Jukka Seppälä

The decreasing water transparency of the Baltic Sea over the last century has been documented by in situ measurements of the Secchi disk depth ( z _SD ) as well as by satellite measurements over recent decades. While the decrease in transparency for the Baltic Sea in general has become negligible from about 2013 onwards, satellite-detected light attenuation of the violet and blue light ( k _d 412) has increased, particularly from 2013 onwards. In the Gulf of Bothnia (particularly in the Bothnian Bay), eastern Gulf of Finland and eastern Gulf of Riga, k _d 412 has increased significantly, indicating a dramatic increase in the concentration of colored dissolved organic matter (CDOM). These findings are partly supported by in situ CDOM data. The increase in CDOM concentration is probably due to climate change induced shifts in hydrology and greening of the land, all related to global warming. The increase in CDOM concentration and its effect on light availability may cause significant ecological changes and deterioration of the aquatic food chains.

Phytoplankton undertake daily vertical migration through the water column to optimize light and nutrient access while avoiding predators. However, diel vertical migration (DVM) patterns remain poorly characterized for many taxa due to limitations of labor-intensive traditional microscopy. Here, we employed high-throughput in situ imaging flow cytometry to investigate DVM. An Imaging FlowCytobot (IFCB) was deployed to continuously profile the vertical water column for ~10 weeks (August–October 2016) at a location in the Skagerrak, eastern North Sea. This revealed significant DVM for several morpho-taxonomic groups, including taxa belonging to ciliates, dinoflagellates, and diatoms, shifting median depth by 2–6 m between night and day. The analysis also revealed that DVM can be inferred from diel pulses in surface water biomass, which we leveraged to study DVM in an extensive IFCB time-series dataset from the central Baltic Sea (June–October in 2020 and 2021). Migratory taxa accounted for 77% and 79% of total phytoplankton biomass (size range <10–150 μm) in the Skagerrak and Baltic Sea, respectively, underscoring the ecological significance of DVM. Most populations peaked near the surface at midday, although other patterns were also observed. While many taxa displayed consistent migration behaviors across both regions, others differed—likely due to population-specific traits or local environmental conditions. Seasonal changes in migration patterns suggest a role for community turnover and shifting environmental conditions. This study highlights the prevalence of DVM in phytoplankton and showcases the power of automated, high-throughput imaging technologies to advance our understanding of plankton ecology.

The impact of chromophoric dissolved organic matter (CDOM) on the spectral underwater light field within photosynthetically active radiation (PAR), and the related responses of spectral phytoplankton light absorption are poorly documented in the mixed layer of lakes. We examined how CDOM influences the spectrum of lake optical properties, underwater light field, photons absorbed by phytoplankton, photoacclimation, and chromatic adaptation of phytoplankton in 127 boreal lakes. In lakes with increasing CDOM content, light intensity decreased steeply with depth, but mixed layer depth and mean light intensity of the mixed layer decreased only moderately. Increasing CDOM content red‐shifted the central wavelengths of lake optical properties, underwater light fields, and the photons absorbed by phytoplankton. In study lakes with increasing CDOM content, the highest light availability shifted stepwise from 580 to 650 nm and close to 700 nm. The ratio of chlorophyll a concentration to phytoplankton biomass decreased slightly in lakes with higher CDOM content. The absorption coefficient of phytoplankton at blue relative to red wavelengths decreased with increasing CDOM content, indicating decreased blue‐absorbing pigmentation in response to decreased availability of blue light. Surprisingly, the number of red photons (600–700 nm) absorbed by phytoplankton increased with CDOM content below the mid‐point of the euphotic zone. Over the entire water column, red light (600–700 nm) accounted for > 50% of PAR absorption by phytoplankton in 69% of the lakes. The high contribution of red photons to the absorption of PAR by phytoplankton may have photobiological consequences, which are poorly understood and require further study.

The depth of the mixed layer is a major determinant of nutrient and light availability for phytoplankton in stratified waterbodies. Ongoing climate change influences surface waters through meteorological forcing, which modifies the physical structure of fresh waters including the mixed layer, but effects on phytoplankton biomass are poorly known. To determine the responses of phytoplankton biomass to the depth of the mixed layer, light availability and associated meteorological forcing, we followed daily changes in weather and water column properties in a boreal lake over the first half of a summer stratification period. Phytoplankton biomass increased with the deepening of the mixed layer associated with high wind speeds and low air temperature relative to the temperature of the mixed layer (T_air−T_mix < 0), whereas heatwave conditions—shallow mixed layer driven by high T_air−T_mix value and low wind speed—reduced the biomass. Improving light availability from low to moderate light conditions increased the phytoplankton biomass, while the highest light availability was associated with low phytoplankton biomass. Our study demonstrates that the climatic impact‐drivers wind speed and T_air−T_mix are major drivers of mixed layer depth, which controlled phytoplankton biomass during the early summer stratification period. Our study suggests that increasing air temperature relative to water temperature and declining wind speeds have potential to lead to reduced phytoplankton biomass due to a shallower mixed layer during the first half of the stratification period in non‐eutrophic lakes with sufficient light availability.

Summer blooms of nitrogen-fixing filamentous cyanobacteria are recurrent phenomena in the Baltic Sea. Salinity, varying from 0 to10 PSU in the surface layer of the Baltic Sea, is among the major factors affecting the basin-scale distribution of various bloom-forming cyanobacterial species. The effects of salinity on the growth rate and cellular carbon, nitrogen, and phosphorus ratios of two major cyanobacterial species that form dense blooms in the Baltic, Aphanizomenon sp. (strain KAC 15) and Nodularia spumigena (strain HEM), were studied. Cells were grown under N₂-fixing conditions in a salinity gradient from 0 to 10 PSU. The growth rates of the species showed contrasting responses to salinity. For Aphanizomenon sp. the maximum growth rates (0.28–0.31 d^–1) were observed at salinities of 0–2 PSU, while for N. spumigena the maximum growth rate occurred at 8–10 PSU (0.14–0.16 d^–1). The latter species did not tolerate low salinities (< 2 PSU). The observed differences in salinity tolerances constrain the distribution patterns of these two species during cyanobacterial blooms, Aphanizomenon sp. being more abundant in the coastal and less saline areas. The variations in growth rates were largely reflected in cellular N : P and C : P ratios, which varied two-fold, and in C : Chla ratios with 5-fold variability. Cellular C : N ratios were rather constant at all salinities and close to the Redfield ratio for Aphanizomenon sp. (on average 5.9 g g^–1) and above the Redfield ratio for N. spumigena (on average 8.0 g g^–1). The relatively higher N : P and lower C : N ratio showed a higher need of N for Aphanizomenon sp. than for N. spumigena. This is partly explained by the higher abundance of N-rich phycobilin pigments in Aphanizomenon sp. as indicated by fluorescence measurements. The observed differences in pigmentation indicate species-specific strategies in light harvesting.