly affected by temperature and nitrogen and phosphorus availability. Temperature was found to have the predicted positive relationship with growth rate and timing of reproduction. Food quality effects were strongest for D. pulex at intermediate temperatures near their growth optimum (15 -25 CC), with high quality food resulting in a significantly higher growth rate at these temperatures. Food quality had no detectably significant effect at the highest and lowest temperatures tested (10 and 30DC). D. magna was found to have the highest growth rate at 30DC, indicating that the optimum or maximum temperature for this species' had not been found in this study; however, food quality resulted in a higher growth rate only at 25DC. Reproductive output for both species responded strongly to temperature, with higher temperatures resulting in earlier reproduction and fewer days between clutches. Food quality had significant effects on D. magna reproduction at 25DC while D. pulex showed strong effects of food quality at the intermediate temperatures (15 and 20DC), just as was observed with this species' growth rate. The effects of these changing environmental conditions on Daphnia can potentially have fitness effects on the organism (e.g., alteration of predation risk) and cascading effects on aquatic food webs. Understanding how changes in temperature and nutrient availability as well as the introduction of exotic species will impact zooplankton communities in manipulated model systems, such as those used in these experiments, can help reveal how these changes may alter aquatic ecosystems. In order to continue to reap the benefits of healthy freshwater systems such as drinking water, food, flood mitigation, and biodiversity, it is imperative to protect the integrity of aquatic systems so critical to sustaining our planet and human communities.