Tiina Paalme

‘In vitro’ and ‘in situ’ decomposition of nuisance macroalgae Cladophora glomerata and Pilayella littoralis

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.

Seasonal changes in situ grazing of the mesoherbivores Idotea baltica and Gammarus oceanicus on the brown algae Fucus vesiculosus and Pylaiella littoralis in the central Gulf of Finland, Baltic Sea

The in situ grazing experiments were performed in the shallow water rocky habitat of the northern Baltic Sea during ice-free season 2002. In the experiments the effects of algal species and choice on the grazing of the mesoherbivores Idotea baltica (Pallas) and Gammarus oceanicus Segerstrale were tested. Salinity, temperature, concentration of nutrients in water and macroalgae and net production of macroalgae were considered as random effects in the analysis. The invertebrate feeding rate was mainly a function of the net photosynthetic activity of Pylaiella littoralis (L.) Kjellman and Fucus vesiculosus L. Feeding rate increased significantly with decreasing algal photosynthetic activity. When the two algal species were incubated together invertebrates fed primarily on P. littoralis. Low selectivity towards P. littoralis coincided with its high photosynthetic activity. The presence of F. vesiculosus did not modify the invertebrate feeding on P. littoralis. The results indicated that (1) the grazing on F. vesiculosus depended on the availability of P. littoralis, (2) the photosynthetic activity of algae explained the best the variation in grazing rate and (3) the grazers are not likely to control the early outbreak of filamentous algae in the northern Baltic Sea by avoiding young and photosynthetically active algae. The likely mechanism behind the relationship is that the increased photosynthetic activity of macroalgae coincides with their higher resistance to herbivory.

Seasonality Pattern of Biomass Accumulation in a Drifting Furcellaria Lumbricalis Community in the Waters of the West Estonian Archipelago, 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, Kõiguste Bay) resulted in significantly lower growth rates during the whole incubation period.

The effect of CO2 enrichment on net photosynthesis of the red alga Furcellaria lumbricalis in a brackish water environment

Anthropogenic carbon dioxide (CO2) emissions to the atmosphere are causing reduction in the global ocean pH, also known as ocean acidification. This change alters the equilibrium of different forms of dissolved inorganic carbon in seawater that macroalgae use for their photosynthesis. In the Baltic Sea, benthic macroalgae live in a highly variable environment caused by seasonality and rapid changes in meteorological conditions. The effect of increasing water CO2 concentration on the net photosynthesis of the red macroalgae Furcellaria lumbricalis (Hudson) Lamouroux was tested in short-term mesocosm experiments conducted in Kõiguste Bay (N Gulf of Riga) in June–July 2012 and 2013. Separate mesocosms were maintained at different pCO2 levels: ca. 2,000, ca. 1,000 and ca. 200 µatm. In parallel, different environmental factors were measured such as nutrients, light and water temperature. Thus, the current study also investigated whether elevated pCO2 and different environmental factors exerted interactive effects on the photosynthetic rate of F. lumbricalis. In addition, laboratory experiments were carried out to determine the optimal temperature for photosynthesis of F. lumbricalis. The results of our field experiments demonstrated that elevated pCO2 levels may remarkably enhance the photosynthetic rate of F. lumbricalis. However, the magnitude of this effect is altered by different environmental factors, mainly by changes in water temperature.

The Influence of CO2 Enrichment on Net Photosynthesis of Seagrass Zostera marina in a Brackish Water Environment

Seagrasses are distributed across the globe and their communities may play key roles in the coastal ecosystems. Seagrass meadows are expected to benefit from the increased carbon availability which might be used in photosynthesis in a future high CO2 world. The main aim of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Zostera marina in a brackish water environment. The short-term mesocosm experiments were conducted in Koiguste Bay (northern part of Gulf of Riga, the Baltic Sea) in June-July 2013 and 2014. As the levels of pCO2 naturally range from ca. 150 μatm to well above 1000 μatm under summer conditions in Koiguste Bay we chose to operate in mesocosms with the pCO2 levels of ca. 2000, ca. 1000 and ca. 200 μatm. Additionally, in 2014 the photosynthesis of Z. marina was measured outside of the mesocosm in the natural conditions. In the shallow coastal Baltic Sea seagrass Z. marina lives in a highly variable environment due to seasonality and rapid changes in meteorological conditions. This was demonstrated by the remarkable differences in water temperatures between experimental years of ca. 8°C. Thus, the current study also investigated the effect of elevated pCO2 in combination with short-term natural fluctuations of environmental factors, i.e. temperature and PAR on the photosynthesis of Z. marina. Our results show that elevated pCO2 alone did not enhance the photosynthesis of the seagrass. The photosynthetic response of Z. marina to CO2 enrichment was affected by changes in water temperature and light availability.