Abstract. Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the
biogeochemical functioning of the Baltic Sea. In its content, the paper
covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field of marine biogeochemistry in the Baltic Sea.


Biogeochemical functioning of the Baltic Sea

The study of marine coastal food webs has a central role in the application of the integrated ecosystem approach for marine management. Changes in food webs caused by natural or anthropic drivers can lead to dramatic shifts in the overall structure and function of coastal marine ecosystems and deterioration of their services. The present review investigates the methodological approaches employed for the assessment of coastal shelf food webs at a global scale and highlights existing gaps and limitations. Out of 1652 published articles that initially met our search criteria, 880 passed the initial screening and 493 were found relevant and were fully analysed. The information extracted included the spatiotemporal coverage of the studies; the main methodological approaches utilized for the assessment of population state variables (i.e. biomass, size, abundance) and trophic levels; the biotic components and driving factors considered; indices used to describe the structure and functioning of coastal food webs; and main knowledge gaps. Results showed that most studies have been conducted at a subnational level, mostly in the Temperate Northern Atlantic marine realm. Overall, 54% of the studies provided quantitative information on food web structure. The most common methodological approach utilized was modelling (40%), followed by non-experimental-based correlations (30%), and natural or manipulative experiments (14%). Information on population state variables was provided by 69% of the studies, while 42% employed some of the following trophic level determination techniques: stable isotopes, gut contents, fatty acids, and molecular analysis, which were either combined or used in isolation. Specific natural or human drivers were incorporated in 76% of the studies, with fishing being the most common driver. Modelling approaches included multiple indices to describe food web attributes and/or the structure and functioning of coastal shelf ecosystems. Despite the great progress achieved through the development of new tools and techniques, food web analysis still suffers from important knowledge gaps and limitations of the methodological approaches, which are extensively discussed. The present review establishes a useful knowledge base to provide guidance for future research and assessments on coastal shelf food webs, and to support ecosystem-based management.

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Global Systematic Review of Methodological Approaches to Analyze Coastal Shelf Food Webs

The loading of terrestrially derived macroelements, such as dissolved organic carbon (DOC), total nitrogen (N), and total phosphorus (P), to inland and estuarine waters is increasing in the northern hemisphere. Thesemacroelements often limit heterotrophic bacterioplankton metabolism (production and respiration), which in turn influence food web structures, exchange of greenhouse gases between the atmosphere and aquatic systems and oxygen consumption in estuarine ecosystems. In order to predict the impact of increasing macroelement inflows on the function and structure of boreal aquatic ecosystems it is vital to understand the potential sources of macroelements and their regulation of bacterioplankton metabolism. In this thesis, I aimed to determine whether terrestrially derived macroelement loadings to inland and estuarine waters can be utilized by bacterioplankton. In addition, I assessed how riverine influxes of DOC, N and P, alone or combined, regulate estuarine bacterioplankton metabolism. For this purpose, I applied a bioassayapproach to measure bacterioplankton respiration (BR) and production (BP) on samples from boreal freshwaters and estuarine systems. Terrestrially derived macroelement availability in brown-water lakes increased from DOC to N to P and averaged 2%, 31% and 49%, respectively. Although extremely low, relativeDOC bioreactivity increased with downstream transit for rivers with long water residence time and high fractions of urban and agricultural land use. Riverine bioreactive DOC was not the primary source of energy supporting bacterioplankton respiration at the Ore estuary, confirming the low bioreactive DOC exports expected for the Ore river. Instead riverine nutrients showed a more important effect on BR through stimulation of primary production, which in turn derived the organic carbon utilized by the bacterioplankton in the estuary.However, BP at the Ore estuary was in general limited by P and BR by carbon. In light of expected increases in riverine loadings of DOC and P, and decreases of estuarine primary production, estuarine BP is expected to increase, as well as estuarine BR of riverine DOC inputs. These bacterioplankton responses may beexacerbated by combined increases of riverine inflows of DOC, N and P. In summary, terrestrially derived macroelements can have a substantial impact on freshwater and estuarine bacterioplankton metabolism. Given the large role of bacterioplankton on the structure and function of aquatic systems, continuous changes in the input of terrestrial macroelements may have large consequences for boreal aquatic ecosystems. (Less)

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Riverine sources of bioreactive macroelements and their impact on bacterioplankton metabolism in a recipient boreal estuary

Estuaries are among the most altered and vulnerable marine ecosystems. These ecosystems will likely continue to deteriorate owing to increased population growth in coastal regions, expected temperature and precipitation changes associated with climate change, and their interaction with each other, leading to serious consequences for the ecological and societal services they provide. A key function of estuaries is the transfer, transformation, and burial of carbon and other biogenic elements exchanged between the land and ocean systems. Climate change has the potential to influence the carbon cycle through anticipated changes to organic matter production in estuaries and through the alteration of carbon transformation and export processes. This review discusses the effects of climate change on processes influencing the cycling of organic carbon in estuaries, including examples from three temperate estuaries in North America. Our goal is to evaluate the impact of climate change on the connectivity of terres...

Habib Rahman

Papers

Allochthonous carbon is a major driver of the microbial food web – A mesocosm study simulating elevated terrestrial matter runoff