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Inflows of coloured terrestrial organic matter cause seawater browning and reduced phytoplankton production in subarctic coastal ecosystems, potentially deteriorating the nutritional quality of marine food webs. We analyzed the fatty-acid (FA) compositions of seston and the zooplankton taxa Eurytemora affinis and cladocerans at three locations of the northern Baltic Sea. At the coastal and northerly locations, salinity and phosphorus concentrations were low, while concentrations of humic substances (i.e., terrestrial organic matter) were high. The southerly location showed the opposite trend. The ratio between alga-specific ?3 polyunsaturated FA and terrigenous monounsaturated FA (MUFA) in Eurytemora decreased from south to north, as did the ratio between the alga-specific docosahexaenoic acid (DHA) and terrigenous MUFA in cladocerans. With increasing humic substances, the biomass of DHA-rich phytoplankton decreased and the zooplankton MUFA content increased. Our results indicate that coloured terrestrial organic matter alters the phytoplankton composition, consequently affecting the zooplankton nutritional quality.

Les influx de matière organique terrestre causent le brunissement de l’eau de mer et réduisent la production duphytoplancton dans les écosystèmes côtiers subarctiques, ce qui peut se traduire par une détérioration de la qualité nutri-tionnelle des réseaux trophiques marins. Nous avons analysé la composition d’acides gras du seston, du taxon de zooplanctonEurytemora affinis et de cladocères dans trois sites de la mer Baltique septentrionale. Dans les sites côtier et nordique, la salinité etles concentrations de phosphore étaient faibles, alors que les concentrations de substances humiques (c.-à-d. matière organiqueterrestre) étaient élevées. Le site plus au sud présentait des tendances inverses. Le rapport des acides gras polyinsaturés x3 spécifiquesaux algues et des acides gras monoinsaturés (AGMI) terrigènes chez Eurytemora diminuait du sud au nord, tout comme le rapport del’acide docosahexanoïque (ADH) spécifique aux algues et des AGMI chez les cladocères. Plus l’abondance de substances humiques étaitélevée, plus la biomasse de phytoplancton riche en ADH était faible et plus le contenu en AGMI du zooplancton était important. Nosrésultats indiquent que de la matière organique terrestre colorée modifie la composition du phytoplancton et a par conséquent uneincidence sur la qualité nutritionnelle du zooplancton. [Traduit par la Rédaction]

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.

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 community on mesozooplankton production and nutritional quality. A low and a high hydrological mixing regime was applied. The δ¹⁵N 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 µm) 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. 

Climate change causes large environmental alterations in subarctic marine ecosystems, which can affect the overall food web production. There is, however, a knowledge gap about longterm changes in the food web efficiency (FWE) and quality in ecosystems such as the northern Baltic Sea. To evaluate these changes, we analyzed the fatty acid (FA) composition of archived herring samples collected in the Gulf of Bothnia from 1995 to 2020, together with data on primary production, heterotrophic bacterial production and estimates of herring biomass production. The FWE was then calculated as the ratio between herring production and the sum of bacterial and primary production. The results showed that the omega-3 and omega-6 polyunsaturated fatty acid (i.e., ω3 and ω6 PUFA) content in Bothnian Bay herring decreased over time, likely due to the long-term decrease in salinity; and increase in dissolved organic carbon. In contrast, omega-3 and omega-6 PUFA content in Bothnian Sea herring did not show any long-term change and variations and did not relate to the environmental changes. In the Bothnian Sea, herring biomass production was stable from 2001-2009 (9.96 ± 1.24 × 10^-7 kg C m^-2 year^-1) while the primary production significantly increased from 0.01 to 0.10 kg C m^-2 year^-1 in 2001-2020. Bacterial production was stable (0.02 ± 0.01 kg C m^-2 year^-1). Consequently, the FWE gradually decreased from 0.0025% in 2001 to 0.001% in 2009, reflecting that the increased primary production that was not transferred to the upper trophic levels. Our results suggest that climate-induced environmental changes likely have a negative impact on the FWE in the Bothnian Sea and food web quality measured in terms of herring PUFA content in the Bothnian Bay