Lake browning counteracts cyanobacteria responses to nutrients: Evidence from phytoplankton dynamics in large enclosure experiments and comprehensive observational data

Lakes worldwide are affected by multiple stressors, including climate change. This includes massive loading of both nutrients and humic substances to lakes during extreme weather events, which also may disrupt thermal stratification. Since multi-stressor effects vary widely in space and time, their combined ecological impacts remain difficult to predict. Therefore, we combined two consecutive large enclosure experiments with a comprehensive time-series and a broad-scale field survey to unravel the combined effects of storm-induced lake browning, nutrient enrichment and deep mixing on phytoplankton communities, focusing particularly on potentially toxic cyanobacterial blooms. The experimental results revealed that browning counteracted the stimulating effect of nutrients on phytoplankton and caused a shift from phototrophic cyanobacteria and chlorophytes to mixotrophic cryptophytes. Light limitation by browning was identified as the likely mechanism underlying this response. Deep-mixing increased microcystin concentrations in clear nutrient-enriched enclosures, caused by upwelling of a metalimnetic Planktothrix rubescens population. Monitoring data from a 25-year time-series of a eutrophic lake and from 588 northern European lakes corroborate the experimental results: Browning suppresses cyanobacteria in terms of both biovolume and proportion of the total phytoplankton biovolume. Both the experimental and observational results indicated a lower total phosphorus threshold for cyanobacterial bloom development in clearwater lakes (10-20 mu g P L-1) than in humic lakes (20-30 mu g P L-1). This finding provides management guidance for lakes receiving more nutrients and humic substances due to more frequent extreme weather events

Cyanotoxin Occurrence and Diversity in 98 Cyanobacterial Blooms from Swedish Lakes and the Baltic Sea

The Drinking Water Directive (EU) 2020/2184 includes the parameter microcystin LR, a cyanotoxin, which drinking water producers need to analyze if the water source has potential for cyanobacterial blooms. In light of the increasing occurrences of cyanobacterial blooms worldwide and given that more than 50 percent of the drinking water in Sweden is produced from surface water, both fresh and brackish, the need for improved knowledge about cyanotoxin occurrence and cyanobacterial diversity has increased. In this study, a total of 98 cyanobacterial blooms were sampled in 2016-2017 and identified based on their toxin production and taxonomical compositions. The surface water samples from freshwater lakes throughout Sweden including brackish water from eight east coast locations along the Baltic Sea were analyzed for their toxin content with LC-MS/MS and taxonomic composition with 16S rRNA amplicon sequencing. Both the extracellular and the total toxin content were analyzed. Microcystin's prevalence was highest with presence in 82% of blooms, of which as a free toxin in 39% of blooms. Saxitoxins were found in 36% of blooms in which the congener decarbamoylsaxitoxin (dcSTX) was detected for the first time in Swedish surface waters at four sampling sites. Anatoxins were most rarely detected, followed by cylindrospermopsin, which were found in 6% and 10% of samples, respectively. As expected, nodularin was detected in samples collected from the Baltic Sea only. The cyanobacterial operational taxonomic units (OTUs) with the highest abundance and prevalence could be annotated to Aphanizomenon NIES-81 and the second most profuse cyanobacterial taxon to Microcystis PCC 7914. In addition, two correlations were found, one between Aphanizomenon NIES-81 and saxitoxins and another between Microcystis PCC 7914 and microcystins. This study is of value to drinking water management and scientists involved in recognizing and controlling toxic cyanobacteria blooms

Kan antalet växtplanktonprovtagningar minskas från sju till fyra gånger per år utan att betydelsefull information förloras?

Växtplankton är en grupp av organismer som reagerar snabbt på både kemiska och biologiska variabler. Det innebär att variationen inom och mellan år kan vara stor. De nationella monitoringprogrammen har under en längre tid provtagit växtplank-ton månadsvis under tillväxtsäsongen vilket innebär sju provtagningar per år. Växtplankton bör provtas ofta för att man inte ska missa förändringar i succes-sionsstadier och produktionsmaxima som kan ge indikationer på vattenkvalitetsför-ändringar. Dessa missar man lätt även med sju provtagningar per år. Med fyra provtagningar fokuserar man på vårens tidiga successionsstadium (april och maj) och sensommarens både biomasse- och strukturmässigt mer stabila stadium (juli och augusti). Antalet växtplanktonprovtagningar i det nationella Trendsjöpro-grammet har sedan 2007 minskat från sju till fyra gånger per år. Inom IKEU-programmet har växtplankton fortsatt att analyseras sju gånger per år och nu finns det en önskan att utreda vilken information man missar om man reducerar antalet provtagningar från sju till fyra gånger per år i alla sjöar. Studien visar att om man vill detaljstudera växtplanktons samhällsstruktur mellan sjöar eller grupper av sjöar som ingår i IKEU-programmet, blir det ingen större skillnad i resultat om man istället för sju tillfällen endast analyserar fyra tillfällen (april, maj, juli och augusti). Många mindre vanliga taxa försvinner med minskat antal prov men alla taxa som kan ses som sjögruppsspecifika (sura referenser, neu-trala referenser, kalkade, överkalkade och kalkavslutade sjöar) finns kvar. Likaså blir det inga stora skillnader i vilka miljövariabler som bäst samvarierar med växt-planktons samhällsstruktur även om förklaringsgraden minskar något när årsme-delvärdena baseras på fyra istället för sju provtagningstillfällen. Analyserar man däremot alla ingående data utan att göra några sammanslagningar av provtagnings-tillfällen (medelvärden) blir förklaringsgraden tvärtom något högre med minskat antal provtagningar, vilket antagligen beror på den stora variationen hos växtplank-ton under försommar och höst

Phytoplankton biomass in northern lakes reveals a complex response to global change

Global change may introduce fundamental alterations in phytoplankton biomass and community structure that can alter the productivity of northern lakes. In this study, we utilized Swedish and Finnish monitoring data from lakes that are spatially (135 lakes) and temporally (1995-2019, 110 lakes) extensive to assess how phytoplankton biomass (PB) of dominant phytoplankton groups related to changes in water temperature, pH and key nutrients [total phosphorus (TP), total nitrogen (TN), total organic carbon (TOC), iron (Fe)] along spatial (Fennoscandia) and temporal (25 years) gradients. Using a machine learning approach, we found that TP was the most important determinant of total PB and biomass of a specific species of Raphidophyceae - Gonyostomum semen - and Cyanobacteria (both typically with adverse impacts on food-webs and water quality) in spatial analyses, while Fe and pH were second in importance for G. semen and TN and pH were second and third in importance for Cyanobacteria. However, in temporal analyses, decreasing Fe and increasing pH and TOC were associated with a decrease in G. semen and an increase in Cyanobacteria. In addition, in many lakes increasing TOC seemed to have generated browning to an extent that significantly reduced PB. The identified discrepancy between the spatial and temporal results suggests that substitutions of data for space-for-time may not be adequate to characterize long-term effects of global change on phytoplankton. Further, we found that total PB exhibited contrasting temporal trends (increasing in northern- and decreasing in southern Fennoscandia), with the decline in total PB being more pronounced than the increase. Among phytoplankton, G. semen biomass showed the strongest decline, while cyanobacterial biomass showed the strongest increase over 25 years. Our findings suggest that progressing browning and changes in Fe and pH promote significant temporal changes in PB and shifts in phytoplankton community structures in northern lakes

Declining calcium concentration drives shifts toward smaller and less nutritious zooplankton in northern lakes

Zooplankton community composition of northern lakes is changing due to the interactive effects of climate change and recovery from acidification, yet limited data are available to assess these changes combined. Here, we built a database using archives of temperature, water chemistry and zooplankton data from 60 Scandinavian lakes that represent broad spatial and temporal gradients in key parameters: temperature, calcium (Ca), total phosphorus (TP), total organic carbon (TOC), and pH. Using machine learning techniques, we found that Ca was the most important determinant of the relative abundance of all zooplankton groups studied, while pH was second, and TOC third in importance. Further, we found that Ca is declining in almost all lakes, and we detected a critical Ca threshold in lake water of 1.3 mg L-1, below which the relative abundance of zooplankton shifts toward dominance of Holopedium gibberum and small cladocerans at the expense of Daphnia and copepods. Our findings suggest that low Ca concentrations may shape zooplankton communities, and that current trajectories of Ca decline could promote widespread changes in pelagic food webs as zooplankton are important trophic links from phytoplankton to fish and different zooplankton species play different roles in this context.Among five environmental variables tested, we found that low lake calcium (Ca) concentrations shape most zooplankton communities in northern lakes. When lake water Ca falls below a threshold of 1.3 mg L-1, the relative abundance of zooplankton shifts toward dominance of Holopedium and small cladocerans at the expense of Daphnia and cyclopoid and calanoid copepods. The current trajectory of Ca declines found in northern lakes implies community shifts toward dominance of smaller and less nutritious zooplankton. Ca declines potentially have strong repercussions for lake food webs and productivity as zooplankton are important trophic links from phytoplankton to fish.imag

Effects of trophic status, water level, and temperature on shallow lake metabolism and metabolic balance: A standardized pan‐European mesocosm experiment

Important drivers of gross primary production (GPP) and ecosystem respiration (ER) in lakes are temperature, nutrients, and light availability, which are predicted to be affected by climate change. Little is known about how these three factors jointly influence shallow lakes metabolism and metabolic status as net heterotrophic or autotrophic. We conducted a pan‐European standardized mesocosm experiment covering a temperature gradient from Sweden to Greece to test the differential temperature sensitivity of GPP and ER at two nutrient levels (mesotrophic or eutrophic) crossed with two water levels (1 m and 2 m) to simulate different light regimes. The findings from our experiment were compared with predictions made according the metabolic theory of ecology (MTE). GPP and ER were significantly higher in eutrophic mesocosms than in mesotrophic ones, and in shallow mesocosms compared to deep ones, while nutrient status and depth did not interact. The estimated temperature gains for ER of ~ 0.62 eV were comparable with those predicted by MTE. Temperature sensitivity for GPP was slightly higher than expected ~ 0.54 eV, but when corrected for daylight length, it was more consistent with predictions from MTE ~ 0.31 eV. The threshold temperature for the switch from autotrophy to heterotrophy was lower under mesotrophic (~ 11°C) than eutrophic conditions (~ 20°C). Therefore, despite a lack of significant temperature‐treatment interactions in driving metabolism, the mesocosm's nutrient level proved to be crucial for how much warming a system can tolerate before it switches from net autotrophy to net heterotrophy

Challenges in supplying empirical proof for predictions derived from Species Distribution Models (SDMs): the case of an invasive cyanobacterium

Species distribution models (SDMs) calibrated with bioclimatic variables revealed a high probability for range expansion of the invasive toxin producing cyanobacterium, Raphidiopsis raciborskii to Sweden, where no reports of its presence have hitherto been recorded. While predictions focused on the importance of climate variables for possible invasion, other barriers to dispersal and successful colonization need to be overcome by the species for successful invasion. In this study, we combine field-based surveys of R. raciborskii (microscopy and molecular analysis using species-specific primers) of 11 Swedish lakes and in-silico screening of environmental DNA using 153 metagenomic datasets from lakes across Europe to validate the SDMs prediction. Field-based studies in lakes with high/low predicted probability of occurrence did not detect the presence of R. raciborskii, and in-silico screening only detected hints of its presence in 5 metagenomes from lakes with probability ranging from 0.059 to 0.825. The inconsistencies between SDMs results and both field-based/in-silico monitoring could be due to either sensitivity of monitoring approaches in detecting early invasions or uncertainties in SDMs that focused solely on climate drivers. However, results highlight the necessity of proactive monitoring with high temporal and spatial frequency

Fokus på Mälaren 2021 : sammanfattande resultat från miljöövervakning och forskningsprojekt knutna till samarbetet mellan SLU och Mälarens vattenvårdsförbund

Mälarens vattenvårdsförbund och Sveriges lantbruksuniversitet fortsätter att samarbeta med fokus på Mälaren. Under 2021 påverkade pandemin möjligheten att ses under årsstämman på våren men Mälarseminariet på hösten kunde hållas som hybridmöte med ungefär 40 deltagare på plats på SLU i Uppsala och lika många deltog via videolänk. Digitala Mälarinarier hölls vid fyra tillfällen och var även de välbesökta. Faruk Djodjic är ny kontaktperson tillsammans med Stina Drakare från SLU. Vid vinterprovtagningen hade alla stationer is vilket är första gången sedan detta samarbete startade 2017. Nederbördsrika månader blev maj och augusti och sommarmånaderna juli och augusti var varma. Statusbedömningen visar att stationer med måttlig status dominerar och att det finns flera stationer med otillfredsställande (7) och dålig status (5). Bara två stationer hade god status och ingen hade hög status. Antalet stationer med bra status växlar mellan två och fyra stycken mellan åren vilket beror på vilka stationer ingår just det året. Dessa statusbedömningar kopplar främst till övergödningsproblematik vilket provtagningsprogrammet är designat för att följa. Tre doktorander med ämnen som berör Mälaren har försvarat sina avhandlingar under perioden. Två av dem berör just övergödningsproblematik och behandlade ämnen som bra verktyg för att minska transport av näringsämnen som fosfor från land till Mälaren och för att inaktivera fosfor på sjöbottnar för att minska internbelastning från bottensediment. Den tredje avhandlingen gav verktyg för att förbättra dricksvattenrening när man har problem med organiska miljöföroreningar. Andra vetenskapliga publikationer av intresse för Mälaren som presenteras kortfattat är t.ex. ett verktyg för att identifiera potentiella högrisksubstanser i akvatisk miljö kopplat till påverkan från avloppsreningsverk, att förbruningen av våra vatten verkar ha avstannat där det finns en tydlig brytpunkt år 2010. Ytterligare en publikation visar med en första studie att antibiotikaresistensgener är vanliga och täcker in flera typer av antibiotika i vatten runt Västerås, Uppsala, Eskilstuna och Stockholm. Årets kandidatarbeten gjordes i våtmarker med fokus på sådana som används som fosforfällor och i hästhagar för att ta reda på om näringsläckage från dessa kan minskas genom att mocka i hagen. På masternivå handlade det om att ta reda på vid vilka förhållanden kvävefixerande cyanobakterier trivs och att mäta algblomningar med sensorer för högre tids- och djupupplösning