Phytoplankton functions and community properties in the pelagic food web

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http://urn.fi/URN:ISBN:978-951-51-6047-8
Title: Phytoplankton functions and community properties in the pelagic food web
Author: Lehtinen, Sirpa
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences
Doctoral Programme in Wildlife Biology
Marine Research Centre, Finnish Environment Institute (SYKE)
Publisher: Helsingin yliopisto
Date: 2020-05-08
Belongs to series: Dissertationes Schola Doctoralis Scientiae Circumiectalis, Alimentariae, Biologicae - URN:ISSN:2342-5431
URI: http://urn.fi/URN:ISBN:978-951-51-6047-8
http://hdl.handle.net/10138/314233
Thesis level: Doctoral dissertation (article-based)
Abstract: Primary producing phytoplankton forms the basis of pelagic food webs. Within this thesis, certain phytoplankton functions (food source for grazers, harmfulness, nitrogen (N2) –fixation) and community properties (diversity, productivity, biomass, resource use efficiency (RUE), taxonomic and functional composition), as well as their relation to available resources (nutrients) were studied. The aim was to further clarify the role of phytoplankton in the pelagic food web. Such information is in high demand also for applied purposes, e.g. in understanding how phytoplankton monitoring data can be interpreted in environmental assessments. Both experimental approaches and multi-decadal monitoring data from the Baltic Sea were used. Experiments showed that phytoplankton communities can be directly shaped by selective mesozooplanktonic grazing. Study of cultured algae demonstrated the relationship between resources and harmfulness; in nutrient deficient conditions, the haptophyte Prymnesium parvum displayed increased allelopathy and toxicity. A data set of natural phytoplankton communities in experimental bioassays was used to study effects of nutrient availability and imbalance on N2-fixation, diversity, productivity, biomass, RUE, taxonomic community composition, and diversity-productivity-function relationships. The biomass of N2-fixing cyanobacteria was primarily regulated by other factors than ambient nutrient imbalance, which implied that the build-up of N2-fixing cyanobacterial bloom biomass levels depends on longer-term nutrient dynamics. As expected, phytoplankton biomass and productivity were the highest when more resources (especially total phosphorus (TP)) were available. The Species Energy Theory was supported, since resource availability affected species richness positively. However, evenness was not explained by resource availability. Results did not support the Resource Ratio Theory predicting that species richness would be the highest when more than one resource is limiting. Species richness did not explain biomass, primary productivity or RUE, but there was a highly significant inverse relation between evenness and biomass. The result suggests that high productivity and RUE in natural phytoplankton communities are supported by recurrently shifting, narrow time windows suitable for events of temporary resource monopolization by a few opportunistic species. The resource-related ambient local environmental variables explained only 8 % of the variability in the coastal phytoplankton community composition (taxonomic biomass composition). As species richness was the highest in the study area which is most exposed to water exchange with adjacent areas, the role of metacommunity dynamics in shaping community composition and ensuring species turnover was supported. When examining coastal data, these underlying regional differences in phytoplankton composition should be taken into account. Based on analyses on multi-decadal monitoring data, phytoplankton communities were functionally more similar in spring than in summer. The functional composition of phytoplankton was primarily driven by the abiotic environment (instead of biotic interactions), since ca. 75 % of the observed communities did not depart significantly from the simulated prediction, which had been constructed based on environmental preferences of the species (season, location, temperature, and salinity). However, ca. 25 % of the communities departed significantly from the simulated prediction, signaling a notable effect of also biotic interactions in the phytoplankton communities. Multi-decadal monitoring data was used to develop an approach to assess the potential effects of phytoplankton community composition on the next trophic level. The following functional characteristics of taxa were specifically considered: potential suitability or quality as food for micro- and mesozooplanktonic grazers, harmfulness, size, and trophic properties (auto- or mixotrophy). These functional characteristics were selected since they are common functions for phytoplankton in all aquatic environments, even though species identities vary. Since the Baltic Sea was used as demonstration area, also N2-fixation was considered. In conclusion, the results of this thesis contribute to the knowledge on phytoplankton community data serving as a versatile source of information describing the first level of the food web, including availability as food items for grazers, potentially direct harmful effects, N2-fixation, and biodiversity. Results also add to the knowledge on how the functional and community properties of phytoplankton are connected to nutrient resources and point out the importance of regionality especially when studying coastal phytoplankton communities.Perustuottajana toimiva kasviplankton muodostaa meriekosysteemeissä ulapan ravintoverkon perustan. Väitöskirjatyössäni tutkin kokeellisten menetelmien ja seuranta-aineistojen avulla tiettyjä kasviplanktonin toimintoja (ravinnonlähde laiduntajille, haitallisuus, typensidonta) ja yhteisöominaisuuksia (monimuotoisuus, perustuotanto, ravinteiden käyttötehokkuus, koostumus) sekä niiden suhteita ravinneresursseihin Itämerellä. Tietoa tarvitaan muun muassa meriympäristön tilan arvioinnissa. Mesoeläinplanktonin laidunnus vaikutti suoraan kasviplanktonin koostumukseen, ja ravinnerajoitteisuus lisäsi tarttumalevän muille leville haitallisia (allelopaattisia) ja kaloille myrkyllisiä ominaisuuksia. Ravinnerajoitteisuus ei vaikuttanut merkittävästi typpeä sitovien sinilevien samanaikaiseen biomassaan. Kasviplanktonin biomassa ja tuotanto olivat odotetusti suurimpia ravinnemäärän ollessa suurin. ”Species Energy” -teorian mukaisesti ravinteiden määrä vaikutti lajimäärään myönteisesti, mutta lajiston tasaisuusindeksiin (evenness) se ei vaikuttanut. “Resource Ratio” -teoria ei saanut tukea, koska lajimäärä ei ollut suurin typen ja fosforin yhteisrajoitteisuuden vallitessa. Lajimäärä ei selittänyt biomassaa, perustuotantoa eikä ravinteiden käyttötehokkuutta. Lajiston tasaisuusindeksi oli suurin biomassan ollessa pienin. Metayhteisödynamiikan tärkeyttä kasviplanktonyhteisön taksonomiseen biomassakoostumukseen ilmensi se, että alueelliset ympäristötekijät selittivät vain 8 % rannikon kasviplanktonyhteisöjen koostumuksesta, ja että lajimäärä oli suurin alueella, joka on suoraan yhteydessä ympäröiviin vesialueisiin. Kasviplanktonyhteisön toiminnallinen monimuotoisuus oli suurempi kesällä kuin keväällä, ja kasviplanktonyhteisön toiminnalliseen koostumukseen vaikuttivat pääasiassa abioottiset tekijät bioottisten eli eliöiden välisten vuorovaikutussuhteiden sijaan. Seuranta-aineistoa hyödyntäen kehitettiin menetelmä, jota voidaan käyttää arvioitaessa kasviplanktonkoostumuksen vaikutuksia ylemmille ravintoverkon tasoille. Kasviplanktonin toiminnalliset ominaisuudet, joita menetelmässä tarkasteltiin, olivat sopivuus mikro- ja mesoeläinplanktonin ravinnoksi, haitallisuus, koko ja trofia (auto- tai miksotrofia). Koska esimerkkiaineisto oli Itämereltä, myös typensidonta oli alueellisen merkittävyytensä vuoksi tarkasteltavien ominaisuuksien joukossa. Väitöskirjan tulokset osoittavat kasviplanktonin yhteisöaineiston tarjoavan monipuolista tietoa ravintoverkon perustuottajatasosta, mukaan lukien eläinplanktonille saatavilla olevasta ravinnosta, haitallisuudesta, typensidonnasta ja monimuotoisuudesta.
Subject: bio- ja ympäristötieteet
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