Mediterranean seagrasses as carbon sinks: Methodological and regional differences

The increasing rates of CO2 due to anthropogenic activities are causing important potential climate threats for the Mediterranean Sea: ocean acidification and warming. In this region, two seagrass species, Posidonia oceanica and Cymodocea nodosa can play a crucial role in climate change mitigation. Through their metabolic activity, they can act as carbon sinks; buffer lowering pH values during the day and store carbon in the sediment underneath their meadows. In this study we analyse the metabolism synthesized from published data on seagrass community metabolism and from own results to evaluate trends through time of these two species comparing two methodologies: benthic chambers and multiparametric sensors. Furthermore, we analysed seasonal trends of both seagrass species´ metabolic rates and their variation between the Eastern and Western Mediterranean basins, with no significant results despite the clear visual trends. Our analysis revealed that there are significant differences between methodologies, with multiparametric sensors estimating higher rates, but unable to differentiate between habitats and useful to assess seagrass metabolism at a community level whereas benthic chambers are capable to evaluate rates at a seagrass species level. We found significant differences between the two Mediterranean regions for both methodologies, with highest rates of Net Community Production found in the Easter basin. At a species level, we found that Posidonia was more productive compared to Cymodocea. Furthermore, 86.7 % of the metabolic values reflected that the meadows were acting as carbon sinks in the Western basin.

Impacts of the non-native alga Sargassum horneri on benthic community production in a California kelp forest

The arrival of Sargassum horneri throughout the Southern California Bight and the Baja Peninsula has raised concern regarding kelp forest resilience and ecosystem function following the invasion of this non-native species. To understand how S. horneri impacts native algal abundance and community production, we removed S. horneri from experimental plots over a period of 11 mo. We measured impacts on native algal communities and community productivity using SCUBA surveys and benthic chambers equipped with oxygen, temperature, and light sensors. We observed a nearly 4-fold increase in recruitment of Macrocystis pyrifera and a 9-fold increase in adult M. pyrifera stipe density in S. horneri removal plots, but no discernable changes in net community production among treatments. We found ephemeral increases in gross community production and community respiration in the non-removal plots that coincided with periods of peak S. horneri biomass. To understand the temporal dynamics of community production, we deployed benthic chambers across a rocky reef dominated by S. horneri. Here, temporal variation in community production was most strongly related to corresponding variation in water temperature and changes in S. horneri biomass related to its annual lifecycle. Overall, our study indicates that S. horneri presence contributed to ephemeral increases in gross community production and community respiration, but it did not affect net community production. Moreover, S. horneri removal can lead to increases in native algal abundances given favorable abiotic conditions. We suggest that S. horneri thrives in a disturbed ecosystem rather than being a driver of ecosystem change.

Oxygen consumption and nutrient fluxes in coastal marine sediments off the Mejerda River delta (Gulf of Tunis)

The authors studied benthic flux of oxygen, alkalinity, and nutrients in situ at three points in the Mejerda River Delta at depths of 10m, 20m and 40m in March and August 2012. Three sedimentary cores were simultaneously drilled at the same locations to determine the diffusive flux of NO2-, NO3-, NH4+ and PO43- and to estimate diagenetic mechanisms occurring below the sediment-water interface. Photosynthesis was not sufficiently high during the day, and the oxygen consumption at sediment-water interface was about 1.7 to 10mmol/m²/day, essentially controlled by the degradation of organic matter and oxidation of reduced elements. Nitrate contents are relatively high in the sediment (above 140μM for NO3-)and their production is not always in conformity with the general scheme of early diagenesis, moreover, benthic flux between water and sediment are not clearly established. The diffusive flux of NH4+ and PO43- are always directed to the water column, at averages of 1.27μmol/m²/day for PO43- and 96.5μmol/m²/day for NH4+, complying with those measured by benthic chambers, but representing less than 30% of benthic fluxes for NH4+ and less than 5% for PO43-.

Artificially induced migration of redox layers in a coastal sediment from the Northern Adriatic

Long-term experimental studies suggest that, under transient anoxic conditions, redox fronts within the sediment shift upwards, causing sequential rise and fall of benthic fluxes of reduced species (Mn(II), Fe(II) and S(-II)). Infaunal benthic organisms are associated with different redox fronts as micro-habitats and must be affected by such changes during natural hypoxia events. In order to document the geochemical evolution of the sediment during prolonged anoxia in the framework of an in situ experiment designed to mimic natural conditions, benthic chambers were deployed on the seafloor of the Northern Adriatic and sampled after 9, 30 and 315 days of incubation. Oxygen and sulfide were measured continuously in the early stages (9 days) of the experiment. High-resolution pore water profiles were sampled by DET probes and redox-sensitive species (S(VI), Mn(II) and Fe(II)) and alkalinity were measured. Starting oxygen saturation was about 80% within the chamber. After 7 days, anoxia was established in the bottom waters within the chambers. Mn(II) and Fe(II) started diffusing towards the anoxic water column until they reached the surficial sediment. Being reoxidized there, Mn and Fe reprecipitated, giving a rusty coloration to the seafloor. Infaunal species appeared at the sediment surface. After 20 days, all macro-organisms were dead. Decomposition of macro-organisms at the sediment–water interface generated S(-II) within the entire height of the chamber, leading to a downward flux of sulfides into the sediment, where they were quickly oxidized by metallic oxides or precipitated as FeS. S(-II) was below detection in the water column and pore waters at the end of the experiment. Our results suggest that S(-II) enrichment in the water column of coastal systems, which are episodically anoxic, is strongly controlled by the biomass of benthic macrofauna and its decay during anoxia, whereas its residence time in the water column is controlled by iron availability (as solid oxides or as dissolved reduced cations) within the sediment, even without water circulation.

Foraminiferal species responses to in situ, experimentally induced anoxia in the Adriatic Sea

Anoxia was successfully induced in four benthic chambers installed at 24 m depth in the northern Adriatic Sea for periods varying from 9 days to 10 months. During the 10-month period, species richness significantly decreased. Although no significant change in Shannon diversity and evenness was observed, the composition of the foraminiferal assemblages changed with time. This change is due to interspecific differences in tolerance to anoxia. Reophax nanus, Textularia agglutinans and Quinqueloculina stelligera all showed a significant decrease with time, strongly suggesting they are sensitive to anoxia. Conversely, Eggerella scabra, Bulimina marginata, Lagenammina atlantica, Hopkinsina pacifica and Bolivina pseudoplicata appeared to be resistant to the experimental conditions. Quinqueloculina seminula was apparently sensitive to anoxia but showed a clear standing stock increase during the first month of the experiment, which we interpret as an opportunistic response to increasing organic matter availability due to the degradation of the dead macrofaunal organisms. None of the anoxia-sensitive species is able to accumulate intracellular nitrates. Nitrate accumulation could be shown for some tested specimens of the dominant anoxia-tolerant species E. scabra and B. marginata. However, tests on the denitrification capacity of these taxa yielded negative results, suggesting that their resistance to long-term anoxia is not due to their ability to denitrify.