Exploring the diversity patterns of benthic diatoms along environmental, spatial and temporal gradients in the Baltic Sea

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http://urn.fi/URN:ISBN:978-951-51-6773-6
Title: Exploring the diversity patterns of benthic diatoms along environmental, spatial and temporal gradients in the Baltic Sea
Author: Virta, Leena
Contributor: University of Helsinki, Faculty of Science, Department of Geosciences and Geography
Doctoral Programme in Geosciences
Faculty of Biological and Environmental Sciences, Tvärminne Zoological Station
Publisher: Helsingin yliopisto
Date: 2020-11-20
URI: http://urn.fi/URN:ISBN:978-951-51-6773-6
http://hdl.handle.net/10138/320530
Thesis level: Doctoral dissertation (article-based)
Abstract: During the ongoing rapid environmental change, different aspects of biodiversity and its effects on ecosystem functioning need to be resolved. A lot has already been learned regarding the value of biodiversity, but due to the complexity of natural environments many aspects are still unresolved, especially about the patterns and effects of microorganismal diversity in marine and brackish environments. This is surprising, given that microorganisms play key roles in many ecosystem functions and the marine microorganisms are estimated to, e.g., provide the majority of Earth’s oxygen. To facilitate better understanding, studies conducted along large gradients and considering the functional diversity of communities can be useful. Large gradients provide insights into the variation of ecological patterns relative to the environment and can indicate the consequences of environmental change on community responses, and the functional diversity may describe the community characteristics and diversity-ecosystem functioning relationships more effectively than taxonomic diversity and allow the generalizations of results between organisms and ecosystems to be made. In this thesis, biodiversity patterns of benthic diatoms, a highly diverse and productive microorganismal group in all aquatic systems, were resolved along different environmental, spatial and temporal gradients in the coastal ecosystems of the Baltic Sea. Being one of the world’s largest brackish water ecosystems with a naturally strong gradient of salinity and climate and with a unique mixture of marine and freshwater species, the Baltic Sea provides an ideal platform for biodiversity research. Biodiversity patterns resolved here included analyzing the effects of benthic diatom diversity on ecosystem productivity, investigating spatial and temporal beta diversity patterns, i.e. the change in community composition between sites or sampling occasions, and examining the effects of environment on the distribution and diversity of diatoms. All studies were conducted as field studies to increase knowledge on real-world processes. The results revealed some significant new insights and showed that the diversity, especially functional diversity, of benthic diatoms may set the lower boundary for ecosystem productivity. Thus, productivity could be high even when the diatom diversity was low, but high diatom diversity seemed to consistently support high productivity. This positive relationship may be due to several reasons, such as more complete resource use or the facilitative effect in diverse communities, or complex ecological interactions. However, the diversity of diatoms varied substantially among different habitats, highlighting the need to consider environmental heterogeneity and large environmental gradients in biodiversity research. Spatial beta diversity studies conducted at different spatial scales indicated a general pattern: across steep environmental gradients, the taxonomic beta diversity was consistently high while the functional beta diversity remained considerably lower. This suggests that the ecosystem requirements for the functional characteristics of microphytobenthic organisms are highly similar in different environments, and that diatoms are able to meet these requirements in variable environments, which may indicate an insurance effect against environmental change. However, simulated species loss of communities significantly increased the functional beta diversity, suggesting that the deterioration of diversity may decrease resilience, and thus emphasizing the importance of biodiversity for the stable functioning of benthic ecosystems. Despite some similarities in the environmental drivers of diatom communities in different environments, environmental variables controlling the communities varied between and within gradients. Thus, the effect of environment on communities seems to be context-dependent and variable between regions, which emphasizes the need for large-gradient studies and the consideration of region-specific differences in, e.g., environmental management and conservation efforts. The seasonal and inter-annual variation in the composition and diversity of communities was investigated along a temporal gradient of two years. The taxonomic and functional composition of communities changed significantly between seasons and years, while diversity remained fairly stable. This refers to either an ample seed bank, i.e. locally occurring resting stages of species, or a large regional species pool and effective dispersal of species, which rescue the populations. However, diversity decreased during an exceptionally warm winter with a short ice-cover duration, which may suggest that climate warming affects the diversity of benthic communities. To conclude, this thesis has increased the knowledge on the diversity and importance of benthic diatoms in complex real-world environments. Some of the diversity patterns were general and non-dependent of spatial scale, whereas others were highly variable between regions and gradients. The results emphasize the need to consider the effect of benthic diatoms when modelling and designing the management of coastal areas, and indicate the usefulness of studies with environmentally and spatially large gradients for the understanding of diversity patterns in natural ecosystems.Piilevät ovat erittäin monimuotoinen ryhmä mikroskooppisia leviä, joilla on koko maapallon kannalta elintärkeitä tehtäviä. Merissä elävät piilevät muun muassa tuottavat suuren osan maapallon ilmakehän hapesta. Merissä ja murtovesissä, kuten Itämeressä, elävien piilevien monimuotoisuudesta ja sen vaikutuksesta elinympäristön toimintaan tiedetään kuitenkin vasta vähän. Tässä väitöskirjassa selvitettiin Itämeren rannikkoalueiden pohjilla elävien piilevien monimuotoisuuden vaikutusta ekosysteemien kykyyn tuottaa biomassaa, monimuotoisuuden vaihtelua ajan ja paikan suhteen erilaisissa ympäristöissä sekä monimuotoisuuteen vaikuttavia ympäristötekijöitä. Piilevien monimuotoisuus vaihteli alueellisesti mutta oli tärkeää koko elinympäristön toiminnalle, sillä se muodosti alarajan ekosysteemin kyvylle tuottaa biomassaa. Toisin sanoen piileväyhteisön monimuotoisuuden ollessa pieni ekosysteemien tuottavuus oli vaihteleva, mutta kun monimuotoisuuden oli suuri, ekosysteemien tuottavuus oli säännöllisesti korkea. Piilevien ß-diversiteettiä eli sitä, miten erilaisia piileväyhteisöt ovat näytteenottoalueen eri osissa, tutkittiin usealla suuruudeltaan erilaisella alueella. Tulokset pysyivät alueen koosta riippumatta samankaltaisina: näytteenottoalueiden eri päissä piileväyhteisöt olivat lajistoltaan lähes täysin erilaisia, mutta lajipiirteiltään eli toiminnaltaan melko samankaltaisia. Ympäristömuuttujien vaikutusta tutkittaessa havaittiin, että se, mitkä muuttujat olivat tärkeitä piileväyhteisöille, vaihteli alueelta toiselle. Alueiden välisten erojen huomioiminen on siis tärkeää esimerkiksi ympäristön suojelutoimia suunniteltaessa. Ajallista vaihtelua tutkittaessa todettiin, että piileväyhteisöjen koostumus vaihteli huomattavasti vuodenaikojen ja vuosien välillä, kun taas monimuotoisuus pysyi melko samankaltaisena. Monimuotoisuus oli kuitenkin alhaisin sellaisena talvena, jolloin sää oli poikkeuksellisen lämmin ja jääpeiteaika lyhyt. Ilmaston lämpeneminen saattaa siis heikentää pohjalla elävien eliöyhteisöjen monimuotoisuutta. Tämä väitöskirja on tuottanut uutta tietoa pohjalla elävien piilevien monimuotoisuudesta monimutkaisissa ja vaihtelevissa luonnonympäristöissä. Piilevien monimuotoisuuden todettiin olevan tärkeää koko elinympäristön toiminnalle, ja sen vaikutus tulisikin tulevaisuudessa ottaa huomioon esimerkiksi suunniteltaessa rannikkoalueiden hyödyntämistä ja suojelua.
Subject: maantiede
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