The primary production and zooplankton life cycles in the Southern Ocean synchronize with seasonal sea ice melting, during which particulate organic carbon (POC) and nitrogen (PN), including ice algae, are supplied from sea ice into the water column. To understand carbon flow through food webs in productive marginal ice zones, it is necessary to determine the quantity and quality of organic carbon and nitrogen in sea ice. However, knowledge regarding brash sea ice is scarce, even though it is the predominant form at the Antarctic ice edge. In this study, 102 sea-ice samples were collected from the Indian sector (20–160° E) during summer to clarify variations in isotopic baselines and the potential contribution of sea ice-derived carbon and nitrogen to the water column. The δ13C and δ15N values of all sea-ice particulate matter samples were -24.6±3.2 ‰ and +1.5±2.2 ‰ (mean ± standard deviation), respectively, showing greater variability compared to those in seawater (-28.8±1.0 ‰ for δ13C and -0.5±1.4 ‰ for δ15N). We identified correlations of δ13C and δ15N with organic matter and nutrient concentrations and their ratios (e.g., silicate : nitrate), suggesting that nutrient consumption and organic matter decomposition influenced those isotopic characteristics in sea ice. Using freshwater flux from sea ice melt and the measured sea-ice POC concentrations, the POC supply was estimated to be 4.1±0.3 Tg C yr−1, equivalent to 14.7±2.8 % of primary production in the marginal ice zone of the studied region. This result suggests that sea ice melt provides significant portion of organic matters potentially supporting Antarctic ecosystems in the seasonal sea ice zone, which could be traced by difference of sea-ice and seawater stable isotope compositions.
Northern lakes are warming and browning due to increased cDOM inputs, altering water column C:N:P ratios and temperatures that regulate microbial production. Yet the relative importance of warming versus browning, and their combined effects on nutrient stoichiometry and microbial dynamics remain unclear. We experimentally manipulated boreal ponds along a cDOM gradient under ambient and +2 °C warming to quantify impacts on nutrient conditions and microbial production. Browning shifted C:N:P stoichiometry in both total and dissolved pools, with increased C:N and C:P ratios in the total nutrient pools, while reducing inorganic nutrient availability. Despite declining N:P ratios, both pools were strongly carbon-rich and depleted in both N and P. Nutrient-addition assays confirmed NP co-limitation. Browning increased bacterial production, while browning and warming combined reduced primary production, lowering PP:BP ratios. We show that browning -not warming- drives boreal freshwaters toward carbon-rich, N- and P- colimited conditions, while warming and browning together modulate microbial production by amplifying cDOM-driven heterotrophy and suppressing photoautotrophic production. Our study reveals a dual mechanism behind the shifting of brown systems toward heterotrophy. As browning and warming are pervasive trends across Northern lakes, these findings have wide applicability and advance general understanding of how freshwaters respond to ongoing environmental change.
Ocean alkalinity enhancement (OAE) is a carbon dioxide removal strategy with high carbon dioxide (CO2) uptake potential and moderate cost. Long-term implementation has been assessed in modelling studies, but most laboratory experiments have focused on short time frames with strong advection, which may not be representative of natural systems. Hence, the long-term fate of added alkalinity has yet to be fully addressed in laboratory experiments. Furthermore, the role of CO2 ingassing on overall efficiency is still largely overlooked. In a new setup, six-month experiments were conducted using seawater amended with solid Ca(OH)2 and Mg(OH)2 and liquid NaOH, with a constant atmospheric CO2 supply. Bottles were either kept stagnant or exposed to slow circular advection, yielding gas-transfer-velocity equivalent wind speed of ∼3 m s−1. All experiments revealed smaller measured increases in total alkalinity than expected, but in the bottles subjected to advection, alkalinity generation was nearly twice as high in some cases and CO2 ingassing rates were significantly faster. This led to a faster equilibration, decreasing the high pH/low CO2 period, recently identified as a potential threat to living organisms. The critical alkalinity period (CAP) is introduced here to represent this critical period of altered carbonate chemistry during which secondary calcium carbonate (CaCO3) formation may occur, significantly altering the CDR potential of the approach. CaCO3 precipitated in most bottles, either as needles when adding Ca(OH)2 or as broccoli-like structures when adding Mg(OH)2. These broccoli-like structures most likely resulted from precipitation on slowly dissolving Mg(OH)2 particles. After the CAP ended, conditions remained stable and freshly precipitated CaCO3 did not redissolve. In addition, when NaOH was used as the alkaline feedstock, Mg(OH)2 flakes were observed but these eventually redissolved in the bottles exposed to advection. The study emphasises the role of water advection on OAE's efficiency and the data presented here may be used to derive lower thresholds for OAE implementation in global models.
Human activities are increasingly affecting coastal ecosystems, with shoreline stabilization structures becoming a prevalent response to sea-level rise and extreme climatic events. While these structures aim to protect coastal communities and infrastructures, their effects on ecosystem functioning, and particularly nitrogen cycling and its removal processes such as denitrification, remain poorly understood. To assess the ecological impacts of breakwater construction, we employed a space-for-time substitution approach to examine changes in sediment biogeochemistry, macrobenthic community structure, and nitrogen cycling in an intertidal salt marsh in the St. Lawrence Estuary, Canada. We measured benthic fluxes, denitrification rates, and macrofaunal assemblages at five locations: two landward of the breakwater (impacted sites), one situated immediately seaward of the breakwater (intermediate site) and two reference sites (one vegetated and one unvegetated outside the breakwater system). Landward sites exhibited finer sediment with higher organic carbon content compared to other sites and supported distinct macrobenthic communities dominated by opportunistic Oligochaetes, while reference sites were dominated by Molluscs and Crustaceans. The intermediate seaward site consistently resembled the unvegetated reference site across all variables measured at both sites, including sediment characteristics, oxygen dynamics and benthic fluxes, thereby showing reference-like conditions. Surprisingly, despite these substantial physical and biological changes, dark total benthic oxygen uptake, nitrate and ammonium benthic fluxes showed no consistent separation among impacted, intermediate, and reference sites, while benthic denitrification rates were comparable among the impacted and reference sites where they were measured. This functional similarity reflects an apparent functional convergence arising from distinct combinations of abiotic and biotic factors, including root-mediated rhizosphere oxygenation, macrofauna-mediated sediment irrigation, and contrasting reliance on internally produced versus water-column nitrate. However, this convergence might not apply to all ecosystem services offered by natural salt marshes and may depend on specific biological communities, emphasizing the importance of careful consideration in coastal management decisions.
Seagrass meadows are important blue carbon ecosystems that sequester large amounts of organic carbon but can also emit methane (CH4), potentially offsetting part of their climate mitigation benefits. However, the seasonal and diurnal dynamics of CH4 production and emissions in tropical seagrass ecosystems remain poorly understood. This study presents the first integrated assessment of CH4 cycling in the Haikou seagrass meadow, southern Taiwan. Field campaigns conducted between May 2022 and June 2023 combined surface water sampling, in situ benthic chamber incubations, and porewater profiling in a Thalassia hemprichii–Halophila pinifolia seagrass meadow to quantify CH4 dynamics at both the sediment-to-water and sea-to-air interfaces. The seagrass meadow exhibited higher dissolved CH4 concentrations and sea-to-air CH4 fluxes than adjacent coastal waters, confirming that it functions as a localized atmospheric CH4 hotspot. Nighttime dissolved CH4 concentrations generally exceeded daytime values, reflecting diurnal variations in oxygen availability, whereas seasonal variability in sea-to-air CH4 fluxes was primarily driven by wind speed. Mean sediment-to-water and sea-to-air CH4 fluxes were 88.2±41.2 and 4.9±2.2µmol m−2 d−1, respectively. Despite substantial benthic CH4 production, atmospheric emissions remained comparatively low, indicating a marked decoupling between sediment-to-water and sea-to-air CH4 fluxes. This decoupling suggests that much of the sediment-derived CH4 was transported laterally by tidal exchange and coastal circulation before atmospheric release, with microbial oxidation acting as an additional sink. Compared with adjacent bare sand sediments, seagrass sediments maintained higher baseline CH4 production rates and benthic fluxes owing to greater organic matter availability and favorable microbial conditions, whereas unvegetated sandy sediments generally exhibited low CH4 production but produced episodic CH4 pulses following storm-induced sediment disturbance. Elevated porewater CH4 concentrations during summer and autumn further indicated enhanced methanogenesis associated with organic matter accumulation under calmer hydrodynamic conditions. Overall, these findings demonstrate that CH4 cycling in tropical seagrass meadows is jointly regulated by biological processes and hydrodynamic conditions, highlighting the need to consider both CH4 emissions and carbon sequestration when evaluating the climate benefits of blue carbon ecosystems.
