Relationships between species traits and ecosystem processes in brackish aquatic plant communities

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Title: Relationships between species traits and ecosystem processes in brackish aquatic plant communities
Author: Angove, Charlotte
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences, Tvärminne Zoological Station
Doctoral Programme in Wildlife Biology
Publisher: Helsingin yliopisto
Date: 2020-06-08
Thesis level: Doctoral dissertation (article-based)
Abstract: Aquatic plant meadows provide a variety of global ecosystem services. Their populations are declining globally. To conserve and restore aquatic plant meadows and the services which they provide, it is necessary to understand their ecology. A key approach which allows us to explore plant ecology is to investigate the relationships between plant functional traits and ecosystem processes. By investigating plant functional traits, it is possible to develop insights about functional diversity and plant growth strategies. In this thesis, plant functional traits, functional diversity and plant growth strategies are used to investigate aquatic plant biomass production responses to the environment. A series of manipulative experiments were conducted in situ in submerged aquatic plant meadows of the northern Baltic Sea using SCUBA. Firstly, the role of plant traits, species identity and sediment porewater NH₄⁺ availability for plant nitrogen uptake rates were investigated using a short-term (3.5 h) nitrogen enrichment experiment (Chapter I). Secondly, a 15-week transplant experiment was conducted to explore plant functional trait and functional diversity relationships to productivity (Chapter II). Finally, a similar experiment with additions of the bivalve Limecola balthica (12 weeks) was conducted to investigate infauna effects to plant functional trait–productivity relationships (Chapter III). Chapter I showed that short-term nitrogen uptake rates from the sediment were driven by plant-biomass related demands. Similarly, results suggested that plants likely drained ammonium availability from their adjacent sediment porewater. Overall, Chapter I parameterised the possible unfulfilled potential for larger temperate aquatic plants to cycle nutrients. Chapter II results showed strong relationships between productivity and traits which enhanced light capture (height and leaf area). Leaf tissue δ¹³C and functional richness were also related to community productivity. The relationship between height and productivity was likely exacerbated by a competitive height interaction between the tallest and second tallest species. Overall, functional richness was related to community biomass production, likely by selecting for traits which enhanced light capture (selection effect) with potential consequences to carbon supply. Findings support inferences from previous studies of aquatic plant communities which showed that height is strongly related to aquatic plant productivity and trait identity may be more descriptive for primary production compared to functional diversity indices. Chapter III results showed Specific Root Length (SRL) exhibited the strongest relationship to productivity. Leaf area was also related to community production and Median Maximum Root Length (MMRL) exhibited a marginally non-significant relationship to productivity. SRL exhibited collinearity to species identity, therefore it was not possible to interpret SRL effects separately to other traits which may coincide with species identity. Community SRL was related to community shoot frequency, not aboveground biomass production. SRL and shoot proliferation both represent strategies to enhance nutrient absorption from the sediment. Relationships between plant leaf tissue nutrient concentrations (N (% DW), δ¹⁵N, δ¹³C) and L. balthica condition index suggested that L. balthica affected the sediment nutrient supply and enriched the plants with nutrients. Overall, Chapter III showed that infauna, common in aquatic plant meadows, can change aquatic plant trait-productivity relationships and thus arguably the drivers for submerged aquatic plant community growth. Findings of this thesis can be applied to a variety of other temperate submerged aquatic plant communities. Targeted research questions could contribute to further understanding of submerged aquatic plant ecosystem ecology, including the ecology of monocultures. This thesis summary suggests updating the current description of context-dependent seagrass biomass responses to sediment nutrient enrichment. It proposes a model which, once tested, would help to improve predictive modelling for submerged aquatic plant biomass responses to future change. Also, results of this thesis contribute towards increasing effectiveness of future management by providing insights to infauna effects on plant functioning. This is beneficial to current restoration development because infauna additions to submerged aquatic plant meadows are an option for increasing seagrass restoration success and seagrass resilience to future change. This thesis identifies that there is requirement for further research in seagrass meadows which form dense root-rhizome mattes, and it describes potential options for future research. It also recommends isotope-tracing experiments and compound-specific isotope tracing experiments to better understand mechanisms of nutrient exchange between infauna and temperate submerged aquatic plants. It has empirically shown current limitations of global plant trait syntheses and it identifies constructive steps forward to improve the global perspective of plant trait ecology. Finally, this thesis advocates the value of insights gained from data-rich functional diversity experiments and plant functional trait experiments. To conclude, this thesis has improved the collective understanding of temperate aquatic plant ecosystem functioning.The archipelago of the northern Baltic Sea contains shallow, submerged soft sediments that are colonised by diverse aquatic plant communities. Such diverse communities are valuable assets for investigating the relationships between species traits and ecosystem processes, to understand the ecology of submerged aquatic plants. This thesis constitutes three experiments conducted in situ using SCUBA in the northern Baltic Sea. The purpose of these experiments was to investigate how plant biomass production is related to plant functional traits, growth strategies, and functional diversity, as well as the role of infauna to plant functional trait-productivity relationships. Overall, results showed that plant functional diversity can be related to productivity likely by selecting for light capture traits, that the finite sediment nutrient source was likely affected by plant biomass-driven demands, and finally infauna can affect plant functional trait-productivity relationships. Overall, by using plant trait and functional diversity investigations, this thesis has improved the collective understanding of submerged aquatic plant functioning.
Subject: aquatic Sciences
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