Results of laboratory and field experiments of the direct effect of increasing CO2 on net primary production of macroalgal species in brackish-water ecosystems

Studies on the effects of increasing acidification on marine communities have been previously mostly carried out in truly marine areas whereas brackish-water ecosystems such as the Baltic Sea have been less studied. The current study analyses how acidification induced by elevated atmospheric carbon dioxide affects the photosynthetic net production of different macroalgal species in the brackish Baltic Sea. Research methods include sets of laboratory and field experiments carried out in shallow coastal brackish waters. The aim of the laboratory experiments was to develop the necessary techniques and experience for the mesocosm experiments. Laboratory experiments were carried out using specimens of the red alga Furcellaria lumbricalis collected from Kakumäe Bay. The mesocosm experiments were conducted in Kõiguste Bay during the field season of 2011. Separate mesocosms were operated in each set with different CO₂ concentrations and a control treatment in natural conditions. Field experiments were carried out with three species representing three different morphological and ecological groups: Ulva intestinalis, a fast-growing green alga; Fucus vesiculosus, a perennial brown alga with a slow metabolism; and Furcellaria lumbricalis, a perennial red alga. Photosynthetic activity was used as the response variable. In the laboratory decreasing pH increased the net primary production of F. lumbricalis with the lowest net primary production values measured at pH 8.0 and the highest at pH 6.5. Results of the field experiments indicated that increased CO₂ levels in seawater favoured photosynthetic activity of the macroalgae U. intestinalis and F. lumbricalis, but F. vesiculosus showed no response to elevated CO₂. Elevated CO₂ levels are suggested to favour the production of fast-growing filamentous species, which thus may indirectly enhance the effect of eutrophication in the shallow coastal brackish waters.

A potential tool for high-resolution monitoring of ocean acidification

Current anthropogenic carbon dioxide emissions generate besides global warming unprecedented acidification rates of the oceans. Recent evidence indicates the possibility that ocean acidification and low oceanic pH may be a major reason for several mass extinctions in the past. However, a major bottleneck for research on ocean acidification is long-term monitoring and the collection of consistent high-resolution pH measurements. This study presents a low-power (< 1 W) small sample volume (25 μl) semiconductor based fluorescence method for real-time ship-board pH measurements at high temporal and spatial resolution (approximately 15 s and 100 m between samples). A 405 nm light emitting diode and the blue and green channels from a digital camera was used for swift detection of fluorescence from the pH sensitive dye 6,8-Dihydroxypyrene-1,3-disulfonic acid in real-time. Main principles were demonstrated by automated continuous measurements of pH in the surface water across the Baltic Sea and the Kattegat region with a large range in salinity (∼ 3–30) and temperature (∼ 0–25 °C). Ship-board precision of salinity and temperature adjusted pH measurements were estimated as low as 0.0001 pH units.

Sex in murky waters : anthropogenic disturbance of sexual selection in pipefish

Animals experience variation in their environment because of natural changes. However, due to anthropogenic disturbance, the speed and severity of these changes have recently increased. This thesis investigates how reproductive behaviours may be affected by human induced environmental change. In specific, I investigate how visual and chemical changes in the aquatic environment, caused by eutrophication, affect mating systems and sexual selection in fish. Broad-nosed- and straight-nosed pipefish, which both have been studied in detail for a long period, were used as model organisms. These two species are particularly suitable model organisms since they perform complex courtship behaviours, including the advertisement of ornaments and a nuptial dance. Further, two distinct populations were studied, one on the Swedish west coast and one in the Baltic Sea, as these two locations vary in the degree and extent of environmental disturbance, in particular turbidity. I found that changes in the visual environment had no impact on the development of female sexual ornaments in these sex-role reversed pipefishes, but it hampered adaptive mate choice. Turbidity also had a negative effect on reproductive success in the Baltic Sea population. Changes in the chemical environment in the form of increased pH reduced the probability to mate, while hypoxia did not alter mating propensity. However, hypoxic water delayed the onset of both courting and mating. Hence, human induced change in aquatic environments may alter the processes of sexual selection and population dynamics.

Response of Nodularia spumigena to pCO2 – Part 3: Turnover of phosphorus compounds (update)

Diazotrophic cyanobacteria form extensive summer blooms in the Baltic Sea driving the surrounding surface waters into phosphate limitation. One of the main bloom-forming species is the heterocystous cyanobacterium Nodularia spumigena. N. spumigena exhibits accelerated uptake of phosphate through the release of the extracellular enzyme alkaline phosphatase whose activity also serves as an indicator of the hydrolysis of dissolved organic phosphorus (DOP). The present study investigated the utilisation of DOP and its compounds (e.g., ATP) by N. spumigena during growth under different CO₂ concentrations, in order to estimate potential consequences of ocean acidification on the cell’s supply with phosphorus (P). Cell growth, the phosphorus pool, and four DOP compounds (ATP, DNA, RNA, and phospholipids) were determined in three setups with different CO₂ concentrations (average 341 μatm, 399 μatm, and 508 μatm) during a 15-day batch experiment. The results showed stimulated growth of N. spumigena and a rapid depletion of dissolved inorganic phosphorus (DIP) in all pCO₂ treatments. DOP uptake was enhanced by a factor of 1.32 at 399 μatm and of 2.25 at 508 μatm compared to the lowest CO₂ concentration. Among the measured DOP compounds, none was found to accumulate preferentially during the incubation or in response to a specific pCO₂ treatment. However, at the beginning 61.9 ± 4.3% of total DOP were not characterised but comprised the most utilised fraction. This is demonstrated by the decrement of this fraction to 27.4 ± 9.9% of total DOP during the growth phase with a preference at high pCO₂. Our results indicate a stimulated growth of diazotrophic cyanobacteria at increasing CO₂ concentrations which is accompanied by increasing utilisation of DOP as an alternative P source.

Impact of temperature and species interaction on filamentous cyanobacteria may be more important than salinity and increased pCO2 levels

A future business-as-usual scenario (A1FI) was tested on two bloom-forming cyanobacteria of the Baltic Proper, Nodularia spumigena and Aphanizomenon sp., growing separately and together. The projected scenario was tested in two laboratory experiments where (a) interactive effects of increased temperature and decreased salinity and (b) interactive effects of increased temperature and elevated levels of pCO₂ were tested. Increased temperature, from 12 to 16 °C, had a positive effect on the biovolume and photosynthetic activity (F _v/F _m) of both species. Compared when growing separately, the biovolume of each species was lower when grown together. Decreased salinity, from 7 to 4, and elevated levels of pCO₂, from 380 to 960 ppm, had no effect on the biovolume, but on F _v/F _m of N. spumigena with higher F _v/F _m in salinity 7. Our results suggest that the projected A1FI scenario might be beneficial for the two species dominating the extensive summer blooms in the Baltic Proper. However, our results further stress the importance of studying interactions between species.

Benthic interactions in a high CO2-world

The Baltic Sea is a physically diverse habitat with a generally low species diversity. The blue mussel Mytilus edulis is widely distributed in the benthic macrozoobenthos of the Western Baltic, the main predators of which are the common sea star Asterias rubens and the shore crab Carcinus maenas. Any environmental stress influencing the predator-prey interactions between these species has the potential to shape the entire ecosystem. The current increase in atmospheric pCO2 causes a concurrent increase in the acidification of seawater and can thus pose such an environmental stress. In coastal habitats and specifically the Baltic Sea, the decrease of seawater pH can be much more pronounced than in the open ocean. In order to estimate possible interaction shifts in the macrozoobenthos under conditions of seawater acidification, this work investigates the effect of an increase in water pCO2 on the predators A. rubens and C. maenas and their consumption of M. edulis. The results of three different own studies show an impact of increased seawater pCO2 around 3500 μatm on growth and mussel consumption in adult sea stars and a seawater pCO2 of around 1200 μatm to impact growth, mussel consumption, scope for growth and righting response of juvenile A. rubens. Mussel sizes consumed, metabolism, NH4+-excretion and calcification were, however, not impacted by an increase in seawater acidification and coelomic pH not regulated by means of active bicarbonate accumulation. Crabs were impacted in metabolism, NH4+-excretion, O:N-ratio and metabolic energy loss at a seawater pCO2 of about 3500 μatm. Mussel consumption was only impacted at a pCO2 of 3500 μatm, over intermediate time spans (10 weeks), but not over a longer (six month) time span. Hemolyph pH was regulated to remain at control levels by active bicarbonate accumulation at intermediate (10 week) time spans over all levels of seawater acidification, while hemolymph pH followed the non-bicarbonate buffer line at the intermediate (around 1200 μatm) level and was only regulated at the high (around 3500 μatm) treatment level over the long (six month) time span. Moulting intervals, growth, mussel sizes consumed, carapace thickness, stability, dry weight and calcification were not influenced by seawater acidification These results indicate a change in feeding pressure on the blue mussel M. edulis under future high levels of seawater acidification. Further, A. rubens appears stronger impacted by seawater acidification than C. maenas and juvenile A. rubens even stronger than adult specimen. I conclude that seawater acidification has the potential to reshape the benthic ecosystem of the Western Baltic. This work therefore helps to understand ecosystem responses to environmental stress and contributes to making predictions on future species distributions in the Baltic Sea.

Tolerance of juvenile barnacles (Amphibalanus improvisus) to warming and elevated pCO2

We investigated the impacts of warming and elevated pCO₂ on newly settled Amphibalanus improvisus from Kiel Fjord, an estuarine ecosystem characterized by significant natural pCO₂ variability. In two experiments, juvenile barnacles were maintained at two temperature and three pCO₂ levels (20/24 °C, 700–2,140 μatm) for 8 weeks in a batch culture and at four pCO₂ levels (20 °C, 620–2,870 μatm) for 12 weeks in a water flow-through system. Warming as well as elevated pCO₂ hardly affected growth or the condition index of barnacles, although some factor combinations led to temporal significances in enhanced or reduced growth with an increase in pCO₂. While warming increased the shell strength of A. improvisus individuals, elevated pCO₂ had only weak effects. We demonstrate a strong tolerance of juvenile A. improvisus to mean acidification levels of about 1,000 μatm pCO₂ as is already naturally experienced by the investigated barnacle population.

Diurnal changes in seawater carbonate chemistry speciation at increasing atmospheric carbon dioxide

Natural variability in seawater pH and associated carbonate chemistry parameters is in part driven by biological activities such as photosynthesis and respiration. The amplitude of these variations is expected to increase with increasing seawater carbon dioxide (CO₂) concentrations in the future, because of simultaneously decreasing buffer capacity. Here, we address this experimentally during a diurnal cycle in a mesocosm CO₂ perturbation study. We show that for about the same amount of dissolved inorganic carbon (DIC) utilized in net community production diel variability in proton (H⁺) and CO₂ concentrations was almost three times higher at CO₂ levels of about 675 ± 65 in comparison with levels of 310 ± 30 μatm. With a simple model, adequately simulating our measurements, we visualize carbonate chemistry variability expected for different oceanic regions with relatively low or high net community production. Since enhanced diurnal variability in CO₂ and proton concentration may require stronger cellular regulation in phytoplankton to maintain respective gradients, the ability to adjust may differ between communities adapted to low in comparison with high natural variability.

Modeling the dissolved CO2 system in the redox environment of the Baltic Sea

Generation and depletion of total alkalinity (A_T) were added to a Baltic Sea numerical model. The vertical distribution of generation and depletion of total alkalinity were described and attributed to different processes in the Eastern Gotland basin at the Gotland deep station (BY15) during the 1995–2004 period. At this site, the mean annual generation (28.2 µmol kg⁻¹ yr⁻¹) and depletion (−25.8 µmol kg⁻¹ yr⁻¹) were almost balanced, though the transient rates were much faster (+125/−340 µmol kg⁻¹ yr⁻¹). The mean volume-integrated A_T content increased up to 50 µmol kg⁻¹ when generation and depletion were added to the model. The A_T changes were coupled to oxidation–reduction (redox) reactions and the model budget indicates that internal generation and depletions is as important as lateral transports, including riverine input, at this site. Model predictive capability in marine environments with strong biogeochemical gradients was improved by coupling within the dissolved CO₂, and the biogeochemical, systems. This enables evaluation of eutrophication, acidification, and climate change simultaneously, and is important specifically in regions with permanent or periodic anoxia.

Future changes in the Baltic Sea acid-base (pH) and oxygen balances

Possible future changes in Baltic Sea acid–base (pH) and oxygen balances were studied using a catchment–sea coupled model system and numerical experiments based on meteorological and hydrological forcing datasets and scenarios. By using objective statistical methods, climate runs for present climate conditions were examined and evaluated using Baltic Sea modelling. The results indicate that increased nutrient loads will not inhibit future Baltic Sea acidification; instead, the seasonal pH cycle will be amplified by increased biological production and mineralization. All examined scenarios indicate future acidification of the whole Baltic Sea that is insensitive to the chosen global climate model. The main factor controlling the direction and magnitude of future pH changes is atmospheric CO₂ concentration (i.e. emissions). Climate change and land-derived changes (e.g. nutrient loads) affect acidification mainly by altering the seasonal cycle and deep-water conditions. Apart from decreasing pH, we also project a decreased saturation state of calcium carbonate, decreased respiration index and increasing hypoxic area – all factors that will threaten the marine ecosystem. We demonstrate that substantial reductions in fossil-fuel burning are needed to minimise the coming pH decrease and that substantial reductions in nutrient loads are needed to reduce the coming increase in hypoxic and anoxic waters.