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Daphnia galeata performance along food quantity gradients of mixed Rhodomonas, Pseudomonas and terrestrial particulate organic material
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Unproductive lakes are often characterized by high inputs of allochthonous dissolved and particulate organic carbon compared to autochthonous carbon, resulting in low phytoplankton abundances and high heterotrophic bacterial biomass. Thus, metazooplankton in these lakes feed on a mixture of phytoplankton, bacteria and terrestrial particulate organic material (t-POM) where the proportions of these components can be highly variable. As the nutritional value of the different food components varies, the dilution of phytoplankton (high-quality food) with high proportions of bacteria and/or t-POM (low-quality food) is expected to affect the performance of unselective filter feeders such as Daphnia. We tested how different combinations of constant concentrations of phytoplankton and increasing additions of bacterioplankton or t-POM affect zooplankton performance. We found that increasing the concentrations of both the bacterium Pseudomonas and t-POM relative to phytoplankton (Rhodomonas) negatively affected Daphnia survival, growth and reproduction when Rhodomonas was available at non-limiting concentrations (0.37 and 0.55 mgC L-1). When Rhodomonas quantity was low (0.22 mgC L-1), the addition of Pseudomonas led to better Daphnia performance except at very high concentrations of the bacterium relative to Rhodomonas. In contrast, the addition of t-POM was detrimental for overall Daphnia performance at low Rhodomonas quantity. These results imply that the ability of zooplankton to use low-quality food is affected by the availability of high-quality food. Further, food sources that do not fulfil dietary requirements of grazers may still provide nutritional benefits as long as other complimentary food components are available in sufficient quantities to compensate for mineral and/or biochemical deficiencies.

Keyword [en]
heterotrophic bacteria, phytoplankton, food quality, allochthonous, autochthonous, zooplankton
National Category
Ecology
Research subject
Limnology
Identifiers
URN: urn:nbn:se:umu:diva-62277OAI: oai:DiVA.org:umu-62277DiVA: diva2:576951
Funder
FormasSwedish Research Council, 2007-5523
Available from: 2012-12-14 Created: 2012-12-14 Last updated: 2012-12-17
In thesis
1. The role of terrestrial and phytoplankton-derived organic matter in planktonic food webs
Open this publication in new window or tab >>The role of terrestrial and phytoplankton-derived organic matter in planktonic food webs
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lakes are important global ecosystems and many of them are nutrient-poor (unproductive). Especially in northern boreal latitudes, lakes may be heavily subsidized by terrestrial organic material (t-OM) from peat layers in the catchment. Thus, in addition to heterotrophic bacteria and phytoplankton, zooplankton may also use the particulate fraction of peat layer t-OM (t-POM) as a potential food source in those systems. Inputs of t-OM in northern latitudes are anticipated to increase in the future due to increasing precipitation and temperature. As t-OM is a good substrate for bacterial growth and as bacteria can often outcompete phytoplankton for inorganic nutrients, the proportions of heterotrophic bacteria and phytoplankton are expected to change in unproductive lakes. This may have pronounced impacts on zooplankton population dynamics.

The aim of my thesis was to investigate how changes in food quality and quantity will affect metazoan zooplankton performance in unproductive lakes. Three laboratory studies assessed the quality of specific food components (phytoplankton, bacteria and peat layer t-POM) and their effects on Daphnia survival, growth and reproduction. Further, a mesocosm study with a full natural plankton community tested the predictions of the Light:Nutrient-Hypothesis in an unproductive clear water lake in situ by adding carbon and inorganic nutrients and changing light availability.

I found that pure bacterial (Pseudomonas sp.) or t-POM diets could not sustain Daphnia populations, even though both were readily ingested. Daphnids needed at least 10-20% phytoplankton (Rhodomonas) in the diet to survive and even higher proportions (≥ 50%) were necessary for the production of viable offspring. Further, I showed that the dilution of non-limiting concentrations of Rhodomonas with increasing proportions of Pseudomonas or t-POM led to decreased Daphnia performance. Both Pseudomonas and t-POM lack essential biochemicals (fatty acids and sterols). In contrast, mineral nutrient limitation only occurred on t-POM-dominated diets as evidenced by a labeling experiment that showed Daphnia can incorporate carbon and phosphorus from Rhodomonas and Pseudomonas with similar efficiencies. Thus, peat layer t-POM was a lower quality food than Pseudomonas. This was corroborated by the finding that intermediate additions of Pseudomonas to limiting amounts of Rhodomonas supported increased Daphnia survival, growth and reproduction while t-POM additions had no beneficial effect.

My results suggest that high terrestrial stable isotope signals in metazoan zooplankton are most likely derived from t-OM that is channeled tohigher trophic levels via the microbial loop (i.e. heterotrophic bacteria and/or bacterivorous protozoa) but not from direct metazoan feeding on t-POM. Furthermore, bacteria may serve as an important supplement to zooplankton diets when phytoplankton abundance is low. However, a sufficient proportion of high quality phytoplankton is always necessary to fulfil mineral and especially biochemical requirements of zooplankton in unproductive aquatic systems.

The results of the mesocosm study showed that the Light:Nutrient-Hypothesis is not applicable to unproductive clear water systems in which the phytoplankton community is dominated by mixotrophs. In the face of the theoretical predictions, low light levels led to decreased zooplankton biomass. This was most likely caused by a shift in the algal community composition towards less edible taxa. Another reason may have been a weakening of the microbial loop. This is in line with the results of the laboratory studies that point out the importance of the microbial food web for zooplankton nutrition in unproductive lakes.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2012. 33 p.
Keyword
Daphnia, phytoplankton, heterotrophic bacteria, terrestrial particulate organic material, food quality, food quantity, fatty acids, oligotrophic, Light:Nutrient-Hypothesis
National Category
Ecology
Research subject
Limnology
Identifiers
urn:nbn:se:umu:diva-62287 (URN)978-91-7459-512-3 (ISBN)
Public defence
2013-01-25, KBC-Huset, Lilla Hörsalen (KB3A9), Umeå Universitet, Umeå, 10:15 (English)
Opponent
Supervisors
Available from: 2012-12-20 Created: 2012-12-14 Last updated: 2012-12-17Bibliographically approved

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