Nitrogen-fixing cyanobacterial blooms worldwide have been shown to support production in primary consumers such as zooplankton and benthic invertebrates, but there have been few estimates on how much of the cyanobacterial nitrogen ends up in secondary consumers, such as fish. Using compound specific analysis of δ¹⁵N in amino acids (CSIA-AA), we have investigated if cyanobacterially fixed (diazotrophic) nitrogen (N) can be traced in higher trophic level organisms such as zooplankton and mysids, and observed in juvenile Atlantic herring (Clupea harengus). At the eutrophied station, the biovolume of N fixing cyanobacteria was higher than at the reference station and the autotrophic population was comprised of a higher proportion of cyanobacteria palatable to zooplankton. The δ¹⁵N ‰ of source amino acids in juvenile herring significantly decreased over the season at the eutrophied station. Bayesian mixing models were used to calculate the percent of diazotrophically-fixed N found in herring muscle tissue in relation to pre-bloom N, and results showed that herring at the eutrophied station had a higher percent (~34 %) of diazotrophically-fixed N in their muscle tissue than fish at the reference station (~18%). A significant relationship was found between the percentage diazotrophically-fixed N in herring muscle tissue and the biovolume of N fixing cyanobacteria in the environment two months prior to the capture date of the fish. This study adds to the mounting evidence that cyanobacteria blooms, a global occurrence, support production at higher trophic levels, including fish.

In highly seasonal systems, the emergence of planktonic resting stages from the sediment is a key driver for bloom timing and plankton community composition. The termination of the resting phase is often linked to environmental cues, but the extent to which recruitment of resting stages is affected by climate change remains largely unknown for coastal environments. Here we investigate phyto- and zooplankton recruitment from oxic sediments in the Baltic Sea in a controlled experiment under proposed temperature and light increase during the spring and summer. We find that emergence of resting stage differs between seasons and the abiotic environment. Phytoplankton recruitment from resting stages were high in spring with significantly higher emergence rates at increased temperature and light levels for dinoflagellate and cyanobacteria than for diatoms, which had highest emergence under cold and dark conditions. In comparison, hatching of copepod nauplii was not affected by increased temperature and light levels. These results show that activation of plankton resting stages are affected to different degrees by increasing temperature and light levels, indicating that climate change affects plankton dynamics through processes related to resting stage termination with potential consequences for bloom timing, community composition and trophic mismatch.

I denna rapport presenteras 2023 års resultat från de undersökningar som utförs i Himmerfjärden och angränsande fjärdar för att följa upp effekterna av utsläppen av renat avloppsvatten från Himmerfjärdsverket.

Lufttemperaturens årsmedel 2023 var 1,5 grader högre än jämförperioden 1961-1990 men låg nära medlet för perioden 1991-2020. Efter en varm och så gott som isfri vinter följde en torr, varm vår och försommar, medan juli och augusti blev svalare och augusti nederbördsrik. Även oktober blev regnig vilket medförde att slussen öppnades i Södertälje, med stora flöden från Mälaren i november som följd. Avrinningen från land blev större än de närmaste sju åren, som varit relativt torra, och tillförseln av kväve och fosfor från land blev därmed också större vad den varit de senaste åren. Trots den pågående ombyggnaden av Himmerfjärdsverket blev utsläppet 2023 ungefär på samma nivå som den senaste 10-årsperioden.

Efter att ha ökat vid millennieskiftet och sedan ytterligare kring 2016 tycks salthalten nu ha stabiliserats på en högre nivå. Säsongsvariationen 2023 visar på ett inflöde av saltare djupvatten till Himmerfjärden i slutet av juli och lägre salthalter än normalt i ytan i november-december, vilket kan kopplas till Mälarens avtappning. De lägsta syrehalterna 2023 blev inte så låga som 2022 men det finns en generell trend mot allt lägre syrehalter, både i Himmerfjärden och vid referensstationen vid Askö.

Året inleddes med mycket höga fosforhalter i Himmerfjärden, troligen främst en effekt av omfattande syrebrist sensommaren 2022 och uppblandning av mycket fosforrikt bottenvatten under hösten 2022. Även hösten 2023 var fosforhalterna högre än normalt men inte fullt lika höga som året innan och års- och sommarmedel för fosfor blev lägre 2023 än 2022. Även kvävehalten var relativt hög i början av vintern, men sjönk tillbaka under året. Statusen för kväve och fosfor, bedömd enligt vattenförvaltningens bedömningsgrunder för sommaren, har sedan 1996 försämrats både på referensstationen och i Himmerfjärden, för fosfor skedde försämringen framförallt för omkring 10 år sedan.

Maxvärdet av biovolymen växtplankton på våren blev bland de högsta uppmätta, med dominans av kiselalger, i mitten av april. Medelvärdet för vårmånaderna mars-maj blev dock inte särskilt högt. Ävenbiovolymen under sommaren blev lägre än vad som varit fallet merparten av de senaste åtta åren. Av de kvävefixerande cyanobakterierna under sommaren dominerade Aphanizomenon sp. medan den ofta giftiga Nodularia spumigena förekom ytterst sparsamt. Även potentiellt giftiga dinoflagellater förekom enbart som enstaka celler i proverna.

Statusen för bottenfauna har generellt förbättrats under perioden 1997-2000, och når god status i alla områden utom på djupa stationer i Näslandsfjärden och inre norra Himmerfjärden. Både ökade tätheter av djur samt ändrad artsammansättning kan bidra till förbättringen. Även flera fynd av nya arter har gjorts på senare tid vilket möjligen kan bero på införsel med fartygstrafiken till Södertälje hamn.

Ocean climate change strongly affects organisms and ecosystems, and the causes, consequences, and underlying mechanisms need to be documented. In the Baltic Sea, a marginal sea under severe eutrophication stress, a longer productive season, and changes in the phytoplankton community over the last few decades have likely impacted diet and condition of keystone species, from individual to population level. This study uses stable isotopes (δ¹³C, δ¹⁵N, and derived isotope niche metrics) to trace energy and nutrient flows in archived samples of blue mussel (Mytilus edulis trossulus) spanning 24 yr (1993–2016). We test if long-term changes in isotope and elemental composition in mussels, as well as population abundance and biomass, can be explained by changes in abiotic and biotic variables, using partial least square regressions and structural equation modeling. We found decreasing trends in δ13C and δ15N as well as in mean size and total biomass of mussels, but no unidirectional changes in their stoichiometry or condition index. Changes in isotope composition were best explained by nitrogen-fixing cyanobacteria, by increased terrestrial organic carbon from land runoff (reflecting precipitation) and by decreases in dissolved inorganic nitrogen (indicative of successful eutrophication mitigation) and in biomass of a mixotrophic ciliate species. The trophic niche (assessed from isotope niche) was included as the best predictor for both mussel body condition and the observed decline in their total biomass. This study reveals that altered trophic relationships from climate-induced changes in the productivity base may strongly impact keystone species, with potential knock-on effects on ecosystem functions. 

This study aims to highlight how satellite data can be used for an improved understanding of ecological processes in a narrow coastal bay. The usefulness of the Medium Resolution Imaging Spectrometer (MERIS) data (2003-2011) as a complement to the in situ monitoring in Himmerferdenn (HF) bay is used as an example that can also be applied to other coastal areas. HF bay is one of the most frequently monitored coastal areas in the world, allowing for a rigorous comparison between satellites and ship-based monitoring data. MERIS data was used for the integration of chlorophyll-a (chl-a) over each waterbody in the HF area, following the national waterbody classification by the Swedish Meteorological and Hydrological Institute (SMHI). Chl-a anomaly maps were produced for the bay and its adjacent areas. The maps could be used to show events with high chl-a, both with natural causes (e.g. a Prymnesium polylepis bloom observed in summer 2008) and of anthropogenic causes (e.g. failure in the local sewage treatment plant resulting in a strong spring bloom in 2006). Anomaly maps thereby allow to scan larger coastal stretches to discriminate areas that may require additional sampling by ship, or to identify areas that do not differ much from the median value of the MERIS time series.

Despite cyanobacteria being a key phytoplankton group in the Baltic Sea, the factors governing their community structure are still poorly understood. Here, we studied the occurrence of the orders Chroococcales, Oscillatoriales and Nostocales, and potentially explanatory variables at five locations in the northern Baltic Sea from June September, 1998-2012. Cyanobacteria constituted 1-36% of the total phytoplankton biomass along the north south gradient. In the Bothnian Bay, Chroococcales and Oscillatoriales dominated the cyanobacterial community, whereas in the Bothnian Sea and northern Baltic Proper, Nostocales was the dominant group. The dominance of Chroococcales was coupled to low salinity and low total phosphorus, whereas Oscillatoriales correlated with high total nitrogen and low salinity. Nostocales correlated to high total phosphorus, inorganic phosphorus and salinity. Chroococcales showed an increase over time in the offshore Bothnian Bay, whereas Nostocales increased in the coastal Bothnian Sea and coastal Baltic Proper. The increase of Nostocales in the coastal Bothnian Sea was explained by a rise in total phosphorus and decrease in dissolved inorganic nitrogen compared to an increase of total nitrogen and phosphorus in the coastal Baltic Proper. No significant trends were observed in the cyanobacterial community in the offshore Bothnian Sea and the offshore northern Baltic Proper. We concluded that Chroococcales may be a useful indicator for increased phosphorus levels in waters with low phosphorus concentrations, whereas Nostocales could be used as a quality indicator for increasing phosphorus concentrations in waters with low inorganic N/P ratios (< 20), such as in the coastal Bothnian Sea and Baltic Proper.

We used nitrogen stable isotopes to study the regulation of nitrogen fixation by filamentous cyanobacteria. Nitrogen fixation was found to be almost insensitive to combined nitrogen, along a gradient from the Himmerfjarden sewage treatment plant discharge to the open sea. We found similarly low cyanobacterial (mostly Aphanizomenon sp.) delta N-15-values at all stations, despite significant differences along the bay in both total nitrogen concentrations in water and delta N-15 in seston (particles <10 mu m), the latter used as a proxy for algae growing on combined nitrogen alone. Only late in the productive season, when cyanobacterial biomass was declining or already low, did elevated delta N-15 suggest uptake of combined nitrogen. However, this coincided with an increase in the contribution of Dolichospermum spp. to overall diazotrophic biomass and may indicate uptake of combined nitrogen by this species. These results indicate that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. This study was part of a whole ecosystem experiment analyzing the effects of nitrogen removal in a sewage treatment plant discharging to the Himmerfjarden Bay, northern Baltic Sea Proper.

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90–200 m, mostly copepod nauplii and rotifers; mesozooplankton, >200 m, mostly copepods), and zooplanktivorous fish (herring, size range 5–15 cm and sprat, 9–11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher ¹⁵N/¹⁴N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.