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Density-dependent positive feedbacks buffer aquatic plants from interactive effects of eutrophication and predator loss
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Swedish University of Agricultural Sciences, Sweden.ORCID iD: 0000-0002-8920-9630
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0002-3560-3245
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
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Number of Authors: 62018 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 99, no 11, p. 2515-2524Article in journal (Refereed) Published
Abstract [en]

Self-facilitation allows populations to persist under disturbance by ameliorating experienced stress. In coastal ecosystems, eutrophication and declines of large predatory fish are two common disturbances that can synergistically impact habitat-forming plants by benefitting ephemeral algae. In theory, density-dependent intraspecific plant facilitation could weaken such effects by ameliorating the amount of experienced stress. Here, we tested whether and how shoot density of a common aquatic plant (Myriophyllum spicatum) alters the response of individual plants to eutrophication and exclusion of large predatory fish, using a 12-week cage experiment in the field. Results showed that high plant density benefitted individual plant performance, but only when the two stressors were combined. Epiphytic algal biomass per plant more than doubled in cages that excluded large predatory fish, indicative of a trophic cascade. Moreover, in this treatment, individual shoot biomass, as well as number of branches, increased with density when nutrients were added, but decreased with density at ambient nutrient levels. In contrast, in open cages that large predatory fish could access, epiphytic algal biomass was low and individual plant biomass and number of branches were unaffected by plant density and eutrophication. Plant performance generally decreased under fertilization, suggesting stressful conditions. Together, these results suggest that intraspecific plant facilitation occurred only when large fish exclusion (causing high epiphyte load) was accompanied by fertilization, and that intraspecific competition instead prevailed when no nutrients were added. As coastal ecosystems are increasingly exposed to multiple and often interacting stressors such as eutrophication and declines of large predatory fish, maintaining high plant density is important for ecosystem-based management.

Place, publisher, year, edition, pages
2018. Vol. 99, no 11, p. 2515-2524
Keywords [en]
aquatic vegetation, Baltic Sea, cage experiment, coastal ecosystem, eutrophication, facilitation, multiple stressors, Myriophyllum spicatum, positive feedbacks, predator loss, trophic cascade
National Category
Environmental Sciences Ecology
Identifiers
URN: urn:nbn:se:su:diva-162994DOI: 10.1002/ecy.2501ISI: 000449937900011PubMedID: 30248177OAI: oai:DiVA.org:su-162994DiVA, id: diva2:1270235
Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2021-12-01Bibliographically approved
In thesis
1. Aquatic vegetation in coastal ecosystems: The role of biotic interactions and environmental change for ecosystem functions and resilience in the Baltic Sea
Open this publication in new window or tab >>Aquatic vegetation in coastal ecosystems: The role of biotic interactions and environmental change for ecosystem functions and resilience in the Baltic Sea
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Coastal ecosystems are among the most productive on Earth but subjected to many human pressures. In shallow coastal areas, aquatic vegetation constitutes foundation species that sustain secondary production and act as a nutrient filter, which may buffer human impacts. But little is known about how anthropogenic factors alter biotic interactions in aquatic vegetation, and how these changes affect ecosystem functions and resilience.

The aim of this thesis was to investigate how natural and anthropogenic factors alter aquatic vegetation communities and biotic interactions, and how these in turn affect ecosystem functions and resilience to common stressors. Shallow coastal bays in the Baltic Sea were used as model system. 

A large field survey was conducted to investigate effects of natural and anthropogenic gradients, including bay topographic openness and nutrient runoff, on vegetation communities and ecosystem functions. Results suggest that high vegetation cover can improve water clarity, whereas sediment-driven turbidity can negatively affect vegetation by decreasing the light penetration of the water (Paper I). This dual relationship indicates the potential for two alternative, self-sustaining states in shallow bays; with or without vegetation.

Using data from the same survey I investigated the influence of species richness and cover of rooted aquatic vegetation and drift wrack (Fucus vesiculosus), for ecosystem multifunctionality (MF) (Paper II). MF was estimated as the mean of four variables used as proxies for key functions; large predatory fish recruitment, grazer biomass, inverted ‘nuisance’ algal biomass and water clarity. MF was highest when the two functionally different vegetation types (rooted and drifting) co-occurred at high covers, and high species richness increased multifunctionality by increasing rooted vegetation cover.

To understand in greater detail if and how interactions within and between vegetation species mediate the effects of environmental change, I conducted two experiments. First, a cage experiment to test if intraspecific plant facilitation may buffer effects of altered top-down and bottom-up control (Paper III), then a mesocosm experiment to test if shading alters interspecific interactions between three common plant species (Paper IV). The cage experiment showed that high shoot density of a common plant (Myriophyllum spicatum) increased individual shoot performance, but only when subjected to both fertilization and large predatory fish exclusion (Paper III). The mesocosm experiment showed that individual species’ traits had stronger effect than shading on interspecific competition and community yield (Paper IV).

In conclusion, my thesis shows that single and multiple ecosystem functions benefit from high vegetation cover, with direct and indirect effects of diversity, but are sensitive to anthropogenic stressors (Papers I, II). Further, shading alters biotic interactions among vegetation species in a eutrophic coastal ecosystem by increasing the competitive advantage of dominant species (Paper IV), while intraspecific facilitation increases resilience to interacting stressors (Paper III). Together, the results highlight the need for ecosystem-based management where efforts to reduce anthropogenic influence (e.g. by nutrient reduction and fishing restrictions) are combined with improved protection and restoration of the ecologically and economically valuable aquatic vegetation communities.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2021. p. 89
Keywords
biodiversity, ecosystem functions, ecosystem multifunctionality, water clarity, fish recruitment, functional traits, feedbacks, biotic interactions, competition, facilitation, foundation species, seagrass, macrophytes, lagoons, brackish water, Baltic Sea
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-199296 (URN)978-91-7911-710-8 (ISBN)978-91-7911-711-5 (ISBN)
Public defence
2022-01-21, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20 and online via Zoom, public link is available at the department website, Stockholm, 09:30 (English)
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Available from: 2021-12-21 Created: 2021-12-01 Last updated: 2021-12-14Bibliographically approved

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Donadi, SerenaAustin, Åsa N.Hansen, Joakim P.Eklöf, Johan S.

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