Blue carbon refers to the carbon accumulation capacity of vegetated coastal habitats, including salt marshes, mangroves forests and seagrass meadows. Here we present estimates of organic carbon ...(Corg) and calcium carbonate (CaCO₃) burial rates from 4 seagrass species (Halophila ovalis, Posidonia australis, Ruppia megacarpa, Zostera muelleri) in 3 temperate estuaries on the east coast of Australia. The Corg burial rates (mean ± SE) varied by an order of magnitude across the seagrass communities (16 ± 3 to 130 ± 40 g m−2 yr–1). The δ
13Corg and Corg:N ratios suggest that the seagrass communities buried variable mixtures of seagrass, algal and mangrove/terrestrial material. CaCO₃ burial rates ranged from 15 ± 11 to 188 ± 122 g m−2 yr−1, which, if precipitated by calcifying organisms in these or nearby habitats, may offset up to 89% of the Corg burial across the 8 seagrass communities. Our results highlight a large range in both Corg and CaCO₃ burial rates, and the provenance of the carbon sequestered in seagrasses, factors that need to be considered when assessing the role of seagrasses in blue carbon and climate change mitigation strategies.
Groundwater is often overlooked as a source of nutrients to estuaries and most previous groundwater-surface water exchange studies did not consider the input of dissolved organic nutrients. Here, we ...hypothesize that groundwater is contributing to high dissolved inorganic and organic nutrient concentrations in an eutrophic subtropical tidal river and estuary (Caboolture River, Queensland, Australia). Several spatial radon (222Rn, a natural groundwater tracer) surveys indicated that the majority of groundwater discharge occurred in the tidal river just upstream of the estuary, and that the radon hotspot did not necessarily coincide with the nutrient hotspot. A radon mass balance revealed that groundwater discharge into the tidal river was equivalent to about 50% of the gauged river flow in February 2012. Groundwater discharge apparently contributed 85% of ammonium and 35% of phosphate entering the estuary. In spite of significant correlations between radon and nitrate and dissolved organic nitrogen (DON) during spatial surveys, groundwater could account for only 7% of nitrate and 9% of DON inputs due to low groundwater concentrations and other sources (i.e., apparently a sewage treatment plant for nitrate and floodplain tributaries for DON). Because total dissolved nitrogen (TDN) was dominated by DON (69%) and nitrate (23%), the groundwater ammonium inputs were a minor source to the TDN pool within the tidal river and estuary. This study demonstrated that correlations between a groundwater tracer and nutrient concentrations do not necessarily illustrate causation. To assess how groundwater drives nutrient dynamics in estuaries, it may be important to include the tidal river (not only the estuarine salinity gradient) in field investigations, consider DON (not only ammonium and nitrate), and perform detailed budgets that include minor tributaries.
•We conducted high resolution radon surveys and time series on a subtropical tidal river and estuary.•The majority of groundwater discharge occurred in the tidal river upstream of the estuary.•Groundwater contributed 85% of ammonium, 7% of nitrate, and 9% of DON exported from the estuary.•Groundwater flow was ∼50% of the upstream flow and 10% of the total flow into the estuary.•Radon versus nutrient correlations do not necessarily illustrate causation.
Large rivers, including the Murray River system in southeast Australia, are disturbed by many activities. The arrival of European settlers to Australia by the mid-1800s transformed many floodplain ...wetlands of the lower Murray River system. River impoundment and flow regulation in the late 1800s and, from the 1930s, resulted in species invasion, and elevated nutrient concentrations causing widespread eutrophication. An integrated palaeoecology, and palaeo-and-modern food web approach, incorporating mixing models, was undertaken to reveal changes in a regulated wetland (i.e. Kings Billabong). The lack of preserved sediment suggests the wetland was naturally intermittent before 1890. After this time, when used as a water retention basin, the wetland experienced net sediment accumulation. Subfossil cladocerans, and δ
C of Daphnia, chironomid, and bulk sediment, all reflected an early productive, likely clear water state and shifts in trophic state following river regulation in the 1930s. Food web mixing models, based on δ
C and δ
N in subfossil and modern Daphnia, fish, and submerged and emergent macrophytes, also indicated a shift in the trophic relationships between fish and Daphnia. By the 1970s, a new state was established but a further significant alteration of nitrogen and carbon sources, and trophic interactions, continued through to the early 2000s. A possible switch from Daphnia as a prey of Australian Smelt could have modified the food web of the wetland by c. 2006. The timing of this change corresponded to the expansion of emergent macrophytes possibly due to landscape level disruptions. The evidence of these changes suggests a need for a broader understanding of the evolution of wetlands for the management of floodplains in the region.
Marine macroalgae are a key primary producer in coastal ecosystems, but are often overlooked in blue carbon inventories. Large quantities of macroalgal detritus deposit on beaches, but the fate of ...wrack carbon (C) is little understood. If most of the wrack carbon is respired back to CO
2
, there would be no net carbon sequestration. However, if most of the wrack carbon is converted to bicarbonate (alkalinity) or refractory DOC, wrack deposition would represent net carbon sequestration if at least part of the metabolic products (e.g., reduced Fe and S) are permanently removed (i.e., long-term burial) and the DOC is not remineralised. To investigate the release of macroalgal C via porewater and its potential to contribute to C sequestration (blue carbon), we monitored the degradation of
Ecklonia radiata
in flow-through mesocosms simulating tidal flushing on sandy beaches. Over 60 days, 81% of added
E. radiata
organic matter (OM) decomposed. Per 1 mol of detritus C, the degradation produced 0.48 ± 0.34 mol C of dissolved organic carbon (DOC) (59%) and 0.25 ± 0.07 mol C of dissolved inorganic carbon (DIC) (31%) in porewater, and a small amount of CO
2
(0.3 ± 0.0 mol C; ca. 3%) which was emitted to the atmosphere. A significant amount of carbonate alkalinity was found in porewater, equating to 33% (0.27 ± 0.05 mol C) of the total degraded C. The degradation occurred in two phases. In the first phase (days 0–3), 27% of the OM degraded, releasing highly reactive DOC. In the second phase (days 4–60), the labile DOC was converted to DIC. The mechanisms underlying
E. radiata
degradation were sulphate reduction and ammonification. It is likely that the carbonate alkalinity was primarily produced through sulphate reduction. The formation of carbonate alkalinity and semi-labile or refractory DOC from beach wrack has the potential to play an overlooked role in coastal carbon cycling and contribute to marine carbon sequestration.
Graphical abstract
The effect of nutrient enrichment on mangrove sediment accretion and carbon accumulation rates is poorly understood. Here we quantify sediment accretion through radionuclide tracers to determine ...organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) accumulation rates during the previous 60 years in both a nutrient‐enriched and a pristine mangrove forest within the same geomorphological region of southeastern Brazil. The forest receiving high nutrient loads has accumulated OC, TN, and TP at rates that are fourfold, twofold, and eightfold respectively, higher than those from the undisturbed mangrove. Organic carbon and TN stable isotopes (δ13C and δ15N) reflect an increased presence of organic matter (OM) originating with either phytoplankton, benthic algae, or another allochthonous source within the more rapidly accumulated sediments of the impacted mangrove. This suggests that the accumulation rate of OM in eutrophic mangrove systems may be enhanced through the addition of autochthonous and allochthonous nonmangrove material.
Key Points
High OC, N and P accumulation from impacted compared to pristine mangrove
Carbon and N stable isotopes indicate diversified source of OM
Mangrove migration may increase global OC, N, and P burial rates
Atmospheric dimethylsulfide (DMSa), continually derived from the world's oceans, is a feed gas for the tropospheric production of new sulfate particles, leading to cloud condensation nuclei that ...influence the formation and properties of marine clouds and ultimately the Earth's radiation budget. Previous studies on the Great Barrier Reef (GBR), Australia, have indicated coral reefs are significant sessile sources of DMSa capable of enhancing the tropospheric DMSa burden mainly derived from phytoplankton in the surface ocean; however, specific environmental evidence of coral reef DMS emissions and their characteristics is lacking. By using on-site automated continuous analysis of DMSa and meteorological parameters at Heron Island in the southern GBR, we show that the coral reef was the source of occasional spikes of DMSa identified above the oceanic DMSa background signal. In most instances, these DMSa spikes were detected at low tide under low wind speeds, indicating they originated from the lagoonal platform reef surrounding the island, although evidence of longer-range transport of DMSa from a 70 km stretch of coral reefs in the southern GBR was also observed. The most intense DMSa spike occurred in the winter dry season at low tide when convective precipitation fell onto the aerially exposed platform reef. This co-occurrence of events appeared to biologically shock the coral resulting in a seasonally aberrant extreme DMSa spike concentration of 45.9 nmol m−3 (1122 ppt). Seasonal DMS emission fluxes for the 2012 wet season and 2013 dry season campaigns at Heron Island were 5.0 and 1.4 µmol m−2 day−1, respectively, of which the coral reef was estimated to contribute 4 % during the wet season and 14 % during the dry season to the dominant oceanic flux.
Coral mass-spawning represents a spectacular annual, short-term, fertilization event of many oligotrophic reef communities. The spawning event in 2005 at Heron Island, Great Barrier Reef, was ...followed by an intense bloom of benthic dinoflagellates. Within a day from the first observed spawning, the primary production of the water column and the benthic compartment increased by factors of 4 and 2.5, respectively. However, the phototrophic communities were intensively grazed by macrozoans, and after 4-5 d the net photosynthesis (P) returned to the pre-spawning background level. The heterotrophic activity (R) mirrored the phototrophic response: a short term of elevated activity was followed by a rapid decline. However, the net autotrophic microbial communities exhibited a marked increase in the P : R ratio just after coral mass-spawning, indicating a preferential phototrophic recycling of nutrients rather than a microbial exploitation of the release of labile organic carbon. The heterotrophic and phototrophic activity of the benthic community exceeded the pelagic activity by ~2- and ~5-fold, respectively, underlining the importance of benthic activity for coral reef ecosystem function. Mass balance calculations indicated an efficient recycling of spawn-derived nitrogen (N) and carbon (C) within the benthic reef community. This was presumably facilitated by advective solute transport within the coarse, permeable, carbonate sand.
Tritium dating of groundwater and a radon mass balance was used to assess the contribution of submarine groundwater discharge (SGD) to a nutrient budget in a tropical reef lagoon (Rarotonga, Cook ...Islands). Open ocean exchange accounted for the largest percentage of potential lagoon exports (nitrogen (N)=61%, phosphorous (P)=22%) with N export mostly in the form of dissolved organic nitrogen (DON) (93% of total N). SGD accounted for 29% and 11% of exported N and P respectively. Overall we could account for 92% of dissolved N but only 36% of dissolved P leaving the lagoon. However, if DON is assumed to be recalcitrant, SGD would be the major driver of lagoon N and would account for 81% of dissolved inorganic N (DIN) inputs and 47% of DIN exported form the lagoon. A small scale (~50m), high definition survey indicated areas of higher NH4+ towards the middle of the lagoon. This is indicative of SGD input away from the seepage face and may mean that SGD measurements made at the beach face underestimate the broader contribution of SGD to nutrient fluxes. Time series observations during a 100mm rain event indicated that these episodes can deliver high nutrient loads to the lagoon and may contribute to closing the phosphorous budget. Analysis of tritium concentrations in groundwater, surface waters and springs showed that old, deep groundwater (10–93years old) was the main source of SGD derived nutrients to the lagoon. This study demonstrates that there may be a long time lag between nutrient infiltration into aquifers and the discharge of nutrient loaded groundwater into the lagoon. As such, potential mitigation measures which decrease terrestrial nutrient loads may not result in decreased SGD nutrient fluxes for decades to come.
•SGD accounted for 81% of DIN inputs and 47% of DIN exported from the lagoon.•The budget accounted for 92% of TDN but only 36% of TDP leaving the lagoon.•Old, deep groundwater (10–93years) was the main source of SGD N and P.•Decreasing nutrient inputs may not reduce SGD nutrient fluxes for decades to come.
Bottom trawling and eutrophication are large stressors that are critically coupled. Here we show, using a before‐after control‐effect design, the significant reduction in denitrification as a result ...of experimental bottom trawling in a shallow coastal system. Trawl disturbance destroys the complex three‐dimensional redox structures in surface sediments that maximize denitrification potential, resulting in up to a 50% reduction in net denitrification. The decrease in net denitrification also increased after each trawling event suggesting a declining resilience to trawling and eutrophication. Bottom trawling occurs at such a large scale that it could result in significant amounts of nitrogen being retained on the continental shelf. As such, impacts on the global ocean nitrogen cycle and associated eutrophication should be counted among the many negative consequences of extensive seafloor trawling.
Coccolithophores are unicellular marine phytoplankton and important contributors to global carbon cycling. Most work on coccolithophore sensitivity to climate change has been on the small, abundant ...bloom-forming species Emiliania huxleyi and Gephyrocapsa oceanica. However, large coccolithophore species can be major contributors to coccolithophore community production even in low abundances. Here we fit an analytical equation, accounting for simultaneous changes in CO
and light intensity, to rates of photosynthesis, calcification and growth in Scyphosphaera apsteinii. Comparison of responses to G. oceanica and E. huxleyi revealed S. apsteinii is a low-light adapted species and, in contrast, becomes more sensitive to changing environmental conditions when exposed to unfavourable CO
or light. Additionally, all three species decreased their light requirement for optimal growth as CO
levels increased. Our analysis suggests that this is driven by a drop in maximum rates and, in G. oceanica, increased substrate uptake efficiency. Increasing light intensity resulted in a higher proportion of muroliths (plate-shaped) to lopadoliths (vase shaped) and liths became richer in calcium carbonate as calcification rates increased. Light and CO
driven changes in response sensitivity and maximum rates are likely to considerably alter coccolithophore community structure and productivity under future climate conditions.
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