Global climate change and the Baltic Sea ecosystem : 1 direct and indirect effects on species , communities and 2 ecosystem functioning 3 4

14 15 Climate change has multiple direct and indirect potentially synergistic effects on Baltic Sea species, 16 organism communities, and on ecosystem functioning, through physical and biogeochemical 17 environmental characteristics of the sea. Associated indirect and secondary effects on species 18 interactions, trophic dynamics and ecosystem function are expected to be significant. Evidence on 19 effects of climate are compiled from and reviewed for field studies, experimental work, as well as 20 modelling studies primarily from published literature after 2010. 21 The responses vary within and between species groups, even between sibling species. Such 22 subtle differences, as well as secondary feedbacks and altered trophic pathways, make projections 23 difficult. Some common patterns arise from the wealth of recent studies, however. It is likely that the 24 combined effects of increased external nutrient loads, stratification and internal loading will improve 25 the conditions for cyanobacterial blooms in large parts of the Baltic. In the northernmost areas the 26 increasing allochtonous DOM may further complicate the picture by increasing heterotrophy and by 27 decreasing food web efficiency. This effect may, however, be counteracted by the intensification of 28 the bacteria-flagellate-microzooplankton-mesozooplankton link, which may change the system from a 29 bottom-up controlled one to a top-down controlled one. 30 In deep benthic communities, continued eutrophication may promote higher sedimentation of 31 organic matter and increase zoobenthic biomasses, but eventually increasing stratification and 32 hypoxia/anoxia will disrupt benthic-pelagic coupling, leading to reduced benthic biomass. In the 33 photic benthic systems warmer winters with less ice and nutrient increase enhances eutrophication. 34 The projected salinity decline suppresses marine species, and temperature increase overgrowth of 35 perennial macroalgae by annual filamentous alga throughout the growing-season, and major changes 36 in the marine entire ecosystem are expected. The changes in environmental conditions probably also 37 lead to increased establishment of non-indigenous species, potentially affecting food web dynamics in 38 large areas of the Baltic Sea. 39 However, several modelling studies have concluded that nutrient reductions according to the 40 Baltic Sea Action Plan of Helsinki Commission may be a stronger driver for ecosystem functions in 41 the Baltic Sea than climate change. Such studies highlight the importance of studying the Baltic Sea 42 as an interlinked socio-ecological system. 43 Knowledge gaps include uncertainties in projecting the future salinity level as well as 44 stratification under different climate forcings. This weakens our ability to project how overall 45 biodiversity, pelagic productivity, fish populations, and macroalgal communities may change in the 46 future. Experimental work must be better integrated into studies of food web dynamics, to get a more 47 comprehensive view of the responses of the pelagic and benthic systems to climate change, from 48 bacteria to fish. Few studies have holistically investigated the shallow water ecosystems holistically. 49 There are complex climate-induced interactions and multiple feedbacks between algae, grazers and 50 https://doi.org/10.5194/esd-2021-73 Preprint. Discussion started: 10 September 2021 c © Author(s) 2021. CC BY 4.0 License.