Historically, phosphorus has been considered the limiting nutrient for primary production in the northern Baltic Sea, whereas further south in the Baltic Proper nitrogen is the limiting nutrient. In this study, we investigated the nutrient distribution during the past 30 years using Swedish and Finnish monitoring data. The results show comprehensively for the first time that the entire Bothnian Sea has switched to nitrogen limitation and that the Bothnian Bay is rapidly heading towards similar conditions. This is due to increasing inorganic phosphorus concentrations, possibly resulting from phosphorus rich water inflowing from the Baltic Proper. Inorganic nitrogen concentrations decrease or are unchanged. The increase of phosphorus led to increased primary production and, in the Bothnian Sea, to increased occurrence of filamentous cyanobacteria. The resulting shift emphasizes the need to revise management strategies of these basins, where treating both basins for phosphorus and nitrogen is advisable.

Phytoplankton depend on light, and while intensity effects are well known, responses to light spectrum are less studied. The northern Baltic Sea appears increasingly brown due to CDOM, which absorbs blue light. Using incubation experiments and DNA barcoding on communities from Kattegat, the Central Baltic, and the Bothnian Bay, we show that browner, red-shifted light promotes mixotrophs and heterotrophs. Green algae from the Bothnian Bay tolerated red light, unlike those from southern regions, indicating local adaptation. These results reveal CDOM-driven spectral selection shaping northern Baltic phytoplankton communities as browning intensifies.

Abstract Methylmercury (MeHg) accumulation in aquatic food webs is a key pathway for human exposure. We examined how terrestrially derived organic matter (tOM) influences MeHg bioaccumulation in coastal planktonic food webs by measuring MeHg bioaccumulation factors (BAFs) for a MeHg isotope tracer added to mesocosms receiving four tOM levels. Combined with two previous studies, the results showed that tOM alters MeHg bioaccumulation through opposing processes. All treatments produced highly heterotrophic food webs, with bacterial production contributing 68–87% of basal production and supporting high MeHg accumulation (log BAFs up to 6) due to complex trophic structure. However, average BAFs declined with increasing tOM among individual treatments, attributed to higher concentrations of thiol compounds associated with the dissolved organic matter (DOM-RSH) that suppress MeHg bioavailability. Together, these findings demonstrate that tOM inputs exert dual effects on MeHg bioaccumulation: enhancing it by promoting heterotrophy while simultaneously reducing it via increased DOM-RSH. Understanding the net outcome of these processes is essential for predicting MeHg dynamics under changing tOM inputs.

Marine phytoplankton monitoring has long relied on microscopy, but DNA metabarcoding has recently emerged as a complementary approach. This study assessed the applicability of DNA metabarcoding of the 18S ribosomal RNA gene in marine monitoring and compared its results with conventional microscopy. We analyzed data from 232 surface water samples from 17 monitoring stations in the Baltic Sea, Kattegat, and Skagerrak. Metabarcoding detected more orders, genera, and species than microscopy, with a 43% overlap in the most common genera identified by both methods. Despite attempts to normalize sequence reads to spike-in DNA or DNA concentrations, the correlations between abundances derived from the two methods were weak, though varied considerably between taxonomic groups and geographical areas. Correlations were consistently stronger when using carbon and biovolume concentrations than cell abundances. Our results highlight the potential of metabarcoding to expand biodiversity assessments and advance our understanding of microbial biodiversity in marine ecosystems. As a complement to microscopy, it can enhance existing monitoring efforts. Future improvements in reference database completeness, adoption of long-read sequencing technologies, and better characterization of gene copy number variability per cell are needed to further extend the applicability of metabarcoding for quantitative analyses.

Climate change is projected to cause elevated precipitation in northern Europe, leading to increased runoff of terrestrial matter to coastal areas. The consequences for food web production and ecosystem function remain unclear. A mesocosm experiment was performed to investigate the impacts of elevated terrestrial matter input, using a natural plankton community from the northern Baltic Sea with added young-of-the-year perch as planktivorous top consumer. Addition of terrestrial matter caused water browning and increased dissolved organic carbon and inorganic nutrient concentrations. Phytoplankton primary production showed a negative response to terrestrial matter due to decreased light availability, while heterotrophic bacterial production increased. The trophic balance, calculated as the difference between primary production and heterotrophic bacterial production, indicated that net-heterotrophy was triggered by terrestrial matter enrichment. Primary production was identified as the main basal energy source for fish. Addition of terrestrial matter reduced the food web efficiency, calculated as the ratio between fish production and basal production (primary production + heterotrophic bacterial production). Furthermore, stable isotope analysis of seston and fish indicated that the added terrestrial matter was not efficiently incorporated in the food web and only marginally altered the food web trophic positions. The results suggest that the main food chain consisted of phytoplankton, mesozooplankton, and fish, and that the ecosystem production was overall light driven. Under a changing climate, several negative effects can be expected, including a poorer light climate, reduced ecosystem production and net-heterotrophy. These alterations have potentially significant consequences for ecosystem functioning, fish production, and thus ecosystem services.

Cyanobacterial blooms in marine and freshwater ecosystems have increased in magnitude, frequency, and distribution worldwide during recent decades. Filamentous cyanobacteria are of unsuitable size for mesozooplankton feeding and of poorer nutritional quality than other phytoplankton taxa. The production and quality of higher trophic levels are therefore expected to decrease when cyanobacterial blooms increase. We conducted a mesocosm experiment using natural seawater from the northern Baltic Sea to contrast the effects of cyanobacteria‐ ( Aphanizomenon flosaquae ) and diatom‐dominated phytoplankton communities on mesozooplankton production and nutritional quality. A low and a high hydrological mixing regime was applied. The δ 15 N isotopic signal of seston and mesozooplankton was lower in the cyanobacteria‐based food web, demonstrating that Aphanizomenon fixed atmospheric nitrogen, which was transferred in the food web. The biomass of edible‐sized phytoplankton (2–50 μ m) was lower in the cyanobacteria‐based food web. The fatty acid quality, indicated by the ω3:ω6 ratio, was lower in the cyanobacteria‐based food web for both phytoplankton and mesozooplankton. Together, this resulted in 75–80% lower copepod production and food web efficiency (FWE) in the cyanobacteria‐based food web than in the diatom‐based food web. The hydrological mixing regime did not affect the biological production and quality. The results demonstrate that copepod production and FWE were driven by the quality and production of edible‐sized phytoplankton. The study implies that climate‐induced increases of filamentous cyanobacterial blooms will cause decreased production and nutritional quality of higher trophic levels.

The perfoliate pondweed, Potamogeton perfoliatus, is a common macrophyte in freshwater and subarctic coastal areas. This species builds extensive meadows that play a role as a filter removing nutrients traversing from land to sea and maintain essential ecosystem functions. Here, we investigated the function of perfoliate pondweed as a filter of potentially pathogenic bacteria by combining culture-dependent and 16S rRNA metabarcoding approaches. Our results suggest no significant nutrient reduction in the meadow region but the enrichment of potentially pathogenic bacteria, such as Vibrio, Legionella and Leptospira, particularly attached to macrophyte leaves. The bacterial community composition differed between seawater and macrophyte habitats, with higher relative abundances of Cyanobacteriia attached to macrophytes, without affecting alpha-diversity. The metabolic pathways of bacteria for aromatic and polymer compound degradation were enriched in the macrophytes, attributed to members of the genera Pseudorhodobacter, Novosphingobium and Erythrobacter. Functions related to such degradation suggest that the bacteria may be able to remove complex organic compounds coming from land. We argue that the macrophyte meadows may be relevant to both animal and human health, as these habitats can be hot spots for potentially pathogenic bacteria.

Mixing of entire microbial communities represents a frequent, yet understudied phenomenon. Here, we mimicked estuarine condition in a microcosm experiment by mixing a freshwater river community with a brackish sea community and assessed the effects of both environmental and community coalescences induced by varying mixing processes on microeukaryotic communities. Signs of shifted community composition of coalesced communities towards the sea parent community suggest asymmetrical community coalescence outcome, which, in addition, was generally less impacted by environmental coalescence. Community stability, inferred from community cohesion, differed among river and sea parent communities, and increased following coalescence treatments. Generally, community coalescence increased alpha diversity and promoted competition from the introduction (or emergence) of additional (or rare) species. These competitive interactions in turn had community stabilizing effect as evidenced by the increased proportion of negative cohesion. The fate of microeukaryotes was influenced by mixing ratios and frequencies (i.e. one-time versus repeated coalescence). Namely, diatoms were negatively impacted by coalescence, while fungi, ciliates, and cercozoans were promoted to varying extents, depending on the mixing ratios of the parent communities. Our study suggests that the predictability of coalescence outcomes was greater when the sea parent community dominated the final community, and this predictability was further enhanced when communities collided repeatedly.

The Baltic Sea is one of the largest brackish water environments on earth and is characterised by pronounced physicochemical gradients and seasonal dynamics. Although the Baltic Sea has a long history of microscopy-based plankton monitoring, DNA-based metabarcoding has so far mainly been limited to individual transect cruises or time-series of single stations. Here we report a dataset covering spatiotemporal variation in prokaryotic and eukaryotic microbial communities and physicochemical parameters. Within 13-months between January 2019 and February 2020, 341 water samples were collected at 22 stations during monthly cruises along the salinity gradient. Both salinity and seasonality are strongly reflected in the data. Since the dataset was generated with both metabarcoding and microscopy-based methods, it provides unique opportunities for both technical and ecological analyses, and is a valuable biodiversity reference for future studies, in the prospect of climate change.

Microplastics (MPs) have the potential to modify aquatic microbial communities and distribute microorganisms, including pathogens. This poses a potential risk to aquatic life and human health. Despite this, the fate of 'hitchhiking' microbes on MPs that traverse different aquatic habitats remains largely unknown. To address this, we conducted a 50-day microcosm experiment, manipulating estuarine conditions to study the exchange of bacteria and microeukaryotes between river, sea and plastisphere using a long-read metabarcoding approach. Our findings revealed a significant increase in bacteria on the plastisphere, including Pseudomonas, Sphingomonas, Hyphomonas, Brevundimonas, Aquabacterium and Thalassolituus, all of which are known for their pollutant degradation capabilities, specifically polycyclic aromatic hydrocarbons. We also observed a strong association of plastic-degrading fungi (i.e., Cladosporium and Plectosphaerella) and early-diverging fungi (Cryptomycota, also known as Rozellomycota) with the plastisphere. Sea MPs were primarily colonised by fungi (70%), with a small proportion of river-transported microbes (1%-4%). The mere presence of MPs in seawater increased the relative abundance of planktonic fungi from 2% to 25%, suggesting significant exchanges between planktonic and plastisphere communities. Using microbial source tracking, we discovered that MPs only dispersed 3.5% and 5.5% of river bacterial and microeukaryotic communities into the sea, respectively. Hence, although MPs select and facilitate the dispersal of ecologically significant microorganisms, drastic compositional changes across distinct aquatic habitats are unlikely.

Mixing of entire microbial communities represents a frequent, yet understudied phenomenon. Here, we mimicked estuarine condition in a microcosm experiment by mixing a freshwater river community with a brackish sea community and assessed the effects of both environmental and community coalescences induced by varying mixing processes on microeukaryotic communities. Signs of shifted community composition of coalesced communities towards the sea parent community suggest asymmetrical community coalescence outcome, which, in addition, was generally less impacted by environmental coalescence. Diatoms were negatively impacted by coalescence, while fungi, ciliates, and cercozoans were promoted to varying extents, depending on the mixing ratios of the source (i.e., river or sea) communities. Community stability, inferred from community cohesion, suggests that the more stable parent community (i.e., community with greater negative cohesion attributed to competitive interactions) dominates the final, coalesced community, but the fate of its community members is influenced by mixing ratios and frequencies (i.e., one-time versus repeated coalescence). Generally, community coalescence increased alpha diversity and promoted competition from the introduction (or emergence) of additional (or rare) species. These competitive interactions in turn had community stabilizing effect as evidenced by the increased proportion of negative cohesion. Our study suggests that the predictability of coalescence outcomes was greater when the more stable parent community (i.e., sea microbes) dominated the final community and this predictability was further enhanced when communities collided repeatedly.

Climate change is having many negative impacts worldwide. Increased rainfall caused by climate change has become a serious issue in the northern parts of the world. With more rainfall, a larger amount of brown-colored decaying plant material is transported from the land to oceans, making the seawater browner. The brown color reduces the amount of sunlight that penetrates into the seawater, which can decrease the growth of microscopic plant-like organisms called phytoplankton that rely on sunlight to grow. Phytoplankton are an important food source for ocean animals, such as tiny creatures called zooplankton. This study explored the effects of seawater browning on phytoplankton and zooplankton in the northern Baltic Sea.

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L−1; median: 8.0 µmol N L−1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

Planktonic microorganisms in coastal waters form the foundation of food webs and biogeochemical cycles while exposed to pronounced environmental gradients, especially brackish salinities. Yet, commonplace ecological assessment overlooks most of their diversity. Here, we analyzed the protist and bacterial diversity from new and publicly available DNA metabarcoding data collected alongside the Swedish marine monitoring program. We show that salinity, unlike other environmental factors, had a stronger effect on bacterial than protist community composition. The seasonality of protist but not bacterial families showed high geographic variation. Bacterial alpha diversity increased with dissolved inorganic nitrogen, while protist alpha diversity was highest in near-marine salinities. Microbial community composition patterns displayed interannual stability despite technical differences affecting the detection of rare taxa. Co-occurrence analysis identified clusters of potentially interdependent microorganisms. Bayesian modeling showed that the same bacterial lineages were less likely than protists to occur in both lower (<9 PSU) and higher (>15 PSU) brackish salinities. We propose that protists are less ecologically sensitive to salinity due to the disconnection of basic metabolic processes from the cell membrane through compartmentalization. Ultimately, incorporating DNA metabarcoding into an environmental monitoring program allowed us to connect ecological and biogeographic processes with understudied taxa and biodiversity dynamics. ### Competing Interest Statement The authors have declared no competing interest.

Climate change is projected to cause alterations in northern coastal systems, including humification and intensified nutrient loads, which can lead to ecosystem imbalances and establishment of new bacterial species. Several potential pathogens, such as different species of Legionella , hide in the environment between infections, some by living inside protozoan host cells. Knowledge about the occurrence of Legionella in natural waters is missing, which disable risk assessments of exposure. We performed a study of the species diversity of Legionella in the northern Baltic Sea (Gulf of Bothnia) during early summer to map their occurrence and to identify possible environmental drivers. We detected Legionella and potential protozoan hosts along gradients of the Gulf of Bothnia. We also for the first time present third generation full-length 16S rRNA amplicon sequencing (Nanopore) to resolve environmental species classification of Legionella , with a method suitable to study all bacteria. Our data show that full length 16S rRNA sequences is sufficient to resolve Legionella while the standard short Illumina sequences did not capture the entire diversity. For accurate species classification of Legionella , harmonization between the Nanopore classification methods is still needed and the bias toward the well-studied Legionella pneumophila need to be resolved. Different Legionella species occurred both in the Bothnian Sea and in the Bothnian Bay and their abundance were linked to humic substances and low salinity. The relative abundance of Legionella was higher in the humic-rich northern waters of the Bothnian Bay. The link between Legionella species and humic substances may be indirect via promotion of the heterotrophic microbial food web, allowing Legionella species and similar bacteria to establish. Humic substances are rich in iron, which has been shown crucial for growth of Legionella species and other pathogens. Considering climate change projections in this regional area, with increased humification and freshwater inflow, this bacterial niche containing potential pathogens might become more widespread in the future Baltic Sea. This study demonstrates the significance of DNA sequencing to monitor public health relevant bacteria like Legionella species in the environment. Including sequencing of bacteria and protozoa in the environmental monitoring programs could be used to identify ecosystem imbalances, which enable appropriate responses to emerging diseases.

Bacteria are major utilizers of dissolved organic matter in aquatic systems. In coastal areas bacteria are supplied with a mixture of food sources, spanning from refractory terrestrial dissolved organic matter to labile marine autochthonous organic matter. Climate scenarios indicate that in northern coastal areas, the inflow of terrestrial organic matter will increase, and autochthonous production will decrease, thus bacteria will experience a change in the food source composition. How bacteria will cope with such changes is not known. Here, we tested the ability of an isolated bacterium from the northern Baltic Sea coast, Pseudomonas sp., to adapt to varying substrates.We performed a 7-months chemostat experiment, where three different substrates were provided: glucose, representing labile autochthonous organic carbon, sodium benzoate representing refractory organic matter, and acetate - a labile but low energy food source. Growth rate has been pointed out as a key factor for fast adaptation, and since protozoan grazers speed-up the growth rate we added a ciliate to half of the incubations. The results show that the isolated Pseudomonas is adapted to utilize both labile and ring-structured refractive substrates. The growth rate was the highest on the benzoate substrate, and the production increased over time indicating that adaptation did occur. Further, our findings indicate that predation can cause Pseudomonas to change their phenotype to resist and promote survival in various carbon substrates. Genome sequencing reveals different mutations in the genome of adapted populations compared to the native populations, suggesting the adaptation of Pseudomonas sp. to changing environment.

Bacteria are major consumers of dissolved organic matter (DOM) in aquatic systems. In coastal zones, bacteria are exposed to a variety of DOM types originating from land and open sea. Climate change is expected to cause increased inflows of freshwater to the northern coastal zones, which may lead either to eutrophication or to increased inputs of refractory terrestrial compounds. The compositional and functional response of bacterial communities to such changes is not well understood. We performed a 2-day microcosm experiment in two bays in the coastal northern Baltic Sea, where we added plankton extract to simulate eutrophication and soil extract to simulate increased inputs of refractory terrestrial compounds. Our results showed that the bacterial communities responded differently to the two types of food substrates but responded in a similar compositional and functional way in both bays. Plankton extract addition induced a change of bacterial community composition, while no significant changes occurred in soil extract treatments. Gammaproteobacteria were promoted by plankton extract, while Alphaproteobacteria dominated in soil extract addition and in the non-amended controls. Carbohydrate metabolism genes, such as aminoglycan and chitin degradation, were enriched by plankton extract, but not soil extract. In conclusion, the coastal bacterial communities rapidly responded to highly bioavailable substrates, while terrestrial matter had minor influence and degraded slowly. Thus, in the northern Baltic Sea, if climate change leads to eutrophication, large changes of the bacterial community composition and function can be expected, while if climate change leads to increased inflow of refractory terrestrial organic matter the bacterial communities will not show fast compositional and functional changes. Degradation of terrestrial organic matter may instead occur over longer periods of time, e.g. years. These findings help to better understand the ability of bacterial communities to utilize different carbon sources and their role in the ecosystem.

Climate change is expected to alter global temperature and precipitation patterns resulting in complex environmental impacts. The proposed higher precipitation in northern Scandinavia would increase runoff from land, hence increase the inflow of terrestrial dissolved organic matter (tDOM) in coastal regions. This could promote heterotrophic bacterial production and shift the food web structure, by favoring the microbial food web. The altered climate is also expected to affect transport and availability of organic micropollutants (MPs), with downstream effects on exposure and accumulation in biota. This study aimed to assess climate-induced changes in a Bothnian Sea food web structure as well as bioaccumulation patterns of MPs. We performed a mesocosms-study, focusing on aquatic food webs with fish as top predator. Alongside increased temperature, mesocosm treatments included tDOM and MP addition. The tDOM addition affected nutrient availability and boosted both phytoplankton and heterotrophic bacteria in our fairly shallow mesocosms. The increased tDOM further benefitted flagellates, ciliates and mesozooplankton, while the temperature increase and MP addition had minor effect on those organism groups. Temperature, on the other hand, had a negative impact on fish growth and survival, whereas tDOM and MP addition only had minor impact on fish. Moreover, there were indications that bioaccumulation of MPs in fish either increased with tDOM addition or decreased at higher temperatures. If there was an impact on bioaccumulation, moderately lipophilic MPs (log Kow 3.6 – 4.6) were generally affected by tDOM addition and more lipophilic MPs (log Kow 3.8 to 6.4) were generally affected by increased temperature. This study suggest that both increased temperatures and addition of tDOM likely will affect bioaccumulation patterns of MPs in shallow coastal regions, albeit with counteracting effects.

Algae-produced long-chain polyunsaturated fatty acids (LC-PUFA; with >= 20 carbon atoms) are key biomolecules for consumer production and animal health. They are transferred to higher trophic levels and accumulated in food chains. However, LC-PUFA accumulation in consumers and their trophic transfer vary with the diet quality and the physiological demand for LC-PUFA of consumers. The goal of this study was to investigate spatial and taxonomic differences in LC-PUFA retention of coastal fish predators that potentially differ in their habitat use (benthic versus pelagic) and prey quality. We analyzed the fatty acid (FA) composition of common fish species, namely roach and European perch, as well as their potential prey from benthic and pelagic habitats in three bays of the northern Baltic Sea. We then assessed whether the fish LC-PUFA retention differed between species and among the study bays with different diet quality, that is, LC-PUFA availability. Our data indicated taxon-specific differences in the retention of LC-PUFA and their precursor FA in fish (i.e., short-chain PUFA with <20 carbon atoms). Perch did not show any spatial variation in the retention of all these FA, while roach showed spatial differences in the retention of docosahexaenoic acid (DHA) and their precursor FA, but not eicosapentaenoic acid (EPA). Data suggest that diet quality and trophic reliance on benthic prey underlay the DHA retention differences in roach. Although the PUFA supply might differ among sites, the low spatial variation in LC-PUFA content of perch and roach indicates that both fishes were able to selectively retain dietary LC-PUFA. Climate change together with other existing human-caused environmental stressors are expected to alter the algal assemblages and lower their LC-PUFA supply for aquatic food webs. Our findings imply that these stressors will pose heterogeneous impacts on different fish predators. We advocate further investigations on how environmental changes would affect the nutritional quality of the basal trophic level, and their subsequent impacts on LC-PUFA retention, trophic ecology, and performance of individual fish species.

Climate change has been projected to cause increased temperature and amplified inflows of terrestrial organic matter to coastal areas in northern Europe. Consequently, changes at the base of the food web favoring heterotrophic bacteria over phytoplankton are expected, affecting the food web structure. We tested this hypothesis using an outdoor shallow mesocosm system in the northern Baltic Sea in early summer, where the effects of increased temperature (+ 3°C) and terrestrial matter inputs were studied following the system dynamics and conducting grazing experiments. Juvenile perch constituted the highest trophic level in the system, which exerted strong predation on the zooplankton community. Perch subsequently released the microbial food web from heavy grazing by mesozooplankton. Addition of terrestrial matter had a stronger effect on the microbial food web than the temperature increase, because terrestrial organic matter and accompanying nutrients promoted both heterotrophic bacterial production and phytoplankton primary production. Moreover, due to the shallow water column in the experiment, terrestrial matter addition did not reduce the light below the photosynthesis saturation level, and in these conditions, the net-autotrophy was strengthened by terrestrial matter enrichment. In combination with elevated temperature, the terrestrial matter addition effects were intensified, further shifting the size distribution of the microbial food web base from picoplankton to microphytoplankton. These changes up the food web led to increase in the biomass and proportion of large-sized ciliates (>60 µm) and rotifers. Despite the shifts in the microbial food web size structure, grazing experiments suggested that the pathway from picoplankton to nano- and microzooplankton constituted the major energy flow in all treatments. The study implies that the microbial food web compartments in shallow coastal waters will adjust to climate induced increased inputs of terrestrial matter and elevated temperature, and that the major energy path will flow from picoplankton to large-sized ciliates during the summer period.