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Climate change is likely to have large effects on the Baltic Sea ecosystem. Simulations indicate 2-4 °C warming and 50-80 % decrease in ice cover by 2100. Precipitation may increase ~30 % in the north, causing increased land runoff of allochthonous organic matter (AOM) and organic pollutants and decreased salinity. Coupled physical-biogeochemical models indicate that, in the south, bottom-water anoxia may spread, reducing cod recruitment and increasing sediment phosphorus release, thus promoting cyanobacterial blooms. In the north, heterotrophic bacteria will be favored by AOM, while phytoplankton production may be reduced. Extra trophic levels in the food web may increase energy losses and consequently reduce fish production. Future management of the Baltic Sea must consider the effects of climate change on the ecosystem dynamics and functions, as well as the effects of anthropogenic nutrient and pollutant load. Monitoring should have a holistic approach, encompassing both autotrophic (phytoplankton) and heterotrophic (e.g., bacterial) processes.

A common and established view is that increased inputs of nutrients to the sea, for example via river flooding, will cause eutrophication and phytoplankton blooms in coastal areas. We here show that this concept may be questioned in certain scenarios. Climate change has been predicted to cause increased inflow of freshwater to coastal areas in northern Europe. River waters in these areas are often brown from the presence of high concentrations of allochthonous dissolved organic carbon (humic carbon), in addition to nitrogen and phosphorus. In this study we investigated whether increased inputs of humic carbon can change the structure and production of the pelagic food web in the recipient seawater. In a mesocosm experiment unfiltered seawater from the northern Baltic Sea was fertilized with inorganic nutrients and humic carbon (CNP), and only with inorganic nutrients (NP). The system responded differently to the humic carbon addition. In NP treatments bacterial, phytoplankton and zooplankton production increased and the systems turned net autotrophic, whereas the CNP-treatment only bacterial and zooplankton production increased driving the system to net heterotrophy. The size-structure of the food web showed large variations in the different treatments. In the enriched NP treatments the phytoplankton community was dominated by filamentous >20 µm algae, while in the CNP treatments the phytoplankton was dominated by picocyanobacteria <5 µm. Our results suggest that climate change scenarios, resulting in increased humic-rich river inflow, may counteract eutrophication in coastal waters, leading to a promotion of the microbial food web and other heterotrophic organisms, driving the recipient coastal waters to net-heterotrophy.

Summary Endothermic organisms can adapt to short growing seasons, low temperatures and nutrient limitation by developing high growth rates and high gross growth efficiencies (GGEs). Animals with high GGEs are better at assimilating limiting nutrients and thus should recycle (or lose) fewer nutrients. Longer guts in relation to body mass may facilitate higher GGE under resource limitation. Within the context of ecological stoichiometry theory, this study combines ecology with evolution by relating latitudinal life‐history adaptations in GGE, mediated by gut length, to its ecosystem consequences, such as consumer‐mediated nutrient recycling. In common garden experiments, we raised Rana temporaria tadpoles from two regions (Arctic/Boreal) under two temperature regimes (18/23 °C) crossed with two food quality treatments (high/low‐nitrogen content). We measured tadpole GGEs, total nutrient loss (excretion + egestion) rates and gut length during ontogeny. In order to maintain their elemental balance, tadpoles fed low‐nitrogen (N) food had lower N excretion rates and higher total phosphorous (P) loss rates than tadpoles fed high‐quality food. In accordance with expectations, Arctic tadpoles had higher GGEs and lower N loss rates than their low‐latitude conspecifics, especially when fed low‐N food, but only in ambient temperature treatments. Arctic tadpoles also had relatively longer guts than Boreal tadpoles during early development. That temperature and food quality interacted with tadpole region of origin in affecting tadpole GGEs, nutrient loss rates and relative gut length, suggests evolved adaptation to temperature and resource differences. With future climate change, mean annual temperatures will increase. Additionally, species and genotypes will migrate north. This will change the functioning of Boreal and Arctic ecosystems by affecting consumer‐mediated nutrient recycling and thus affect nutrient dynamics in general. Our study shows that evolved latitudinal adaption can change key ecosystem functions.

Climate change scenarios project that precipitation will increase in northern Europe, causing amplified inflows of terrestrial matter (tM) and inorganic nutrients to coastal areas. How this will affect the plankton community is poorly understood. A mesocosm experiment was carried out to investigate the influence of two levels of tM inputs on the composition, size-structure and productivity of a natural coastal phytoplankton community from the northern Baltic Sea. The tM addition caused browning of the water and decreased underwater light levels, while the concentrations of dissolved organic carbon (DOC) and inorganic nutrients increased. Microphytoplankton were promoted by tM addition, while in the controls picophytoplankton dominated the phytoplankton community. Inorganic nutrient availability was instrumental in defining the phytoplankton community composition and size-structure. As a response to tM addition, the phytoplankton increased their chlorophyll a content. This physiological adaptation helped to maintain high primary production rates at the low tM enrichment, but at the high tM load the primary production decreased as did the biomass of mesozooplankton. The ciliate biomass was high when the mesozooplankton biomass was low, indicating that a trophic cascade occurred in the system. Structural equation modeling showed that tM borne DOC promoted ciliates, while primary and bacterial production were disfavored. Thus, DOC originating from soils had an indirect negative effect on the mesozooplankton by reducing their food availability. Although, a positive correlation between heterotrophic bacteria and phytoplankton suggested coupling between phytoplankton produced carbon and heterotrophs growth. The results from our study indicate that river-borne DOC and inorganic nutrients have a large impact on the phytoplankton community, driving the system to the dominance of large diatoms. However, since river-borne humic substances cause browning of the water, phytoplankton increase their light harvesting pigments. At moderate inflow this helps to support the primary production, but at high inflows of terrestrial material the primary production will decrease. As high river inflows have been projected to be a consequence of climate change, we foresee that primary production will decrease in coastal areas in the future, and the impacts of such changes on the food web could be significant.

The influence of nutrient availability and light conditions on phytoplankton size-structure, nutritional strategy and production was studied in a phosphorus-poor estuary in the northern Baltic Sea receiving humic-rich river water. The relative biomass of mixotrophic nanophytoplankton peaked in spring when heterotrophic bacterial production was high, while autotrophic microphytoplankton had their maximum in summer when primary production displayed highest values. Limiting substance (phosphorus) only showed small temporal variations, and the day light was at saturating levels all through the study period. We also investigated if the phytoplankton taxonomic richness influences the production. Structural equation modelling indicated that an increase of the taxonomic richness during the warm summer combined with slightly higher phosphorus concentration lead to increased resource use efficiency, which in turn caused higher phytoplankton biomass and primary production. Our results suggest that climate warming would lead to higher primary production in northerly shallow coastal areas, which are influenced by humic-rich river run-off from un-disturbed terrestrial systems.

Climate change predictions indicate that coastal and estuarine environments will receive increased terrestrial runoff via increased river discharge. This discharge transports allochthonous material, containing bioavailable nutrients and light attenuating matter. Since light and nutrients are important drivers of basal production, their relative and absolute availability have important consequences for the base of the aquatic food web, with potential ramifications for higher trophic levels. Here, we investigated the effects of shifts in terrestrial organic matter and light availability on basal producers and their grazers. In twelve Baltic Sea mesocosms, we simulated the effects of increased river runoff alone and in combination. We manipulated light (clear/shade) and carbon (added/not added) in a fully factorial design, with three replicates. We assessed microzooplankton grazing preferences in each treatment to assess whether increased terrestrial organic matter input would: (1) decrease the phytoplankton to bacterial biomass ratio, (2) shift microzooplankton diet from phytoplankton to bacteria, and (3) affect microzooplankton biomass. We found that carbon addition, but not reduced light levels per se resulted in lower phytoplankton to bacteria biomass ratios. Microzooplankton generally showed a strong feeding preference for phytoplankton over bacteria, but, in carbon-amended mesocosms which favored bacteria, microzooplankton shifted their diet towards bacteria. Furthermore, low total prey availability corresponded with low microzooplankton biomass and the highest bacteria/phytoplankton ratio. Overall our results suggest that in shallow coastal waters, modified with allochthonous matter from river discharge, light attenuation may be inconsequential for the basal producer balance, whereas increased allochthonous carbon, especially if readily bioavailable, favors bacteria over phytoplankton. We conclude that climate change induced shifts at the base of the food web may alter energy mobilization to and the biomass of microzooplankton grazers.