Differential effects of nutrient enrichment on carbon metabolism and dissolved organic carbon (DOC) fluxes in macrophytic benthic communities

Effects of a chronic impact on Cymodocea nodosa community carbon metabolism and dissolved organic carbon fluxes

Seagrass communities have been degraded worldwide experiencing elevated shoot density reduction by anthropogenic chronic pressures. This study aims to assess how a chronic (i.e., low intensity but long-lasting) impact that promotes reduced shoot density in a temperate seagrass population may affect community components and functioning. To this end, shoot density was reduced (0, 40, and 75 %) for three months in contrasting seasons (winter and summer), and assessed its effects on biotic components (i.e., seagrasses, macroalgae, macrofauna, and microphytobenthos), as well as on community carbon metabolism, dissolved organic carbon (DOC) fluxes and sediment organic matter (OM) content. Lower shoot densities enhanced the presence of macroalgae and microphytobenthos in the community, while macrofauna remained unchanged. Net community production was significantly reduced with the simulated reduction in shoot density in both seasons (up to 10-fold lower), which shifted the community in winter from being largely autotrophic (CO2 sink) to heterotrophic (CO2 source). This was due to the expected reduction in gross primary production, but also to the unexpected increase in community respiration (up to 2.2-fold higher). Since OM in the sediment was reduced in the simulated shoot density reduction treatments, the increase in sediment bacterial activity may help explain the increase in community respiration. DOC fluxes were also greatly reduced in both seasons (up to 5.5-fold lower), which coupled with the reduced net community production and loss of OM in the sediment may have a continued silent effect on blue carbon capture and storage capacity in this chronically stressed community. This study therefore highlights the importance of chronic impacts that promote the degradation of seagrass communities that may reduce their ability to provide highly valuable ecological services, including the ability to cope with the effects of climate change.

Changes in carbon metabolism and dissolved organic carbon fluxes on seagrass patches (Halodule wrightii) with different ages in Southern Gulf of California

Seagrass meadows are highly dynamic, particularly in sites where extreme climatological events may produce a mosaic of patches with different ages. This study evaluates the community carbon metabolism, dissolved organic carbon (DOC) fluxes and associated fauna in patches of Halodule wrightii with different ages since establishment. Net community production declined as patch age increased, probably due to the increase in non-photosynthetic tissues, higher respiration rates of the community assemblage and a likely increase in self-shading of the canopy. The export of DOC was significantly higher in the youngest patches, mainly as a consequence of the lower seagrass net production recorded in older meadows. We concluded that 'colonizers' seagrass species may show higher production rates and DOC release during the first stages of colonization, which suggest that, the production, organic carbon exportation and their role as relevant blue carbon communities may be higher than expected.

Effects of herbivore on seagrass, epiphyte and sediment carbon sequestration in tropical seagrass bed

Herbivores strongly affect the ecological structure and functioning in seagrass bed ecosystems, but may exhibit density-dependent effects on primary producers and carbon sequestration. This study examined the effects of herbivorous snail (Cerithidea rhizophorarum) density on snail intraspecific competition and diet, dominant seagrass (Thalassia hemprichii) and epiphyte growth metrics, and sediment organic carbon (SOC). The growth rates of the herbivorous snail under low density (421 ind m-2) and mid density (842 ind m-2) were almost two times of those at extremely high density (1684 ind m-2), indicating strong intraspecific competition at high density. Herbivorous snails markedly reduced the epiphyte biomass on seagrass leaves. Additionally, the seagrass contribution to herbivorous snail as food source under high density was about 1.5 times of that under low density, while the epiphyte contribution under low density was 3 times of that under high density. A moderate density of herbivorous snails enhanced leaf length, carbon, nitrogen, total phenol and flavonoid contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. However, high density of herbivorous snails decreased leaf glucose, fructose, detritus carbon, and total phenols contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. Therefore, the effects of herbivorous snail on seagrass, epiphyte and SOC were density-dependent, and moderate density of herbivorous snail could be beneficial for seagrasses to increase productivity. This provided theoretical guidance for enhancing carbon sink in seagrass bed and its better conservation.

Resistance and recovery of benthic marine macrophyte communities to light reduction: Insights from carbon metabolism and dissolved organic carbon (DOC) fluxes, and implications for resilience

A crucial factor in the long-term survival of benthic macrophyte communities under light-reduction stress is how they balance carbon metabolism during photosynthesis and respiration. In turn, the dissolved organic carbon (DOC) released by these communities, which can be highly light-dependent, stands as a source of carbon, fuelling marine communities and playing an important role in the ocean carbon sequestration. This is the first study to evaluate light-reduction stress and recovery in the seagrass Zostera noltei and the macroalga Caulerpa prolifera. Light reduction led to a significant decrease in the production of both communities from autotrophic to heterotrophic. Results indicated that most of the DOC released by vegetated coastal communities comes from photosynthetic activity, and that the net DOC fluxes can be greatly affected by shading events. Finally, both communities showed resilience underpinned by high recovery but low resistance capacity, with C. prolifera showing the highest resilience to unfavourable light conditions.

Leaf Senescence of the Seagrass Cymodocea nodosa in Cádiz Bay, Southern Spain

Leaf decay in seagrasses is enhanced in some seasons since large green senescent beach-cast seagrass leaves are frequently recorded during autumn and winter seasons. Here, we explore if senescence is operating in seagrass leaf decay or if hydrodynamic stress is responsible for the seasonal leaf abscission. A seasonal study on the temperate seagrass Cymodocea nodosa was carried out in four locations with contrasting hydrodynamic regimes. The morphological, biomechanical and material properties of C. nodosa were measured. The force required to break the ligule was always lower than that required to break the blade. This could be considered an adaptive strategy to reduce acute drag forces and thus lessen the chance of plant uprooting. The absolute force needed to dislodge the blade at the ligule level varied with season and location, with the lowest forces recorded in autumn. This may indicate that senescence is operating in this species. On the other hand, the minimum estimated failure velocities for leaf abscission were also recorded in autumn. Consequently, this may cause the premature shedding of leaves in this season before the senescence process has finished and can probably explain the occurrence of green beach-cast seagrass leaves usually found during autumn and winter.

Carbon metabolism and bioavailability of dissolved organic carbon (DOC) fluxes in seagrass communities are altered under the presence of the tropical invasive alga Halimeda incrassata

Seagrass beds act as blue carbon sinks globally as they enhance the trapping of recalcitrant (i.e., low biodegradability) organic carbon in their sediments. Recent studies also show that the recalcitrant fraction of the dissolved organic carbon (DOC) pool in seawater has an important role as long-term carbon sequestration in oceans. Although seagrasses are known for the large amount of DOC they export, little attention has been given to its biodegradability, which ultimately determinates its fate in the coastal carbon cycle. In turn, invasive algae are a major global concern in seagrass ecosystems since they can deeply modify their structure and functions, which may affect carbon metabolism and DOC release. This work assesses how the presence of Halimeda incrassata, an invasive tropical calcareous macroalga, modifies carbon metabolism and DOC fluxes in invaded areas dominated by the seagrass Cymodocea nodosa. Our results show that stands with the presence of this seagrass (i.e., both monospecific and mixed meadow) had the highest production values, acting as high DOC producers in both winter (mainly of labile DOC; DOC_L) and summer (mainly as recalcitrant DOC; DOC_R). In contrast, monospecific H. incrassata beds exhibited low production values, and the presence of this macroalga (either as monospecific beds or mixed with C. nodosa) triggered the shift from a net DOC-producing-system in summer (mainly DOC_L) to a net DOC-consuming-system in winter. This work thus suggests that C. nodosa meadows have the potential to export a significant fraction of both labile and recalcitrant DOC, and that the spread of this invasive alga might decrease the C export capacity of seagrass meadows. Such shift would imply the reduction of a quick and efficient transfer of carbon and energy to higher trophic levels, and might reduce the blue carbon potential of seagrasses as dissolved form in the water column.

Whole System Analysis Is Required To Determine The Fate Of Macroalgal Carbon: A Systematic Review

The role of marine primary producers in capturing atmospheric CO₂ has received increased attention in the global mission to mitigate climate change. Yet, our understanding of carbon sequestration performed by macroalgae has been limited to a relatively small number of studies that have estimated the ultimate fate of macroalgal-derived carbon. This systematic review was conducted to provide a timely synthesis of the methods used to determine the fate of macroalgal carbon in this rapidly expanding research area. It also aimed to provide suggestions for more effective future research. We found that the most common methods to estimate the fate of macroalgal carbon can be categorized into groups based on those that quantify: (i) export of macroalgal carbon to other environments-known as horizontal transport; (ii) sequestration of macroalgal carbon into deep-sea sediments-known as vertical transport; (iii) burial of macroalgal carbon directly beneath a benthic community; (iv) the loss of macroalgal carbon as particulate carbon or dissolved carbon to the water column; (v) the loss of macroalgal carbon to primary consumers; and finally (vi) those studies that combined multiple methods in one location. Based on this review, several recommendations for future research were formulated, which require the combination of multiple methods in a whole system analysis approach.

Migration and Transformation of Multiple Heavy Metals in the Soil–Plant System of E-Waste Dismantling Site

E-waste generation has become a major environmental issue worldwide. Heavy metals (HMs) in e-waste can be released during inappropriate recycling processes. While their pollution characteristics have been studied, the migration and transformation of different multi-metal fractions in soil–plant system of e-waste dismantling sites is still unclear. In this study, pot experiments were conducted to investigate the migration and transformation of different multi-metal fractions (Cu, Pb, Zn and Al) in the soil–plant system using two Chinese cabbage cultivars (heavy metals low-accumulated variety of Z1 and non-low-accumulated Z2) treated with or without biochar. The result showed that the acid-soluble fraction of Cu, Pb, Zn and Al in soil decreased by 5.5%, 55.7%, 7.8% and 21.3%, but the residual fraction (ResF) of them increased by 48.5%, 1.8%, 30.9% and 43.1%, respectively, when treated with biochar and plants, compared to that of the blank soil (CK). In addition, Pb mainly existed as a reducible fraction, whereas Cu existed as an oxidisable fraction. Biochar combined with plants significantly increased the ResF of multi-metals, which reduced the migration ability of Pb among all other metals. The relative amount of labelled ¹³C in the soil of Z1 was higher than that of Z2 (25.4 fold); among them, the Gram-negative bacteria (18-1ω9c, 18-1ω7c) and fungi (18-2ω6c) were significantly labelled in the Z1-treated soil, and have high correlation with HM migration and transformation. In addition, Gemmatimonadete were significantly positive in the acid-soluble fraction of HMs, whereas Ascomycota mostly contributed to the immobilisation of HMs. Therefore, the distribution of fractions rather than the heavy metal type plays an important role in the HM migration in the soil–plant system of e-waste dismantling sites.

Rate and fate of dissolved organic carbon release by seaweeds: A missing link in the coastal ocean carbon cycle

Dissolved organic carbon (DOC) release by seaweeds (marine macroalgae) is a critical component of the coastal ocean biogeochemical carbon cycle but is an aspect of seaweed carbon physiology that we know relatively little about. Seaweed-derived DOC is found throughout coastal ecosystems and supports multiple food web linkages. Here, we discuss the mechanisms of DOC release by seaweeds and group them into passive (leakage, requires no energy) and active release (exudation, requires energy) with particular focus on the photosynthetic "overflow" hypothesis. The release of DOC from seaweeds was first studied in the 1960s, but subsequent studies use a range of units hindering evaluation: we convert published values to a common unit (μmol C · g DW^-1 · h^-1 ) allowing comparisons between seaweed phyla, functional groups, biogeographic region, and an assessment of the environmental regulation of DOC production. The range of DOC release rates by seaweeds from each phylum under ambient environmental conditions was 0-266.44 μmol C · g DW^-1 · h^-1 (Chlorophyta), 0-89.92 μmol C · g DW^-1 · h^-1 (Ochrophyta), and 0-41.28 μmol C · g DW^-1 · h^-1 (Rhodophyta). DOC release rates increased under environmental factors such as desiccation, high irradiance, non-optimal temperatures, altered salinity, and elevated dissolved carbon dioxide (CO₂ ) concentrations. Importantly, DOC release was highest by seaweeds that were desiccated (<90 times greater DOC release compared to ambient). We discuss the impact of future ocean scenarios (ocean acidification, seawater warming, altered irradiance) on DOC release rates by seaweeds, the role of seaweed-derived DOC in carbon sequestration models, and how they inform future research directions.