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Marine eutrophication is a pervasive and growing threat to global sustainability. Macroalgal cultivation is a promising circular economy solution to achieve nutrient reduction and food security. However, the location of production hotspots is not well known. In this paper the production potential of macroalgae of high commercial value was predicted across the Baltic Sea region. In addition, the nutrient limitation within and adjacent to macroalgal farms was investigated to suggest optimal site-specific configuration of farms. The production potential of Saccharina latissima was largely driven by salinity and the highest production yields are expected in the westernmost Baltic Sea areas where salinity is >23. The direct and interactive effects of light availability, temperature, salinity and nutrient concentrations regulated the predicted changes in the production of Ulva intestinalis and Fucus vesiculosus. The western and southern Baltic Sea exhibited the highest farming potential for these species, with promising areas also in the eastern Baltic Sea. Macroalgal farming did not induce significant nutrient limitation. The expected spatial propagation of nutrient limitation caused by macroalgal farming was less than 100–250 m. Higher propagation distances were found in areas of low nutrient and low water exchange (e.g. offshore areas in the Baltic Proper) and smaller distances in areas of high nutrient and high water exchange (e.g. western Baltic Sea and Gulf of Riga). The generated maps provide the most sought-after input to support blue growth initiatives that foster the sustainable development of macroalgal cultivation and reduction of in situ nutrient loads in the Baltic Sea.

Reduced seawater pH due to elevated carbon dioxide (CO2), a process known as ocean acidification (OA), is a globally significant environmental issue. OA is predicted to influence a range of ecosystem processes, but little is known about how changing seawater carbon chemistry could influence the extent and impacts of epiphytism. In the brackish Baltic Sea, increased epiphytism is associated with coastal eutrophication and the potential for OA to interact with this relationship remains unclear. This study focuses on slow-growing perennial algae Fucus vesiculosus—which is one of the most important habitat-forming species in the Baltic Sea—and two of its most common and abundant filamentous epiphytes Ceramium tenuicorne and Pylaiella littoralis. Material for this study was collected from Estonian coastal waters. The aim of the research was to determine which carbon acquisition strategies these species possess, which could indicate how they respond to predicted changes in seawater chemistry due to elevated CO2. Carbon-use strategies in macroalgae were determined by analyzing natural carbon isotope signatures (δ13C), pH drift experiments, and photosynthesis vs. dissolved inorganic carbon (P vs. DIC) curves. Our results showed that although F. vesiculosus and its filamentous epiphytes all possess a carbon concentrating mechanism (CCM), the potential species-specific variation in the CCMs operation will favor C. tenuicorne over F. vesiculosus and P. littoralis in a future high CO2 world.

Wild harvest of seaweed supports small-scale, high-value industries in a number of regions in the world. Information is lacking on how increasing carbon dioxide (CO2) concentrations in seawater could impact seaweeds in wild harvest situations. This study focuses on a mixed unattached loose-lying red algal community of Furcellaria lumbricalis in close association with Coccotylus truncatus that is found in the West Estonian Archipelago Sea, NE Baltic Sea. In Estonian coastal waters, the wild harvest of F. lumbricalis started in 1960s and it has since been used as raw material for furcellaran production. The aim of this study was to determine how ocean acidification may impact the balance of these two red algal species in the community. Mechanistic assessment of the carbon physiology of F. lumbricalis and C. truncatus was used to predict productivity and competitive interactions between these species in a high-CO2 world. Carbon use strategies in macroalgae were determined by analysing the natural abundances of carbon isotopes (δ13C), pH drift experiments and photosynthesis vs. dissolved inorganic carbon (DIC) curves. Photosynthesis of F. lumbricalis (operating with a CO2 concentrating mechanism or CCM) performed worse along the broader range of DIC concentrations compared to C. truncatus (non-CCM), especially those characterized under future climate conditions. Therefore, changing seawater carbon chemistry through ocean acidification has the potential to influence the balance of F. lumbricalis and C. truncatus in the community and the efficiency of the wild harvest of this community and the quality of product provided.

Furcellaran (β/κ-carrageenan hybrid) samples were extracted from the drifting (n = 36) and attached (n = 46) forms of red algae Furcellaria lumbricalis, collected from the Estonian waters in the Baltic Sea. The extracted polysaccharides were characterised with 1H NMR and size-exclusion chromatography. Additionally, the gelling points, storage moduli and melting points of furcellaran samples were determined with a dynamic rheometer. Results showed that the extracted furcellaran samples contained a sulphate ester group per every 3.3 to 4.5 monosaccharide residues. On average, the furcellarans had a sulphate group for every 3.9 monomers, indicating nearly equimolar amounts of β- and κ-carrageenans in the tested polysaccharide samples. The estimated peak average molecular weights of the samples were higher for the furcellarans extracted from attached specimens. Furcellaran gels exhibited a three-time decrease in storage moduli (from ~30000 Pa to ~9300 Pa) when KCl concentrations were reduced from 40 mM to 10 mM. Also, on average, the furcellaran gels (1.5% w/w) in 40 mM KCl solutions exhibited a degree hysteresis of 31 °C.

Phycobiliproteins, red phycoerythrins and blue (allo)phycocyanins, are one of the most highly valued components extracted from cyanobacteria and red algae. In the given study, 95 Furcellaria lumbricalis and 92 Coccotylus truncatus samples were collected from different locations and depths from Kassari Bay, Estonia. Firstly, a strong, positive exponential correlation between the R-phycoerythrin and allophycocyanin concentrations and the collection depth (R > 0.75 and R > 0.61, respectively) was found. Secondly, a preparative scale purification was performed, and the proteins were further characterised by spectrophotometry, fluorimetry and native mass spectrometry. The obtained purity indices (A566/A280) for the R-phycoerythrins were 1.6 for F. lumbricalis and 2.6 for C. truncatus, thus making them suitable for cosmetics and food industries. Purified R-phycoerythrins from the two algal species had subtle differences in absorption and fluorescence maxima. Based on their slightly different absorption characteristics, R-phycoerythrin from F. lumbricalis would categorise as type III (RIII-PE) and R-phycoerythrin from C. truncatus, alongside with the Sigma 52412 standard, as type II (RII-PE). The native mass spectrometry analysis supported the results by showing different intact complexes with distinct γ-subunits for the R-phycoerythrin types. The R-phycoerythrin hexamers linked to various γ-subunits, which are in turn coupled to different chromophores, could explain the similarities (analytical standard and C. truncatus) and differences (F. lumbricalis) in absorption and fluorescence characteristics among the tested species.

Primary production is of special importance in ecology, since it fuels other trophic levels with energy and biomass. In aquatic ecosystems, almost all primary production is performed by algae. The primary production of aquatic macrophytes has been mainly quantified using detached phyto-elements (fronds and algal thalli) or whole plants, whereas the community level is usually ignored. In a field experiment we evaluated macroalgal photosynthesis at individual and community levels. We used natural communities dominated by the ephemeral green alga Cladophora glomerata (L.) Kutzing and the perennial brown alga Fucus vesiculosus L. Fucus vesiculosus had temporally stable and C. glomerata highly variable photosynthetic production. On a biomass basis, net and gross photosynthetic production was always higher at the individual level than at the community level. The ratio of individual photosynthetic production to community photosynthetic production increased with light intensities. The observed relationships give clear evidence that in shallow water, where light is supposedly plentiful, macroalgae are nevertheless strongly light-limited. Although photosynthetic production estimates at individual levels are traditionally extrapolated to the community level, such estimates largely overestimate reality and give a false impression of the potential photosynthetic production of macroalgae.

There is a growing need for diversification of seaweed aquaculture practices in Europe. In the Baltic Sea, very few seaweed species are utilised commercially, and only the kelp

The effects of local nutrient loading, harvesting, regional temperature patterns and weather induced light conditions were assessed on the interannual dynamics of the commercially important loose-lying red algal community of Furcellaria lumbricalis and Coccotylus truncatus . Higher algal biomasses were due to higher temperatures and number of calm days (i.e. clear water phases) during the active growing season in spring and summer. In addition, higher biomasses were associated with a higher number of stormy days in November–February, which is linked to a reduced ice cover and longer growing season. Commercial harvesting and nutrient load did not appear to drive the interannual dynamics of these red algae. Thus, the dynamics of the loose-lying red algal community is constrained by regional rather than local environmental factors.

Increased eutrophication has caused a bloom of opportunistic macroalgae and development of drift algal mats in coastal seas worldwide including the Baltic Sea. To date the patterns of the drift algae have been described at one scale (usually at a sampling unit scale i.e.

Mesograzers are known to reduce the biomass of their host plant and modify the structure of the whole macrophyte community in many ecosystems. Thus, the introduction of an efficient mesograzer may destabilize macrophyte community and also affect the native grazers. We estimated how large proportion of macrophyte production are consumed by the alien gammarid G. tigrinus and the native gammarid G. salinus in the species poor ecosystem of the northern Baltic Sea. We analysed whether G. tigrinus consumes different diet as the native G. salinus and whether the effect of G. tigrinus on the survival of the native G. salinus is macrophyte species specific. Grazing experiments showed that there was a clear difference in the grazing rates of gammarids among the studied macrophyte species in summer and autumn but not in spring. The grazing rates were significantly higher in the prevailing macrophyte Pilayella littoralis as compared to other macrophytes. The grazing was inversely related to the diurnal net photosynthetic values of macrophytes. The gammarid amphipods potentially removed only a minor part of plant primary production except for summer and autumn when grazing of a few perennial species exceeded macrophyte production. Macrophyte species and presence of G. salinus had no effect on the survival of G. tigrinus. The presence of G. tigrinus, however, reduced the survival of the native gammarids within P. littoralis in summer. To conclude it is likely that both native and alien gammarid amphipods do not exert significant pressure on the macroalgal communities in the northern Baltic Sea. Competitive interactions between G. tigrinus and G. salinus within the prevailing macrophyte P. littoralis is the likely explanation of the decline of the native gammarid amphipods after the establishment of G. tigrinus in the northern Baltic Sea.

A free-floating, loose form of Furcellaria lumbricalis (Huds.) Lamour is rare in the Baltic Sea area. Kassari Bay, situated in the West Estonian Archipelago Sea area contains the largest known community of this kind. Here the free-floating mixed Furcellaria lumbricalis-Coccotylus truncatus (Paela) M. J. Wynne et J. N. Heine community inhabits sandy bottom, covering up to 120 km2. Commercial exploitation of the community started in 1966 and has led to regular monitoring surveys for the quantification of the commercial resource. The aim of the present study was to determine the potential growth rates of the two community-forming species as well as to test different environmental factors affecting their growth. Results showed that the highest growth rates were measured in shallower depths (4 m) for both species. The seasonal growth pattern was also very similar for both species, showing the highest growth rates during the beginning of summer. Incubation of both species in another sea area with apparently similar basic environmental conditions (the northern part of the Gulf of Riga, Koiguste Bay) resulted in significantly lower growth rates during the whole incubation period.

Two ecologically and morphologically distinct forms of Furcellaria lumbricalis and loose form of Coccotylus truncatus were experimentally tested to obtain information on the growth rates and influence of the habitat depth on their eco-physiological activity. Incubations were carried out in the area inhabited by a loose F. lumbricalis–C. truncatus community in Kassari Bay of the West-Estonian Archipelago Sea, the NE Baltic Sea. During the incubation period (20.04–21.10.2002) loose forms of F. lumbricalis and C. truncatus showed similar dynamics of both growth rate of biomass and primary production while attached form of F.␣lumbricalis had, as a rule, significantly lower growth rate and primary production values. The highest eco-physiological activity was recorded from the shallowest incubation depth (4 m) for all three algal forms. All three tested algal forms had similar pattern of growth during the incubation period – highest growth rates were detected in spring and early summer while during the rest of the incubation period algal biomass was in a steady state when production balanced degradation processes.

Followed by the massive increase in the biomass of Pilayella littoralis, Fucus vesiculosus has almost disappeared in Koiguste Bay, north-eastern Baltic Sea during 1995‐1998. Consequently, Idotea baltica, which previously fed on Fucus, switched to new food items ‐ Furcellaria lumbricalis and Pilayella littoralis. Idotea grazed more Pilayella than Furcellaria whereas Furcellaria was highly preferred as a habitat. Maximum 2.2% and 4.7% of the production of Furcellaria and Pilayella were removed by grazing. We conclude that the grazing pressure by Idotea is not sufficient to control the outbreak of Pilayella. However, Idotea has an advantageous effect on the persistence of Furcellaria by preferentially removing fast growing epiphyte Pilayella.

Following climate change and elevated nutrient loads the proliferation of annual filamentous macroalgae results in the accumulation of drifting algal mats in many coastal areas worldwide. The occurrence of the mats may significantly affect the structure and functioning of the benthic communities. We tested the importance of the benthic herbivores Idotea balthica, Gammarus oceanicus and the suspension feeder Mytilus trossulus on the decomposition of the common drift algae Pilayella littoralis and Cladophora glomerata in a brackish water ecosystem of the Baltic Sea. The field experiments were performed in April-May (29 d) and July-August (29 d, last checkpoint 70 d in autumn). The changes in dry weight, total nitrogen and phosphorus content of the macroalgae were examined with an average of 7 d interval. The results showed that algal decomposition was related to temperature and their net photosynthetic rates. The benthic invertebrates significantly affected the algal decomposition. However, the character of the effect depended on the algal and invertebrate species and the physiological state of algae. When algae were photosynthetically active M. trossulus, through the fertilizing effect of biodeposition, promoted the growth of the algae and hence, retarded the algal decomposition. On the other hand, in the presence of herbivores the complete breakdown of the drift algae was reached already within 30-40 d as compared to the control value of 70 d. G. oceanicus had stronger effect on P. littoralis whereas I. balthica contributed mostly to the decomposition of C. glomerata. It is concluded that herbivores significantly accelerate the decomposition of the drift algae whereas suspension feeders postpone the decomposition as far as macroalgae remain photosynthetically active.

The composition, structure, and thermal stability of carrageenans from unattached Coccotylus truncatus (the Baltic Sea, Estonia) were investigated. The complex polysaccharide was characterized by 13 C NMR and FTIR spectroscopy, ICP-OES and gel permeation chromatography methods. The main components of C. truncatus galactan are 3,6-anhydro-α- d -galactose-2-sulfate (30 ± 1.5%) and β- d -galactose-4-sulfate (45.3%), indicating a ι-carrageenan backbone. As the minor components, α- d -galactose-2,6-disulfate (12 ± 2%) from ν-carrageenan and 4′,6′-pyruvated β- d -galactopyranosyl residues (1.4%) from pyruvated α-carrageenan are found to be present, latter being responsible for the undersulfated nature of the galactan. The native polysaccharide with the average molecular weight of about 1500 kDa is highly susceptible to thermal degradation. The high-temperature treatment of this galactan gives products with 3,6-anhydro-α- d -galactose units predominantly at the reducing end. The carrageenan extraction from C. truncatus gives characteristically low yields (12–17%); weak gelling ability of the polysaccharides from this seaweed species (gel strength 30–40 g/cm 2 ) does not depend significantly on extraction conditions.

The decomposition of two macroalgal species Cladophora glomerata (CHLOROPHYTA) and Pilayella littoralis (PHAEOPHYTA) was studied in the laboratory and field conditions. These species are known to cause the extensive ‘macroalgal blooms’ in the whole coastal range of the Baltic Sea. The objective of the experiments was to determine decomposition rates of the macroalgae, follow the changes in tissue nutrient content and validate the role of benthic invertebrates in this process. In the laboratory conditions, the differences in the decomposition rates of the algae were mainly due to the oxygen conditions. The weight loss of C. glomerata was slightly higher in anaerobic conditions than in aerobic conditions. If 99% of initial dry weight of P. littoralis was lost in aerobic conditions then only 20% was lost in anaerobic conditions. In general, the loss of phosphorus and nitrogen in algal tissues followed the weight loss. As an exception, the amount of nitrogen changed very little during the decomposition of C. glomerata. In field conditions, the photosynthetic activity exceeded the decomposition rate of C. glomerata at lower temperatures in spring. The decomposition of P. littoralis was estimated at 49% of its initial dry weight. The addition of benthic invertebrates had no effect on the decomposition process. In summer, the decomposition rates were estimated at 65% for C. glomerata and 68% for P. littoralis being in the same order of magnitude as observed in laboratory conditions. If the decomposition of C. glomerata was faster at the end of the experiment, the most significant losses of weight of P. littoralis took place during the first 2 weeks of deployment. Idotea baltica significantly contributed to the loss of C. glomerata. The decomposition rate of P. littoralis was reduced by the presence of Mytilus edulis and increased by Gammarus oceanicus.

The West Estonian Archipelago Sea hosts a loose-lying red macroalgal community dominated by Furcellaria lumbricalis and Coccotylus truncatus. The community is truly unique in European seas. In factorial field experiments we evaluated separate and interactive effects of algal density and the proportions of F. lumbricalis and C. truncatus on their growth rates in a set of monospecific and mixed communities. Our experiment demonstrated that the growth of red algae was density dependent and that increased algal densities resulted in a fall of daily growth rates. An elevated growth of the red algae was observed at their lower proportions in the community. A potential mechanism behind the observed patterns is the light availability for photosynthesis, i.e. light utilization is more efficient in mixed communities.