Universität zu Köln

Iannino, Alessandra ORCID: 0000-0001-6098-9840 (2020). Effects of phosphorus enrichment on the control of stream periphyton. PhD thesis, Universität zu Köln.

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Abstract

The release of nutrients into aquatic ecosystems has been dramatically increasing over the past decades as a consequence of human activities, often leading to an excessive growth of primary producers. While the effects of harmful algal blooms in lakes have been widely studied, less is known about the consequences of nutrient enrichment for stream food webs, which are strongly dominated by benthic habitats. The growth of benthic algal communities (periphyton) in streams is controlled by nutrient availability (bottom-up control) and by the grazing activity of herbivores (top-down control), which is known to counteract the effects of nutrient enrichment on algal biomass. However, nutrient availability may also affect the nutritional quality of periphyton, which can in turn influence the growth and behaviour of grazers and hence the strength of top-down control. Furthermore, nutrient enrichment may alter periphyton spatial heterogeneity, an important ecosystem aspect that mediates trophic interactions and favours biodiversity. In the present thesis I investigate the effects of phosphorus enrichment on the interaction between bottom-up and top-down control of stream periphyton, with a focus on periphyton nutritional quality for herbivores and periphyton spatial heterogeneity. In a microcosm experiment with the rheophilic gastropod Ancylus fluviatilis, I demonstrate that phosphorus enrichment may lead to a weaker top-down control of stream periphyton, because grazers need lower amounts of nutrient-rich food to maintain an optimal soft body stoichiometry, compared to nutrient-poor food. In addition, I demonstrate that, in the long term, phosphorus enrichment may lead to an overall lower periphyton nutritional quality for grazers by favouring the growth of cyanobacteria and chlorophytes over diatoms. I demonstrate for the first time that, when such a taxonomic shift occurs, phosphorus enrichment decreases the relative phosphorus content of periphyton, because cyanobacteria and chlorophytes generally have higher C:P ratios than diatoms. I observed both positive and negative effects of phosphorus enrichment on periphyton heterogeneity, depending on other factors. In the presence of grazers, phosphorus enrichment resulted in a more homogeneous periphyton biomass, possibly because it led to a homogeneous nutrient content of periphyton and therefore a more even grazer distribution. However, in an environment with heterogeneous light conditions, phosphorus enrichment resulted in a more heterogeneous periphyton biomass, while the heterogeneity of periphyton relative phosphorus content was not affected by phosphorus availability, because periphyton patches of different sizes acquired nutrients in proportion to their biomass. The present thesis shows that nutrient enrichment can have a significant impact on the control of stream periphyton. In addition to directly increasing algal production, it can alter periphyton nutritional quality and spatial heterogeneity, and therefore affect the growth and behaviour of grazers. Nutrient enrichment can thus interfere with the balance between bottom-up and top-down control of stream periphyton, with possibly harmful consequences for stream food webs.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Iannino, Alessandra alessandra.iannino@ufz.de orcid.org/0000-0001-6098-9840 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-298255
Date:	8 December 2020
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Biology > Zoologisches Institut
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language eutrophication English periphyton English stream English
Date of oral exam:	17 August 2020
Referee:	Name Academic Title Fink, Patrick PD Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/29825