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This study uses dual‐labeled (13C and 15N) stable isotope applications to examine microbial uptake and short‐term processing of carbon (C) and nitrogen (N) from organic and inorganic compounds in subtropical intertidal sediment. Four treatment applications were applied: (1) algal dissolved organic matter (DOM), (2) amino acid mixture, (3) glucose and NH4+, and (4) NaHCO3 and NH4+ to assess bioavailability effects on processing (1 vs. 2) and short‐term processing for OM fixed via microphytobenthos only (pennate diatom dominated) (4) vs. material taken up by the entire microbial community (3) across 24 h. 13C from algal‐DOM was preferentially used by the microbial community vs. 15N. At 24 h more 13C from algal‐DOM remained in microbial biomass indicating use of labeled precursor molecules to form biomass. Conversely, 13C from the amino acid treatment was not incorporated into biomass and was either rapidly respired to DIC or discarded as the in situ microbial community preferentially used and retained 15N from amino acids. Short‐term export of 13C as CO2 from glucose was lower than from microphytobenthos‐C, while retention of 15N from NH4+ was similar between treatments (3 and 4) despite doubling the application N concentration, suggesting potentially higher glucose‐stimulated 15N export via nitrification–denitrification that was not confirmed via flux measurements in this study. Despite careful site selection for similar tidal exposure and sediment types among the three estuaries, the uptake and processing of labeled substrate varied substantially between replicates and sites which challenged traditional statistical analysis. Disproportionate processing of substrates occurring in sediment hotspots of microbial activity can cause variability spanning orders of magnitude which was found to be widespread through comparison of our results against 19 previous studies in intertidal settings. Development of robust analytical techniques to handle variability from abiotic and biotic factors will allow greater clarity surrounding in situ biogeochemical processing in intertidal environments. [ABSTRACT FROM AUTHOR]

Carnivorous plants obtain nutrients from arthropod prey (carnivory) and their environment. However, little is known about the seasonal diet shifts between carnivory versus environment nutrient acquisition among co‐occurring carnivorous plant species. Although studies reporting captured prey species for carnivorous plants are abundant, studies investigating the composition of assimilated prey species are scarce. Here, we report spatiotemporal dynamics in carnivory versus environmental nutrient acquisition and compositional analysis of assimilated prey species for carnivorous plants of New England (USA). We measured the natural abundance stable isotope ratios of aquatic Utricularia inflata (Lentibulariaceae), terrestrial Sarracenia purpurea (Sarraceniaceae) and Drosera intermedia (Droseraceae) and their nutrient sources during their growing season in two oligotrophic ponds in New England (USA). MixSIAR, a Bayesian stable isotope mixing model, was used to estimate the extent of carnivory for each plant species and the composition of prey species assimilated in terrestrial carnivorous plants. Aquatic U. inflata acquired more nutrients from carnivory (c. 70%–100%) than terrestrial species (c. 30–60%). Sarracenia purpurea acquired nutrients mainly from Formicidae. The nutrient sources for D. intermedia varied spatiotemporally between arthropod species. The large contribution of carnivory to the nutrient budget of U. inflata could be due to its lack of roots and high metabolism. Sarracenia purpurea and D. intermedia could benefit from dissolved organic nitrogen in the environment. Our findings on prey compositions can be used as the first available prior information for the Bayesian approach in future studies. Understanding the sources of nutrients in carnivorous plants provides invaluable insight into their ecophysiology and may inform the conservation of their fragile wetland ecosystems. [ABSTRACT FROM AUTHOR]

Estuarine sediments make an important contribution to the global carbon cycle, but we do not know how this will change under a future climate, which is expected to have lower pH oceans and frequent high‐temperature days. Six combinations of warming and partial pressures of CO2 (pCO2) were chosen to investigate the combined and individual effects of short‐term pressures on the diel metabolic response of shallow unvegetated sediments ex‐situ. Whereas warming significantly increased respiration, making sediments more heterotrophic, high‐pCO2 increased net primary productivity, resulting in less heterotrophic sediments. As a result, warming decreased the carbon burial potential of estuarine sediments and high‐pCO2 had the opposite effect. High‐pCO2 mitigates the negative effects of warming on benthic metabolism under the combined scenario, with carbon burial similar to that expected under high‐pCO2 conditions alone. Climate scenarios also changed the diurnal pCO2 variation, with ranges increasing by 33% with warming, and almost doubling under high‐pCO2 conditions. An additive response in pCO2 variability was observed under the combined scenario, increasing to 2.3× the current diel‐pCO2 range, highlighting the reduced buffering capacity of the water associated with a high CO2 climate. Future carbon burial and export under increased frequencies of unseasonably warm days projected for mid and end of century (30% and 50% of days‐per‐year, respectively) were estimated with and without ocean acidification. By 2100, warming alone could decrease annual estuarine sediment burial potential by 25%. However, ocean acidification could mitigate the negative effects of more frequent high‐temperature days and increase carbon burial potential over current conditions by ~18%. [ABSTRACT FROM AUTHOR]

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of N assimilated by MPB. We used an in situ 15N pulse‐chase experiment in intertidal, nonpermeable, sandy mud to trace the assimilation, transformation, and loss from the sediment of MPB‐N over 31 d. Following assimilation, 15N was tightly retained in surface sediments, with only up to 8.1% transported to 2–10 cm sediments over 31 d. MPB accounted for a considerable portion of the 15N in surface sediments throughout the study (59% ± 13%). Bacteria rapidly assimilated 15N but accounted for only up to 17% of the 15N within surface sediments. Of the assimilated 15N, 78.9% was lost from the sediment over 31 d. Resuspension was the dominant loss pathway (74.4%) and was primarily associated with minor flooding following a rainfall event at 8 d. Biological pathways for N export were far less important. Denitrification, during both sediment exposure and inundation, was the main biological pathway for 15N loss (2.6% total), whereas there was little loss through ammonification and nitrification (NH4+ or NOx efflux; 1.4% combined) and dissolved organic nitrogen efflux (0.5%). It was estimated that the N incorporated by MPB within one emersion period would take ~ 40 d to be entirely removed from the sediment. This study highlights the potential importance of MPB and intertidal sediments for the uptake and longer‐term storage of coastal N, and the need to better quantify the impact of episodic flooding on coastal N budgets. [ABSTRACT FROM AUTHOR]

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of this N after it has been assimilated. We used an in situ 15N pulse-chase experiment in subtidal sands to follow the assimilation, trophic transfer, transformation, and flux pathways of MPB-N over 33 d. Throughout the study MPB dominated 15N uptake, on average representing only 18.1% of the biomass but 63.9% of the 15N within 0-2 cm sediment. Following assimilation, 15N was rapidly transferred to deeper sediment, with 32.1% below 2 cm and 16.5% below 5 cm after 60 h. In contrast to MPB, bacteria represented 39.5% of sediment biomass but accounted for only up to 27.3% of assimilated 15N. Foraminifera accumulated and stored 15N more than bacteria; their contribution to the 15N remaining in 0-2 cm sediment at the end of the study was more than double their biomass contribution. Thirty-three days after the 15N was assimilated by MPB 27% remained in the sediment, 16.5% had been effluxed as [ABSTRACT FROM AUTHOR]

Microphytobenthos (MPB) at higher latitudes has been poorly studied. This study used pulse-chase 13C-labeling to investigate the production, processing, and fate of MPB-derived carbon (MPB-C) in sub-Arctic intertidal sediments over 31 d. Gross primary production (2.1 mmol C m−2 h−1 ± 0.4 mmol C m−2 h−1) was comparable to that reported for temperate regions. Some of the 13C fixed by sub-Arctic MPB was rapidly (within 0.5 d) transferred to deeper sediments (below 2 cm), but most was retained within surface sediments (>70.2% of the 13C present at any time during the study). At any time, MPB accounted for ≥ 49.8% of this 13C. The 13C content of sediment organic carbon declined over time, but > 31% of the 13C fixed within the first tidal cycle remained after 31 d, suggesting that sub-Arctic MPB may contribute to coastal carbon retention during the productive season. Over 21 d, 10.6% of the fixed 13C was removed via DIC fluxes and 0.3% via DOC fluxes from inundated sediment, and 0.6% as CO2 from exposed sediment. The greatest loss of 13C (38.2%) was via unmeasured pathways, including resuspension and/or removal by mobile consumers. The rates of MPB-C production and the relative importance of the pathways for MPB-C loss were similar to that observed for comparable lower latitude sediments, demonstrating that MPB at higher latitudes are not necessarily distinct from MPB at lower latitudes and probably play a similarly important role in ecosystem functioning. Apparently, local environmental conditions are more important than climate differences for determining the processing and fate of MPB-C. [ABSTRACT FROM AUTHOR]

Microphytobenthos (MPB) in photic sediments are highly productive but the fate of this production remains uncertain. Over 33 d, tracing of 13C from added bicarbonate in subtropical shallow subtidal sand showed rapid transfer of MPB-derived carbon to deeper sediment; below 2 cm (31% within 60 h) and 5 cm (18%). Despite their high turnover (5.5 d) and only representing ~ 8% of sediment organic carbon, MPB represented up to 35% of the 13C retained in sediments, demonstrating substantial carbon recycling. Carbon was rapidly transferred to heterotrophs, but their contribution to sediment 13C was similar to their biomass contribution (< 0.1% to 3.8%), with the exception of the foraminifera Cellanthus craticulatus, which accounted for up to 33% of the 13C within sediment. There was little loss of MPB-derived carbon via dissolved organic carbon (DOC) effluxes (3%) or resuspension (minimal). Respiration was the major loss pathway (63%), reflecting the high microbial biomass typical of lower latitudes. Given that MPB take up dissolved inorganic carbon (DIC) from overlying water, and the carbon they fix is released as DIC, MPB in subtropical sands are unlikely to substantially alter the form of carbon transported offshore (i.e., there is no conversion to DOC), but processing within the sediment may alter its d13C value. Given that 31% of fixed carbon remained in sediments after 33 d, subtropical sands may act as a carbon sink, thereby affecting the quantity of carbon transported offshore. [ABSTRACT FROM AUTHOR]

Additions of 13C-labeled macroalgal detritus (Caulerpa taxifolia) and seagrass detritus (Zostera capricorni)t oa vegetated intertidal mudflat in subtropical Australia provided insight into the mechanisms and ecosystem effects of C. taxifolia invasions in seagrass beds. Despite the high lability typical of macroalgae, carbon from seagrass detritus was removed from sediments and transferred to benthic compartments (microphytobenthos, bacteria, mud whelks, and live C. taxifolia and Z. capricorni) at a faster rate than carbon from macroalgal detritus. This preference was more pronounced for live Z. capricorni than live C. taxifolia. Whereas rates of dissolved inorganic carbon production and utilization were similar for seagrass and macroalgal detritus, higher fluxes of dissolved organic carbon (DOC) from macroalgal detritus early in the experiment reflected the lower utilization of this carbon pool. Although both seagrass and algae can use DOC as a source of carbon, caulerpenyne may comprise part of the DOC pool leached from C. taxifolia detritus during early degradation. This compound can have allelopathic effects on seagrass and may, therefore, be inaccessible to Z. capricorni. The availability of an additional carbon source for C. taxifolia, generated from its own detritus, represents a competitive advantage for this invasive macroalga over existing seagrass, particularly because seagrass productivity can be carbon-limited. A similar mechanism may exist for other species of invasive macroalga, particularly those that produce toxic secondary metabolites. [ABSTRACT FROM AUTHOR]

Benthic metabolism, nutrient fluxes, and denitrification were measured in Caulerpa taxifolia communities that had recently invaded (between 2003 and 2008) bare sediments following the loss of Zostera capricorni (prior to 2003), and in adjacent Z. capricorni communities and bare sediments, to determine the likely changes in ecosystem function following C. taxifolia colonization. Areas colonized by C. taxifolia had higher gross primary production (GPP), net primary production (NPP), and dark benthic N2 effluxes than adjacent bare sediments, but lower dark benthic Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. effluxes. C. taxifolia invasion of bare sediments had a beneficial role in removing excess nitrogen from the system through enhanced denitrification. In contrast, areas occupied by C. taxifolia had similar GPP and NPP relative to Z. capricorni beds and similar benthic N2 and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. effluxes. Thus, C. taxifolia appears to play a comparable role to Z. capricorni in terms of some ecosystem functions, although the degree of similarity is likely to be affected by the relative densities of the two macrophytes. However, replacement of Z. capricorni communities by C. taxifolia appears to significantly alter the quantity and quality (higher ratio) of benthic dissolved organic carbon and dissolved organic nitrogen fluxes. [ABSTRACT FROM AUTHOR]