Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species

Limited trait responses of a tropical seagrass to the combination of increasing pCO2 and warming

Understanding species-specific trait responses under future global change scenarios is of importance for conservation efforts and to make informed decisions within management projects. The combined and single effects of seawater acidification and warmer average temperature were investigated by means of the trait responses of Cymodocea serrulata, a tropical seagrass, under experimental conditions. After a 35 d exposure period, biochemical, morphological, and photo-physiological trait responses were measured. Overall, biochemical traits mildly responded under the individual exposure to high temperature and increasing pCO2 values. The response of C. serrulata was limited to a decrease in %C and an increase in the sucrose content in the rhizome under the high temperature treatment, 32 °C. This suggests that this temperature was lower than the maximum tolerance limit for this species. Increasing pCO2 levels increased %C in the rhizome, and also showed a significant increase in leaf δ13C values. The effects of all treatments were sublethal; however, small changes in their traits could affect the ecosystem services they provide. In particular, changes in tissue carbon concentrations may affect carbon storage capacity, one key ecosystem service. The simultaneous study of different types of trait responses contributes to establish a holistic framework of seagrass ecosystem health under climate change.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 μmol L^-1, 20 μmol L^-1 and 40 μmol L^-1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g^-1, 67.03 ± 5.33 mg g^-1, 49.14 ± 3.48 mg g^-1, and 18.51 ± 2.09 mg g^-1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root^-1 h^-1, 5.11 ± 0.42 mg g DW root^-1 h^-1, 4.08 ± 0.23 mg g DW root^-1 h^-1, and 3.78 ± 0.74 mg g DW root^-1 h^-1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

Interactive effects of global warming and eutrophication on a fast-growing Mediterranean seagrass

Coastal ecosystems, such as seagrasses, are subjected to local (e.g. eutrophication) and global (e.g. warming) stressors. While the separate effects of warming and eutrophication on seagrasses are relatively well known, their joint effects remain largely unstudied. In order to fill this gap, and using Cymodocea nodosa as a model species, we assessed the joint effects of warming (three temperatures, 20 °C, 30 °C and 35 °C) with two potential outcomes of eutrophication: (i) increase in nutrients concentration in the water column (30 and 300 μM), and (ii) organic enrichment in the sediment). Our results confirm that temperature in isolation clearly affects plant performance; while plants exposed to 30 °C performed better than control plants, plants exposed to 35 °C showed clear symptoms of deterioration (e.g. decline of photosynthetic capacity, increase of incidence of necrotic tissue). Plants were unaffected by high ammonium concentrations; however, organic enrichment of sediment had deleterious effects on plant function (photosynthesis, growth, demographic balance). Interestingly, these negative effects were exacerbated by increased temperature. Our findings indicate that in addition to the possibility of the persistence of C. nodosa being directly jeopardized by temperature increase, the joint effects of warming and eutrophication may further curtail its survival. This should be taken into consideration in both predictions of climate change consequences and in local planning.

Losing a winner: thermal stress and local pressures outweigh the positive effects of ocean acidification for tropical seagrasses

Seagrasses are globally important coastal habitat-forming species, yet it is unknown how seagrasses respond to the combined pressures of ocean acidification and warming of sea surface temperature. We exposed three tropical species of seagrass (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri) to increasing temperature (21, 25, 30, and 35°C) and pCO₂ (401, 1014, and 1949 μatm) for 7 wk in mesocosms using a controlled factorial design. Shoot density and leaf extension rates were recorded, and plant productivity and respiration were measured at increasing light levels (photosynthesis-irradiance curves) using oxygen optodes. Shoot density, growth, photosynthetic rates, and plant-scale net productivity occurred at 25°C or 30°C under saturating light levels. High pCO₂ enhanced maximum net productivity for Z. muelleri, but not in other species. Z. muelleri was the most thermally tolerant as it maintained positive net production to 35°C, yet for the other species there was a sharp decline in productivity, growth, and shoot density at 35°C, which was exacerbated by pCO₂ . These results suggest that thermal stress will not be offset by ocean acidification during future extreme heat events and challenges the current hypothesis that tropical seagrass will be a 'winner' under future climate change conditions.

Nitrogen uptake kinetics and saltmarsh plant responses to global change

Coastal wetlands are important carbon sinks globally, but their ability to store carbon hinges on their nitrogen (N) supply and N uptake dynamics of dominant plant species. In terrestrial ecosystems, uptake of nitrate (NO3-) and ammonium (NH4+) through roots can strongly influence N acquisition rates and their responses to environmental factors such as rising atmospheric CO2 and eutrophication. We examined the 15N uptake kinetics of three dominant plant species in North American coastal wetlands (Spartina patens, C4 grass; Phragmites australis, C3 grass; Schoenoplectus americanus, C3 sedge) under ambient and elevated CO2 conditions. We further related our results to the productivity response of these species in two long-term field experiments. S. patens had the greatest uptake rates for NO3- and NH4+ under ambient conditions, suggesting that N uptake kinetics may underlie its strong productivity response to N in the field. Elevated CO2 increased NH4+ and NO3- uptake rates for S. patens, but had negative effects on NO3- uptake rates in P. australis and no effects on S. americanus. We suggest that N uptake kinetics may explain differences in plant community composition in coastal wetlands and that CO2-induced shifts, in combination with N proliferation, could alter ecosystem-scale productivity patterns of saltmarshes globally.

Structural and physiological responses of Halodule wrightii to ocean acidification

Coastal areas face high variability of seawater pH. Ocean acidification (OA) and local stressors are enhancing this variability, which poses a threat to marine life. However, these organisms present potential phenotypic plasticity that can offer physiological and structural tools to survive in these extreme conditions. In this study, we evaluated the effects of elevated CO₂ levels and consequent pH reduction on the physiology, anatomy and ultrastructure of the seagrass Halodule wrightii. A mesocosm study was conducted in an open system during a 30-day experiment, where different concentrations of CO₂ were simulated following the natural variability observed in coastal reef systems. This resulted in four experimental conditions simulating the (i) environmental pH (control condition, without CO₂ addition) and (ii) reduced pH by - 0.3 units, (iii) - 0.6 units and (iv) - 0.9 units, in relation to the field condition. The evaluated population only suffered reduced optimum quantum yield (Y(II)), leaf width and cross-section area under the lowest CO₂ addition (- 0.3 pH units) after 30 days of experiment. This fitness commitment should be related to carbon concentration mechanisms present in the evaluated species. For the highest CO₂ level, H. wrightii demonstrated a capacity to compensate any negative effect of the lowest pH. Our results suggest that the physiological behaviour of this primary producer is driven by the interactions among OA and environmental factors, like irradiance and nutrient availability. The observed behaviour highlights that high-frequency pH variability and multifactorial approaches should be applied, and when investigating the impact of OA, factors like irradiance, nutrient availability and temperature must be considered as well.

Nutrient Loading Fosters Seagrass Productivity Under Ocean Acidification

The effects of climate change are likely to be dependent on local settings. Nonetheless, the compounded effects of global and regional stressors remain poorly understood. Here, we used CO₂ vents to assess how the effects of ocean acidification on the seagrass, Posidonia oceanica, and the associated epiphytic community can be modified by enhanced nutrient loading. P. oceanica at ambient and low pH sites was exposed to three nutrient levels for 16 months. The response of P. oceanica to experimental conditions was assessed by combining analyses of gene expression, plant growth, photosynthetic pigments and epiphyte loading. At low pH, nutrient addition fostered plant growth and the synthesis of photosynthetic pigments. Overexpression of nitrogen transporter genes following nutrient additions at low pH suggests enhanced nutrient uptake by the plant. In addition, enhanced nutrient levels reduced the expression of selected antioxidant genes in plants exposed to low pH and increased epiphyte cover at both ambient and low pH. Our results show that the effects of ocean acidification on P. oceanica depend upon local nutrient concentration. More generally, our findings suggest that taking into account local environmental settings will be crucial to advance our understanding of the effects of global stressors on marine systems.

Seagrass metabolism and carbon dynamics in a tropical coastal embayment

Net ecosystem metabolism and subsequent changes in environmental variables were studied seasonally in the seagrass-dominated Palk Bay, located along the southeast coast of India. The results showed that although the water column was typically net heterotrophic, the ecosystem as a whole displayed autotrophic characteristics. The mean net community production from the seagrass meadows was 99.31 ± 45.13 mM C m^-2 d^-1, while the P/R ratio varied between 1.49 and 1.56. Oxygen produced through in situ photosynthesis, exhibited higher dependence over dissolved CO₂ and available light. Apportionment of carbon stores in biomass indicated that nearly three-fourths were available belowground compared to aboveground. However, the sediment horizon accumulated nearly 40 times more carbon than live biomass. The carbon storage capacities of the sediments and seagrass biomass were comparable with the global mean for seagrass meadows. The results of this study highlight the major role of seagrass meadows in modification of seawater chemistry. Though the seagrass meadows of Palk Bay are increasingly subject to human impacts, with coupled regulatory and management efforts focused on improved water quality and habitat conservation, these key coastal ecosystems will continue to be valuable for climate change mitigation, considering their vital role in C dynamics and interactions with the overlying water column.