Spring bloom dynamics in the Baltic Sea : from the environment to macroelements and microbial interactions

The Baltic Sea is a substantial brackish water system, and due to its shallow average depth, physicochemical dynamics affecting, for instance, phytoplankton growth, are controlled by atmospheric forces. This unique ecosystem is threatened by environmental changes, with implications for the water quality. Eutrophication remains the major challenge for the Baltic Sea ecosystem. The increases in nitrogen (N) and phosphorus (P) are not proportional to the carbon (C) input, which might cause imbalances in seston C:N:P stoichiometry and affect biogeochemical cycles. In turn, plankton growth, food web dynamics, and nutrient- / C-cycling, which plays a pivotal role in climate change, will be affected. The ongoing eutrophication supports the formation of high phytoplankton biomass, enhancing the spreading of anoxic bottom waters, affecting benthic communities, which might be worsened by an extended annual growing season caused by increasing sea surface temperatures (SST). Higher SST and effects of eutrophication are known to support harmful algal blooms such as the occurrence of toxic dinoflagellates and cyanobacteria in the Baltic Sea during summer. The spring bloom is the most productive period of the year (~50 % annual C-fixation), driven by high inorganic nutrient concentrations and improving light conditions. The co-existence of cold-water diatoms and dinoflagellates during this period is characteristic of the Baltic Sea. Heterotrophic bacteria will become more abundant due to increased concentrations of allochthonous organic matter, which could reduce the phototrophic contribution already in spring. The health of arctic and sub-arctic ecosystems is highly dependent on environmental factors such as temperature affecting, for example, sea ice conditions. Climatically driven changes have already caused variations in the plankton community structure during the Baltic Sea spring bloom. These alterations have led to lower diatom-dinoflagellate-proportions (DDP), affecting food web dynamics and biogeochemical cycles. Therefore, shifts in the community composition may have far-reaching ecosystem consequences. The dominant phytoplankton group defines the quantity and quality of bioavailable (labile) dissolved organic carbon (DOC), providing different niches for heterotrophic bacteria. Furthermore, environmental factors affect the abundances and community compositions of both phototrophic and heterotrophic plankton. Microzooplankton is particularly crucial in linking the microbial loop with higher trophic levels, and in the recycling of inorganic nutrients. For example, heterotrophic ciliates become more abundant at elevated temperatures and can significantly reduce the biomass of bacteria and phytoplankton. The Baltic Sea spring bloom is dominated by phytoplankton, while the grazer community is not well developed yet. Thus, considering the ongoing climatic changes, it can be expected that the share of microzooplankton and, in turn, the partially unaccounted biomass will be higher at elevated temperatures (e.g., in summer). Temporal and spatial shifts of zooplankton populations, affecting higher trophic levels, could be the consequence. The main objectives of this study: • The identification of environmental drivers shaping the nano- and microplankton community composition. • To study the effects of changing communities and bloom phases on seston nutrient stoichiometry. • The investigation of bacterioplankton responses to changing dominance patterns of diatoms and dinoflagellates. • To reveal the proportion of microzooplankton during the spring bloom. For this, data from research cruises (chapters 1 and 3) in the Baltic Sea and experiments carried out at Tvärminne Zoological Station (TVZ, University of Helsinki) on the south-west coast of Finland (chapter 2) were used. Surface water samples (z = 3 m) for the analysis of nutrients (inorganic and organic), biomass, community composition, and primary production amongst others were collected from seven sub-basins and different spring bloom phases (April / May) during four cruises on-board R/V Aranda (2013 to 2016). The Gulf of Finland (GOF) and the Baltic Proper were the main study areas. The two experiments (2012 and 2013) at TVZ (also GOF) took place in February / March. The water was collected from 20 m under the sea ice and featured low biomass and high inorganic nutrient concentrations (winter conditions). In addition to nutrients and phototrophic variables, samples were taken to determine bacterial abundances, production, and community composition (BCC). The main difference between these two approaches was the fact that the cruise data comprise natural mixed communities, and samples from different bloom phases represent an extract of the spring bloom. In contrast, dominance patterns were artificially shifted by adding cultured strains to the natural pre-bloom community for the experiments. Furthermore, the bacterial growth phase was induced by increasing the temperature, and the course of the bloom was followed in 20 L mesocosms. The determination of nutrient and chlorophyll a (Chl a) concentrations as well as the microscopy counting strategy (field data) were based on in-house protocols of the Finnish Environment Institute (SYKE), certified by the Finnish Accreditation Service (FINAS). Some of the main methods were explicitly chosen for the study area (Baltic Sea), the sampling season (spring bloom), and the database (Hertta, SYKE) that was used (plankton community composition, upper mixed layer depth, definition of bloom phases, bacterial production, and gross primary production). Various multivariate and other statistical analyses were applied to study different aspects of the presented studies. In addition to the different chapters, three cold-water dinoflagellates, usually not identified to species-level, were distinguished, and their biomass-contributions investigated (cruise data, unpublished). The corresponding species (Gymnodinium corollarium, Biecheleria baltica, and Apocalathium malmogiense) can co-occur during the Baltic Sea spring bloom but are not separable by traditional microscopy of Lugol-preserved samples due to their similar appearance. Thus, this group is referred to as DinoComplex throughout this summary. Two different methods for the identification and quantification of the three species were applied: inverted epifluorescence microscopy of stained (fluorescence brightener) cells and quantitative polymerase chain reaction (qPCR), using species-specific primers and fluorescent (Taqman®) probes. Environmental factors, mostly temperature and the concentration of inorganic nutrients, defined the nano- and microplankton community composition (phototrophs and heterotrophs) and affected the collective physiology as well as the biomass. Combined, Mesodinium rubrum, Peridiniella catenata, the DinoComplex, and heterotrophic ciliates dominated the biomass in the cruise data (n = 119). The experiments have shown that lower DDP´s reduce the bacterial activity (protein- and DNA-synthesis) and alter the BCC due to decreasing availability of diatom-derived dissolved organic matter (DIADOM). Additionally, a predominant diatom-species has shown specific effects on bacterial responses, i.e., Chaetoceros wighamii dominance yielded the highest bacterial activity. In the field data, the community composition explained 19 % (average) of shifts in seston C:N:P ratios, which were significantly different between some of the bloom phases. The presented findings highlight the importance of species-level identification (e.g., DinoComplex) and the introduction of a microzooplankton monitoring program. The fact that the most relevant diatom (Thalassiosira baltica) contributed <10 % to the biomass (relative contribution, n = 119), combined with high dinoflagellate-proportions throughout the bloom, agrees with previous findings on the increase of dinoflagellates over diatoms in several sub-basins. Not only variations in the community composition, but also different growth phases (physiology) of the bloom affected the interactions between phyto- and bacterioplankton. For instance, high proportions of T. baltica (equivalent to ~6 µg Chl a L-1) were associated with low primary production (field data, presented study) as well as low bacterial activity (field data, not shown). At the bloom peak, the cells are less active, but not decaying and excreting larger amounts of labile DOM, yet. Thus, a decreasing availability of DIADOM will cause a decrease in the energy efficiency of the microbial food web. Comparing the C:N:P ratios from the field study and the experiments revealed that the absolute dominance of either diatoms or dinoflagellates lead to more clear connections to the intracellular element composition. However, there was no significant relationship between the DDP and seston C:N:P ratios, and thus, it was concluded that the overall community activity supersedes the effect of different dominance-patterns on seston stoichiometry in mixed communities. Species-specific effects on bacterial responses and seston stoichiometry, with implications for biogeochemical cycles, are challenging to study in diverse communities. Seston ratios that were clearly affected by the community composition, namely Chl a:C (and its fixed range), C:Si, and N:Si, could be predicted with relatively high certainty for the spring bloom. The Chl a:C ratio can be used as a proxy for phototrophic and heterotrophic production and to study C-budgets. For example, C:Si indicates the diatom-proportion of the seston, which could be interesting for modeling studies. Due to a high inter-annual variability in seston ratios, season-specific findings should be combined to estimate future developments of, for example, the annual primary production and C-fluxes.Itämeri on laaja murtovesialue, jonka ainutlaatuista ekosysteemiä uhkaavat veden laatuun vaikuttavat ympäristömuutokset. Rehevöityminen on Itämeren suurin ongelma. Se voi aiheuttaa epätasapainoa sestonin (vesipatsaan elollinen ja eloton orgaaninen aines) C:N:P-stoikiometriassa ja vaikuttaa biogeokemiallisiin kiertoihin. Vaikutukset yltävät ilmastonmuutoksen kannalta keskeisiin mekanismeihin kuten planktonin kasvuun, ravintoverkkojen dynamiikkaan sekä ravinteiden ja hiilen kiertoon. Rehevöityminen lisää kasviplanktonbiomassan muodostumista, minkä seurauksena hapettomien pohjasedimenttien pinta-ala kasvaa. Samalla pohjaeläinyhteisöjen elinmahdollisuudet heikkenevät, ja rehevöitymisen vaikutukset saattavat yhä pahentua tulevaisuudessa. Ilmaston lämpenemisen ja rehevöitymisen vaikutusten tiedetään lisäävän haitallisten leväkukintojen, kuten myrkyllisten panssarisiimalevien ja syanobakteerien, esiintymistä Itämerellä kesäisin. Kevätkukinta on vuoden tuottavin jakso, jota epäorgaanisten ravinteiden pitoisuus ja valaistusolosuhteet säätelevät. Kylmän veden piilevien ja panssarisiimalevien rinnakkaiselo kevätkukinnan aikaan on ominaista Itämerelle. Toisenvaraisten bakteerien määrä kasvaa valuman mukana tulevan orgaanisen aineksen lisääntyessä, mikä voi vähentää yhteyttävien levien osuutta jo keväällä. Arktisten ja subarktisten ekosysteemien hyvinvointi on riippuvainen ympäristötekijöistä, kuten lämpötilasta, joka vaikuttaa esimerkiksi merijään muodostumiseen. Ilmastomuutos on jo aiheuttanut muutoksia Itämeren keväisen planktonyhteisön lajikoostumuksessa. Nämä muutokset ovat johtaneet matalampiin pii- ja panssarisiimalevien välisiin suhteellisiin osuuksiin, vaikuttaen ravintoverkkoon ja biogeokemiallisiin kiertoihin. Siksi yhteisökoostumuksen muutoksilla voi olla ekosysteemitason seurauksia. Hallitseva kasviplanktonryhmä määrittelee biologisesti käyttökelpoisen liuenneen orgaanisen hiilen määrän ja laadun tarjoten erilaisia elinmahdollisuuksia toisenvaraisille bakteereille. Ympäristötekijät vaikuttavat sekä oma- että toisenvaraisen planktonin määrään ja yhteisöjen lajikoostumukseen. Mikroeläinplankton on erityisen tärkeä linkki ravintoverkossa mikrobien ja korkeampien ravintoketjun tasojen välillä sekä epäorgaanisten ravinteiden kierrossa. Kasviplankton hallitsee kevätkukinnan aikaista eliöyhteisöä, kun laiduntajayhteisö ei ole vielä ehtinyt kehittyä. Ripsieläimet voivat kuitenkin olla tärkeitä bakteerien ja kasviplanktonin laiduntajia erityisesti lämpötilan kohottua. Siksi vielä toistaiseksi piilossa olevan mikroeläinplanktonin osuus kokonaisyhteisöstä oletettavasti kasvaa elinympäristön lämmetessä, mikä aiheuttaa ajallisia ja alueellisia muutoksia eläinplanktonpopulaatioissa ja vaikuttaa korkeampiin ravintoketjun tasoihin. Tutkimuksen päätavoitteet: • Tunnistaa planktonyhteisön (<100 µm) koostumukseen vaikuttavat ympäristötekijät. • Tutkia muuttuvien eliöyhteisöjen ja kukintavaiheiden vaikutuksia sestonin ravinnestoikiometriaan. • Tutkia bakteeriplanktonin vastetta muuttuvaan piilevien ja panssarisiimalevien suhteeseen. • Selvittää mikroeläinplanktonin osuutta kevätkukinnassa. Lisäksi, erotettiin toisistaan kolme kylmän veden panssarisiimalevälajia (Gymnodinium corollarium, Biecheleria baltica ja Apocalathium malmogiense), joita ei tavanomaisesti määritetä lajitasolle, ja näiden kolmen lajin biomassaosuudet selvitettiin. Ympäristötekijät, erityisesti lämpötila ja epäorgaanisten ravinteiden pitoisuus, määrittivät planktonyhteisön (